Technical Paper-cement Chemistry

8/19/2019 Technical Paper-cement Chemistry

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CEMENT CHEMISTRY

SOURCE: theconcreteportal

Hydration of cement

The reaction of cement hydration is exothermic. Measurements using a conduction

calorimeter can give the rates of heat evolution at various stages.

A typical heat evolution pattern from cement hydration is presented in Figure 1. There arethree characteristic peaks for ordinary Portland cement. The initial heat burst corresponds to

the instantaneous high rate of heat evolved when cement is brought into contact with water.

This is due to the heat of wetting (Heat of wetting = Surface energy – Energy required for

interface creation). Hydration of C3S and C3A also contribute to this peak.

Figure 1: Heat evolution during cement hydration

The initial burst is followed by a slowdown of the heat evolution rate. The rate does not

become negative or zero at any stage, implying that although slowly, the reactions do

continue. This is termed as the ‘dormant’ or the ‘induction’ period. This period is followed by

the main peak of cement hydration, which is associated with the rapid dissolution of C3S to

form CSH and CH, and formation of ettringite (AFt) from C3A.

A slowdown of the hydration process beyond the main peak leads to lower rates of heat

evolution. A broader peak is associated with the conversion of ettringite to monosulphate

(AFm).


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The latest calorimeters can also detect an extra endothermic peak in the beginning that

corresponds with the dissolution of potassium sulphate (when it is present in the cement).

It is difficult to obtain the correct relationship between heat evolution and temperature unless

the system is perfectly insulated. Another problem is the dependence on the water to cement

ratio. Water has a much higher specific heat than cement, thus when more water is present, ahigher degree of heat will be required to increase the temperature of the system.

Cement contains highly soluble alkali oxides (Na2O and K 2O). The dissolution of these

compounds is responsible for the high alkalinity (pH 12 – 13) of the pore solution. Thus, the

hydration of cement actually takes place in the pore solution, and not in water.

Dormant Period

Various theories have been proposed for the existence of the dormant period. As stated

earlier, the rate of heat evolution during this stage is low. The slowdown of the hydration

process has been explained using the following ideas:

1. Formation of an impermeable hydrate layer (CSH) on the surface of the C3S

particle precludes the further dissolution of C3S.

2. The hydrate layer has a lower C/S ratio compared to C3S. As a result Ca2+ is

released into the liquid phase (which contains OH-), and a silica rich layer

forms on the surface of the C3S particle. This electrical double layer thus

formed prevents any reaction to form CSH by impeding the passage of ions.

3.

Liquid phase gets supersaturated with respect to CH. As a result CH starts

precipitating and this stops the further dissolution of C3S.

The end of the dormant period can come about in many ways:

1. The barrier can weaken due to ageing.

2. Diffusion of ions can occur across the barrier by osmosis.

3. A gradual weakening of the electrical double layer may occur.

4. Nucleation of CH can get slowed down when the nuclei start approaching their

critical size.

Reactions during hydration

Reactions involving the silicates

2 C3S + 6 H —›C3S2H3 + 3 CH (1)

2 C2S + 4 H —›C3S2H3 + CH (2)

The above reactions are perfectly stoichiometrically balanced. However, C-S-H does not

have a well defined stoichiometry. The C/S of C-S-H can vary from 1.5 to 2, and commonly

is around 1.8. The main difference in the hydration of the two silicates lies in the amount of

CH formed in the reaction. It is evident from the above equations that 3 times as much CH is

formed from C3S hydration as in C2S hydration.


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C-S-H does not have a definite structure is thus termed as a gel. CH deposits as hexagonal

crystals, generally oriented tangentially to pore spaces and aggregates along the longitudinal

axis.

Reactions involving the aluminates

In the absence of gypsum, calcium aluminate hydrates form from C3A, resulting in a flash set

of the cement paste.

2 C3A + 21 H —›C4AH13 + C2AH8 (3)

C2AH8 is a metastable phase that deposits as hexagonal platelets (similar to CH). Above 30oC, it is converted to cubic hydragarnet (C3AH6).

In the presence of gypsum, ettringite formation occurs.

C3A + 3 C S H2 + 26 H —›C6A S 3H32 (4)

Ettringite (or the AFt phase) gets deposited as acicular, columnar, hexagonal crystals. The

presence of tubular channels in between the columns can lead to high water absorption and

swelling by ettringite. This is one of the theories explaining the expansion caused by

ettringite formation.

