building materials chemistry

7/30/2019 Building materials chemistry

1/31

Dr. Kimberly KurtisSchool of Civil Engineering

Georgia Institute of TechnologyAtlanta, Georgia

Structure of theHydrated Cement Paste

Structure of the Hydrated Cement Paste

What do we mean by structure?

Type, amount, size, shape, and

distribution of phases present

macrostructure can be seen

unaided (200 m or larger)

microstructure must been

observed with the aid of a

microscope


2/31

Structure of Concrete

Macroscopically, concrete may be

considered to be composed of 2

phases coarse aggregate and

mortar (paste + fine aggregate) or

aggregate and paste.

heterogeneous distribution

At the microscale, we see that

these 2 phases are not

homogenous themselves!

Aggregate 60-75% of the solid volume of most concretes

The aggregate is principally responsible for the

unit weight, elastic modulus, and dimensional

stability of the concrete because these

properties depend on the physical

characteristics (strength, and bulk density) of the

aggregate.

In addition, porosity, shape and texture of the

aggregate are important for workability,

durability, and strength.

The chemical and mineralogical composition ofthe aggregate is usually less important, with the

exception of some deleterious and some

advantageous reactions.

Aggregate phase is generally stronger, than

the other 2 phases, with some exceptions.


3/31

Hydrated Cement Paste

SolidsC-S-H

CH

Ettringite

Monosulfate hydrate

Residual unhydrated cement

Voids Entrapped air (>1mm)

Entrained air (75-500um)

Capillary pores (macromeso)

Interlayer space(micropores)

Water

Capillary water

Adsorbed water Interlayer water

Chemically combinedwater

Important 3rd Phase in Concrete!In addition to the coarse

aggregate, fine aggregate and

paste (together the mortar

fraction), an important 3rd

phase generally exists the

transition zone (TZ) or

interfacial transition zone

(ITZ)

the interfacial region

between the coarseaggregate and the hcp

10-50 um thick

the weakest link


4/31

Structure of Concrete

Each of the phases may be heterogeneous in its

composition (both solids and voids)

Relative proportions and characteristics of the phases

vary with mixture composition, time, environment, etc.

All of these factors make predictions of

concrete behavior more challenging than

predictions for other materials.

Microstructure

Solids

C-S-H

CH

Ettringite

Monosulfate hydrate


Voids

Entrapped air (>1mm)

Entrained air (75-500um

Capillary pores (macro

meso)


Water

Capillary water Adsorbed water

Interlayer water



5/31

Cement Hydration Reactions

2C3S + 11H C3S2H8 + 3CH

2C2S + 9H C3S2H8 + CH

C3A + 26H + 3CSH2 C6AS3H32

2C3A + 4H + C6AS3H32 3C4ASH12

3C3A + 12H + CH C4AH13

C4AF + 10H + 2CH C6AFH12

Pozzolanic Reaction

Reaction of silica in pozzolan with calcium hydroxide:

xCH + yS + zH CxSyHX+ZHydration

WaterCalcium-silicate

hydrateCalcium

hydroxide

Silica in

pozzolan

Add it ional cemen ti tious C-S-H

In alumino-siliceous pozzolans (e.g. fly ash, slag and metakaolin)

the alumina also participates in reactions with calcium hydroxide

producing various calcium-aluminate hydrates (C-A-H) and calcium-

alumino-silicate hydrates (C-A-S-H).


6/31

Solids: Review


7/31

Microstructure

In solids, microstructural inhomogeneities can lead to serious effects on

strength and other related mechanical properties because these properties

are controlled by the microstructural extremes, not by the average

microstructure.

Thus, the presence of voids, cracks, and other defects play an important

role in determining the performance of the composite material.

Why do these defects exist in concrete?

Why do these defects exist in concrete?

Some voids result from the intrinsic nature of the

cement hydration process

Other voids are introduced intentionally or

unintentionally during mixing and/or placing

Microcracks and cracks can develop due to

mismatch between the components (i.e., different

CTE, E)

Microcracks and cracks can develop due to

loading and environment


8/31

Microstructure

SolidsC-S-H

CH

Ettringite

Monosulfate hydrate


Voids

Entrapped air (>1mm)

Entrained air (75-500um

Capillary pores (macromeso)


Water

Capillary water

Adsorbed water Interlayer water


Microstructure


9/31

Anhydrous cement

Water

Development of MicrostructureDevelopment of Microstructure

C-S-H

CH

Ettringite



10/31

C-S-H

CH

Ettringite


C-S-H

CH

Ettringite



11/31

C-S-H

CH

Ettringite


C-S-H

CH

Monosulfate



12/31

0 5 30 1 2 6 1 2 7 28 90

Minutes Hours Days

Amount

0 5 30 1 2 6 1 2 7 28 90

Minutes Hours Days

Amount

Porosity

CH

Ettringite


13/31

0 5 30 1 2 6 1 2 7 28 90

Minutes Hours Days

Amount

Porosity

CHEttringiteC-S-H

0 5 30 1 2 6 1 2 7 28 90

Minutes Hours Days

Amount Porosity

CH

Ettringite

C-S-H

C-(A,F)-H


14/31

0 5 30 1 2 6 1 2 7 28 90

Minutes Hours Days

Amount

Porosity

CH

Ettringite

C-S-H

C-(A,F)-H

Monosulfate

0 25 50 75 100

Degree of Hydration

0

25

50

75

100

RelativeVolume(%)

