Aluminate-gypsum reaction

In the conduction calorimetric curves of the aluminate-gypsum system,the first stage is characterized by a peak at 30 minutes for the formation of ettringite. In the second stage, the peak occurs at 24-48 hours when the monosulfate phase forms by the reaction between ettringite and aluminate. 

Fig. 5. The early hydration reactions of tricalcium aluminate in the presence of gypsum and calcium hydroxide. Initial molar proportions l-C A ...

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO and the tricalcium aluminate keeps the SO level near zero concentration for the first 10-20 min causing rapid stiffening and quick set (approximately 15 min). It is also reported that chemical... 

Carbon dioxide dissolves in the pore solution of cement paste, producing CO3", which reacts with Ca to produce CaCOj. The OH and Ca ions required by these reactions are obtained by the dissolution of CH and decomposition of the hydrated silicate and aluminate phases. C4AH. is quickly converted into C ACH, and ultimately into CaCOj and hydrous alumina monosulphate and ettringite yield CaCOj, hydrous alumina and gypsum. C-S-H is decalcified, initially by lowering of its Ca/Si ratio, and ultimately by conversion into a highly porous, hydrous form of silica. The... 

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. 

Another method consists in the water processing of calcirrm aluminate mixture with gypsum and, if it is necessary, with calciiun oxide, according to the following reaction schemes ... 

A rapid reaction of C3 A with water is oidy possible after gypsum consumption this occurs commonly after 24 h of hydration. Then the hexagonal hydrates are formed and react with ettringite to form AFm. The direct reaction of CjAwith ettringite or rather locally present aluminate ions with ettringite is also possible. 

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. 

As it was shown by Mehta [46], the rate of ettringite formation in the mixture of anhydrous calcium aluminates, calcium hydroxide and gypsum was the highest in the case of CA. The reaction in the mixture with C4 AjS was slower, while the reaction of C3A was hampered, because of the impermeable ettringite layer formation (see Sect. 3.3). The C,2A.7 is also not a good source of ettringite, because it reacts... 

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