Nearly all the SO42- gets combined to form ettringite in an ordinary Portland cement. If there

is still C3A left after this reaction, it can combine with ettringite to form monosulphate (or

AFm phase) which has a stoichiometry of C4A S H12-18. If there is sufficient excess C3A, then

C4AH13 can also form as a hydration product, and can exist in a solid solution with AFm.

C4AF produces similar hydration products as C3A, with the Al3+ being partly replaced by

Fe3+. The final hydration product depends on the availability of lime in the system. In the

presence of gypsum, C4AF produces an iron-substituted ettringite. Higher the ratio

C4AF/C3A, lower is the conversion of ettringite to monosulphate.

Kinetics of cement hydration

The progress of cement hydration depends on:

Rate of dissolution of the involved phases (in the initial stages), and at later stages,

Rate of nucleation and crystal growth of hydrates

Rate of diffusion of water and dissolved ions through the hydrated materials already

formed

The factors affecting the kinetics of hydration are:

1. The phase composition of cement

2. The amount and form of gypsum in the cement: Whether gypsum is present in the

dihydrate, hemihydrate, or the anhydrite form.

3.

Fineness of cement: Higher the fineness, higher the rate of reaction due to availabilityof a larger surface area.


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4. w/c of mix: At high w/c, hydration may progress till all of the cement is consumed,

while at low w/c the reaction may stop altogether due to lack of water.

5. Curing conditions: The relative humidity can have major effects on the progress of

hydration.

6. Hydration temperature: Increase in temperature generally causes an increase in the

rate of the reaction, although the hydrated structure can be different at differenttemperatures.

7. Presence of chemical admixtures: For example, set controllers, and plasticizers.

Stages in Cement Hydration

1. Pre-induction period (first minutes):

Rapid dissolution of ionic species (alkali sulphates contribute K +, Na

+, and SO4

2-;

CaSO4 dissolves until saturation, contributing Ca2+ and SO4

2-)

C-S-H forms on the surface of dissolving C3S. C/S of C-S-H is lesser than of C3S,

thus an increase of the Ca2+ concentration in the liquid phase occurs. Formation ofelectrical double layer, and the precipitation of CH leads to the dormant period.

C3A dissolves, and reacts with SO42- to form AFt, which forms a surface barrier. C4AF

also reacts to form AFt.

Only very small % of C2S reacts at this stage.

2. Induction (dormant) period (first few hours):

CH concentration in the liquid phase reaches a maximum and then starts to decline.

The concentration of SO42- remains constant as the amount consumed due to AFt

formation is balances by the amount dissolved from gypsum.

3. Acceleration stage (3 – 12 hours after mixing):

Nucleation and growth of C-S-H (often termed as the ‘second-stage CSH’) and CH

occurs. C2S also starts hydrating substantially.

Ca2+ concentration in the liquid phase declines as Ca(OH)2 starts precipitating.

SO42- concentration starts to decline with increasing AF t formation, and adsorption of

SO42- on C-S-H.

4. Post-acceleration period:

Slow down due to decline in non-reacted material, and because the process becomes

diffusion controlled.

The contribution of C2S increases steadily, leading to a decline in the rate of

formation of CH.

Consumption of SO42- leads to a conversion of AFt to AFm.

The progress of hydration, both in terms of the unhydrated compounds consumed, as well the

hydration products formed, has been presented in Figure 2. In the first few minutes, about 2 –

10 % of C3S hydrates, and a significant fraction is consumed within 28 days. The rate of

hydration depends upon the reactivity of alite (i.e. the amount of foreign ions present within

the alite structure). With an increase in the amount of SO3, the C3S reaction becomes faster.However, beyond a limit, SO3 can start causing retardation.


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The hydration of C2S is a slow process, and does not pick up for many hours. On the other

hand, 5 – 25% of C3A reacts in the first few minutes of hydration. The initial reactivity

depends on the quantity and quality of alkalis present (K + increases reactivity, while Na+

decreases it).The reactivity of C4AF is dependent on the A/F of the cement.

The method of grinding cement may also influence the hydration kinetics. Cements ground in

high pressure roller mills set faster than in ball mills, because of higher reactivity of C 3A andC3S phases, and a lowered rate of decomposition of CaSO4.

Figure 2: Progress of cement hydration

Composition of pore solution

The evolution of pore solution composition for a typical cement (0.6% equivalent Na2O, 3%

SO3, 0.5 w/c) is shown in Figure 3. By 1 week, the only ions remaining in appreciable

concentration are Na+, K +, and OH-. The concentration of OH- is almost a mirror image of

that of SO42-

, due to considerations of ionic balance within the pore solution. Ground clinkerwould typically have a lower ionic concentration in the pore solution due to the absence of

SO42-.


