forced drying of hemp lime and the effects on its hygrothermal properties

7/25/2019 Forced Drying of Hemp Lime and the Effects on its Hygrothermal Properties

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Forced Drying of Hemp Lime and the

Effects on its Hygrothermal Properties

Student: Victor Delegrego

Supervisor: Mike Lawrence

Department of Architecture and Civil Engineering

The University of Bath

2016

AR30315 – BEng dissertation



AR30315 Victor Delegrego

1

Abstract

Hemp lime is a biocomposite made from lime and hemp (Cannabis sativa) shives. The material is used

as a non-structural shuttering for its interesting thermal and acoustic insulating properties, its air tightness

and its ‘breathability’. A common problem experienced is that the hemp shives, that is the woody core of thehemp plant, absorbs very large amounts of water during the mixing process. The release of this water is going

not be immediate, such that its thermal conductivity will rise and its breathability properties will be impaired,

while already installed in the building. The drying process can take months or even years to cease.

This research tested a new fabrication processes for hemp lime and the effects it had on the

hygrothermal properties of the material. The method consists of forced drying hemp lime as soon as it is cast

into the molds. After this process, the moisture content inside the material should be low enough to allow it

to work under project specification from day one.

In order to test these hygrothermal characteristics two different hemp lime formulations were

developed. One using high surface area lime (HSA lime) and calcium sulfoaluminates (CSA), as a setting

additive. The other using the commercial Tradical® PF70 binder. The two formulations went through bothforced (rapid) and normal (slow) drying methods.

The hygrothermal parameters tested were thermal conductivity, moisture buffer value and vapour

permeability. These results showed some sensible differences between the two drying methods. However,

it was clear that those differences were not enough to hinder the use of the forced drying method and that

this method can be considered a promissory solution for the drying problem. From both formulations, the

HAS lime with CSA was the one which seemed to perform better.




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Table of contents

Abstract ................................................................................................................................................. 1 Table of contents ................................................................................................................................... 2

Table of figures ...................................................................................................................................... 3

List of tables .......................................................................................................................................... 4

1.0 - Introduction ................................................................................................................................... 5

1.1 - Hemp lime ............................................................................................................................................. 5

1.1.1 - Modern uses ....................................................................................................................................... 5

1.1.2 – Hemp plant ........................................................................................................................................ 6

1.1.3 - The drying problem ............................................................................................................................ 61.1.4 - Proposed solution............................................................................................................................... 7

1.2 - Aims and objectives ............................................................................................................................... 7

2.0 – Literature review ........................................................................................................................... 8

2.1 - Forced drying of hemp lime .................................................................................................................. 8

2.2 - Hygrothermal properties ....................................................................................................................... 8

2.2.1 - Thermal conductivity .......................................................................................................................... 8

2.2.2 - Hygric properties .............................................................................................................................. 10

2.2.2.1 - Moisture buffer capacity ............................................................................................................... 102.2.2.2 - Vapour permeability ...................................................................................................................... 12

3.0 - Materials and methods ................................................................................................................. 14

3.1 - Preparing the samples ......................................................................................................................... 14

3.1.1 – Nomenclature .................................................................................................................................. 17

3.2 - Thermal conductivity testing ............................................................................................................... 17

3.2.1 - Equipment and testing procedure ................................................................................................... 17

3.2.2 – Thermal conductivity value ............................................................................................................. 18

3.3 - Moisture buffer testing ....................................................................................................................... 19

3.3.1 – Test procedure ................................................................................................................................. 19

3.3.2 - Moisture buffer value ....................................................................................................................... 21

3.4 - Vapour permeability testing ................................................................................................................ 23

3.4.1 - Testing procedure ............................................................................................................................ 23

3.4.3 - Determination of steady state ......................................................................................................... 24

3.4.4 – Vapour permeability value .............................................................................................................. 25

4.0 – Discussion.................................................................................................................................... 28

5.0 - Conclusion .................................................................................................................................... 29

References ........................................................................................................................................... 30




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Table of figures

Figure 1 – ‘Definition of quasi-steady state (the 3 cycles inside the ellipse) and the moisture uptake and

release’ (Rode et al. 2006) ............................................................................................................................... 12

Figure 2 – Mold with a perforated timber bottom.......................................................................................... 14Figure 3 – Shives and binders inside pan mixer .............................................................................................. 15

Figure 4 – Demolding of forced dried samples ............................................................................................... 16

Figure 5 – Very fragile sample with detached layer on top ............................................................................ 16

Figure 6 – Samples being tested with the hot wire machine .......................................................................... 17

Figure 7 – Crumbled PA sample....................................................................................................................... 20

Figure 8 – Samples placed inside the climate chamber .................................................................................. 20

Figure 9 – The cutting of a sample’s bottom face ........................................................................................... 23

Figure 10 – Wax sealing of the sample for the dry cup test ............................................................................ 24




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List of tables

Table I – Hemp lime formulations ................................................................................................................... 15

Table II – Results from the firs testing day, with TC in W/(m*K) .................................................................... 18

Table III - results from the second testing day, with TC in W/(m*K)............................................................... 18Table IV – results from the third testing day, with TC in W/(m*K) ................................................................. 19

Table V – MBV of samples and the moisture uptake variation of the three last cycles.................................. 21

Table VI – MBV of each type of hemp lime and statistical dispersion of samples’ MBVs ............................... 22

Table VII – Moisture buffer value (MBV) class ranges..................................................................................... 22

Table VIII – Variation from G of the last three moisture uptake measurements ............................................ 25

Table IX - Vapour permeability (δ) of each sample ......................................................................................... 26

Table X – Vapour permeability (δ), vapour diffusion resistance factor (μ) and the differences between

drying methods................................................................................................................................................ 26




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1.0 - Introduction

This dissertation aimed to investigate a forced drying process for manufacturing hemp lime and the

effect it has on the material’s hygrothermal properties. The prospects of using this method as a solution for

hemp lime’s drying problem were also discussed.

To investigate these effects, two different hemp lime formulations were prepared and subjected to

both forced and normal drying processes. Their hygrothermal characteristics were then assessed by a

number of tests and the results were compared between themselves and the ones found in the literature.

1.1 - Hemp lime

The growing concern with the depletion of natural resources and release of CO2 by the modern

construction industry, responsible for one third of the world’s energy consumption (Rahim 2015), is one of

the main motivations behind a return to more sustainable and natural building practises. Walking together

with this growing interest, research and development on natural building materials and components is

growing, with the intention of finding new, environmental friendly solutions for engineering (Benfratello et

al. 2013). It is in within this context that hemp lime, a relatively new natural building material with interesting

properties, is taking its share in the market (Shea et al. 2012) (Rahim et al. 2015).

Hemp lime, also known as hempcrete, lime hemp, hemp concrete and green concrete (Bruijn,

Johansson 2014), is a form of biocomposite concrete. Its basic formulation consists of a mixture of lime and

hemp shiv, the first being responsible for providing the mineral matrix that binds the aggregates, while the

second for providing natural fibre aggregates to the mixture (Benfratello et al. 2013). Shiv is the name given

to the woody core of the hemp plant. More information about hemp shives is described later in the text.

