Construction and Building Materials 59 (2014) 39–50

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Construction and Building Materials

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Review

Composition and compatibility requirements of mineral repair mortarsfor stone – A review

http://dx.doi.org/10.1016/j.conbuildmat.2014.02.0200950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +32 65374532.E-mail addresses: aurelie.isebaert@umons.ac.be (A. Isebaert), laurent.vanpary-

s@umons.ac.be (L. Van Parys), veerle.cnudde@ugent.be (V. Cnudde).

Aurélie Isebaert a,⇑, Laurent Van Parys a, Veerle Cnudde b

a Civil Engineering Department, Faculty of Engineering, University of Mons, Place du Parc 20, 7000 Mons, Belgiumb Geology and Soil Science Department, Faculty of Sciences, Ghent University, Krijgslaan 281 S8, 9000 Ghent, Belgium

h i g h l i g h t s

� The aggregates, binders and their ratios determine the mortar’s properties.� Developing repair mortars should be with respect to the ranking of its properties.� Surface repair mortars can be advantageous but are now empirically made or adapted.

a r t i c l e i n f o

Article history:Received 5 September 2013Received in revised form 3 February 2014Accepted 8 February 2014Available online 12 March 2014

Keywords:RestorationNatural stoneMineral mortarsCompatibility

a b s t r a c t

When designing repair mortars, it is important to consider all its components such as the binder and theaggregates since they have a great influence on the mortar’s final properties. The binder, and the aggre-gates’ angularity and chemical composition determine the properties of the mortar, properties critical fora good compatibility and durability of a restoration intervention. In this article, some mineral repair mor-tar design philosophies are approached, followed by the requirements set for a plastic repair mortar forstone. Up to which point is an intervention compatible? An answer was found when examining severalarticles that discuss the compatibility tolerance plane. This article aims to give the reader a hands-onapproach in mineral repair mortar design and how this can be used to make a mineral repair mortar morecompatible with the substrate.

� 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402. Influence of the main components of a mineral mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.1. Nature and influence of different mineral binders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.2. Impact of aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.3. Complementary aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3. Mortar design philosophies: case-related development of repair mortars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454. Compatibility requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.1. Critical parameters for compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.2. Incompatibility features and following alterations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5. Compatibility tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476. Discussion and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Fig. 2. Left is a sketch of a damaged column for which a plastic repair mortar mightbe used to fill in the missing parts, as suggested in the right sketch.

40 A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50

1. Introduction

This article is a review, and has as purpose to create a link be-tween the scientific research on binders and mortars in general,and the stone repair practice. The focus lies on the specific needsand required demands needed for mineral repair mortars i.e. mor-tars used for replicating lost parts of stone elements. In the firstpart, the different components of a mortar will be discussed, aswell as their influence on the final properties of a repair mortar.Next, the compatibility requirements that are expected from min-eral repair mortars are listed, including possible evaluation meth-ods for conservation materials. Finally, the current approaches forformulating compatible repair mortar for stone are discussed.

During restoration of heritage buildings, mortars are frequentlyused for the repointing of joints or for the ‘‘plastic’’ repair of stone,which are designed to fill in missing parts of stone. Stone repairmortar, reconstitution mortar or ‘plastic’ repair mortar as it is alsocalled, is a moldable mortar that can be applied in situ, and ‘setsinto place by its own adhesion to the substrate’ [1] (see Figs. 1and 2). They consist generally of a binder, aggregates and some-times additives or adjuvants. Empirical results show they can at-tain a life expectancy of 30 years [2,3]. Plastic repair mortars areoften subdivided based on their binders [1,4]. Ashurst and Dimesand Feilden both discuss repair mortars for stone with cement,lime, or a combination of both [3,5]. [3] mention resin-based repairmortars as well. Also [1] divide repair mortars in inorganic and or-ganic repair mortars. [4] divides them into resin-based mortars,mineral mortars and chemical mortars. Resin-based mortars haveorganic polymers as binders such as epoxies or acrylics. Eventhough they imitate alabaster and marble very well, due to theircompletely different chemical composition, they behave andweather differently than stone [1,3,6,7]. Chemical mortars areready-made zinc hydroxychloride mortars, wherein two compo-nents, a zinc chloride fluid and a zinc oxide coated aggregate react[8,9]. The group of mineral binders with lime and cement are closerto stone in composition and they have been used the longest forthe repointing and reconstituting of stone. Hydraulic mortars havebeen used since roman period for the repair of buildings. In the19th century, (Portland) cement mortars are manufactured and

Fig. 1. Restoration of architectural details at the entrance of the St. Anne church,Bottelare, Belgium. �Aurélie Isebaert.

used for the repair of stone [1]. This group of mineral binders willbe discussed more thoroughly in this article, including possibleweak points in terms of compatibility.

Interpreting the philosophical and ethical guidelines of both theVenice Charter and the Nara Document, an ideal repair mortar fornatural stone should be durable enough, but self-sacrificing on thelong run [10–12]. The properties of the mortar should be close tothe stone it is repairing, but overall, less durable than the stone.One of the questions this article tries to answer is which propertiesare considered important, and which elements of a mortar deter-mine these properties. Considering that the values of each propertyone would like to obtain for its mortar vary from project to project,the answer is not simple and universal. Taking into account the dif-ferential properties of various stone types and stone layers, it is dif-ficult to define exact values, and some binder–aggregatecombinations will be more applicable for these or other stones.One of the advantages of repair mortar is that it allows preservingas much as original material as possible [3]. Although rougheningor cutouts are required in order for the mortar to attach itselfmechanically to the stone, more original material can be saved. An-other advantage that is often claimed is the lower cost comparedwith other approaches [3]. Steenmeijer agrees, certainly whenthese repair mortars are used for casting replicas of repetitivearchitectural elements [13]. Another advantage is that they canbe adapted to the condition and appearance of the stone, and canhereby increase the lifetime expectancy of the original material.Moreover, the restoration of missing and weathered parts andthe filling in of holes, will allow the architectural lines and detailsto be understood better by the visitor, and it will therefore,together with other interventions, positively influence the percep-tion and estimation of the building.

