Hydration & Supplementary Materials

When cement meets water, a series of exothermic reactions produce the binding phases that give concrete its strength. Understanding hydration kinetics is essential for predicting setting time, strength development, and durability.

Alite (C₃S) hydration — the main strength reaction:
2(3CaO·SiO₂) + 6H₂O → 3CaO·2SiO₂·3H₂O (C-S-H) + 3Ca(OH)₂

This produces C-S-H gel — the strong, dense binding matrix — and calcium hydroxide (Ca(OH)₂), a weak, soluble byproduct. C-S-H accounts for 50–60% of hydrated cement paste volume and provides most of the strength. Ca(OH)₂ accounts for 20–25% and is a durability liability (soluble, reactive with sulfates, vulnerable to acid).

Belite (C₂S) hydration — slower, same products:
2(2CaO·SiO₂) + 4H₂O → 3CaO·2SiO₂·3H₂O (C-S-H) + Ca(OH)₂

Belite produces less Ca(OH)₂ per unit of C-S-H, contributing to long-term durability.

SCMs are added to cement or concrete to replace a portion of the clinker. They react with Ca(OH)₂ from cement hydration (the pozzolanic reaction) to form additional C-S-H — consuming the weak byproduct and producing more strong binder.

Class F Fly Ash: From coal power plants. Low calcium, high reactive silica. Slow early strength but excellent long-term strength and sulfate resistance. Typical replacement: 15–30%.

Class C Fly Ash: From sub-bituminous coal. Self-cementing (contains its own CaO). Faster setting, higher early strength. Typical replacement: 15–40%.

Ground Granulated Blast Furnace Slag (GGBFS): From iron production. Latent hydraulic material — reacts with water slowly but with Ca(OH)₂ activator, forms C-S-H. Excellent sulfate and chloride resistance. Typical replacement: 30–70%.

Silica Fume: From silicon metal production. Extremely fine (100x finer than cement), nearly pure reactive SiO₂. Dramatically increases strength and reduces permeability. Used in high-performance concrete at 5–10% replacement.