Experimental Investigation of the Mechanical Performance of Low-Calcium Fly Ash and Slag-Based Geopolymer Concrete

Abstract

Geopolymer concretes produced using fly ash and slag (FAGPC and SGPC) were investigated in terms of their mechanical properties and short-term durability performance. An alkaline activating solution was prepared by combining sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. Conventional ordinary Portland cement (OPC) concrete was also prepared for comparison. The primary objective of the study was to assess the potential of geopolymer concrete as an alternative to conventional Portland cement concrete (PCC) for structural applications. The research also aimed to support the development and standardisation of geopolymer concrete for use in the construction industry. Concrete specimens incorporating slag-based geopolymer concrete (SGPC), fly ash-based geopolymer concrete (FAGPC), and OPC concrete were prepared and exposed to different aggressive environments, including 5% sulfuric acid, 5% magnesium sulfate (MgSO₄), and 3.5% sodium chloride (NaCl) solution representing seawater conditions. The durability performance was evaluated through acid-resistance testing, visual examination, and measurements of changes in specimen weight. Compressive strength tests were conducted before and after chemical exposure to determine the effect of these aggressive environments on the mechanical performance of the geopolymer concretes. The results indicated that SGPC containing 100% slag exhibited higher strength and durability than FAGPC containing 100% fly ash. This superior performance was attributed to the formation of a denser and stronger cross-linked aluminosilicate polymer network in the slag-based geopolymer concrete. Furthermore, the 100% fly ash-based geopolymer concrete demonstrated better durability than conventional OPC concrete under the investigated chemical environments. However, its mechanical performance was lower than that of OPC concrete, mainly due to the relatively low reactivity of fly ash, its lower calcium content, and the more porous internal structure of the geopolymer matrix. Among all the chemical exposure conditions, sulfuric acid produced the most severe deterioration in all types of concrete.   Keywords: Fly ash–slag-based geopolymer concrete (FAGPC/SGPC); mechanical properties; durability; sulfuric acid; magnesium sulfate; seawater; chemical attack.