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Figure 3: Evolution of the pore solution composition in cement paste

Structure of hydrated cement paste

The following micrographs (obtained by Scanning Electron Microscopy) reveal the typical

features of a hydrated cement paste.


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Figure 4: Fracture surface of a PC mortar showing the gel-like nature of C-S-H


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Figure 5: Image of polished surface of a PC mortar; the bright particles are that of unhydrated

cement; the grayish background is the C-S-H, while the white rims around the aggregate

pieces are deposits of CH


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Figure 6: Polished surface of a C3S mortar showing hydrating grains of C3S; the darker

shades are C-S-H deposits, while the lighter shades, especially as rims around aggregates are

deposits of CH


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4. Bound water: This is chemically bound to the hydration product, and can only be

removed on ignition. Also called ‘non-evaporable’ water.

2 and 3 are together called ‘gel’ water.

Calculation of the structure of hydrated cement

Theoretically, 0.23 g of bound water is required to completely hydrate 1 g of cement. The

remaining water fills up the pores within the structure of the hydrated cement paste (hcp),

called the gel pores, as well as the pores external to the hcp, called the capillary pores.

Upon hydration, a volume decrease in the amount of 25.4% of the bound water occurs in the

solid hydration product. The characteristic porosity of the hydrated gel is 28%.

Using the above data, some sample calculations are provided below.

Scenario 1: w/c = 0.50; Assume 100% hydration and no drying; Calculate the volume ofcapillary pores.

Let mass of cement = 100 g. Hence, Vcem = 100/3.15 = 31.8 ml

Mwater = 50 g, therefore Vw = 50 ml.

V bound-w = 23 ml

Hence, Vsolid-hcp = 31.8 ml + 23 ml – 0.254 x 23 ml = 48.9 ml

Porosity = 28% = 0.28 = Vgel-pores / (48.9 ml + Vgel-pores) —›Vgel-pores = 19.0 ml

Hence total hcp volume = 48.9 + 19.0 = 67.9 ml

Total reactant volume = 31.8 + 50 = 81.8 ml.

Therefore, volume of capillary pores, Vcap-pores = 81.8 – 67.9 = 13.9 ml

Of these, (50-23-19) = 8 ml will be filled with water, and the remaining (5.9 ml) will be

empty.

From the above scenario, 23 ml + 19 ml = 42 ml of water is required for complete conversion

of 100 g of cement to the hydration product. In other words, a w/c of 0.42 is required. What

would happen if the w/c is less than 0.42? Consider the next scenario.

Scenario 2: w/c = 0.30; Cement = 100 g, water = 30 g; Assume that p grams of cement

hydrates.

Hence Vsolid-hcp = p/3.15 + 0.23p – 0.254 x 0.23p = 0.489p

Porosity = 0.28 = Vgel-pores / (0.489p + Vgel-pores) (5)Total water = 30 ml = 0.23p + Vgel-pores (6)

Solving (5) and (6), p = 71.5 g, and Vgel-pores = 13.5 ml

Thus, Vhcp = 0.489 x 71.5 + 13.5 = 48.5 ml

Vunhyd-cem = (100 – 71.5)/3.15 = 9.1 ml

Hence, Vcap-pores = (100/3.15 + 30) – (48.5 + 9.1) = 4.2 ml

That means there are 4.2 ml of empty capillary pores. If this cement paste gets any externalmoisture (for example, from curing) more cement will hydrate and fill up this space.

Structure of cement hydration products


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The structure of C-S-H is best described by the Feldman-Sereda model, shown in Figure 8. It

consists of randomly oriented sheets of C-S-H, with water adsorbed on the surface of the

sheets (adsorbed water) , as well as in between the layers (interlayer water), and in the spaces

inside (capillary water). Such a model implies a very high surface area for the gel. This is

indeed found to be true. Using water sorption and N2 sorption measurements, a surface area

of 200000 m2

/kg is reported (ordinary PC has a fineness in the order of 225 – 325 m2

/kg).Small angle X-ray scattering measurements show results in the range of 600000 m2/kg. The

corresponding figure for high pressure steam-cured cement paste is 7000 m2/kg, which

suggests that hydration at different temperatures leads to different gel structures. The

structure of C-S-H is compared to the crystal structure of Jennite and Tobermorite. A

combination of the two minerals is supposed to be the closest to C-S-H.

Figure 8: Feldman-Sereda model for CSH

Calcium hydroxide deposits as hexagonal crystals. These crystals are typically aligned in the

long direction inside pores and around aggregate surfaces.

The structure of ettringite consists of tubular columns with channels in between the columns.

