Effect of chemical activation method and curing conditions on strength development of high PFA content mortars

Despite the proven economic, environmental and technical benefits, long-term strength development and durability performance, the utilisation of PFA (pulverised-fuel ash, fly ash) in many countries concrete remains at a moderate level. Wider use of PFA in concrete is inhibited by a number of factors, including its relatively slow initial strength gain. Activation of PFA has potential to enhance this by accelerating the early reaction rate. Mechanical, thermal and chemical methods may all be effective in promoting the pozzolanic reaction. In this study, grinding, calcination, and chemical activation of PFA with NaOH, Ca(OH)2 , CaSO4 ·1/ 2H2O, Na2SO4 , CaCl2 , and various combinations thereof were applied and compared. Breakdown of the glassy phase of PFA was found to be strongly influenced by the curing conditions applied. These included wet curing at 20°C or 50°C as well as a regime reflecting that used in some pre-cast concrete work, i.e. 50°C for 24 hours followed by wet curing at 20°C. The best results were obtained with simultaneous addition of gypsum and NaOH; and by a combination of lime, gypsum and Na2 SO4 , especially when pre-cast cured.