Cement Hydration and Pozzolans

Hydration is the result of a chemical reaction that occurs between water and the chemical compounds present in portland cement. Portland cement is predominately composed of two calcium silicates which account for 70 percent to 80 percent of the cement. The two calcium silicates are dicalcium silicate (C2S) and tricalcium silicate (C3S). The other compounds present in portland cement are tricalcium aluminate (C3A), tetracalcium aluminoferrite (C4AF) and gypsum. For the sake of simplicity this discussion focuses only on the reaction between the calcium silicates and water.
The reaction of dicalcium silicate and tricalcium silicate with water (abbreviated as “H”) produces calcium silicate hydrate (C-S-H) and calcium hydroxide (CH), as illustrated in the following chemical equations.

C-S-H accounts for more than half the volume of the hydrated cement paste while CH accounts for about 25% of the paste volume. The remainder of hydrated portland cement is predominantly composed of Calcium Sulfoaluminates (ettringite) and capillary pores.

C-S-H is a poorly crystalline material with a variable composition that forms extremely small particles less then 1.0 µm in size. C-S-H is the main cementitious compound, or glue, that gives concrete its inherent strength. The structure of C-S-H becomes much more stable and resistant to subsequent environmental changes upon prolonged moist curing or curing at elevated temperatures. Calcium hydroxide, on the other hand, is a well-crystallized material with a fixed composition. CH contributes somewhat to concrete’s inherent strength because it will form large crystals inside voids, thereby reducing porosity. However, CH is a soluble compound, meaning it will move throughout the pore system in the presence of water, making it extremely vulnerable to chemical attack.

All you really need to remember is that C-S-H is a superior reaction product because it creates a denser microstructure that increases strength, reduces the permeability of the concrete and improves its resistance to chemical attack. The formation of CH, on the other hand, increases the concrete’s porosity and is susceptible to sulfate attack. The pozzolanic reaction converts the soluble CH to C-S-H, increasing the overall strength and durability of the concrete.