Introduction

The hydration of Portland cement involves the reaction of the anhydrous calcium silicate and aluminate phases with water to form hydrated phases.

These solid hydrates occupy more space than the anhydrous particles and the result is a rigid interlocking mass whose porosity is a function of the ratio of water to cement (w/c) in the original mix.

Provided the mix has sufficient plasticity to be fully compacted, the lower the w/c, the higher will be the compressive strength of the hydrated cement paste/mortar/concrete and the higher the resistance to penetration by potentially deleterious substances from the environment.

Cement hydration is complex and it is appropriate to consider the reactions of the silicate phases (C3S and C2S) and the aluminate phases (C3A and C4AF) separately. The hydration process has been comprehensively reviewed (Taylor, 1997).

Hydration of silicates

Both C3S and C2S react with water to produce an amorphous calcium silicate hydrate known as C–S–H gel which is the main ‘glue’ which binds the sand and aggregate particles together in concrete.

The reactions are summarized in Table 1.3. C3S is much more reactive than C2S and under ‘standard’ temperature conditions of 20°C approximately half of the C3S present in a typical cement will be hydrated by 3 days and 80% by 28 days. In contrast, the hydration of C2S does not normally proceed to a significant extent until ~14 days.

The C–S–H produced by both C3S and C2S has a typical Ca to Si ratio of approximately 1.7.

This is considerably lower than the 3:1 ratio in C3S and the excess Ca is precipitated as calcium hydroxide (CH) crystals. C2S hydration also results in some CH formation.

The following equations approximately summarize the hydration reactions:

C3S + 4.3H ⇒ C1.7SH3 + 1.3CH

C2S + 3.3H ⇒ C1.7SH3 + 0.3CH

An important characteristic of C3S hydration is that after an initial burst of reaction with water on first mixing it passes through a dormant, or induction, period where reaction appears to be suspended.

This is of practical significance because it allows concrete to be placed and compacted before setting and hardening commences.

Several theories have been developed to explain this dormant period. The most favoured is that the initial reaction forms a protective layer of C–S–H on the surface of the C3S and the dormant period ends when this is destroyed or rendered more permeable by ageing or a change in structure.

Reaction may also be inhibited by the time taken for nucleation of the C–S–H main product once water regains access to the C3S crystals.

Hydration of C3A and C4AF

The reactions of laboratory-prepared C3A and C4AF with water, alone or in the presence of calcium sulfate and calcium hydroxide have been extensively studied (Odler,1998).

However, the findings should be interpreted with caution as the composition of the aluminate phases in industrial clinker differs considerably from that in synthetic preparation and hydration in cements is strongly influenced by the much larger quantity of silicates reacting and also by the presence of alkalis.

In the absence of soluble calcium sulfate C3A reacts rapidly to form the phases C2AH8 and C4AH19, which subsequently convert to C2AH6. This is a rapid and highly exothermic reaction.

If finely ground gypsum (CaSO4·2H2O) or hemihydrate (CaSO4·0.5H2O) is blended with the C3A prior to mixing with water then the initial reactions are controlled by the formation of a protective layer of ettringite on the surface of the C3A crystals. The reaction can be summarized as:

C3A + 3C + 3_ + 32H ⇒ C3A.3C_. 32H

where in cement chemists’ notation _ represents SO3 and H represents H2O, i.e.

C3A + dissolved calcium (Ca2+) + dissolved sulfate (SO4 )

2– + water ⇒ ettringite

The more rapid dissolution of dehydrated forms of gypsum ensures an adequate supply of dissolved calcium and sulfate ions and will be more effective in controlling the reaction of finely divided or highly reactive forms of C3A.

Table 1.3 Hydration of calcium silicates

In most commercial Portland cements there will be insufficient sulfate available to sustain the formation of ettringite. When the available sulfate has been consumed the ettringite reacts with C3A to form a phase with a lower SO3 content known as monosulfate.

The reaction can be summarized as:

C3A.3C_.32H + 2C3A + 4H 3(C3A.C_.12H)

Many studies have shown that the hydration of C4AF (or more correctly the C2A – C2F solid solution) is analogous to that of C3A but proceeds more slowly (Taylor, 1997).

The iron enters into solid solution in the crystal structures of ettringite and monosulfate substituting for aluminium. In order to reflect the variable composition of ettringite and monosulfate formed by mixtures of C3A and C4AF they are referred to respectively as AFt (alumino-ferrite trisulfate hydrate)and AFm (alumino-ferrite monosulfate hydrate) phases.

The hydration reactions of C3A and C4AF are summarized in Table 1.4.

Table 1.4 Hydration of aluminates