Capillary

porosity

C-S-H

Calcium

hydroxide

AFt/AFm

calcium

sulfate

C4AF

C3A

C2S

C3S


15/31

0 25 50 75 100

Degree of Hydration

0

25

50

75

100

RelativeVolume(%)

Capillary

porosityC-S-H

Calcium

hydroxide

AFt/AFm

calcium

sulfate

C4AF

C3A

C2S

C3S

Water-filled porosity

C3S

C4AF

C3A

C2S

CSH2

Hydration ProductsHydration Products

0 25 50 75 100

Degree of Hydration

0

25

50

75

100

RelativeVolume(%)

Capillary

porosity

C-S-H

Calcium

hydroxide

AFt/AFm

calcium

sulfate

C4AF

C3A

C2S

C3S

C-S-H

CH

AFt/AFm


16/31

0 25 50 75 100

Degree of Hydration

0

25

50

75

100

RelativeVolume(%)

Capillary

porosityC-S-H

Calcium

hydroxide

AFt/AFm

calcium

sulfate

C4AF

C3A

C2S

C3S

0 25 50 75 100

Degree of Hydration

0

25

50

75

100

RelativeVolume(%)

Capillary

porosity

C-S-H

Calcium

hydroxide

AFt/AFm

calcium

sulfate

C4AF

C3A

C2S

C3S


17/31

0 25 50 75 100

Degree of Hydration

0

25

50

75

100

RelativeVolume(%)

Capillary

porosityC-S-H

Calcium

hydroxide

AFt/AFm

calcium

sulfate

C4AF

C3A

C2S

C3S

Development of Microstructure

50

40

30

20

10

0

CapillaryPorosity(%)

0.30 0.40 0.50 0.60 0.70 0.80 0.90

W/CM

100% Hydration

Young et al. 1998

Voids: Capillary Porosity Capillary porosity

results from the excess

water used for economy

and workability in the

vast majority of concrete

mixtures.

Al+3

Al+3

Al+3

Ca+2

Ca+2

Ca+2

OH-

OH-

OH-

SiO-

SiO-


18/31

100

50

0

0 20 20 30 40 50 60

Curing time (days)

20

30

40

50

60

CapillaryPorosity(%)

DegreeofHydration(%)

Capillary p orosity

Degree of hydration

Young et al. 1998

Development of Microstructure

Volume ofcapillaryporosity inconcrete is alsorelated todegree ofhydration,which isaffected bycuring (time,Temp, RH)

VoidsThe presence of voidsaffects

Strength

Stress distribution(concentrations)

permeability

freeze/thawresistance

Inverse relationshipbetween strength (fc)

and porosity (p)fc=k(1-p)

3

k=strength of voidlessmortar ~ 34,000 psi


19/31

Transport/Permeability

Capillary

porosity, which

is a function of

w/c, is an

important factor

in determining

the permeability

of the paste.

Permeability = porosity

Solids and Porosity

Powers developed a simple model to estimate the

amount of capillary porosity in a cement paste with

varying degrees of hydration and at different water-

to-cement ratios.

Based on the assumption that 1cm3 of cement

produces 2cm3 hydration product on full hydration.


20/312

Case A: Increasing Degree of Hydration

Consider a paste with w/c of 0.63.

What is the capillary porosity at:

7 days assuming the cement is 50% hydrated?

28 days, 75% hydrated?

365 days, 100% hydrated?


21/312

Case B: Increasing w/c

Assuming 100% hydration, what is the capillary

porosity for w/c=0.70, 0.60, 0.50, and 0.40?

Capillary PorositySome other, simple models describing capillary porosity depend critically

on the volume fractions of water-filled W(t) and total capillary porosity

T(t) and unhydrated cement (t), as a function of time, t. Based on

Power's model for cement hydration, for an ordinary portland cement

paste, these quantities are given by:

T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened portland cement paste. Bulletin 22,

Research Laboratories of the Portland Cement Association, Chicago, 1948.

K.A. Snyder, D.P. Bentz, Cem Concr Res, 34 (11) (2004) 2045-2056.

where (w/c) is the water-to-cement mass ratio

is the degree of hydration (reacted fraction)

of the cement at time t,

cem is the specific gravity of cement,

exp is the volumetric expansion coefficient for

the "solid" cement hydration products relative

to the cement reacted (often taken to be=1.15),

CS is the chemical shrinkage per gram of

cement (= 0.07 mL/g for sealed conditions and

=0 for saturated conditions)


