Mechanical Properties and Reproducibility of One-Part Ambient-Cured Slag and Fly Ash-Based Geopolymer Concrete

The cement industry is a major source of anthropogenic CO2 emissions due to its energy-intensive production process and calcination of limestone. Producing one ton of cement emits approximately one ton of CO2, and cement accounts for about 5% to 8% of global CO2 emissions. In this context, cement-less one-part (“just-add-water”) ambient-cured geopolymer concrete (GPC) has gained attention due to its environmental friendliness and practicality for large-scale cast-in-situ construction. However, field adoption remains limited, mainly due to the scarcity of data on mechanical properties and durability, as well as the lack of widely accepted standards and specifications. This paper is part of the larger research on tensile performance of anchors embedded in GPC. It is well understood that the tensile performance of anchors installed in concrete substrate is largely influenced by their effective embedment depth and the substrate’s mechanical properties, particularly the fracture energy and modulus of elasticity. Therefore, prior to the investigation of the tensile performance of anchors in GPC, it is crucial to understand the mechanical behaviour of the GPC substrate itself. This study examined key parameters that influence the compressive strength of one-part ambient-cured slag/fly ash-based GPC. The alkali content, slag content, water-to-solid (W/S) ratio, and aggregate content were investigated. Additionally, various mechanical properties such as uniaxial tensile strength, splitting tensile strength, elastic modulus, and fracture energy of the hardened GPC are presented. The test results revealed that higher slag and activator content enhanced compressive strength, whereas a higher aggregate content reduced the strength. The strength gain was also attributed to higher alkali content, lower W/S ratio, and increased binder content; however, excessive alkali and an overly low W/S ratio caused rapid setting due to accelerated reaction kinetics. The 7-day compressive strength ranged from 62% to 78% of the 28-day strength, while there was no notable strength gain after 28 days of curing. The developed GPC attained compressive strengths of over 40 MPa at 28 days and 50 MPa at 56 days. The uniaxial tensile strength test demonstrated a ratio of 0.87 relative to splitting tensile strength. The findings also indicated that the aggregate conditions and curing regimes (whether using as-is aggregates with moisture curing or oven-dried aggregates with sealed curing) had no meaningful effect on the mean compressive strength of GPC and its reproducibility.