Lime is the most basic and standard choice of binder for hemp lime, but it can also be combined with

other materials. Because of that, some commercial brands offer binders designed specifically for hemp lime.

Those are usually a combination of lime with other components such as cement and pozzolans, which serve

to increase the hydraulic characteristics of the material. This hydraulicity is responsible for providing some

initial setting strength to the mixture, due to the quick hydration forming components, as opposed to the

following slow strength increase associated to carbonation (Sutton et al. 2011), (Hirst et al. 2010), (Shea et

al. 2012).

The use of hemp lime developed in France by the end of the 1980’s and beginning of the 1990’s for

conservation purposes. In the French region of Champagne there was a large number of historical timber

framed buildings infilled originally with a wattle and daub system, being at that time in need of repair. The

poor choice of materials in earlier restorations had only further damaged the buildings. Hemp lime was thenbrought as a possible solution and it was tested there for its first time (Hirst et al. 2010).

Hemp lime buildings are nowadays present in large numbers in France and the UK, the former having

more than 200 building by 2007 (Hirst et al. 2010). Research on the material is also carried out by other

countries, located both in Europe and North America (Colinart et al. 2012).

1.1.1 - Modern uses

Hemp lime is used as a non-structural insulation material for roofs, wall and floors (Latif et al. 2015).It possesses excellent hygric capacities, good thermal properties, good airtightness and causes a low impact

on the environment (Bruijn, Johansson 2013). It is advertised as having the ability to regulate internal air




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humidity, or as it is technically named, for having an excellent moisture buffer capacity. This property can

even be seen as the main advantage of hemp lime over other insulation materials (Bart et al. 2014).

There are currently three major techniques for building with hemp lime:

Casting: hemp lime is cast between a temporary shuttering, which can be removed from 12 to 24

hours later. The wall is usually 300 mm thick and can be plastered after the shuttering is removed.This procedure is usually done for smaller projects, where a less expensive labour force can be used

( Ahlberg et al. 2014) (Hemcrete nd).

Spraying: hemp lime is sprayed over a temporary or permanent single sided shuttering. After

spraying, the surface can be flattened and the shuttering, if temporary, can be removed 24 hours

later. The spraying option is more suited for larger projects (Hemcrete nd).

Pre-fabrication: the production of blocks or panels containing hemp lime in an industrial context

(Shea et al. 2012). An important problem in the logistics of pre-fabricating hemp lime is the drying

process, discussed later on this text.

1.1.2 – Hemp plant

The hemp plant is an industrial variety of Cannabis sativa. It belongs to the same species of

marijuana, with both plants being visually very similar. The main difference between those plants is the

content of tetrahydrocannabinol (THC). This component is responsible for marijuana’s psychoactive effects

and it is found in high concentrations inside that plant. Hemp on the other hand, is a crop designed for

commercial purposes and contains by law a very low THC concentration, bellow 0.2%. Under this limit, it is

impossible for the plant to give the ‘high’ effect, associated with marijuana. The cultivation of hemp is

therefore approved by the EU ( Ahlberg et al. 2014).

Hemp crops are suited to various climates, with the potential of being cultivated both in Southern

and Northern Europe. It reaches 3 to 4 metres height within 4 months, about the time at which it is harvested(Shea et al. 2012). It does not require pesticides (Sutton et al. 2011) and due to its rapid growth, its leaves

block the sun light from reaching the ground, acting as a natural weed suppressor ( Ahlberg et al. 2014).

To be commercially utilized, hemp goes under a mechanical separation process. The ‘plants are cut

and dried in the sun for two weeks; then they are swingled for separating the bast (that is fibres located in

the outer stalk) from shiv, that is the wooden inner part’ (Benfratello et al. 2013). The fibres are the most

expensive part of the plant and have multiple uses, such as paper manufacture and biocomposites production

for the car industry. (Bruijn, Johansson 2014). For hemp lime, only the shiv is interesting. Shiv composes 40

to 60% of the hemp stalk and is usually marketed as animal bedding (Bruijn, Johansson 2014).

The high porosity of the shiv is of major importance to the hygrothermal behaviour of hemp lime. Some

of the shiv’s important characteristics are:

Very high water absorption, up to 406% its own weigh after 48 hours under submersion (Nguyen et

al. 2010);

A low bulk density of about 103 kg/m3 (Nguyen et al. 2010);

Low thermal conductivity (Benfratello et al. 2013);

Good acoustic insulation properties (Benfratello et al. 2013);

1.1.3 - The drying problem

As explained before, the hemp shives absorb multiple times its own weight in water while

submerged. Research conducted by Nozahic found a weight gain of around 300% in only 5 minutes. Walker




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obtained a lower but also very high value of 225% (Walker, Pavía 2012). A significant problem is the

occurrence of this process during the hemp lime mixing. As the shives fill themselves with the mixing water,

the binder requires more to be added in order to have enough water to react. After the end of this mixing

process, hemp lime carries a considerable amount of unreacted water inside the shives, which will be slowly

released to the external environment during the following months or years (Colinart et al. 2012).

This water absorption is the root cause of a problem experienced in hemp lime construction: the

newly constructed buildings do not achieve project specifications in terms of moisture buffering and thermal

insulation. As research has shown, a rise in hemp lime’s moisture content results in a significant,

approximately linear increase in thermal conductivity of this material (Bruijn, Johansson 2013) (Korjenic et

al. 2011). At the same time, the humidity regulation properties attributed to hemp lime will not exist for

many months after construction, because the saturated shives are only able to release moisture to the

environment. Although this is a significant problem for the hemp lime industry, the current building practice

is to just let the walls dry naturally.

1.1.4 - Proposed solution

In a context of production of insulating panels containing hemp lime, a possible method was

proposed in order to deal with the drying problem. This method consists of submitting hemp lime to a forced

drying process after its casting. The idea is that after being forced dried, the hemp shives will not hold excess

water anymore and the material would be ready for use, fulfilling design specifications from day one.

Although simple, this method cannot be applied without paying attention to the materials used. That

is because the strength gain of pure hydraulic lime comes from the product of the carbonation reaction. In

this reaction the Ca(OH)2 molecules of lime react with the CO2 found in air, producing CaCO3. The problem is

that this reaction is slow. With the few hours it takes to force dry hemp lime, the reaction does not have

sufficient time to produce any sensible increase in strength, thus making hemp lime unable to support itselfstructurally. To counteract this problem and provide the initial mechanical strength needed, a proposed

solution was the addition of small amounts of calcium sulfoaluminates (CSA) to the mixture.

CSA is a low cost and eco-friendly binder with an early mechanical strength onset. Its properties come

mainly from ettringite (C6AS3H32), produced from the hydration of C4A3S. It has been used for decades as a

setting accelerator additive for lime and other binders (Telesca et al. 2014).

