Influence of the Calcium Sulphate and W/C Ratio on the Hydration of Calcium Sulfoaluminate Cement

摘要

Calcium sulfoaluminate cements (CSA) have been known for several decades, but they have recently been considered as a promising low-CO2 alternative to Ordinary Portland cements and an opportunity for the wastes recycling. The key component of CSA cements is the compound calcium sulfoaluminate (C4A3$), due to its ability of generating ettringite when it reacts with water, lime and/or calcium sulphate. Various applications of these binders have been developed, for instance low-shrinkage, self-stressing and high early strength cements, depending upon the composition and on the reactivity of the cement. However the relationships between these factors and-mechanical properties have not been yet deeply studied in terms of mechanisms and kinetic of the hydration process and microstructure and composition of the resulting hydrated phases. In this study, both the hydration kinetic and products microstructure of an industrial CSA clinker were investigated. In particular the influence of the type of calcium sulphate (anhydrite and gypsum), the calcium sulphate addition and water/binder ratio on the hydration of CSA clinker have been exploited. The hydration kinetic is strongly influenced by the type of calcium sulphate, namely gypsum and anhydrite. The main differences are observed during the first day of hydration, in particular in the first 8 hours. The high solubility of gypsum leads to a rapid formation of hydrates in an amorphous form that lead to a more dense paste in the first 24 hours and to a high compressive strength. The slow dissolution kinetic of anhydrite favours the formation of a well crystallised ettringite network that lead to a rapid paste plasticity loss (lower setting time) and to the formation of a less dense paste with lower compressive strength. At later ages (2, 7 and 28days) the degree of hydration results to be similar in both the samples as the microstructure of the paste. The compressive strength is as a consequence similar, but some differences in the shrinkage behaviour are observed. Monosulfate is formed only after 2 days of hydration, when calcium sulphate is consumed. The presence of strtlingite was observed only in the case of low amount of calcium sulphate and high w/c ratio.