Repair mortars could be used where replacement would only bepossible with an unsuitable stone. Although generally, repair mor-tars can co-exist next to the replacement of stones: repair mortarsare used when damage to the original material is limited, andreplacement occurs where whole blocks or larger parts should berestored. However, the border line indicating when to use a mortarand when a replacement stone can vary greatly from practitioner,and even from country to country [5].

2. Influence of the main components of a mineral mortar

2.1. Nature and influence of different mineral binders

The nature of each part of the mortar determines the behaviourof the whole mortar mix. Binders are discussed more elaborately,and previous studies are crossed and combined in order to makethe influence of each material more clear. In the following part,the differences between lime and cement binders are summedup, as well as their (dis)advantages for their use in mortars.

Table 2Synthetic table of natural hydraulic lime.

Raw material Chemicalcomposition

Setting

(siliceous) limestone Ca(OH)2 –CH

(a) Hydraulic setting: C–S–H

Burned below clinkeringpoint (800–1200 �C)

2 CaO�SiO2 –C2S (CaO)

2 (2CaO�SiO2) +4H2O ? 3CaO�SiO2�3H2O + Ca(OH)2

(3 CaO�SiO2

– C2S)(b) Carbonation

Ca(OH)2 + H2CO3 ? CaCO3 + H2O

A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50 41

Air hardening lime is composed of portlandite (Ca(OH)2 or CHin mortar chemistry) (see Table 1).

Portlandite, the main component of air hardening lime, under-goes a change in shape when placed in water; the hexagonal pris-matic crystals turn into thin hexagonal plates [14,15]. Lime puttytherefore has a high water retention capacity and a high sand car-rying capacity [15]. Aged putty (putty of more than one year old)also carbonates faster than non-aged lime putty [16]. The carbon-ation process is slow mainly due to the fact that the hydrogen car-bonate has to be able to reach the portlandite crystals through thepore network [15].

According to research by Lawrence et al. calcite crystals seem tobe smaller than portlandite crystals, and attach themselves to sur-rounding aggregates, which might explain why a carbonated mor-tar contains smaller pores than uncarbonated mortar [17] (seeFig. 3). Calcite also attaches to unreacted portlandite, and due tothe small pores, carbonation through CO2 of portlandite is veryslow and may be self-limiting [17]. Because of its high water reten-tion capacity, air hardening lime is able to take up more water, andit requires a higher water–binder ratio (W/B) to make a workablemortar. This combined with slow carbonation can induce thecracks which are so characteristic for this mortar. It depends onapplication skills and environmental conditions if the subsequentoccurring cracks are limited to micro-scale cracks or if they are vis-ible on a macro-scale. In Mercury Intrusion Porosimetry (MIP) data,air hardening lime mortar presents a bimodal pore size distribu-tion, which is due to the two types of pores present: shrinking fis-sures and air voids [18]. Mortars with this binder have a large porenetwork, a low density and a low compressive strength, which liestypically below 2 or even 1 MPa [19].

Natural hydraulic lime (NHL) consists of portlandite and cal-cium silicate hydrates (see Table 2).

Due to the presence of portlandite as well as calcium silicates,the lime will set by two means. First, a calcium–silicate–hydrate(C–S–H) is formed, then on a long-term, calcite is formed: a similarreaction as in air hardening lime. The portlandite also splits the C–S–H into calcite and amorphous silica [14]. Since a part of NHL alsoconsists of portlandite, the water retention capacity is high, but not

Table 1Synthetic table of air hardening lime.

Raw material Chemicalcomposition

Setting

Pure limestone Ca(OH)2 – CH Carbonation: slowBurned below

clinkering point(800–1200 �C)

Portlandite aspowder orputty

Ca(OH)2 + H2CO3 ? CaCO3 + H2O

Fig. 3. Proposed model from Lawrence et al. (2007) the interaction of air hardeninglime with aggregate particles: (left) portlandite crystals surround the aggregateparticle, (right) after carbonation, smaller calcite crystals are more densely packedaround the aggregate particle [17].

so elevated as in air hardening lime. Failure due to large shrink istherefore lower, but is still present.

Classification concerning hydraulic lime types is sometimes dif-ficult, and therefore it can be unclear to which type of lime is re-ferred. In this review, the classification of EN 459-1 will befollowed [19], wherein natural hydraulic lime (NHL) is lime withhydraulic properties, which was made due to the burning of (sili-ceous) limestone below clinkering point (between 800 and1200 �C) and does not contain any additives. Research in repairmortar discusses largely only NHL lime, most likely because, asSmith et al. (2005) describe, NHL is regarded as the ‘true’ and‘authentic’ hydraulic lime as used in the past [20]. Other typesare formulated lime (FL) and hydraulic lime (HL).

Because of their composition, air lime and hydraulic lime mor-tars have a large connected pore network, with high water vapourdiffusivity and permeability. The presence of portlandite allows abetter plastic deformation of the mortar, but their large pore net-work makes them less resistant to compressive and flexuralstrength [22]. The presence of the C–S–H will give the NHL a fasterand higher resistance than the air hardening lime. EN 459 standardsets the compressive strength of NHL after 28 days between 2(minimal limit NHL2) and 15 MPa (maximal limit NHL5) [19].Although the standard gives these values for NHL binders, this isabsent for air hardening lime.

Natural cement is a natural material, and the binder composi-tion is quite variable (see Table 3).

