Tricalcium aluminate hydration in additivated systems. A crystallographic study by SR-XRPD.

Synchrotron radiation X-ray powder diffraction has been used to monitor the evolution of ettringite in C3A–gypsum synthetic mixture and in commercial cement systems during the first hours of the hydration process. The hydration of the paste was achieved using a remote controlled system, in order to collect data as soon as water is added to the system. The use of full-profile Rietveld method during the analysis of the diffractograms collected allowed us to monitor the evolution of phases weight fraction. The rigorous measurement of the lattice parameters and of the diffraction peak shape proved to be very useful to obtain information on the structural evolution of ettringite and on the mean grain size of the crystal phases. Depending on the admixture added to the system, the precipitation of well crystalline ettringite takes some hours. During this “induction” period we observe a significant variation of a and c lattice parameter values for ettringite. In particular a increases from 11.8 Å to 11.24 Å, the value for pure ettringite. The c parameter decreases from 22 Å to 21.48 Å. The lattice parameter variation could be related to small crystallite size effect, but the large variation more likely reflects also crystallographic changes, such as defect re-organization during the nucleation and growth process or also changes in the SO3 and H2O content in the ettringite channel. Not surprisingly the amount and the grain dimensions of crystalline ettringite are affected by the chemistry of the system. We observed the same evolution trend during ettringite formation also in shrinkage-compensating commercial cements (composed by mixture of Ca–Al cements, Portland cement and bassanite), in which ettringite is the main hydrous phase present.