Gypsum reactions with calcium aluminate cements

The alkali content in clinker is usually on the level of 1 % Na O with dominating soluble alkali sulphates. In the solution rich in CH, they increase the solubility of gypsum, while the solubility of calcium hydroxide decreases. In this condition the SO4 ions can be bonded with C-S-H. That is why the gypsum addition should be higher. When the specific surface area of cement is high and the reaction of tricalcium aluminate is therefore accelerated, the gypsiun addition should be higher too. 

This reaction is not associated with expansion. The extent of expansion of a cement on the basis of calcium aluminate clinker and gypsum may be increased by adding to it opal, microsilica, and especially hydrated lime (Stark and Chartschenko, 1997). 

The most widely used cement is Portland cement (OPC). Its principal components, listed in Table 8-1, are calcium silicates (C3S and C2S), calcium aluminates (C3A), calcium-iron aluminates (C4AF) and alkali oxides (Na20, K2O). The composition varies depending on the site and the clinker minerals used to prepare the Portland cement. In order to slow down the rapid reaction of C3A with water, small proportions of gypsum are added. 

Gypsum is important as it reacts with aluminate to give etringite - calcium aluminium sulphate hydrate, Ca6Al2S30ig 32H2O - on the surface of the aluminate grains. This slows the reaction of aluminate with water and allows the wet cement paste to be worked for longer. 

Alite reacts with water to form calcium silicate hydrate and calcium hydroxide, which is also known as portlan-dite. The hardened paste has high strength when the reaction is completed, and because alite is the most abundant compound in cement, it also makes the dominant contribution to the mechanical properties of the final product. The hydration reaction proceeds at an appreciable rate a few hours after the addition of water and lasts up to about 20 days. The reaction of alite with water is accelerated by aluminate and gypsum. 

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