The calcium sulphoaluminate hydrates are important components of Portland cement and constitute the principal matrix formers of some sulphoaluminate cements. Their practical importance lies in the involvement as intermediates and products of the hydration of portland cements under geothermal conditions. Ettringite is a complex mineral (Ca₆[Al(OH)₆]₂(SO₄)₃.26H₂O) formed during the initial stages of Portland cement hydration at ambient temperature, by reaction of sulphate ions released by gypsum (CaSO₄.2H₂O) with tricalcium aluminate (Ca₃Al₂O₆). After exhaustion of gypsum, the remaining tricalcium aluminate in solution reacts with already formed ettringite transforming to monosulphate (Ca₄Al₂(SO₄)(OH)₁₂.xH₂O). At higher temperature (>100°C), ettringite is unstable and transforms to monosulphate. Monosulphates are known to exist is at least four different hydrate forms (x = 8,10,12,14). In this study the stability of calcium sulphoaluminate hydrates were mapped in various environments (variable relative humidity, temperature and alkalinity). The monosulphate hydrates were obtained by hydrothermal synthesis using microwave radiation at 120°C. Their formation is via ettringite thermal decomposition in autoclave conditions under autogenous pressure. It has been shown that a series of calcium sulphoaluminate hydrates can be obtained depending on temperature and water activity. The interconversion of the calcium sulphoaluminate hydrates was found to be an easy and rapid process, whereby metastable phase are readily formed, indicating the lability of Ca-OH-Al-SO₄ system. The kinetics and the mechanism of growth of calcium sulphoaluminate hydrates are known to influence the development of mechanical properties and the characteristics of cements. The ettringite crystal growth process was evaluated from the point of view of its influence on crystal morphology. General crystallisation methods for ettringite synthesis were developed starting from supersaturated solutions of pure phases and its morphology was found to vary with crystallisation factors (temperature for instance); ettringite crystals are generally hexagonal rods with different aspect ratios.