Although more research into the differences between XRD andSEM-EDX results is required [23], presume that the main phasesin hardened natural cement are calcium silicate hydrate, followedby calcium aluminate hydrate (C–A–H) and partially carbonatedhydrated calcium aluminate phases. Examples for natural cementare available from Vicat or Lillienfeld cement [24]. Natural cementwas a product used frequently for rendering façades in the 19thcentury in Europe (France, Austria, Poland), and in that aspect, itis also known as ‘Roman cement’, although this can cause confu-sion with the ‘cement’ from the Roman period. Natural cement var-ies in colour from yellow to red-brown [23]. The pore structure ofnatural cement is variable, depending on water:binding ratios (W/B) and curing conditions [25], but a dual-phased hydrating mech-anism can be detected through a characteristic development of thepore structure [26]. The C–A–H gel is the first to be developed and

Table 3Synthetic table of natural cement.

Raw material Chemicalcomposition

Setting

Argillaceous limestone C2S, (C3S) Hydraulic setting: C–A–HBurned below clinkering

point (800–1200 �C)C3A2 3 CaO�Al2O3 + 6 H2O ? 3

CaO�Al2O3�6H2OC3S2 Hydraulic setting: C–S–HCS 2 (2CaO�SiO2) + 4 H2O ? 3

CaO�SiO2�3H2O + Ca(OH)2CaO

42 A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50

produces a relative open pore structure (0.2–0.8 lm threshold porediameter in MIP) and only in the second hydration phase thethreshold pore width shifts to about 0.02 lm, due to the formationof the more voluminous C–S–H gel. Samples taken from renderingsfrom the 19th century showed low formation of C–S–H gel, andlarge air pores, with a bi- or tri-modal pore size distribution inMIP-analysis [25]. [24] refers especially to natural cement suitedas binder for repair mortars for natural stone due to its high poros-ity and possibility of water to evaporate; typical porosity values liebetween 22–32%. Natural cement attains a compressive strengthhigher than 10 or 25 MPa (depending on the classification type)[27]. Maximum values are not given in this work, but research per-formed for the European Rocare project let to understand a typicalcompressive strength value between 20 and 60 MPa [27].

Portland cement appears to have been used more for repairmortars than any other cement type.

Portland cement always contains calcium sulphate (CaSO4� 2H2O) (<5%) that is added to slow down the reaction (see Table 4).The gypsum reacts with a part of the C3A, creating primary ettring-ite. Primary ettringite may not be confused with secondary ettring-ite, created after hardening of cement, and which causes damagedue to volume expansion. This primary ettringite reacts on a latertime with calcium aluminate hydrate into calcium aluminasulphate. The larger the percentage of cement in the mortar, thelower the porosity: the calcium silicate hydrate gel takes up allthe capillary pore spaces and encloses large air voids in their ma-trix. Since the matrix is very dense, permeability is very low tonon-existent, and these mortars present high compressive and

Table 4Synthetic table of Portland cement.

Raw material Chemical composition Settin

Argillaceous limestone/clay-limestone mix C3S HydraBurned above clinkering point C3A 3 CaO

C2S, 4 CaOC4AF HydraCaSO4� 2 H2O 2 (2Ca

2 (3CaPrima3 CaOPrima3CaO�

Table 5Technical properties of mortar binders versus the classification of the mortar. The arrows(high), after [29].

Technical requirements Binder type

Air hardening lime Hydraulic lime Pozzola

Adhesion 3

Strength (comp./flex./tens.) 2

E-modulus 1

Water penetration resistance 3

Freeze–thaw resistance 2Thermal dilatation 1

Vapour transmission 5

Aesthetics Depends on specific requirements

flexural strength. The sulphate elements in the matrix can crystal-lize [28]. Cement also undergoes carbonation upon ageing. Carbondioxide slowly penetrating in the dense matrix turns calciumcompounds into calcite and the silicates and aluminates intoamorphous silica and alumina [14]. The carbonation lowers thepH-value of cement from 12 to 8, and will therefore affect thedurability of mortars with reinforcements.

Based on the information given above we can state that all thebinders behave quite differently, even though they may be pro-duced from similar materials. Each binder has its own strengthsand weaknesses, and Hughes et al. assembled the values of thedifferent binders in Table 5 [29].

Blended lime–cement mortars harden due to hydration of ce-ment particles and carbonation of portlandite [22]. Mosquera et al.found that air lime-cement mortars approach hydraulic lime (NHL3,5) in pore size range and water vapour diffusivity values, and thatan air lime-cement mortar (8:1) was considered more suitablethan the tested hydraulic lime [21]. An increase in cement percent-age means a decrease in porosity, while an increase in lime meansan increase in porosity [18,22]. Nevertheless, Arizzi and Cultrone[18] stated that the addition of cement in low percentages still cre-ates mortars that were more vapour diffusive than the naturalstones tested. Sébaïbi et al. stated that the addition of less than10 m% lime to a cement mortar does not alter the microstructureof the mortar, but that the addition of a higher percentage inducesmicro-cracks in the matrix [30]. Advantages of blended lime-ce-ment mortars are the early stage strength development, the resis-tance which is lower than pure cement mortar, and which can be

g

ulic setting: C–A–H�Al2O3 + 6 H2O ? 3 CaO�Al2O3�6H2O�Al2O3�Fe2O3 + 2 Ca(OH)2 + 10 H2O ? 3 CaO�Al2O3�6H2Oulic setting: C–S–HO�SiO2) + 4 H2O ? 3 CaO�SiO2�3H2O + Ca(OH)2

O�SiO2) + 6 H2O ? 3 CaO�SiO2�3H2O + 3Ca(OH)2

ry ettringite:�Al2O3 + 3 CaSO4 + 32 H2O ? 3CaO�Al2O3�3CaSO4�32 H2Ory ettringite + C–A–H:Al2O3�3CaSO4�32 H2O + 2 (3 CaO�Al2O3 + 6 H2O) ? 3 (3CaO�Al2O3�CaSO4) + 2 H2O

indicate the direction of the increase in values, on a relative scale from 1 (low) to 6

n lime Calcium silicate cements Calcium sulphate based Clay earth

6 5 1

6 4 1

6 4 1

6 2 1

6 1 11 1 1

3 3 5

Fig. 4. Representation of variability in compressive and flexural strength anddynamic E modulus when using the same ingredients in a mortar mix, but changingthe proportions. Ingredients are cement, limestone powder and hydraulic lime.Tested at 28 days, (left) compressive strength (MPa), (middle) flexural strength(MPa) and (right) dynamic elastic modulus (GPa), by [50].

