Properties and characterization of a metakaolin phosphate acid-based geopolymer synthesized in a humid environment

To synthesize a metakaolin phosphate acid-based geopolymer (PAG), calcined kaolin and phosphoric acid solutions with phosphorus-to-aluminium molar ratios (P/Al) of 0.6 to 1.0 were prepared. Fresh PAG was cured in a high-humidity environment at 50 degrees C for 7 days. High humidity is helpful for generating AlPO4 during the geopolymerization of PAG. Si and Al are depolymerized from metakaolin upon attack of H3PO4, and the amounts of leached Si and Al are directly proportional to P/Al. Leached Al increases with time, while leached Si decreases and then becomes stable. Pyrophosphate and hexa-coordinate Al could be components of the basic structural unit of PAG. Porosity and cracking increased with increasing P/Al and reduced the strength of PAG. The degree of shrinkage and the pore size are directly proportional to P/Al. The concentrated pores in PAG are gel pores, transitional pores and capillary pores corresponding to different P/Al ratios.