HYDROTHERMAL LIGHTWEIGHT CALCIUM-PHOSPHATE CEMENTS - USE OF POLYACRYLNITRILE-SHELLED HOLLOW MICROSPHERES

Calcium phosphate cement (CPC) slurries with a very low density of les s than 1.0 g cm(-3) were prepared by incorporating polyacrylnitrile (P AN)-shelled hollow microspheres with calcite sizing into CPC pastes co nsisting of sodium metaphosphate, high alumina cement and water. Their characterizations were then investigated to assess their value as lig ht-weight CPC cementing materials for use in geothermal wells at hydro thermal temperatures up to 300 degrees C. This light-weight cement sho wed the following four main features: firstly the chemical inertness o f the PAN shells to CPC served to extend thickening time of the slurry ; secondly the microsphere surfaces preferentially absorbed Al ions fr om among the various ionic species in the interstitial fluid of CPC at 100 degrees 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 decomposi tion of PAN shells around 200 degrees C generated numerous voids in th e cement body, these open spaces were filled by well-grown wardite cry stals formed by the in-situ phase transformation of amorphous sodium a luminate phosphate hydrates, thereby preventing a serious loss in stre ngth of the light-weight calcium phosphate cement (LCPC) specimens; fo urthly the major phase composition of CPC matrix at 200 and 300 degree s C consisted of well-crystallized hydroxyapatite and boehmite compoun ds which can be categorized as alkali carbonation-resistant phases. Th e integration of these characteristics was responsible for maintaining the compressive strength of greater than 0.6 MPa for LCPC specimens d erived from a very-low-density (0.98 g cm(-3)) slurry exposed for 6 mo nths to a 0.05 M Na2CO3-laden solution at 250 degrees C.