Cement Hydration - Summary of Powerpoint Slide 1A (Colour) PDF

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Summary of Powerpoint Slide 1A (Colour)...

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Cement Hydration Overview • • •

Hydration: describes chemical reaction (very long process) between Portland cement and water. Steps of cement hydration: dissolution and precipitation. Exothermic reaction.

Degree of Hydration (DOH)

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Percentage materials reacted (measure amount water used up during hydration or microscopy). Compressive strength increases with DOH.

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Use 0.23 g/g of cement.

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Water to Cement Ratio (w/c) • •

w/c : ratio of water to cement by weight. Most important parameter in mixing concrete (largely determines strength and concrete durability).

Evaporable and Non-Evaporable Water • •

Evaporable water (free water): lost up to 105°C. Non-evaporable water: chemically bounded water (lost between 105°C and 1000°C), increases over time due to hydration.

Hydration Process • • •

Cement grain size decreases with hydration process (small grains completely hydrates, large grains don’t). Each component has different hydration rates and produces different hydration products. Reactions rates (fast to slow): C 3A, C3S, C4AF, C2S.

C3S (fast) and C2S (very slow) Hydration I. II. III. IV. V.

Rapid heat evolution and reactions, Ca2+ and OH- released, pH rise, small change in cement grain size. Plastic state and slow reactions, Ca2+ and OH- reach critical value, C-S-H and CH forms at surface of C3S. Rapid reactions, more C-S- H and CH forms, nucleation and growth. Slower reactions, C-S-H shell thickens and slows hydration. Very slow reactions through thick C-S-H shell.

C3A Hydration • •

When gypsum is plentiful C 3A produces AFt. When sulfate concentration drops C 3A produces AFm.

C-S-H • •

C-S-H: Calcium silicate hydrates. Major source of cohesive strength.

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Multiple microscopic shapes (morphology).

CH • •

CH: Calcium hydroxide. Variable morphology from thin to thick elongated crystals.

AFt (Ettringite) •

Needle-like morphology interlocking contributes to mixture stiffening....