1.2 - Aims and objectives

Research conducted in the University of Bath showed that by adding CSA to the mixture, the resulting

hemp lime product coming from the forced dried process was acceptable from a self-supporting structural

perspective. However, as hemp lime is an insulation material, it is still necessary to know if this process has

any influences on the hygrothermal behaviour of the material and if yes, what those are. It was with the

intention of discovering these influences that the present research was conducted.

To cover the hygrothermal properties of the material, the parameters investigated and the related

methods used were respectively: thermal conductivity with the hot wire method; moisture buffer value with

the Nordtest protocol; vapour permeability with the dry cup method.

It was decided to use two binder formulations to have a better understanding on the effects of forced

drying. The first formulation used a high surface area (HSA) lime with CSA addition as binders. The secondused formulation used the commercial PF70 Tradical® binder. The hemp shives used were the same for all

cases. Both forced and normal dried samples of these compositions were tested.




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2.0 – Literature review

This section describes concepts and study findings that are important to the understanding of the

experiments and results contained in this text.

2.1 - Forced drying of hemp lime

There is very little research on forced dried hemp lime and none could be found that investigated its

hygrothermal properties. Thus, the papers used as basis for this research dealt only with normally dried hemp

lime.

2.2 - Hygrothermal properties

‘Insulation is a seemingly simple product but its actual performance within a building and within the

environment is far from straightforward’ (Tucker et al. 2014). To model the performance of a building it is

then necessary to have some specific experimental parameters, relating to the hygrothermal characteristic

of the material. Some of those parameters were investigated and used as means of comparison between the

different drying processes and formulations examined. The explanation and the meaning of each of those

experiments along with their importance to the research is explained in this section.

2.2.1 - Thermal conductivity

Thermal conductivity, represented by the letters k or λ, is a number that indicates the effectiveness

of a material to conduct heat ( Ahlberg et al. 2014). In technical terms, it is the ‘rate of steady-state heat flow

(W) through a unit area of 1 m thick homogeneous material in a direction perpendicular to isothermal planes,

driven by unit (1 K) temperature difference across the material sample ’ (Shea et al 2013). This heat flow

always occurs in the direction of higher to lower temperatures, because ‘energy is transferred when

neighbouring molecules collide and higher temperature equates to a higher molecular energy, or more

molecular movement’ ( Ahlberg et al. 2014). The lower the thermal conductivity of the material, the better

are its insulation properties and thus, more desirable it is ( Ahlberg et al. 2014).

From thermal conductivity it is possible to obtain the U-value, another important parameter for thebuilding sector. The U-value can be defined as the ‘reciprocal of the sum of all thermal resistances of the

layers of the building element including resistance due to a film of air at the inner and outer surfaces’ (Shea

et al 2013), with thermal resistance being the thickness of the material divided by its thermal conductivity.

The U-value serves as a comparison parameter for building elements and takes away the need to consider

the individual properties of each material ( Ahlberg et al. 2014). It is also used as the ‘basis of many regulatory

frameworks aimed at conserving the use of fuel and power in buildings and are a fundamental part of EU

members’ methodologies for demonstrating compliance with the Energy Performance of Buildings Directive’

(Shea et al 2013).

The standard values of thermal conductivity of hemp lime are observed over a wide range. Literature

shows that these values can stay within 0.04 and 0.19, but are usually found between 0.07 and 0.09 W/mK

(Benfratello et al. 2013) (Sutton et al. 2011). The main reason for this variation is the density of the material.

By increasing the hemp to lime ratio there is a weight reduction (due to the lower density of hemp) and a




9

following decrease of the material’s thermal conductivity, due to the large superiority of hemp as an

insulating material, in an almost linear tendency (Walker, Pavía 2014) (Benfratello et al. 2013). Conversely,

when the binder to hemp ratio is increased there is a rise in thermal conductivity (Walker, Pavía 2014). In the

UK hemp lime compositions with a density lower than 300 kg/m3 have been favoured as a way of reducing

wall thickness while complying with the country’s building regulations (Barclay et al. 2014). Other parameter

which research shows to influence thermal conductivity to a lesser extent is the type of binder, with thosehaving higher hydraulicity tending to present lower thermal conductivities (Walker, Pavía 2014).

By its thermal conductivity alone, hemp lime can be considered a good material for insulation

purposes. However, recent research has shown that this parameter does not tell everything about hemp

lime’s insulation properties. This over simplification is unrepresentative of hemp lime and underestimates

the advantages of its use (Kinnane et al. 2015) (Shea et al. 2012).

Hemp lime is a material with high thermal inertia, a characteristic connected to both thermal

conductivity and volumetric heat capacity (the capacity of a material’s given volume to store heat under a

defined temperature variation). A high thermal inertia means that the modelling of a material’s thermal

properties in a steady state condition is going to give inaccurate results and that the dynamic effects are most

important to be analysed (Kinnane et al. 2015). It is also advantageous to asses a material’s insulatingproperties in a more similar situation to the external environment of a building, with its constantly varying

conditions, rarely achieving a prolonged steady state behaviour (Kinnane et al. 2015).

Research on the dynamic properties of hemp lime are being carried out by different researchers and

different parameters such as the Q24h and ts-s were found to better describe the behaviour of the material

(Shea et al. 2012).

With the aforementioned advantages of testing for the dynamic thermal properties, it may seem

reasonable to adopt them while researching hemp lime. However, most available researches on this topic

are quite recent and more would be necessary in order to stablish reference values. Thus, most studies,

including the present one, still test for the thermal conductivity value (or U-value), as those parameters are

very good for comparison purposes and are widely used by the construction industry (Walker, Pavía 2014).

Thermal conductivity can be measured by different methods and a variety of those were used in

hemp lime research. It is important to take it in consideration, as it was shown that the method employed

may have a sensible effect on the results, especially when dealing with an anisotropic, non-homogenous

material such as hemp lime (Latif et al. 2011).

For this research it was decided to use the transient hot-wire method for determining thermal

conductivity. This method consists of embedding a linear heat source, the hot wire, inside the material to be

analysed. By knowing the heat transmitted by the wire and the temperature change in a time interval, it is

possible to obtain the thermal conductivity value. ‘The mathematical model of hot wire method is based on

the assumption that hot wire is a continuous line source and by providing constant heating power through

thermal impulses it generates cylindrical coaxial isotherms in an infinite homogenous medium with initialequilibrium condition’ (Latif et al. 2011).

The hot wire method works well for materials with low thermal conductivity and is able to provide

quick and accurate results for small samples (when compared to other methods). However, a unidirectional

analysis may not be satisfactory for hemp lime (Latif et al. 2011). When hemp lime is being cast and

compacted into its shuttering or molds, it is possible for the shives to get organized in a preferential way,

perpendicular to the direction of compaction. This in turn, generates a tendency for a stratification of the

material, with intercalated layers of hemp shives and binder. As the binder matrix is the most conductive

component of the material, the measured value of thermal conductivity will tend to be lower along the

compaction direction when compared to the one perpendicular to it (Nguyen et al. 2010). It is thus necessary

to measure thermal conductivity along both directions and consider them separately.




