Acid-resistant polydimethylsiloxane additive for geothermal well cement in 150°C H₂SO₄ solution

The resistance of sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal cement to H₂SO₄ solution (pH 1·2) at 150°C is enhanced by incorporating an emulsion of silanol-terminated polydimethylsiloxane (PDMS) into slurries. The crystalline phase composition of both 150°C-autoclaved PDMS-modified and unmodified cements consisted of three hydration reaction products: hydroxyapatite (HOAp), boehmite, and Na-P type zeolite; one carbonation product, calcite; and three unconsumed reactants: monocalcium aluminate, perovskite and quartz from calcium aluminate cement and fly ash. When unmodified cement was exposed for 20 days to hot acid, its surface was covered with the crystalline bassanite (CaSO4:1/2H2O) phase as the acid corrosion product of cement. Bassanite was formed not only by reactions between H₂SO₄ and calcite or monocalcium aluminate, but also by reaction of HOAp with acid. In contrast, PDMS protected the cement against acid corrosion in two ways: it considerably reduced the formation of calcite and it abated the rate of reaction of HOAp with acid.