Effect of Curing Conditions on the Porosity Charcteristic of Metakaolin-Fly Ash Geopolymers

Abstract : The porosity characteristics of metakaolin (MK)- and fly ash (FA)-based geopolymers cured under ambient, thermal, and moist conditions were investigated using nitrogen adsorption porosimetry. In general, surface areas were higher for materials prepared from FA (29.0-59.2 m(2)/g) than for MK (5.3-12.8 m(2)/g). Total pore volumes ranged from 0.034-0.104 cc/g for MK-based materials and from 0.077-0.089 cc/g for FA-based materials. For MK-based geopolymers, curing under ambient conditions resulted in a single, broad pore size distribution (PSD) between 250-500 A. PSDs for MK-based geopolymers cured using thermal or moist methods were bimodal with broad peaks between approximately 60-250 A and 250-500 A. For FA-based geopolymers, a single, broad PSD was observed and ranged from 20-100 A for ambient temperature cured materials and 40-200 A for materials cured under thermal or moist conditions. The observed pore size distributions were accompanied by a shift to higher wavenumber in the position of the main geopolymer band in the A TR-FTIR spectra of geopolymer materials after thermal or moist curing, indicating a higher concentration of Si-0-Si bonds formed in the materials cured at elevated temperatures. These results suggest that the observed PSDs result from Si-0-Al and Si-0-Si bond formation during curing and are affected by the curing conditions.