Hydrothermal light-weight calcium phosphate cements: use of polyacrylnitrile-shelled hollow microspheres

Abstract

Calcium phosphate cement (CPC) slurries with a very low density of less than 1.0 g cm-3 were prepared by incorporating polyacrylnitrile (PAN)-shelled hollow microspheres with calcite sizing into CPC pastes consisting of sodium metaphosphate, high alumina cement and water. Their characterizations were then investigated to assess their value as light-weight CPC cementing materials for use in geothermal wells at hydrothermal temperatures up to 300°C. This light-weight cement showed the following four main features: firstly the chemical inertness of the PAN shells to CPC served to extend thickening time of the slurry; secondly the microsphere surfaces preferentially absorbed Al ions from among the various ionic species in the interstitial fluid of CPC at 100°C, thereby forming amorphous Al-enriched sodium phosphate hydrates as interfacial intermediate layers which tightly linked the microspheres to the CPC matrix; thirdly although the thermal decomposition of PAN shells around 200°C generated numerous voids in the cement body, these open spaces were filled by well-grown wardite crystals formed by the in-situ phase transformation of amorphous sodium aluminate phosphate hydrates, thereby preventing a serious loss in strength of the light-weight calcium phosphate cement (LCPC) specimens; fourthly the major phase composition of CPC matrix at 200 and 300°C consisted of well-crystallized hydroxyapatite and boehmite compounds which can be categorized as alkali carbonation-resistant phases. The integration of these characteristics was responsible for maintaining the compressive strength of greater than 0.6 MPa for LCPC specimens derived from a very-low-density (0.98 g cm-3) slurry exposed for 6 months to a 0.05 M Na2CO3-laden solution at 250°C.