Study was focused upon formulating sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal well cements with advanced anti-carbonation and anti-acid corrosive properties. At a low hydrothermal temperature of 90 C, to improve these properties, we investigated the effectiveness of various calcium aluminate cement (CAC) reactants in minimizing the rate of carbonation and in abating the attack of H2SO4 (pH similar to 1.6). We found that the most effective CAC had two major phases, monocalcium aluminate (CA) and calcium bialuminate (CA(2)), and a moderate CaO/Al2O3 ratio of 0.4. The reaction between sodium polyphosphate (NaP) and CA or CA(2) at room temperature led to the formation of amorphous dibasic calcium phosphate hydrate and anionic aluminum hydroxide caused by the decalcification of CA and CA(2). When SFCB cement made with this CAC was exposed to 4% NaHCO3-laden water at 90 degreesC, some carbonation of the cement occurred, forming calcite that was susceptible to the reaction with H2SO4. This reaction resulted in the deposition of gypsum gel scales as the acid corrosion product on the cement surfaces. The scale layer clinging to the cement protected it from further corrosion. Under such protection, the amorphous dibasic calcium phosphate hydrate crystal hydroxyapatite and anionic aluminum hydroxide crystal boehmite phase transitions were completed in acid solution. Meanwhile, the further chemical and hydration reactions of NaP with fly ash led to the formation of additional crystalline Na-P type zeolite phases. Thus, we propose that passivation of the surface of the cement by deposition of gypsum, following the formation of these reaction products, which are relatively inert to acid, are the acid corrosion-inhibiting mechanisms of the SFCB cements.