Hydration of Cement

When water contacts cement particles, anhydrous cement compounds dissociate and release ions into the solution. These ions eventually form hydrates of silicates and aluminates that precipitate, holding cement grains in position. Subsequent hydration occurs on the cement particle surfaces as long as unreacted grains and free water remain in contact.

Both C3S and C2S react with water to form calcium silicate hydrate and calcium hydroxide, but in different proportions. C3S hydration produces 61% C-S-H and 39% calcium hydroxide, while C2S hydration yields 82% C-S-H and 18% calcium hydroxide. Since C-S-H is crucial for strength of cement, increasing C2S content relative to C3S enhances ultimate concrete strength.

Gypsum is added to control the rapid hydration of aluminates, which release significant heat immediately upon contact with water. Gypsum releases sulfate ions that reduce aluminate solubility, slowing the reaction and controlling the setting time of cement. This prevents premature hardening and allows proper workability during concrete placement.

Tricalcium aluminate reacts with water and gypsum to form ettringite, which later transforms into calcium monosulfoaluminate hydrate. Additionally, aluminate hydration produces calcium aluminate hydrate and calcium aluminoferrite hydrate. These products contribute to the early strength development and setting characteristics of cement.

Through-solution hydration involves anhydrous compounds dissolving into constituents, with hydrates forming in solution and then precipitating from the supersaturated solution. Topochemical hydration occurs through solid-state reactions at the cement particle surface. The through-solution process dominates during early hydration stages, while both mechanisms contribute to overall cement hydration.

Tricalcium silicate (C3S) hydrates more rapidly than dicalcium silicate (C2S) due to differences in their chemical reactivity with water. Although both consume similar amounts of water, C3S releases its hydration products more quickly, contributing to faster early strength development. This faster reaction rate makes C3S the primary driver of initial strength and heat of hydration.

Calcium silicate hydrate (C-S-H) is the major hydration product of silicates and is crucial for cement strength development. C-S-H forms from the reaction of both C3S and C2S with water, with C2S producing higher proportions of C-S-H. The quantity and quality of C-S-H directly determine the long-term strength and durability of concrete structures.