Acid-Resistant Geopolymer Based on Fly Ash–Calcium Aluminate Cement

Abstract

This paper focuses on the production of high-strength geopolymer binder by alkali activation of binary mix of fly ash and calcium aluminate cement (CAC). The fly ash was partially replaced with CAC (8, 16, and 24%) to increase the reactive phases of source materials, and the resulting blends were activated by aqueous solution of NaOH and sodium silicate with different Na2O contents and SiO2/Na2O ratios and were cured hydrothermally at 95°C for 2 h. The compressive strength of the geopolymer mortar increased up to 59 MPa by optimizing the process parameters. The optimized geopolymer specimens were then immersed in HCl and H2SO4 solutions (pH=1) for 6 months to monitor changes in the physical and mechanical properties of the mortars. These experiments were repeated with Portland cement and high alumina cement mortars as control specimens. Microstructural and characterization tools were also employed to examine the compositional and microstructural changes of the geopolymer paste. The findings showed the superior performance of geopolymer mortar against both acidic solutions compared to the control mortars due to the more stable cross-linked aluminosilicate structure formed in the geopolymer samples. XRD studies also revealed that gypsum is the dominant product in the geopolymer sample exposed to sulfuric acid as a result of reaction between calcium compounds and sulfate ions.