The imbibing of water in these channels can lead to substantial expansions. Ettringite

demonstrates a trigonal structure, while monosulfate is monoclinic.

Figure 9 depicts the relative sizes of pores in concrete. At one end of the scale are entrapped

air voids, while on the lower extreme are the interparticle spaces between sheets of CSH.

Figure 9. Ranges of pore sizes in concrete


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Chemistry of special cements

Expansive cement

These are cements based on mixtures of Portland cement clinker with expansive compounds.

Upon hydration, the typical products that form and cause expansion are ettringite and calciumhydroxide (such as that resulting from free CaO). Many different types of expansive cements

are available:

Type K: PC clinker + expansive clinker + gypsum (or gypsum-anhydrite mixture)

The expansive clinker in Type K cement is fired separately. It is composed of a mixture of

Alite, Belite, Ferrite, anhydrite and Klein compound, which has the formula C4A3S . In the

early stages of hydration, Klein compound reacts faster than C3A to form ettringite. The

formation of extra ettringite in the plastic state leads to the initial expansion, which is able to

overcome the shrinkage that results from drying.

Type M: PC clinker + Calcium aluminate cement + calcium sulphate (anhydrite)

Type S: High C3A Portland cement with additional gypsum. In general, it is difficult to

control the rate of ettringite formation from C3A.

Type O: Portland cement clinker + mixture of Alite, CaO, and anhydrite. In this case, the

expansion results from the hydration of the free lime. The CaO is present largely as

inclusions within the alite grains and undergoes hydration more slowly as the alite hydrates,

resulting in controlled expansive properties.

Supersulphated cement

This cement is a mixture of blast-furnace slag, PC clinker, and calcium sulphate. The amount

of blast furnace slag is usually in the range of 80 to 85 % (not less than 75%), while calcium

sulphate is added in the amount of 10 – 15%. Overall, the SO3 content of this cement is

controlled to be always greater than 4.5%.

This cement requires more water for hydration compared to Portland cement. It is also more

susceptible to deterioration during storage due to carbonation. The heat of hydration is lower

than PC. When the temperature in service exceeds 50 oC, these cements show a drop in

strength, possibly as a result of some changes to the crystal structure of ettringite, which is

primarily responsible for the initial strength and stiffening.

The absence of CH in the hydration product and conversion of all aluminous compounds into

ettringite during the initial stages makes this cement highly resistant to sulphate attack.

The formation of ettringite is affected by the quantity of lime (CH) available. For a proper

reaction, neither too high nor too low of an amount of lime is required. When the amount of

lime is too low, carbonation might combine all of the lime available, which is not good. Too

much lime (as when there is too much PC in the cement) will interfere with the reaction

between slag and calcium sulphate.

Calcium Aluminate cement (or High Alumina cement)


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This cement contains 32 – 45% Al2O3, about 15% iron oxides, and 5% SiO2, with the

remainder composed of CaO. The primary phase present is Calcium Aluminate, or CA. This

cement is produced by sintering a mixture of aluminous (typically bauxite) and calcareous

components, and grinding to a fine powder. A complete fusion of all the compounds occurs

in the kiln itself, and thus this cement is also called ‘Ciment fondu’ in French.

The types of hydration products that form are dependent on the temperature of the system.

When the temperature is less than 10 oC, CAH10 is the hydration product, while between 10

and 27 oC, CAH10 and C2AH8 form. Both these phases, however, are unstable, and a

conversion to the stable phase C3AH6 occurs when the temperature exceeds 27oC. In the long

term, gibbsite (AH3) also forms.

The setting time of this cement is similar to PC. The initial strength gain is much faster than

PC. For hydration at ambient temperatures, the strength is due to the filling up of pore spaces

by the metastable hydration products such as CAH10 and C2AH8. There is a decline in

strength when the temperature increases and a conversion to C3AH6 occurs. This conversion

can also occur as a result of ageing. The loss in strength due to conversion is a result of anincrease in the porosity of the system. The long term strength is due to C3AH6 and AH3.

Apart from strength, the durability of the cement is also compromised due to this conversion.

CAH10 is inert with respect to sulphates, but C3AH6 can react with SO42- in the presence of

lime to form ettringite. The increase in porosity also increases the permeability of the system.

The degree of strength loss is dependent on the w/c of the system. At higher w/c, the strength

loss is greater.

At extremely high temperatures (such as those found in furnaces and kilns), a ceramic bond

can develop between the hydration products and fine aggregate. This lends a very high

durability at high temperatures. Thus CA cement is a popular choice for refractory linings.

Technical Paper-cement Chemistry

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