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The hydration of P-CaZSiO, was carried out a t room temperature by three methods: ( a ) in a small steel ball mill, (b) ill paste form, and (c) in a polyethylene bottle rotated on a wheel. The probable stoichiometry is represented by the equations 2p-CazSiOd + 4Hz0 = CaaSizO7.3HaO + Ca(0H)Z; 2CaaSiOa + 6H20 = Ca3Si207.3HzO + 3Ca(OH)z. The same calcium silicate hydrate was produced in all five cases, and it was similar to the natural mineral tobermorite. The Ca(0H)z was obtained in two forms: the normal crystalline one and as an amorphous phase. The densities of tobermorites of composition CaaSit07.2H20 and Ca3Siz07.2.80Hz0 were 2.86 and 2.73 g./cc., respectively. Thus, the apparent density of the water in excess of 2 moles was 1.39 g./cc. The calculated density of CasSi2-07.3H20 was 2.71 g./cc. The X-ray diffraction patterns showed only the (110), (200) and (020) spacings of tobermorite. The (001) spacing was calculated from these and from density data. Relationships between surface area, particle dimensions, crystal structure and certain swelling properties of tobermorite are discussed.Ca3Si05 was hydrated by methods (b) and (c),

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