The zero point of charge of tobermorite

Synthetic tobermorite with a molar ratio CaO/SiO2 = 1.08 was studied as a model substance for the hydration products of Portland cement. By electrophoresis measurements it was found that at hydroxyl ion concentrations ranging from 10−3 to 2.10−2 M the sign of the surface charge is negative at low Ca concentrations and positive at high Ca concentrations. The zero point of charge is situated at 4 × 10−3 M Ca++, independent of the hydroxyl ion concentration. Under conditions prevailing in hardening normal cements, the resulting Ca hydrosilicate probably has a positive surface charge.