Processes in the Solidification of Cement

Upon mixing cement powder with water, part of the tricalcium aluminate C3A and gypsum dissolve and react together forming ettringite  

The ettringite crystals on the surface of the cement particles are so fine that they cannot bridge the gap between the cement particles and therefore do not form a solid structure. This is the basis of the delay in solidification upon adding gypsum to cement. Without gypsum the tricalcium aluminate immediately reacts with water to calcium aluminate hydrate, which fills the space between the cement particles with its large crystals and leads to very rapid solidification of the cement slurry. 

Portland cement (containing gypsum) solidifies after 1 to 3 hours. The needle-shaped ettringite crystals recrystallize forming larger needles which bond the cement particles together and solidify the crust. 

The hydration of tricalcium silicate C3S and dicalcium silicate C2S (for abbreviations see below Table 5.3-6) are responsible for the further. solidification of Portland cement. This reaction only begins in earnest after ca. 4 hours. Initially long needles of calcium silicate hydrate are formed, which bond the cement particles together. Later, smaller needles of calcium silicate hydrate fill the gaps left. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate. 

The calcium aluminate still present reacts with the calcium hydroxide formed to tetracalcium aluminate hydrate. Calcium aluminum ferrite forms the analogous hydration product, which reacts more slowly. Part of the aluminum is replaced by iron  

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