Studies of strength mechanism in newly developed chemically bonded ceramics in the system CaOSiO₂P₂O₅H₂O

Strength data are presented on totally new room temperature setting cements in CaO-SiO₂-P₂O₅-H₂O system. These compositions lie in the range of CaO:45-65, P₂O₅:29-50, SiO₂:4-20 (wt.%) and were prepared by sol-gel techniques using calcium nitrate and lactate, phosphoric acid and Ludox as precursors. Pressed pellets of the calcined powders were hydrated for times ranging from 1 day to 90 days at room temperature, 85° and 100°C. Typical values of splitting tensile strength near 15 MPa were common at room temperature, and 28 MPa at 100°C; maximum strengths of 31 MPA were achieved at 100°C for 1 day of hydration in some compositions. Microstructural analysis by SEM, XRD and EDAX showed the presence of unusually well crystallized CSH-1, Ca-hydroxy apatite and a well crystallized hydrated silicocarnotite phase together with gels of CSH and/or CSPH. The high strengths are attributed to the ultrafine powders used and the formation of well developed gels of CSH and CSPH which serve as the bond interlinking with crysalline grains of Ca(OH)₂, Ca-hydroxy apatite, CSH and CSPH.