Properties of high-calcium fly ash-based geopolymer concretes improved with high-silica sources

Abstract

In this study, the Afşin-Elbistan fly ash (FA), which is high in calcium content (≈43%) and which does not meet the standards to be used in concrete, was used as a geopolymer raw material. In order to increase the utilization of this high-calcium FA as a geopolymer raw material, two high-silica source industrial by-products, namely silica fume (SF) and waste glass powder (GP), were evaluated. These by-products were used at the concentrations of 5%, 15%, 20%, 25% and 50% in accordance with the weight and in place of FA. Sodium silicate (SS) and sodium hydroxide (SH) solutions were used together as alkali activators. SH was used in three different concentrations: 10 M, 12 M and 14 M. The activator/binder (Ac/B) ratio was selected as 0.45, 0.55 and 0.65. The ratios of SS/SH were 0.75 and 1.5. The curing temperature was 85 °C. The fresh and hardened unit weights, ultrasonic pulse velocities, compressive strengths and splitting tensile strengths of the samples were measured. In addition, changes in the microstructure were investigated by scanning electron microscope, energy dispersive X-ray analyses and, X-ray diffraction analysis. The highest compressive strength result for SF containing series (at 15%, with 0.65 Ac/B and 1.5 SS/SH) was 27.2 MPa at 14 M. For GP containing series, the highest value was 22.2 MPa (at 25% with 0.65 Ac/B and 1.5 SS/SH) at 14 M, where the reference value was 9.4 MPa. The increment of Ac/B from 0.45 to 0.65 and SS/SH from 0.75 to 1.5 were increased the mechanical properties due to the increment in geopolymerization reactions. Besides, UPV results also increased due to the denser structure. These results also supported by microstructure investigations. The experimental results revealed that the Afşin-Elbistan FA, which cannot be used in the cement and concrete industry, can be utilized as a geopolymer raw material with the support of a high-silica source.