A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50 43

varied by adapting the cement:lime ratio [22] (see Fig. 4). Theamount of lime also makes a certain elastic–plastic deformationpossible before failure [22]. Winnefeld et al. tested lime andlime-cement mortars on their sulphate resistance with a testingmethod where 15 m% of gypsum was added to the binder [31].After precuring, both samples without and with gypsum werestored under water for 8 days. After 28 days, the length change be-tween the reference sample and the sulphate-attacked sampleswas measured. [31] admit it is a severe test to study sulphateresistance, but they showed that lime-cement mortars were moreresistant to sulphate attack and freeze–thaw actions than limemortars. Some distinction should be made, however. Arandigoyen

et al. point out that the decrease or increase in compressivestrength is not proportional to the percentage lime–cement: add-ing cement (<40%) to a lime mortar creates a slightly higherstrength, while the addition of 25% lime to a cement mortar meansa decrease in strength by 50% [32]. [32] also remarks that the morecement is added, the more problems can rise concerning the diffu-sion of fluids, creating a similar situation to that of pure cementmortar.

2.2. Impact of aggregates

Next to the binders, the aggregates play an important role in themortar’s characteristics. Depending on the aggregates, an air limemortar can be made more resistant to compressive strength, or acement mortar can be made less resistant. Most of the (natural)aggregates added to mortar are siliceous or calcareous of nature.Their origin (river, quarry, ect.) and possible processing method(crushing) also affects the form and behaviour of each aggregate.The influence of aggregates works on two different levels: theinfluence of the material (mineralogical composition, porosity,resistance) and the influence of the size and the shape of the aggre-gate. [33] suggested that with a low binder: aggregate ratio (B/A),one could easily adapt the properties of the mortar through aggre-gate variation.

In general, increased angularity and fineness of the aggregatewill increase the compressive strength and the bond strength be-tween binder and aggregate. Fine aggregates have a strong influ-ence on the water demand and workability of the mortar.Westerholm writes that plasticity increases due to increased parti-cle friction [34]. Fine aggregates should also give a higher elasticmodulus and higher frost resistance [33]. In terms of angularity,Ingham states that rounded aggregates are easier to work with[35]. Caliskan and Karihaloo noted that the surface roughness ofthe aggregate determines the interfacial bond strength, in particu-lar for smaller sized aggregates [36]. The more porous and moreabsorbing an aggregate is, the rougher the aggregate is at thesurface, and thus the higher the bond strength. The impact of thesurface roughness decreases when the aggregate size increases.

The grain size distribution of the repair mortar can be linkedwith the aimed workability and structural appearance. For exam-ple, a repair mortar for a coarse grained stone will have to becoarser grained as well in order to show compatibility in visualappearance. Allen refers to the standard sand grading curve fromthe European Standard EN 13139:2002 or one of its predecessorsand Ashurst and Dimes state that ‘sand should be well graded, rang-ing from fine to coarse’ and that this may require mixing aggregatesfrom different sources [3,37]. The sole use of fine grained aggre-gates in a mortar could lead to water retention on the substrate’ssurface when applied, while only coarse grained aggregates in amortar will lower the interfacial bond strength, and might prohibitthe desired visual appearance. Grain size distribution influences theporosity and pore size distribution and therefore also the water va-pour transmission. Von Konow [33] suggested the use of an Aggre-gate Index (AI) in order to clarify the influence of aggregates inmortar. An equation showing the relationship between grain sizeswas formulated, taking into account the smaller mass of fine parti-cles (Eq. (1)). A general coarser grain size lowers the AI and a finegrain size increases it. An AI of 40 was considered a well-dispersedgrain size distribution. Lower AI tested mortars proved to have alower modulus of elasticity and a higher capillary absorption rate.Frost resistance was highest for tested mortars with a high AI.

Eq. (1) for the formulation of an aggregate index, after [33]

Aggregate index ¼ ð1=aÞ � ððc � bÞ=ðb� aÞÞ ð1Þ

a = grain size at 10% sieve passage, b = grain size at 50% sieve pas-sage, c = grain size at 80% sieve passage.

44 A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50

The mineralogical composition of the aggregates is also a deter-mining factor. Mortars with clay minerals (<63 lm) and air hard-ening or hydraulic lime, show formation of ettringite on the clayminerals after sulphate attack, and therefore, clayey fines can bean alumina source favouring secondary ettringite in the mortar[31]. The EN13139:2002 standard about additives for mortar forexample allows the presence of maximum 3% clays in sand usedfor (masonry) mortars. Because of the high specific surface of clays,a large amount of water is needed to make the mortar and whenmortar is hardened, mortars with a high amount of clays show ahigh water retention capacity. This increase in water demand leadsto:

� A decrease in dynamic elastic modulus, flexural and compres-sive strength up to 50%.

� An increase in capillary pore content, but also in shrinkage.� An increase in water absorption coefficient and vapour diffu-

sion, but a decrease in drying rate.� Consequently, a decrease in freeze–thaw resistance.

