Physical and mechanical properties of metakaolin based heat-treated stone powder waste substitution colemanite and borax penta hydrate additive geopolymer mortars

Due to their advantages, the geopolymers are produced to be an alternative to traditional concrete and mortars frequently used in today's world, and they are extremely eco-friendly materi-als with high strength values, low energy consumption and low CO2 emissions resulting from the activation of the binders having high pozzolanic activity with the alkaline activators. In this study, the metakaolin having an extremely high pozzolanic activity and obtained by the calcination of the kaolin clay was used as the binder of the geopolymer production. The heat-treated stone powder waste having the quality of waste material obtained from asphalt worksites was used as the substi-tute material. The boron minerals colemanite and borax penta hydrate having extremely strategic importance for our country were used as the additives. Within the scope of the study, geopolymer mortars were produced using the main binder metakaolin and taken as the reference. Additionally, the heat-treated stone powder waste was replaced as the substitute material instead of metakaolin at 10%, 20% and 30% replacement rates, respectively. As the additive materials, colemanite and borax penta hydrate were added to metakaolin at 5%, 10% and 15% additive ratios, respectively. Sodium hydroxide (SH) and sodium silicate (SS) were used as alkali activators in all geopolymer mortar mixtures. Before starting the main experiments, the optimum values of the mortar components such as the amount of binder, liquid/binder ratio, SS/SH ratio and the molarity of sodium hydroxide were obtained by using the Taguchi optimization technique and used in all mixtures. In order to deter-mine the physical properties of the produced geopolymer mortars, the spreading test and the tests to determine the unit volume weight, setting time, apparent specific gravity, volumetric specific gravi-ty, water content, water absorption rate and the void ratio were carried out. In order to determine the mechanical properties, the flexural and compressive strength tests of 7, 28 and 90 days were per-formed. Moreover, various imaging techniques were used to determine the microstructural proper-ties. As a result of the tests applied on the produced geopolymer mortars, the highest flexural and compressive strength values of 7th, 28th and 90th days were obtained in the metakaolin binder mortars among all the mixtures. When the mechanical strength values and the physical properties are considered together, it was observed that 20% of the heat-treated stone powder waste can be substituted in metakaolin binder geopolymer mortars, and 10% colemanite and 5% borax penta hydrate can be used as additives. Considering the physical properties of the mortars, it was seen that the samples having the best workability values were produced with the heat-treated stone pow-der waste. In addition, it was observed that the boron minerals, especially the colemanite additives, provided a significant benefit to the geopolymer mortar by reducing the water absorption rate and the void percentage. SEM images also confirmed the contribution of boron minerals. From the microstructural point of view, it was seen that the characterization of the mortar was good in general and it formed a good geopolymerization bond. The mixtures negatively affected the microstructural characterization of the geopolymer mortar were observed to be the samples using the heat-treated stone powder waste. It was also observed that the boron minerals presented a positive effect on the characterization of the mortar for the mixtures where the boron minerals were used as additives. Keywords: Geopolymer mortar, metakaolin, heat-treated stone powder waste, colemanite, alkaline activator