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2.2.2 - Hygric properties

As discussed before, the thermal insulating properties of hemp lime, along with the acoustic

insulation and air tightness it provides are considered good in comparison to other, more conventional

insulating materials. However, what makes it stand out is its interaction with indoor air humidity (Bart et al.

2014).

Due to hemp’s pore structure, hemp lime walls have the ability to regulate humidity in the air. When

indoor air has a decrease in its relative humidity, a vapour pressure difference between the material and the

air is generated. Then, the moisture trapped inside the hemp shives moves through hemp lime and is released

to the air. Conversely, when humidity increases in the air, the vapour pressure difference makes the hemp

fibres absorb and retain this extra moisture. Both desorption and adsorption phenomena have the effect of

damping the air’s relative humidity variation. This humidity regulation is called ‘moisture buffer effect’ and

is seen in all vapour permeable materials containing natural fibres, but it is significantly accentuated in hemp

lime (May 2005).

Due to the aforementioned effect, hemp lime is frequently categorised as a ‘breathable’ material.Behind this term are two different properties. The first one is named vapour permeability and refers to the

ability of the material to let moisture in form of water vapour pass through. The second one, called

hygroscopicity, is the total amount of this transported water that the material’s pore structure is able to store

(May 2005).

The moisture buffer effect of hemp lime is a non-negligible parameter in the hygrothermal modelling

of a building. Consequently, ignoring this hygric behaviour might result in ‘an incorrect prediction of direct

and indirect energy demands for heating/cooling because of latent heat effects, comfort condition

modifications, and heat transport parameter dependence on moisture content’ (Dubois et al. 2014).

2.2.2.1 - Moisture buffer capacity

In order to compare the effectiveness of materials to buffer variations in air humidity, the parameter

used is the Moisture Buffer Value (MBV), with higher values representing better buffering capacity and being

thus more desirable (Dubois et al. 2014).

Many methods for obtaining the MBV are used. ‘Among those, Nordtest method is the pioneering

method and is mostly used in the European context’ (Latif et al. 2015). It was therefore the one chosen for

this research. The Nordtest protocol defines three values related to moisture buffer capacity. Those are

described below, as seen in (Latif et al. 2015):

- Moisture effusivity (bm):

It is the measurement of the ability of the material to exchange moisture with its surroundings when

the surface of the material is exposed to sudden change in humidity. The equation for moisture effusivity is:

= ∗∗ (1)

Where:

- bm = moisture effusivity [kg/(m2.Pa.s1/2)];




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- dp = water vapour permeability [kg/(m.s.Pa)];

- 0 = dry density of the material (Kg/m3);

- u = moisture content (kg/kg);

- = relative humidity (e);

- Ps = saturation vapour pressure (Pa).

- Ideal moisture buffer value (MBVideal):

Ideal Moisture Buffer Value is the theoretical determination of moisture buffer value (MBV) based

on its moisture effusivity, time period of moisture uptake and saturation vapour pressure. Ideal Moisture

Buffer Value expresses the upper limit of the moisture buffer capacity. The equation for Ideal Moisture Buffer

Value is:

≈

∆ = 0.00568 ∗ ∗ ∗ √ (2)

Where: G(t) = accumulated moisture uptake (kg/m2) and the corresponding moisture release during a time

period tp (s). The ideal moisture buffer value is measured in [g/(m2.% RH)].

- Practical moisture buffer value (MBV practical):

Practical moisture buffer value, MBV practical, is defined as the amount of moisture content that

passes through the unit open surface of the material when the material is exposed to variation in relative

humidity of the surrounding air. MBV can be expressed as:

Equation (3):

= ∆∗(ℎℎ−) (3)

The practical moisture buffer value is, as suggested by its name, obtained by practical experiments

in a laboratory. The ideal and the practical values are only comparable when the material tested is

homogeneous (Rode et al. 2006). Because of that, only the practical value is of use for hemp lime.

To test the samples according to Nordtest specifications, the samples are placed in climate chamberwith only one vapour open face. The samples are then subjected to continuous high and low humidity cycles

until the moisture uptake after each moisture absorption cycle stabilises in a quasi-steady state. Another

characteristic of the stabilization process is that the moisture uptake and the moisture release of each cycle

have similar values (Rode et al. 2006). Figure 1, obtained from Rode et al. (2006) shows a moisture

uptake/release graph depending on the RH tested, which reached the 3 stable cycles, necessary to

characterise a quasi-steady state according to the Nordtest protocol.




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Figure 1 – ‘Definition of quasi -steady state (the 3 cycles inside the ellipse) and the moisture uptake and

release’ (Rode et al. 2006)

A proposed class division is also offered by Nordtest. Depending on the MBV of the material, it can

be divided into: ‘Negligible (MBV: 0.0-0.2), Limited (MBV: 0.2-0.5), Moderate (MBV: 0.5-1.0), Good (MBV:

1.0-2.0), Excellent (MBV: 2.0-upwards)’ (Latif et al. 2015). It is usual for hemp lime to be categorised as

‘excellent’.

2.2.2.2 - Vapour permeability

Vapour permeability is a value used as parameter to define the velocity at which water vapour can

travel through a material subject to a vapour pressure difference. It can be used as an input value for the

hygrothermal modelling of buildings. It is also associated with the moisture buffer capacity of a material: as

water goes through a medium more efficiently there is a tendency for the material to be able to store more

of it. The concepts relevant to the understanding of vapour permeability are, as explained in BS EN 12086-

2013:

- Water vapour transmission rate (g):

Quantity of water vapour transmitted through unit area in unit time under specified conditions of

temperature, humidity and thickness.

- Water vapour permeance (W):

Quotient of the water vapour transmission rate of the test specimen and the water vapour pressure

difference between the two specimen faces during the test.

- Water vapour permeability (δ):

In hand of the former parameters, it can be obtained by the product of the permeance and the

thickness of the test specimen. The water vapour permeability of a homogeneous product is a property of




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the material, as opposed to water vapour permeance, which us a property of the specimen. It is the quantity

of water vapour transmitted per unit of time through a unit area of the product per unit of vapour pressure

difference between its faces for a unit thickness.

- Water vapour diffusion resistance factor (μ):

Quotient of the water vapour permeability of air and the water vapour permeability of the material

or the homogeneous product concerned; it indicates the relative magnitude of the water vapour diffusion

resistance of the product and that of an equally thick layer of stationary air at the same temperature. In

calculating it both the vapour permeability of the test specimens and the air temperature in which those

were tested are considered. It is then possible to obtain a value that is comparable to others obtained in

different testing conditions.

- Water vapour diffusion equivalent air layer thickness (sd):

Thickness of a motionless air layer which has the same water vapour resistance as the test specimen

with the thickness d .

