Compositions along the Ca2SiO4–Ca3(PO4)2join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca3(PO4)2were fired at 1500°C, forming nagelschmidtite +a1‐CaSiO4,A‐phase and silicocarnotite anda‐Ca3(PO4)2, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca2SiO4≠ 0 and portlandite when Ca2SiO4is>50%. Relative hydration reactivities area‐Ca3(PO4)2>nagelschmidtite>α1‐Ca2SiO4>A‐phase>silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14‐A tobermorite rather than calcium