EXPERIMENTAL INVESTIGATION OF COMPRESSIVE, TENSILE, AND FLEXURAL STRENGTHS OF NORMAL CONCRETE VS. GEOPOLYMER CONCRETE (6 MOLARITY) CONTAINING FLY ASH, NAOH, AND NA₂SIO₃

Abstract

This study experimentally investigates the mechanical performance of 6 Molarity geopolymer concrete (GPC) made with Class F fly ash, sodium hydroxide (NaOH), and sodium silicate (Na₂SiO₃), under ambient curing conditions. The results are compared with M25‑grade ordinary Portland cement (OPC) concrete. Compressive strength (ASTM C39) was measured at 7, 14, and 28 days; split tensile strength (ASTM C496) and flexural strength (ASTM C78) were assessed at 28 days. Workability (slump), hardened density, and estimated CO₂ emissions were also evaluated. The 6M GPC achieved a 28‑day compressive strength of 31.2 MPa, reaching 95 % of the OPC control (32.8 MPa). However, its 7‑day strength was only 12.5 MPa (58 % of OPC), indicating a delayed strength gain. Split tensile strength of GPC (3.45 MPa) exceeded that of OPC (3.21 MPa) by 7.5 %, and the tensile/compressive ratio was higher (0.111 vs. 0.098). Flexural strength of GPC (4.5 MPa) was 9.75 % higher than OPC (4.1 MPa), with a 38 % greater deflection at peak load (0.58 mm vs. 0.42 mm), demonstrating superior ductility and crack bridging. Slump was lower (70 mm vs. 85 mm) but workable with superplasticizer, while hardened density was 3.3 % lower. Estimated CO₂ emissions per cubic metre were slightly higher for GPC (372 kg vs. 355 kg) due to chemical production, but the full lifecycle benefits include eliminating cement and repurposing fly ash.

It is concluded that 6M geopolymer concrete under ambient curing is a viable sustainable alternative for structural applications requiring moderate compressive strength (25–30 MPa) and high tensile/flexural performance, provided early‑age loading is not critical. The material is particularly suitable for foundations, pavements, and green building projects