In this research the ‘dry cup’ method was adopted. It was used by various other authors researching

hemp lime (Latif et al. 2015)(Collet et al. 2013)(Mazhoud et al. 2016) and provides thus a good way of

comparing results. The method consists of creating a vapour pressure difference between two faces of the

samples while isolating the others. One face is maintained with 50% relative humidity while the other with

zero relative humidity, by means of a desiccant material.

Hemp lime is usually considered a material with very good vapour permeability properties when

compared to other more traditional building materials (Walker, Pavía 2014) (Bruijn, Johansson 2014). ‘Thecommon industry figure of water vapour diffusion resistance factor (μ) of lime –hemp concrete is 4.85 ± 0.24’

(Walker, Pavía 2014). Nonetheless, this value is significantly influenced by the binder aggregate ratio and the

hydraulicity of the binder, both parameters raise the resistance factor when increased (Walker, Pavía 2014).




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3.0 - Materials and methods

3.1 - Preparing the samples

The samples were fabricated by using rectangular impermeable wooden molds, being divided in two

categories according to its sizes. The first category corresponded to twelve samples measuring 10x10x10 cm,

used for the moisture buffer and vapour permeability tests. The second category was that of the samples

which measured 15x15x15 cm, being sixteen of these produced and used to assess thermal conductivity. For

the samples that passed through the forced drying process, the only difference in the molds was at their

base, made with perforated timber as shows Figure 2.

Figure 2 – Mold with a perforated timber bottom

The preparing of samples happened in two phases. The first phase dealt only with the ones subjected

to the normal (slow) drying process, while the second one only with the forced (rapid) dried samples. The

reason for this division was a logistic one: the time and equipment available demanded all the

experimentation to happen simultaneously. As it was also necessary to give time for the normal dried

samples to set for an adequate time, it was decided to offset the making of the forced dried ones such that

all samples could be tested together.

The mixing process followed was the same for both phases. The hemp shives, binders and water hadtheir weight individually measured with a scale. The shives and binders were put into a large electrical pan

mixer as shown in Figure 3. This machine was then turned on and the water was gradually added. The process

continued until the distribution was homogeneous. Right after the mixing was over, the material was filled

into molds. The molding was done by gently dispersing layers of hemp lime into the molds, taking care to

leave no voids or over compress the material.




15

Figure 3 – Shives and binders inside pan mixer

Two hemp lime formulations were produced, with the proportions and materials presented in Table

I. The first composition used high surface area lime (HSA lime) and CSA as binders, while the second one used

the wide spread commercial brand PF70 from Tradical® (Dubois et al. 2014). According to Nguyen et al.

(2010), the PF70 binder ‘consists of 75% of hydrated lime Ca(OH)2, 15% of hydraulic lime and 10% pozzolana’.

The materials’ dry densities were measured only by the end of the experiments and it is thus not

possible to know what was the dry weight gain due to carbonation. Nonetheless, the measurements

indicated 182.3 kg/m3 and 200.0 kg/m3 for formulations 1 and 2 respectively, independently of the drying

method. Those densities can be considered low values for hemp lime.

Table I – Hemp lime formulations

Formulation 1 (kg) Formulation 2 (kg)

Hemp HSA Lime CSA Water Hemp PF70 Water

1 1.11 0.088 2.356 1 0.9 2.15

After their casting, the normal dried samples were transferred to a controlled environment of 20°C,

60% relative humidity (RH) and were left there for a week. After that, the molds were removed and later

returned to the controlled environment, where they were left to cure for a total of 36 days.

The samples to be forced dried were conducted to the air drying rig after being cast. The drying

equipment consisted of a table made with perforated timber board, covering an air suction machinery. The

molds, having a perforated timber bottom, were correctly placed above the machine, making sure that the

holes matched, and any air leakage between the molds was sealed with tape.

After placing all the samples, the drying rig was turned on, sucking the air through the hemp lime

into the machine, a process that went on for 16 hours. After that, the samples were demolded as presented

in Figure 4 and were ready to be tested.




16

Figure 4 – Demolding of forced dried samples

While demolding the samples, it was already possible to see the difference between a rapid and a

slow dried sample. The slow dried samples had a good mechanical integrity, for both HSA lime and PF70

formulations, while the rapid dried samples had all inferior strength. Also noticeable was that the rapid dried

HSA lime samples were weaker, but still consistent enough for being handled. However, the forced dried

PF70 samples were so fragile and weak that even a greater pressure with the fingers was enough to break it.

Figure 5 shows a sample that was breaking in layers after very little tensions applied during the demolding

process.

Figure 5 – Very fragile sample with detached layer on top




17

3.1.1 – Nomenclature

For simplification purposes, in the next sections of the text, the samples are referred to by a letter

representing its formulation, followed by another related to its drying method. The samples made with

formulation 1, according to Table I, are referred to by the capital letter ‘H’, while those made from

formulation 2 receive the letter ‘P’. The samples dried naturally receive the letter ‘N’, while those dried with

the air drying rig are identified by the letter ‘A’.

3.2 - Thermal conductivity testing

3.2.1 - Equipment and testing procedure

A total of 16 samples were tested for thermal conductivity with the hot wire machine. The hot wire

method does not require any previous preparation of the samples. The test protocol involved stacking two

samples of the same formulation and drying method in the same direction (either vertical or lateral). The hot

wire was inserted between the samples and a weight was put above the sample on top, to increase

airtightness between the interface of the wire and facilitate the experiment. This procedure is shown in

Figure 6.

Figure 6 – Samples being tested with the hot wire machine

The machine was turned on and the equipment kept measuring the temperature until a stable,

steady-state temperature was found. Then, the hot wire became a heat source and for 120 seconds the

temperature arose in the wire/samples interface. The final temperature was measured and with it the

machine calculated and showed the thermal conductivity value for each test.

Multiple tests were performed for each pair of samples, with the number of tests done depending

on the precision of the results: If the values came consistent at first, two or three tests were performed, if

not up to five measurements were taken.




18

The testing was also divided in three parts. The first one happened right after the drying process had

finished, the second part happened 6 weeks after the first and the third part 7 weeks after the first testing

day. On the first day, right after the forced drying process had finished all samples were tested. On the second

day only the forced dried samples were tested and on the third only the slow dried samples.

All tests ran successfully, but the PA samples were very difficult to be handled because of their

fragility. Two of these PA samples crumbled after the first day of testing, being unusable for the second day.

3.2.2 – Thermal conductivity value

The results obtained at the first testing day are displayed bellow in Table II. When compared to the

literature, all thermal conductivity values are considered low. Due to the samples’ light density, low thermal

conductivity values were expected. The moisture content (MCave) is an approximation, using the weight

measured that day with the final dry density of the samples.

