Calcium ions effect a reversal of the sign of the electrokinetic potential on a- and ß-CaSiO3, a-Ca3Si2O7, ¿-Ca2SiO4, vitreous CaSiO3, xonotlite, and tobermorite in 0.01 N NaOH solution. Points of zero charge range from 1.10-3 to 4.10 M Ca2+. Differences of electrokinetic characteristics between different solids indicate that the surface layers are not identical. Changes in NaOH concentration from 0.005 to 0.02 N NaOH do not shift the point of zero charge of a-CaSiO3. A theory is given relating the point of zero charge to adsorption characteristics of calcium ions and to the number of charges provided to the surface by ions other than Ca2+ through chemisorption and dissociation of surface OH groups. A comparison of the points of zero charge of a-CaSiO3, ß-CaSiO3, and vitreous CaSiO3 indicates that chemisorption of ions other than Ca2+ and dissociation of surface OH groups provide a net negative charge of about 0.5 elementary charge unit per calcium adsorption site.