From C-S-H to C-A-S-H: Experimental study and thermodynamic modelling

It has long been known that the stoichiometry of C-S-H varies with the calcium hydroxide concentration in solution. However, this issue is still far from understood. We revisit it in both experimental and modelling aspects. A careful analysis of the solubility confirms the existence of three different C-S-H phases, defined as Ca4H4Si5O16, Ca2H2Si2O7 and Ca-6(HSi2O7)(2)(OH)(2), respectively. The variation of the Ca/Si ratio of the three phases has been described by surface reactions: the increase of the Si content is accounted for by silicate bridging, the increase of calcium content and the surface charge are accounted for by reactions involving silanol groups via deprotonation and complexation with calcium. In the presence of Al in solution, the uptake of Al by C-S-H is experimentally observed. The Al content increases with Al concentration. C-A-S-H formation is modelled by the competition between silicate and aluminate tetrahedra for the bridging of the dimeric silicates in C-S-H. (C) 2014 Elsevier Ltd. All rights reserved.