Table II – Results from the firs testing day, with TC in W/(m*K)

Samples MCave vertical TC1 lateral TC1 Δ directions

HA 37% 0.0562 0.0595 6%

PA 34% 0.0544 0.0524 4%

HN 26% 0.0759 0.0730 4%

PN 23% 0.0742 0.0683 8%

It can be seen in Table II that the measured thermal conductivity of the forced dried samples was

consistently lower, thus more desirable, than that of the normal dried ones. This happened independently of

the formulation and direction studied and is the opposite of what would be expected from analysing the

moisture contents alone, as an increase of moisture also increases thermal conductivity.

The difference between TC values measured in different directions of a same type of HL ranges from

4 to 8%, which is much lower than the average difference of up to 50% suggested by Nguyen et al. (2010).

The thermal conductivity values in one direction can also be higher or lower than those in the other, as shown

in Table II. It means that during the casting process, the formation of interchanged layers of hemp and lime

did not happen or at least those were not strong enough to make a significant difference on the results.

When comparing the formulations, with both drying methods the P samples had a superior performance.

For the second testing day, the results are exhibited in Table III. The table shows the thermal

conductivity (TC) and the difference between measurements taken in different directions (Δ directions) along

with variations that occurred from the first to this second test day, in each direction (Δ1-2 vertical, Δ1-2 lateral).

Table III - results from the second testing day, with TC in W/(m*K)

Samples vertical TC2 lateral TC2 Δ directions Δ1-2 vertical Δ1-2 lateral

HA 0.0592 0.0623 5% 5% 5%

PA 0.0580 0.0564 3% 7% 7%




19

Both material formulations, in either vertical or horizontal directions, showed an increase of TC

during the time gap between the first two tests. However, even with this increase in thermal conductivity,

the values are still significantly lower than those of the slow dried samples from test day one. The moisture

content of these materials was not measured in this day.

The results from the third day, 7 weeks later than the first one, are show bellow in Table IV. The

values from columns Δ1-3 vertical/lateral show the difference between the first and third test days for the N

samples. The last two columns, Δ2-3 vertical/lateral, make a comparison between these samples and the A

ones tested a week before.

It was tried to obtain the moisture content and new density of the samples, so that it could be

compared with values from the first experiments. However, the samples were already very wearied because

of the excessive handling of the tests, with a good part of material from the edges being lost. Thus, the weight

measurements done were not representative for samples of that size and did not provide coherent results.

Other methods could have been employed to obtain the correct volume of the samples, however the time

did not allow for those to be used.

Table IV – results from the third testing day, with TC in W/(m*K)

Samples vertical TC3 lateral TC3 Δ1-3 vertical Δ1-3 lateral Δ2-3 vertical Δ2-3 lateral

HN 0.0608 0.0577 20% 21% 3% 7%

PN 0.0590 0.0597 21% 13% 2% 6%

It is noticeable that the TC values consistently dropped within the elapsed time from test day 1 to 3,in figures from 13 to 21%. Also, there is a remarkable proximity between these presented values and the

ones obtained a week earlier in the second testing day. It then becomes clear by the information in Table IV

that, when left in the same controlled environment for a sufficient period of time, the differences on TC

between the rapid and forced dried samples greatly diminish. However, it is not possible to assure from these

tests that this conditions would be maintained, as it would be necessary to test the samples for longer periods

of time.

3.3 - Moisture buffer testing

3.3.1 – Test procedure

For the moisture buffer experiments the smaller samples, measuring 10x10x10 cm, were used. First

of all, the samples were prepared for the test. The top part of the samples (according to their position in the

molds) were left uncovered, while all the other faces were tightly sealed with metal tape. No part of the top

face was covered with tape, therefore its dimensions were still the same as the sample’s. The intention of

this procedure was to restrict the moisture exchange between sample and exterior environment to the top

face, with its known defined area.

Figure 7 shows a sample that was lost for crumbling during the sealing process. This was due to thevery low mechanical strength of the forced dried P formulation. In this way, only 11 samples could be tested,




20

being those three HN, three HA, three PN and two PA. After the sealing was done, the samples were labelled

and put into a climate chamber as shown in Figure 8.

Figure 7 – Crumbled PA sample

The Nordtest procedure was followed as described by Rode (Rode, Grau 2008). The climate chamber

machine has the purpose of creating a controlled environment where the air temperature and relative

humidity can be manipulated. As described in the ‘2.2.2.1 - Moisture buffer capacity’ section, the samples

were submitted to periodic moisture sorption and desorption cycles. The sorption cycles lasted for 8 hours,going from 9:00 AM to 5:00 PM, and had the climate chamber set at 23 °C and 33% relative humidity (RH).

The desorption cycles went on for 16 hours and had the climate chamber working at 23 °C and 75% RH. ‘The

reason for the asymmetry in this time scheme is twofold: (1) It replicates the daily cycle seen in many rooms,

e.g. offices or bedrooms, where the load comes in approximately 8 hours, and (2) for practical reasons during

testing if the climatic chamber conditions are changed manually, it is a scheme which is easier to keep than

a 12 h + 12 h shift’ (Rode et al. 2006). After the end of each cycle the samples were briefly moved from the

chamber to have their weights measured by an accurate scale, with precision to 1/100 of a gram.

Figure 8 – Samples placed inside the climate chamber




21

The experiment can end when the weight measurements indicate that the difference of weight

amplitude between the 3 last 24 hour cycles is less than 5%. When this happens, it is said that the samples

achieved a quasi-steady state of moisture exchange. Another result of this state is that the moisture uptake

and moisture release weight have their values closer than in previous cycles. The experiment went on for 15

days, but none of the samples was able to achieve a steady state condition and due to time constraints the

experiment had to be stopped. The moisture uptake variation between the last three cycles is shown for

each of the samples in Table V, with the number after the adopted nomenclature indicating the individual

sample to which it refers.

An explanation for this instability cannot be given with certainty, however it may be related to the

fact that the samples were not always placed in the same position, after they were taken away from the

chamber to have their weights measured. It is possible that there were temperature and humidity variations

inside the chamber, such that an inconsistent displacement of samples would cause them to not reach quasi-

steady state conditions. Only the last two 24 hour cycles were the exception to this procedure, as care was

taken to place the samples in their previous positions.

Table V – MBV of samples and the moisture uptake variation of the three last cycles

Samples Cycle 1-2 Cycle 2-3 Cycle 1-3

MBV

[g/(m2.%RH)]

HA1 13% 5% 8% 1.81

HA2 10% 3% 8% 1.81

HA3 17% 23% 36% 5.06

HN1 6% 1% 7% 2.32

HN2 13% 8% 6% 2.15

HN3 8% 7% 2% 2.44

PA1 18% 6% 25% 1.27

PA2 43% 9% 49% 1.95

PN1 4% 13% 10% 2.64

PN2 13% 5% 19% 1.83

PN3 8% 1% 9% 2.13

Even though the test results did not comply with the Nordtest requirements of a quasi-steady state,

a tendency for stabilization can be seen in Table V, slightly more accentuated in the ‘Cycle 2-3’ column of the

table. According to the protocol, the moisture buffer value of the material should be calculated using theaverage of moisture uptake from the last 3 cycles. However, due to the greater tendency of stability seen in

the last two cycles, it was decided to use only those to compose the moisture buffer values.

