Microsphere-filled lightweight calcium phosphate cements

The incorporation of inorganic and organic microsphere fillers into calcium phosphate cement (CPC) to produce lightweight cementitious materials that could be used under hydrothermal conditions at high temperatures between 200 and 1000 °C was investigated. An aluminosilicate based hollow microsphere, with a density of 0.67 gcm^–3 and a particle size of 75–200 m, was the most suitable having a low slurry density of 1.3 gcm^–3, and a compressive strength greater than 6.89 M Pa. This microsphere-filled lightweight CPC exhibited the following characteristics: 1. after autoclaving at 200 °C, amorphous ammonium calcium orthophosphate (AmCOP) salt and Al₂O₃·xH₂0 gel phases, formed by the reaction between calcium aluminate cement and an NH₄H₂P0₄ based fertilizer, were primarily responsible for the development of strength; 2. at a hydrothermal temperature of 300 °C, the microsphere shell moderately reacted with the CPC to form an intermediate reaction product, epistilbite (EP), while crystalline hydroxyapatite (HOAp) and boehmite (BO) were yielded by the phase transformations of AmCOP and Al₂O₃·xH₂O, respectively; 3. at an annealing temperature of 600 °C, the HOAp phase remained in the cement body, even though an EP anorthite (AN) phase transition occurred; 4. at 1000 °C, the phase conversion of HOAp into whitlockite was completed, while the AN phase was eliminated; and 5. the microsphere demonstrated excellent thermal stability up to temperatures of 1000 °C.This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract No. DE-AC02-76CH00016.