So, although clay-rich sands are frequently used because theycan increase workability and/or help finding the right colour forthe restoration mortar, the durability of the mortar itself is affecteddue to the addition of clays. Winnefeld and Böttger therefore rec-ommend to first examine unwashed sands on type and percentageof clays present before use [31].

Other secondary minerals in mortars have a large influence aswell: glauconite swells and oxidizes, and chalcedony and opalare reactive silica, meaning they can form an alkali silica reaction(ASR) when in contact with alkali minerals such as sodium and

Table 6Figure with combinations of binder–aggregates in a field, with water transfer properties, anhave a higher interfacial bond strength.

AL – Air lime.NHL – Natural hydraulic lime.L – Lime (not specified).C – Portland cement.NC – Natural cement.CaCO3 – Calcite aggregate.SiO2 – Silicate aggregate.

potassium hydroxides [35,38]. These secondary minerals are veryfine, and can therefore be catalogued as silt, aggregates with a par-ticle range from 0.004 mm to 0.0625 mm. They can lead to the for-mation of ASR when used on buildings containing sodium chloride[38]. Marine dredged aggregates contain (sodium) chlorides thatcan favour corrosion in reinforced mortars. Any aggregate contain-ing detectable opal or opaline silica (unstable and therefore reac-tive) should be avoided due to its potential alkali-reactivity [35].Ingham also reported other minerals such as marine shells, chalk,organic matter and mica as unsuitable for cement mortars, follow-ing European standards [35]. Lanzon [39] examined low-densityadditions for (cement) mortar through X-ray micro computedtomography (micro-CT). These additions were expanded perlite,expanded glass and cenospheres, which showed good adhesion,while changing the microstructure and increasing the porosity.[40] found that the use of limestone aggregates in a lime mortar in-creased the strength, – pointing out that it is probably due to a lackof discontinuities in the chemical composition of the mortar. How-ever, the calcite aggregate used was less coarse grained than thequartz aggregates Lanas et al. compared it with. [17] found thatmortars with silicate sand and air hardening lime carbonate fasterthan the same mortars with an oolite limestone powder as aggre-gate. The water absorption and porosity of the mortar itself are notaffected by the nature of the aggregates according to Pavia et al.[41].

2.3. Complementary aspects

In a mortar, variations in mechanical and physical characteris-tics can be made as well by changing some variables. These vari-

d strength and thermal tension on opposite sides of the field. Combinations at the left

A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50 45

ables have to be changed with moderation: a too dry or too wetmortar mix is to be avoided. One can change the following:

� The binder: aggregate ratio (B/A). For example, an increase in B/A in an air hardening lime mortar induces higher porosity andhigher strength [40].

� The water: binder ratio (W/B). An increase in W/B means anincrease in porosity (capillary pore spaces due to evaporationof water), although a too high W/B ratio can induce shrinkingand shrinking fractures can appear. The W/B ratio is one ofthe main factors affecting concrete permeability [42]. In naturalcement mortars a low W/B ratio causes uni-modal pore size dis-tribution, while a high W/B ratio creates a bimodal pore sizedistribution [22].

� Curing conditions influence the porosity of lime (-pozzolan)mortars in terms of open porosity and therefore the life expec-tancy of the restoration [43].

� Mixing method: the method of mixing (by hand or mechani-cally), the speed and the time of mixing, can have an influenceon the behaviour of the mortar and on its properties [44].

In the previous sections, we have seen that not only the bindersplay an important role in the behaviour of fresh and hardened mor-tar, the aggregates are important as well. In Table 6, a sketch ismade based on previous sections, to represent where various mor-tars of different binders, combined with different aggregates,would be situated looking at the expected mortar properties. Interms of water transfer properties and deformability, lime mortarsscore highest, while cement mortars make denser mortars, whichare more resistant to compressive and flexural actions. Their highthermal expansion coefficient and low deformability in combina-tion with large thermal cycles can damage the stone [38]. In termsof interfacial bond strength, the theory of Lanas et al. is followedand the mineralogical compatibility of mortar and binder is takeninto account [40]. This theory-based representation differentiatesthe binder–aggregate combinations on a relative scale. Measure-ments should be performed in the future to indicate whether theinterfacial bond strength of Portland cement + SiO2 mortars is ofabout the same level of lime + CaCO3 mortars. Additionally, thedenser cement mortars have a higher frost resistance and lowersusceptibility for gypsum formation than the calcite lime mortars.In terms of the properties discussed here, natural cement mortarsare somewhere in between lime and cement mortars. The sketch isa simplified representation, and some effects of aggregates are notincluded: only the mineralogical composition is taken into account,and not the fineness or angularity of the aggregate.

Fig. 5. Texture variability due to differential erosion between the repaired elementand the original cast element it is to imitate from [57].

3. Mortar design philosophies: case-related development ofrepair mortars

As discussed in ‘Section 2.2’, the addition of a certain mineral tothe mortar that is also present in the stone could make a mortarmore compatible. The use of certain ingredients for repair mortarsfor stone can often be explained by the mineralogical description ofthe stone. For example, Beck and Al-Mukhtar started from theproperties’ analysis of the stone [45]. They focused on a mortarwith only two ingredients: NHL lime and the stone powder, com-ing from the same stone they wished to repair. The main focus inthe research was to obtain a similar porosity between repair mor-tar and stone, and they did so by focusing on bulk density. Schuere-mans et al. report projects where the old repointing mortar wasanalysed, and a mortar mix was assembled based on wet and drychemical analysis, XRF or XRD [12]. In the area of repointing mor-tar research, this approach by building up mortars ‘from scratch’ ismore common. On basis of the properties from the original mate-