22/312

Microstructure

1-5 nm

Ult. strength

Adsorption

50 nm, more significant for strength, permeability

Entrained air - spherical voids 70-500um in size; added for

freeze/thaw resistance

Entrapped air - irregular in shape; can be large

Pores > 2.5nm may be filled with air, water (pore solution), or a

mixture


23/312

Classification of Voids in the hcp

Water in the hcp

Ratio of mass of water to mass of cement in a mixture is the

water-to-cement ratio or w/c

When SCMs are used, this is the water-to-cementitious

materials ratio or w/cm

w/c or w/cm may range 0.20-0.80, but 0.40-0.60 is typical

Water is

Introduced to the concrete during mixingNecessary for reaction of cement and SCMs

Permeates the concrete during service

Because the water in concrete contains ions, it is usually

called pore solution and has a high pH


24/312

Pore Solution

0

200

400

600

800

1000

0 20 40 60 80 100

Na+K+Ca++

OH-SO3-

ionconcentration

(x10-3

mol/l)

curing time (d)

Based upon Page and Vennesland, Materiaux etConstructionsV16:19-25, 1981

Some model pore solutions:

High alkali (pH ~ 13.8) 0.55M

KOH + 0.16M NaOH

(Lawrence solution)

Low alkali (pH ~ 13.5) 0.24M

KOH + 0.08M NaOH

Saturated Ca(OH)2 + 0.7M

NaOH

Pore Solution


25/312

Water in hcp

Capillary water - water present in voids larger than 2.5nm- In capillaries >50nm, water exists as free water because its removal

does not cause volume change

- In capillaries 2.5-50nm, removal of water results in shrinkage because

new bonds can form between C-S surfaces

Water in hcpAdsorbed water - water physicallyadsorbed to the solid surfaces in C-S-H- can be removed on drying to RH ~ 30%,resulting in shrinkage

Interlayer water- water associatedwith the C-S-H structure- can be removed only on strong drying toRH ~ 11%, resulting in shrinkage

Chemically combined water - waterthat is an integral part of varioushydration products- lost only on decomposition during heating


26/312

Nature of Composite Materials

In virtually all composite materials, defects are present ingreater density at the interface between the different

constituents

Often, the composite properties are governed by the nature of

the interfaces

www.uf-bio-nano-center.org/ electron.aspImage courtesy of Ben Mohr

Effects of the 3rd Phase

Mehta and Monteiro, 1993


27/312

Interfacial Transition Zone

The ITZ is the region 10-50umwide around coarse

aggregate; characterized by:

Higher local porosity

Greater density of pre-

existing microcracks due to

differential shrinkage and

drying

Larger CH crystals that tend

to be oriented and more prone

to cleavage.

Interfacial Transition Zone: NIST models

Agg Agg

Cement particles (red) around a

model square aggregate at

w/c=0.47, before hydration

After 77% hydration.

Color key: unhydrated cement,

CC--SS--HH, CH, porosity

http://ciks.cbt.nist.gov/garbocz/paper43/node2.html#Figure%201.


28/312


The mechanism by which the transition zone is formed isassociated with the development of water films around theaggregate in fresh concrete that , in effect, create a localregion of with higher water-to-cement ratio.

Wall effect packing effect; aggregate surface acts as awall, making packing of the cement particles inefficient =>high porosity region (more important)

One-sided growth when no aggregate is present,

hydration products grow in all directions. close to anaggregate, growth only occurs on the cement side,contributing to porosity.

Wall EffectPorosity fraction near aggregate surface prior to hydration for two

different cements. The median cement particle diameters of the two

cements are: A1 - 28 m, A7 - 11 m.*

Effects of bleeding ignored.

*Bentz, D.P., Garboczi, E.J., and Stutzman, P.E., in Interfaces in Cementitious Composites, Ed. J.C. Maso (E & FN Spon, London,

1992) pp. 107-116.


29/312

Relative influence of 1-sided growth

http://ciks.cbt.nist.gov/garbocz/paper43/node3.html

Wall effect and 1-sided growth

1-sided growth only (effect apparent

only a few um from aggregate)


Stress-strain behavior for

both the aggregate and

cement paste alone are

nearly linear elastic.

But because of the ITZ,

concrete displays some

nonlinear and inelastic

behavior in compression.


30/313


Interconnectivity of microcracks and pores in TZ also

increases permeability, durability suffers.

http://ciks.cbt.nist.gov/~garbocz/paper72/paper72.html

By tailoring the concrete mixture to

reduce the influence of the ITZ,

strength, E, and impermeability are

increased.

lower w/c

higher cement content

use of SCMs

smaller MSA

reactive dolomitic aggregate lightweight aggregate

extended moist curing

Interfacial Transition ZoneInterfacial Transition Zone


31/31


The microstructural features and mechanical effects of thetransition zone are the subject of some debate.

Some researchers report the presence of a duplex film

consisting of thin layer of CH adhering to the aggregate

surface surrounded by thin layer of rod-like C-S-H [Hewlett,

1998]

Others disagree finding C-S-H to be the solid phase most

often in contact with the aggregate surface [Scrivener and

Gartner, 1988]

In addition, some researchers have come to believe that a

weaker interfacial region does not always exist between thehydrated cement paste and the aggregate [Mindness and

Diamond, 1992].

building materials chemistry

Documents