Effect of aerated concrete waste aggregate on engineeringproperties of fly ash and silica fume based geopolymer mortars

In this study, the utilization of autoclaved aerated concrete waste aggregate instead of natural sand in producing fly ash and silica fume based geopolymer mortar and its effect on the engineering properties of the geopolymer mortars were investigated experimentally. It is aimed to prevent carbon and greenhouse gas emissions originating from the cement production by using fly ash and silica fume materials in certain proportions together with alkali activators. At the same time, the use of autoclaved aerated concrete waste as aggregate in place of natural sand would be an environmentally friendly study in terms of recycling of wastes. The effect of partial utilization of autoclaved aerated concrete waste aggregate in the production of fly ash and silica fume based geopolymer mortar on workability, compressive strength, flexural strength, fresh and dry unit weight, apparent porosity, water absorption, capillary water absorption, abrasion, UPV and thermal conductivity properties of geopolymer mortars was studied. To this, two series of geopolymer mortar mixtures were designed using only fly ash and 90% fly ash and 10% silica fume as precursors. A sodium silicate-sodium hydroxide mixture at a ratio of 1.5/1.0 was employed as alkali activator, and the molarity of sodium hydroxide solution in this mixture was determined as 12 M. In each mortar mixture series where the alkali activator/precursor ratio was fixed at 0.5, recycled sand obtained from autoclaved aerated concrete waste with three different particle size fractions (0.125-4 mm, 0.125-2 mm and 0.125-1 mm) was used at volumetric ratios of 10%, 20%, 30%, 40%, 50% and 60% instead of natural sand. Thus, a total of 19 mixtures were designed and produced in each mixture series, one of which was a control mixture and 18 of which contained autoclaved aerated concrete waste sand. It was observed that the 28-day compressive and flexural strengths of geopolymer mortar mixtures decreased when autoclaved aerated concrete waste was used as aggregate instead of natural sand. The reason for this is that autoclaved aerated concrete waste has a softer and more porous structure compared to natural sand. However, it was pointed out that the thermal conductivity coefficients decreased in the mixture series where autoclaved aerated concrete waste was utilized and the substitution rate was increased, i.e. the thermal conductivity performance increased. Again, it was determined that there were systematic decreased in the ultrasonic pulse velocity in geopolymer mortars containing aerated concrete waste. Increment was also observed in the compressive and flexural strengths with the use of silica fume at a rate of 10% instead of fly ash. In terms of thermal conductivity, higher values were obtained in the thermal conductivity coefficients of geopolymer mortars with silica fume. It was determined that there were increases in the ultrasonic pulse velocity values in the mixtures where silica fume was used instead of fly ash.