rial (pointing mortar or natural stone), a binder and aggregates areproposed and several mortar mixes are made varying in ratio or inbinder or aggregate material. Consecutively, the properties, work-ability, ageing, chemical analysis of mortar are tested. Unfortu-nately, not every project disposes of the time and moneynecessary to find the best compatible repair mortar, making thistrial and error process disadvantageous to use. Bromblet [46] stud-ied the possibilities to make repair mortars for several French lime-stones (Tuffeau, St. Maximin and Courville) with air and hydraulichardening lime, concluding that each stone demands its specificbinder: soft air lime for the Tuffeau, stronger NHL for the morecompact stones. Most professionals turn to commercial mortars,which are specifically designed for one stone. The advantage is thatit is prefabricated, and the manufacturer can guarantee that thecontent’s mix is standardized, creating the same workability andproperties for each batch. This is much appreciated by restorationarchitects and contractors [2,43]. However, this advantage can be adisadvantage as well. In cases where the stone is very heteroge-neous, and properties can differ greatly from one stone to another,the standardized process will be less successful when aiming toachieve a compatible mortar. It is generally assumed that thesemortars contain additives to facilitate workability or increaseporosity. Bromblet tested four mortars, including homemade andcommercial mortars, to restore the Fontvieille stone [47]. Afterthe analysis of the properties and the onsite test, one of the com-mercial mortars was preferred, mainly due to its high workabilityand the guarantee from the manufacturer. In restoration studiesusing commercial mortars, aggregates (crushed stone powder,sands with a high concentration of a certain mineral) are fre-quently added to solve the problem of heterogeneity in one stone[48,49]. Szemerey-Kiss [49] mixed additional limestone powderwith an unidentified commercial mortar. Powder from porous oo-lite Hungarian Soskut limestone was used as additional aggregateto have a repair mortar more compatible with this limestone.The amount of powder influenced the mortar’s properties.

Research by Ramge et al. at the German BAM showed an ap-proach trying to combine the advantage of (self-made) mortarswith the commercial mortars’ advantage: a modular system witha base mix. According to the stone type, other aggregates or addi-tives are mixed with the base mix. This tends to allow a combina-tion of the ‘personalised’ mortars from the trial-and-error side anda certain standardization, which is the advantage of the commer-cial variants [50,51]. This research is on going, and first resultsare focused on a repair mortar for one type of sandstone.

To conclude, recent studies into repair mortars for stone givesnew prospects for the development of more compatible mortars.However, in practice approaches remain focused on (adaptations

46 A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50

of) commercial mortars and recipes based on experience. Theseadaptations most often include the addition of a certain type ofaggregates to the mortar.

4. Compatibility requirements

4.1. Critical parameters for compatibility

The end products that can be expected from some combinationsof binders and aggregates are now known, but when preparing amortar also certain parameters need to be taken into account.Though, how should one define the properties of the mortar thatwill be used to repair natural stone? Since all stones are subjectedto a range of weathering processes and since they are spatially var-iable and heterogeneous, establishing their properties is not aneasy task. If the stone’s property values are very regular, usingone single repair mortar could be sufficient. When a natural stoneseems to show a large difference in its properties, ideally someadaptations to the mortar are made. The aspects of a mortar whichlargely determine the durability of stone and mortar, are the phys-ical and mechanical aspects:

� Water transfer properties. The presence of water contributesgreatly to the deterioration of stone, since it can cause stressfractures, erode and favour biological colonization. Therefore,the possibility of the mortar to transfer water (vapour) has tobe equal or possibly higher than the stone since it will allowwater trapped in the stone to migrate faster out of the stonethan a mortar with a lower water vapour transmission rate thanthe stone. A key factor is the pore structure of the mortar [25].

� Modulus of elasticity and deformability: if the repair mortarwill be used for the filling-in of larger or structural parts, a goodbalance between strength and deformability has to be found toavoid the formation of cracks, since historic structures can besubjected to external actions inherent to their use. If in that casethe repair material is too rigid, fractures can arise [52].

� Bonding strength and adhesion: a good adhesion to the sub-strate is crucial. The bonding can be assured mechanicallyand/or chemically, with aid of prime coatings or reinforcementdowels. When looking into adhesion, not only the adhesivestrength needs to be taken into account, but also other aspectswhich can influence the bond strength, e.g. water vapour diffu-sion [53]. Following EN 1015-12:2001, one can conclude thatthe bond strength should be lower than the tensile strength ofthe stone, a parameter that is almost never tested on naturalstone.

� Response on high temperature differences. High temperaturedifferences on sun-faced walls cause the minerals in stones toexpand and contract. The internal stresses in the stone due tothe consequent cycles of temperature differences, can lead todetachment, deformation or cracking of stone and/or mortar

Fig. 6. A repair with a cement-based mortar has failed to smoothen and restore the weatand right show the render loosening from the stone façade, allowing differential weath

[52]. Groot and Gunneweg state that for soft masonry, a largethermal expansion means a low hygric expansion, and viceversa [38]. For stone, however, this might not be the case: Kochand Siegesmund [55] investigated marbles that both show ahigh thermal and hygric expansion. Benavente et al. [54] testedseveral stone samples and concluded that thermal processescomply with hygric expansion and reinforce decay phenomena.In comparison to the other aspects, this one takes place on themineralogical level, but it has a direct influence on the durabil-ity of the whole repair mortar intervention.

Next to physical aspects, also the chemical aspects determinethe durability of the stone after conservation interventions. Repairmortars can contain, create or attract materials that are harmful forthe stone. Repair mortars that are made with organic polymers, orthat contain organic polymers as additive or adjuvant, are moresusceptible for biological organisms and can be a nutrition sourcefor biological colonization on the stone [6]. Ageing tests taking intoaccount biological growth can be an indicator for the durabilityand compatibility of this type of mortar.