3.3.2 - Moisture buffer value

In hands of the average moisture uptake of the last two cycles, it was possible to obtain the moisture

buffer values for each of the samples, shown in the last column of Table V. Those values were obtained by

dividing the average moisture uptake by the area of the samples (0.01 m 2) and by the difference between

the RH of the absorption and desorption cycles (42) (Rode et al. 2006). The final MBV is shown below in TableVI.




22

Table VI – MBV of each type of hemp lime and statistical dispersion of samples’ MBVs

MBV [g/(m2.%RH)] SD CV

HA 2.89 1.88 0.70

HN 2.31 0.14 0.06

PA 1.61 0.48 0.26

PN 2.20 0.41 0.18

Before analysing the moisture buffer value results, it is necessary to check the dispersion of the

experimental data. The HA samples were the ones which presented the highest dispersion, as seen by their

coefficient of variation (the ratio between the average of the samples and their SD) presented in Table VI,

which is close to one and can thus be considered a high value. The PA and PN samples presented intermediate

variations, while the HN had a very low dispersion. Clearly, it is necessary to consider the dispersion seenwithin a set of individual samples’ values, before analysing the moisture buffer value obtained from them at

face value.

On the MBV itself, the results present values that are normal but slightly inferior to what is expected

from hemp lime (Latif et al. 2015) (Rahim 2016). By using the characterization method outlined by Rode

(2006) and reproduced in Table VII, it was possible to define the PA type as good, while the HA, HN and PN

types as excellent. It is important to mention that the air velocity has an influence on the moisture buffer

value of any material, with greater velocities leading to a better MBV (May 2005). As the speed inside the

chamber was measured with an anemometer and the result turned out as zero, it is possible to assume that,

if the anemometer’s result was correct, the moisture buffer values would all be higher if tested in different

conditions. Thus, the values obtained can be compared within themselves, but they may possibly not be

representative in order to be comparable with values from the literature.

Table VII – Moisture buffer value (MBV) class ranges

MBV

class

lower

value

upper

value

Negligible 0.0 0.2

Limited 0.2 0.5

Moderate 0.5 1.0

Good 1.0 2.0Excellent 2.0 -

The HA moisture buffer value was greater than the HN one, while the PA value was inferior to the PN

one. This observation, along with the great dispersion of experimental values shown in Table VI, evidence

that the influence of the drying method on the moisture buffer capacity of hemp lime is not strong enough

for a trend to be stablished. However, the results are consistent in showing that the H formulation presented

better results than the P formulation, independent of the drying method it went through.




23

3.4 - Vapour permeability testing

3.4.1 - Testing procedure

After the end of the moisture buffer tests, the samples were prepared for the dry cup test as

prescribed by BS EN 12086:2013. For this test it was necessary to have the samples’ lateral faces vapour

sealed, with the top and bottom faces open. As the top face was already open because of the previous test,

it was only necessary to cut the metal tape covering the bottom of each sample, as shown in Figure 9. The

cut was made such that some tape still remained on the skirts of the bottom face in order to ensure the

sealing of the samples.

Figure 9 – The cutting of a sample’s bottom face

The test consisted of exposing the top face to a 23 °C, 50% relative humidity (RH) environment while

the bottom face was exposed to a 23 °C, 0% RH. The 50% RH condition was achieved by means of a climate

chamber programed to operate in these conditions.

On the other hand, the 0% RH condition was created by a desiccant material in an environment

isolated from that of the climate chamber. This isolated environment was formed by a rectangular glass, the

‘cup’ of the method’s name, placed at bottom of the sample and sealed from the external environment using

wax and metal tape over the sample/cup interface. The desiccant material put inside the cup was Calcium

Chloride, an inorganic salt. This salt was used in abundance to ensure that the environment’s humidity would

be kept as expected, along with respecting the distance required to the sample by the British Standard. A

picture of the sealing process can be seen in Figure 10.




24

Figure 10 – Wax sealing of the sample for the dry cup test

These two environments were responsible for creating a vapour pressure difference between the

top and bottom of each sample. After the start of the experiment, the samples were briefly moved from the

climate chamber and weighted each 24 hours with accurate scales, precise to either 1/10 or 1/100 of a

milligram, to obtain the amount of moisture uptake. This time, the samples were returned to the same places

they occupied in the climate chamber before the weighting, contrary to what took place while testing the

moisture buffer capacity.

3.4.3 - Determination of steady state

The test ran for 11 days, when it had to be ended due to time constraints. According to the reference

Standard, the test should run until a steady state condition is achieved. To determine the existence of this

condition it is necessary to calculate the changes in mass for the selected time interval (G1,2). As described in

BS EN 12086-2013:

Calculate for each test specimen the change in mass for the selected time interval, G1,2, in milligrams

per hour

using Equation (4):

1,2 = −− (4)

Where:

- M1 = mass of the test assembly at time T1, in milligrams;

- M2 = mass of the test assembly at time t2, in milligrams;

- T1 and T2 = successive times of weightings, in hours.

Calculate G, the mean of five successive determinations of G1,2, in milligrams per hour, for each test

specimen The final value of G is obtained when each of the last five successive determinations of G1,2 is within

± 5 % of G.




25

The experiment did not achieve a steady according to the regulation above. It was also noted that

the last 3 cycles were much closer between each other than the other two before them. It was thus decided

to use only the last three values of G1,2 (instead of the last five) to compose G, in order to obtain a more

precise value for this parameter.

3.4.4 – Vapour permeability value

In Table VIII it is possible to see the percentage variation of each of the last three measurements in

relation to G. The values indicate that the rapid dried samples were faster in achieving a steady state

condition, with a smaller dispersion from G than the other slow dried samples. In fact, some of the normal

dried samples were still far from reaching stability, with variations above 20%. Thus, care was taken in

analysing these more unstable samples in conjunction with more stable ones, so that the results would not

be influenced by false tendencies.

Table VIII – Variation from G of the last three moisture uptake measurements

Samples Δ G-1 Δ G-2 Δ G-3

HA1 4% 1% 5%

HA2 4% 13% 9%

HA3 5% 3% 1%

HN1 8% 1% 10%

HN2 24% 9% 15%

HN3 2% 24% 22%

PA1 1% 5% 4%

PA2 7% 2% 4%

PN1 3% 9% 6%

PN2 6% 24% 30%

PN3 14% 23% 9%

With the values of G, it was possible to obtain the figures of vapour permeability (δ) for each of the

samples, presented in Table IX, by following the equations provided in the 2.2.2.2 - Vapour permeabilitysection of this text.




