Geopolymer concrete blocks, heat cured at 200 degrees Celsius and then immersed in an extreme alkali medium for 14 days at 80 degrees Celsius (a and b), resist the attack significantly better than blocks heat-cured at 600 degrees Celsius and subjected to the same treatment (c and d) in this series of scanning electron microscope images. The blocks show the presence of a gel-like substance, characteristic of alkali attack from the 3M NaOH solution. The heat-curing significantly reduced in the intensity of the attack but could not prevent it.
Fly ash generated by coal power generation can be repurposed into superior-grade geopolymer concrete. However, a critical durability problem has been low resistance to alkali attack.
UJ researchers have found that high temperature heat-treatment at 200 degrees Celsius can halve this harmful mechanism in fly ash geopolymer concretes.