The visual appearance of the mortar in se does not affect thedurability of the stone, and is mostly important for the compatibil-ity. In visual appearance, texture and colour are discussed in con-cerned literature. Compatible colour of the repair mortar is foundan important criterion. Ruling conservation theories demand thatthe intervention can still be distinguished from the original[10,56]. Therefore, the mortar can differ slightly with the stone incolour or it can slightly differ in texture. The difficulty lies not onlyin making the matching colour with the stone’s current condition,but also with the stone’s future colour. Some stones are known todiscolour and this should be taken into account when developingor choosing its repair mortar. Ramge et al. also include the aspectof ‘texture’ in their development for a repair mortar for sandstone(see Fig. 5) [57].

4.2. Incompatibility features and following alterations

Unfortunately, several phenomena can prohibit the develop-ment of a convenient repair mortar, consequently inducing dam-age to the stone. A repair mortar is a man-made, and possibly,standardized material, which is designed to be compatible with anaturally heterogeneous material. This makes it difficult to makea good repair mortar. Each stone type is different, and requires aspecifically adapted restoration mortar, maybe with each time adifferent binder. Each binder-lime, cement, or commercial varie-ties-behaves differently in application and setting and thereforedemands from the restorer to adapt their application techniquesand methods. Due to this, a lot of repair mortars have proven tobe incompatible with the stone they had to repair: the stone isdamaged instead of being restored and preserved for the future.Report has been made of lime mortar damaging granite: a correla-

hered surface of these stone ashlars in Glasgow from [2]. The illustration on the leftering of the stone beneath.

Table 7Evaluation method for repair materials for natural stone after [62].

Stone repair materials

Property Symbol Requirement (%) Requirement after 1 year (%)

Dynamic E-modulus E-modulus 20–100 60Compressive strength bCS 20–100 60Thermal dilatation coefficient aTH 50–150 100Water uptake coefficient w 50–100Value of water vapour resistance l 50–100Pull-off strength bPOS 50–801

The requirements are related to the properties of the substrate.The adhesion is desired to fail in the stone repair material or in the contact area, but not in the stone.bPOS: pull-off strength of the surface; aTH: thermal dilatation; l: value of water vapour resistance.

1 Or to be defined in special cases.

Table 8Critical properties in the development of a repair mortar. (a) Values by [62]. Values in brackets indicate the recommended value after 1 year, (b) suggested values proposed by theauthor, (c) value of minor importance, (d) value is similar as the stone’s value.

Ranking Property Symbol Testing method Recommendation

1 Grain size distribution – c (b)1 Mineral components – c (b)2 Water absorption coefficient WAC EN 1015–18 50–100% (a)2 Water vapour resistance l 50–100% (a)2 Porosity (open, total) P% EN 1936 >80% (b)2 Pore size distribution micro-CT d (b)3 Colour difference DE CIE L * a * b* 50–150% [100%] (b)4 Dyn. E mod. Edyn UPV 20–100% [60%] (a)4 Adhesion Ra EN 1015–12 0.5–0.8% (a)5 Thermal expansion coefficient a TMA 50–150% [100%] (a)5 Hygric dilatation coefficient H d (b)6 Compressive strength Rc EN 1015–11 20–100% [60%] (a)6 Tensile strength Rf EN 1015–11 c (b)7 Ageing tests – d (b)

A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50 47

tion was made between areas of disrupted granite and a source ofcalcium ions in solution (run-offs, mortar joint). The calcium-ionsled to the deposition of gypsum in the granite [58,59].

Report has also been made of cement mortar used for therepointing of gneiss and arkoses: the replacement stone was cho-sen for purely esthetical reasons, and the original stone surface re-ceded 100 times faster in the restored zones (repointing withcement and replacement with a new stone) than in the other zones[58] (see Fig. 6). In [12] compatibility mortar recipes were de-scribed after thermal and chemical analysis of each repointingmortar. However, these new repair mortars did not show a good vi-sual compatibility with the original mortar, and therefore the finalrecipe was then adapted until visual appearance did match. Thechange in recipe will have had an impact on the other propertiesof the mortar as well. Schwengelbeck, finally, draws our attentionto the mode of application and the use of rods in the mortar [60].Although application techniques for repair mortar must not be ne-glected, they are beyond the scope of this research and will not befurther discussed here. These examples of incompatible use of re-pair mortars for natural stone, demonstrate which properties areconsidered important when evaluating the compatibility. Althoughvisual aspects (colour and surface roughness) are inferior in termsof durability than e.g. vapour diffusivity; in terms of compatibility,the visual aspects of the repair mortar are not to be neglected.The repair mortar has to re-evoke a certain type of authenticityto the building and should therefore have a comparable colourand structure in fresh and weathered state.

5. Compatibility tolerance

Several authors have discussed the compatibility requirementsfor repair mortars. Most of the researchers set requirements for re-

pair mortars, but do not discuss up to which level a difference inproperties between mortar and stone is acceptable [12,52,61].Van Balen et al. propose a general study to the buildings’ historyand architecture, which should lead to establishing some concep-tual requirements (harmonization, durability and sustainability)[61]. This makes the listing of functional requirements for thatbuilding possible, together with technical requirements. A mortarmix can then be designed that will hopefully lead to a compatiblerepair mortar. According to [61], the ‘most decisive technical char-acteristics’ for compatibility between a new and old (repointing)mortar are: (1) surface features, (2) composition (including grainsize distribution), (3) strength, (4) elasticity, (5) porosity proper-ties, (6) coefficient of thermal dilation and (7) others. Hayen dis-cusses the compatibility of different types of repair materials(lime-based, cement-based, polymer-based) through mechanical,physical, chemical and esthetical properties. These properties aregiven relative values (from ++ to – or o) [52]. Some researchershave proposed evaluation methods in terms of compatibility ofthe repair mortar with the stone, aiming a preservation of the ori-ginal material. Sasse and Snethlage suggest compatibility estima-tions for conservation treatments such as water repellence,consolidation and stone repair. The original material, i.e. the natu-ral stone, is hereby taken as reference level [62]. The requirementsfor the repair material are divided per property: rigidity, (compres-sive) strength, adhesion to the substrate, water (vapour) diffusionand thermal expansion (see Table 7). This method has appearedas well in Sasse and Schulze and Bromblet [47,63]. Bromblet usedthe method to evaluate six different repair mortars, and addedhydric dilation to the list of properties to be evaluated, but no tol-erance percentage was suggested.

