Hydration Mechanism Of C₄A₃（?）-C（?）H₂-CH

Calcium sulphoaluminate （CSA） cement has shown excellent properties, kinds of materials for special use can be developed based on it, presenting a good market prospect. But the research of hydration mechanism of the cement is not clear enough, which limits its application scope. As the main phase of CSA cement, calcium sulphoaluminate （C4A3（?）） plays a key role in the performance of the cement. To better understand hydration of CSA cement and develop new type special materials, it is very important to explore the hydration of C4A3（?） alone and C4A3（?） with extra calcium sulphate and calcium hydroxide. This will provide theoretical support for further research.This paper used a laboratory synthetic C4A3（?） with high purity to investigate the hydration of C4A3（?） at different water/solid ratios. In order to clarify the hydration mechanism of C4A3（?）, hydration products were examined by means of X-ray diffraction, differential scanning calorimetry-thermogravimetry and scanning electron microscope. A theory hydration reaction range for hydration of C4A3（?） with calcium sulfate and calcium hydroxide was established by theoretical calculation with a hypothetical hydration reaction system. Preliminary experimental validation was made by analyzing hydration products （28d, mw/ms=10） of several composition points in the theory reaction range with the same method. In addition, the order of hydration products precipitated out was calculated by using precipitation dissolution equilibrium theory in thermodynamics, and the hydration process was preliminary described. The main achievements are as folio wings:（1） At low water/solid ratios, there were two kinds of calcium monosulphoaluminate, namely, AFm-14 and AFm-12, and amorphous aluminium gel （AH₃） while there was no ettringite （AFt） in the hydration products. AFm-12 gradually converted to AFm-14 as the extension of hydration duration. In an environment with higher water content, AFt was formed quickly during the early hydration time, and then AFm-12 and AH₃ were formed. Because of the decline of sulfate ion concentration, AFt converted to AFm finally. So the final hydration produces of C4A3（?） were AFm phase and AH₃.（2） The theory reaction range included 1mol C4A3（?）, a mol CSH₂ and b mol CH. With different amount of calcium sulfate and calcium hydroxide in reaction systems, their hydration process, hydration products and stoichiometric water/solid ratio show regular changes. All of the reactants participate in hydration reaction and none is left. After complete hydration, the amounts of AFt, AFm and AH₃ were （a/2-b/6） mol, （1+b/2-a/2） mol and （2-b/3） mol, and the total amount of hydration products was 3mol. The amount of water which was required for hydration was （18+8a-4b/3） mol, and the variation range of stoichiometric water/solid ratio was 0.335 to 0.642.（3） Hydration products of some composition points on the four boundaries of the theory reaction range and change law of hydration products on the same boundary were consistent with theoretical assumptions. Four boundaries of the theory reaction range were validated by experiments. Results of thermodynamic calculation showed that AFt precipitated out firstly in all reaction systems which represent the whole theory reaction range, and a critically supersaturated domain with respect to AFt was existed. The critically supersaturated domain could induce an immediate precipitation of AFt and inhibit the formation of other hydration products, even if the aqueous phase was supersaturated with respect to them. Other hydration products would begin to form only if the solution composition was out of the critically supersaturated domain. When all CaSO4 consumed and sulfate ion concentration of solution decreased, AFt would convert to AFm to release calcium ion and sulfate ion. Thus hydration reaction continued until the end.