Characteristics of underwater cast and cured geopolymers

Underwater cast and cured geopolymers with Si/Al ratios of 1.78 and 2.00, synthesized from Class C fly ash and metakaolin, were characterized by X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and unconfined compression, to understand the interactions between the geopolymer slurry and curing saline solutions and pertinent effects on its strength development. Slurries of geopolymers and Class G oil well cement were cast into porous molds that were immediately submerged into 0, 15, and 35 ppt saline solutions for 28 days curing, during which the pH of curing solutions was monitored. While the oil well cement exhibits a decreased compressive strength in high-salinity solutions, the strength of the two geopolymers increases with the curing solution's salinity. The underlying mechanisms are the chemical exchanges between the curing solution and geopolymer slurry, particularly the leaching or ingress of alkali ions (e.g., Na+, OH-) via diffusion, which depends upon the curing solution's salinity.