CE 6110 – Advanced Concrete Technology

Cement Chemistry

Cement hydration

• Reaction of cement with water

• Exothermic; heat released is called ‘Heat of Hydration’

• Rate of heat evolution is faster if the reaction is quicker

• Heat evolved depends on heat of hydration of individual compounds, and also on the clinker morphology!

Heat evolution

Heat of hydration of pure cement compounds

Compound HOH (J/g)

C3S 502

C2S 260

C3A 867

C4AF 419

Bogue: ½ of total heat is evolved between 1 and 3 days, about ¾ in 7 days, and 83 – 91% in 6 months

Polystyrene cover

Interface Circuits and Relays

Signal Conditioning Unit

Sample Chamber

Secondary Water Tank

0 50 100 150 2000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

C1 C2

Matu

rity H

eat r

ate, W

/kg

t20

hours

Ramesh Babu, 2008

Calorimeter

Adiabatic calorimeter at WITS, South AfricaHeat evolution pattern

Isothermal calorimeter at EPFL, Switzerland

Typical pattern of heat evolution

Taylor, 1996

Suggested new heat evolution pattern that shows an initial endothermic peak due to the dissolution of alkali sulphatesHewlett 2001

Heat peaks

• Peak I: ‘Heat of wetting’ + some early C-S-H formation

• Dormant period: Very slow rate of heat evolution• Peak II: Main peak; 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 (III) is associated with the conversion of ettringite to monosulphate (AFm).

Points to ponder..

• Difficult to obtain the correct relationship between heat evolution and temperature unless the system is perfectly insulated

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

• Cement contains highly soluble alkali oxides (Na2O and K2O). 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

• Several theories proposed• These theories basically point to the fact that a layer

(either of hydrates or ions) is created on the surface of cement particles

• Further wetting of the cement particle is possible only by diffusion across this layer

• Therefore, rate slows down• End of dormant periods occurs when:

- Barrier gets weakened by aging- Rates of diffusion increase, and- Ionic strength around the hydrating particle reduces

Hydration reactions

• Silicates (C3S and C2S) hydrate to produce Calcium-silicate-hydrate (C-S-H) gel and calcium hydroxide (CH)

• 3 times as much CH produced by C3S hydration compared to C2S

• C-S-H does not have a well-defined composition; C/S varies from 1.5 to 2

• Aluminates (C3A and C4AF), in the absence of gypsum, hydrate rapidly to produce Calcium-aluminate-hydrates (C-A-H)

• In the presence of gypsum, ettringite (AFt) and monosulphate (AFm) are produced (depending on the C3A to SO3 ratio)

• Ettringite formation is known to be expansive (numerous mechanisms suggested)

Reactions - Specifics

• 2 C3S + 6 H C3S2H3 + 3 CH

• 2 C2S + 4 H C3S2H3 + CH

• 2 C3A + 21 H C4AH13 + C2AH8

Flash set reaction!

• C2AH8 is a metastable phase that deposits as hexagonal

platelets (similar to CH). Above 30 oC, it is converted to cubic hydrogarnet (C3AH6).

• In the presence of gypsum,

C3A + 3 CSH2 + 26 H C6AS3H32

Aluminate-sulphate reactions

• 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 C4ASH12-18.

• 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.

Evolution of hydration

Young et al. 1998

Evolution of hydration products

Taylor, 1968

‘Inner’ and ‘Outer’ CSH

Inner CSH (Phenograin CSH)

Dense

Contains less Al and S

Formed with large grains

Outer CSH (Groundmass CSH)

Less dense (has more pores)

Contains more Al and S (AFm)

Formed with small grains

Kinetics of 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

Factors affecting hydration rate

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 availability of a larger surface area. 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 different temperatures.

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

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 here.

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 SO4

2-, due to

considerations of ionic balance within the pore solution. Ground clinker would typically have a lower ionic concentration in the pore solution due to the absence of SO4

2-.

Hydrated cement paste

• Hydrated cement paste is composed of capillary pores and the hydration product.

• The pores within the structure of the hydration product are termed ‘gel’ pores. This hydration product includes C-S-H, CH, AFt, AFm, etc.

• Gel pores are included within the structure of hydrated cement.

• According to Powers, 1/3 of the pore space is comprised of gel pores, and the rest are capillary pores.

Some micrographs

Gel-like C-S-H observed Bright particles: Unhydrated cement; Gray regions: C-S-H; white rim around aggregates: CH

Some micrographs

C3S mortar showing white unhydrated cement particles and gray C-S-H, along with white rims of CH

Porosity of paste in concreteis visible in this picture

Water within cement paste

1. Capillary water: Present in voids larger than 50 Ao. Further classified into: (a) free water, the removal of which does not cause any shrinkage strains, and (b) water held by capillary tension in small pores, which causes shrinkage strains on drying.

2. Adsorbed water: Water adsorbed on the surface of hydration products, primarily C-S-H. Water can be physically adsorbed in many layers, but the drying of farther surfaces can occur at about 30 % relative humidity. Drying of this water is responsible for a lot of shrinkage.

3. Interlayer water: Water held in between layers of C-S-H. The drying of this water leads to a lot of shrinkage due to the collapse of the C-S-H structure.

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 hcp structure

• 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%.

Example 1

w/c = 0.50; Assume 100% hydration and no drying; Calculate the volume of capillary pores.

• Let mass of cement = 100 g. Hence, Vcem = 100/3.15 = 31.8 ml• Mwater = 50 g, therefore Vw = 50 ml.• Vbound-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 example.

Example 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) ………………… (1)• Total water = 30 ml = 0.23p + Vgel-pores ..…………………………. (2)• Solving 1 and 2, 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 external moisture (for example, from curing) more cement will hydrate and fill up this space.

Structure of hydration products

SE micrograph of paste

Scanning electron micrograph of 7-day old hardened cement paste (Scanning electron micrograph of 7-day old hardened cement paste (×3500)×3500). . Bottom left: cement particle coated with C-S-H surrounded by ettringite needles; Bottom left: cement particle coated with C-S-H surrounded by ettringite needles; Upper left: platelets of monosulfoaluminate; Right: large crystal of calcium hydroxideUpper left: platelets of monosulfoaluminate; Right: large crystal of calcium hydroxide

Young et al. 1998

CSH

Mehta and Monteiro 1993

Taylor, 1968

CSH Structure – Feldman Sereda model

Young et al., 1998

The structure of C-S-H is best described by the Feldman-Sereda model. 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).

CSH Properties

• The Feldman Sereda model suggests a high surface area for CSH

• 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.

• High covalent/ionic and Van der Waals forces• Occupies a volume of 50-65% of the paste

Calcium Hydroxide

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

• Low surface area (0.5 m2/g)

• Low Van der Waals forces

• Volume – 20-25%

Mehta and Monteiro, 1993

Calcium sulphoaluminates

• Ettringite: Acicular, columnar, hexagonal crystals (seen as prismatic needles). 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.

• Monosulphate – plain hexagonal• Volume occupied is 15 – 20%

Ettringite and monosulphate

Ettringite MonosulfoaluminateEttringite Monosulfoaluminate

Mehta and Monteiro, 1993Mindess and Young, 1981

Pores and Air Voids

Mehta and Monteiro, 1993