A method to evaluate the compatibility of replacement stonesmight also be applicable for the evaluation of restoration mortars.

48 A. Isebaert et al. / Construction and Building Materials 59 (2014) 39–50

The method of Dessandier et al. takes into account the behaviour ofminerals in the presence of water: indication of clays, waterabsorption coefficient (48 h), capillarity and compressive strengthof the stone [64]. Each value range for each property is assigneda number, and the durability and the compatibility index is assem-bled by putting them next to each other (e.g. 186 A: a lowcompressive strength, high WAC, medium capillarity, A: absenceof clays). Veiga et al. describe the characteristics a rendering andrepointing mortar should have, as well as appropriate testingmethods [65]. Some value ranges that should make good repairrenders or repointing mortars are given as well. The focus lies onmechanical resistance, water transfer properties and shrinkingbehaviour of the mortar. Delgado Rodrigues et al. take this evenfurther and rank the properties according to the role they play inthe compatibility of a mortar with a stone [66]. The two ranks ofCompatibility Indicators (CI) are further rated in a rating scaledetermining an ‘incompatibility risk’ (0 for low risk, 5 for medium,10 for high risk). This rating method takes into account the proper-ties of the mortar and of the substrate (mortar, traditionalmasonry). These properties include e.g. the chemical composition,the porosity, the colour and the salt content of the mortar:elements that have not been discussed in previously describedcompatibility evaluating methods. [66]do state that this methodis in its first stage of development, and perhaps ought to beadapted when evaluated. Compatibility requirements for repoint-ing mortars in soft masonry were given absolute values by [38].Since these requirements are specifically etched on repointingmortars for soft masonry, these values are to be reconsidered sinceeach stone is different in properties.

6. Discussion and conclusion

Through the discussion of the main components of a mortar,more insight has been created in the behaviour of repair mortarsfor stone. All binders behave quite differently, even though theymay be made from similar materials. Each binder has its ownstrength and its weakness, and Hughes et al. assembled the valuesof the different binders in Table 5. Advantages of blended lime–cement mortars are the early-stage strength development, theresistance which is lower than average Portland cement, and whichcan be varied, by adapting the cement:lime ratio [29]. The amountof lime allows a certain elastic–plastic deformation [29]. Not onlythe binders play an important role in the behaviour of fresh andhardened mortar, the aggregates are important as well. In Table 6,a sketch situates various mortars of different binders and differentaggregates in terms of properties and assembles the characteristicsof each mortar component. The lime mortars have a higher defor-mability and water (vapour) transfer capacity. The cement mortarsare more resistant to compressive strength, but they are also den-ser and have a lower porosity. Their high thermal expansion coef-ficient and low deformability in combination with large thermalcycles can damage the stone [37].

Critical parameters were discussed next to indicate more clearlywhich and how one of the previously discussed components canhelp fulfilling one of these critical parameters. However, whenassembling or adjusting a repair mortar, not all of these parameterscan be answered at once. Visual appearance proved to be an impor-tant factor for the success rate of a repair mortar in terms of com-patibility, but is less important when looking at the durability.However, it must be taken into account when developing a repairmortar, since several cases are known where the proposed repairmortar was changed to meet the visual demands, not fulfillingother requirements that are important for the stone’s preservation.

In literature, several authors have tried to place these parame-ters in their ‘right’ order. In principle, they all mostly agree on each

other, although the representation is variable. [61] discuss thewhole picture, going from the different authenticity values of abuilding and how they finally lead to the choice of the repair mor-tar. Just like Hayen, the properties mentioned as decisive charac-teristics, are a good starting point [52]. Sasse and Snethlage givevariability ranges between mortar and stone in percentages, whichmakes the use of this method easier where stone is the originalmaterial, due to its heterogeneity [62]. [38,65] include the stan-dards and testing methods that they applied for their evaluationmethod. [64,66] rank each evaluated property, taking into accountthe role they play on compatibility level.

Table 8 is the authors’ interpretation of the ideas and conclu-sions gathered above. In the first column, the ranking of the prop-erty is presented and indicates in which order the properties arepreferentially tested. The lower the ranking, the more importantit is in terms of compatibility. Because of the dual meaning of thisranking, it is possible to test only a part of the list, adapt the mortarand test again: the most important properties are ranked first.Grain size distribution and mineral components are designatedas c, inferior in testing, since they are not tested (directly). How-ever, they are important and therefore fill first ranks: they bothaffect all other properties. It is advisable to start with knowledgeabout the mortar’s grain size and mineral components that shouldallow an estimation of properties. In the column ‘recommendation’the recommended compatibility with the stone is given, herebyfollowing Sasse and Snethlage in their evaluation methods. Thevariable range in the recommendation column allows operatingin between the specifically designed mortars for one stone sample,and the commercial, generalized mortars. All the testing methodsare designed to test mortar samples. When natural stone is tested,the norms and standards for natural stone will be used. However,no real comparison is then possible between stone and mortar tosee if they are indeed compatible. Another possibility can be thetesting of natural stone through natural stone test methods andmortar test methods.

Acknowledgement

The authors would like to thank the anonymous reviewers fortheir comments and suggestions, and they would like to thankthem for helping improving this article.

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