26

Table IX - Vapour permeability (δ) of each sample

Samples δ [kg/(m.s.Pa)]

HA1 8.05E-11

HA2 1.43E-10

HA3 1.38E-10

HN1 9.19E-11

HN2 1.56E-10

HN3 1.25E-10

PA1 7.13E-11

PA2 6.85E-11

PN1 6.47E-11

PN2 7.62E-11

PN3 1.27E-10

After these values were obtained, they were averaged to give the final δ value for each hemp lime

type. The vapour diffusion resistance factor (μ), which is the ratio between the air and the material’s vapour

permeability, was also calculated. The value for air’s vapour permeability was taken from BS EN 12086-2013.

It is an unitless factor, with lower values representing greater permeability, which is suitable to be compared

to test results obtained with different air conditions. The δ, the μ and the percentage difference between

same formulations under different drying methods are described in Table X.

Table X – Vapour permeability ( δ ), vapour diffusion resistance factor ( μ ) and the differences between drying

methods

δ [kg/(m.s.Pa)] μ Δ drying

HA 1.20E-10 1.74 3%

HN 1.25E-10 1.65

PA 6.99E-11 2.80 22%

PN 8.93E-11 2.38

The vapour permeability values in Table X show consistently that the H formulation has a significantlyhigher δ than the P formulation, independent of the drying method employed.

Analysing the drying methods alone within each hemp lime formulation, both the HA and PA

averages of δ are lower than their normal dried counterparts. However, the range of values for both the

forced and the normal dried samples greatly overlap with each other. As shown in Table X, the difference

between the HA and HN samples, of 3%, was much smaller than that observed between the PA and PN

samples, of 22%. However, this greater difference of the latter was heavily influenced by the HN3 sample as

can be seen in Table IX, which presents a significantly higher value than the other PA and PN samples, being

possibly just an outlier.

According to Walker (Walker, Pavía 2014) ‘the common industry figure of water vapour diffusion

resistance factor (μ) of lime –hemp concrete is 4.85 ± 0.24 measured in accordance with EN12572 for samples

with a binder:hemp:water ratio of 2:1:3 and a density of c. 400 kg/m 3’. As the samples presented a lower μ




27

value than the expected by the industry, it means that they all had a greater vapour permeability. Which is a

predictable result, due to the low dry density (less than 200 kg/m3) of the tested hemp limes.




28

4.0 – Discussion

The thermal conductivity results showed that the rapid dried samples had a lower thermal

conductivity in the first tests when compared to the slow dried ones. After all samples being placed in the

same controlled environment (20°C, 60%RH) for more than one and a half months, the influence of the dryingmethod greatly diminished, suggesting a tendency for stabilization at a common value for both drying

methods. However, the failure in correctly measuring the densities and moisture contents in different

occasions led to inconclusive results. What seems clear is that both formulations performed well and similarly

in the tests. Also, with the values obtained it is clear that the forced drying method does not have a sensible

negative effect on thermal conductivity of hemp lime and thus would not hinder this technique from being

used.

The moisture buffer value test did not achieve a quasi-steady state as prescribed by the Nordtest

method. However, the results obtained were still valid and useful for the analysis. From them it was seen

that a hemp lime made from a high surface area lime with a CSA additive had a consistently better moisture

buffer capacity than the hemp lime fabricated with the commercial PF70 binder.

When analysing the drying methods alone, the MBV results are not so conclusive. The samples made

with PF70 binder suggest that the forced drying process reduces the moisture buffer capacity of the material.

However, the HSA lime samples have a greater mean MBV when forced dried. This apparent advantage is

strongly influenced by one dissonant value of a forced dried sample, while all other values obtained suggest

the same behaviour pattern seen with the P formulation. Thus, it is still not possible to state conclusively if

and in what cases the forced drying process may affect either positively or negatively the material. What can

be said is that the forced drying of hemp lime does not have any hindering, incapacitating effect on its

moisture buffer properties.

The dry cup method, employed to assess the vapour permeability of hemp lime, also did not comply

with the requirements of quasi-steady state from the standard. However, the results were more precise than

those obtained for MBV and were also valid for analysis. The P formulation performed worse than the H

formulation, independent of how the material was dried.

When assessing the δ value, both hemp lime formulations had a better performance when slow

dried. The individual results of each sample also follow this tendency. Thus it can be asserted that the forced

drying of hemp lime has a negative effect on the vapour permeability value. However, the results suggest

that this influence is notably more expressive with the P formulation than with the H one.

The fact that the forced drying process lowered hemp lime’s δ would also support the hypothesis

that this drying process has a negative effect on the moisture buffer capacity of hemp lime. Nonetheless, the

lowering of δ in the tests although sensible, was nowhere near to discourage the use of rapid dried hemp

lime. The forced dried material was still very vapour permeable, even more than the standard hemp lime in

the market, showing that factors such as formulation and density may easily compensate for any negative

effects of the drying procedure.

With the results obtained, it can be safely said that there are no impediments for using the rapid

drying method tested in this research in the production of hemp lime, other than guaranteeing the minimum

mechanical strength required. The experiments have shown that hygrothermal parameters of both rapid and

slow dried hemp lime are significantly close to each other and that the differences which may arise from

them can be corrected in the formulation of the material. The forced drying method thus represents a

promising method for improving the use of hemp lime as a building material.

Comparing the two formulations tested, it was seen that the HSA lime and CSA composition provided

better overall characteristics, having either similar or better values of hygrothermal parameters when

compared to the PF70 formulation, while maintaining a much better mechanical integrity than the latter.




29

5.0 - Conclusion

This dissertation intended to provide an understanding of the effects that forced drying hemp lime

had on its hygrothermal properties. Two hemp lime formulations were tested, differing from themselves in

the binder used. One used a high surface area lime (HSA lime) together with a small addition of calciumsulfoaluminates (CSA), while the other used the commercial PF70 binder from Tradical®.

Although mechanical strength was not researched in this paper, it was possible to notice from the

forced dried samples that those made with HSA lime had much better structural integrity when compared to

those made with PF70 binder.

Thermal conductivity was assessed with the hot-wire method and each set of samples was analysed

on two different dates. The results were initially better for the forced dried hemp lime, but in the second

testing the results of both drying methods were very close. A problem in the research was that the moisture

content and dry densities were not obtained correctly and thus the thermal conductivity values obtained

offer only limited conclusions.

To obtain the moisture buffer valued the Nordest protocol was followed. The samples were not able

to achieve the quasi-steady state described by the method, however the results were still analysed. They did

not show a consistent tendency for the behaviour of forced dried hemp lime. However, a likely hypothesis is

that this process reduces the moisture buffer value of hemp lime.

The vapour permeability results were obtained by the dry cup method following BS EN 12086-2013

and were also unable to achieve stability. Nonetheless, the results were still analysed and there was a

concrete tendency for a reduction of hemp lime’s water vapour permeability when the samples were forced

dried.

It was concluded that, although some observed differences were sensible, they did not present any

hindrance for the use of forced drying in hemp lime production, which showed to be an interesting method

to be applied in practice. From the formulations tested, the HSA lime and CSA mixture was the one judged

to perform the best overall.




30

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