Optimization Studies on One-Part Geopolymer Mixes (Pastes, Mortars and Concretes)

Abstract

The consumption of ordinary Portland cement (OPC) to meet the enormous need for concrete production all over the world, is a global threat for climate change. To reduce massive carbon dioxide emissions associated with the manufacturing of OPC, the geopolymerization process has given rise to the transformation of industrial wastes into strong and durable construction materials such as geopolymer binders. However, these geopolymer binders are based on aluminosilicate by-products and alkali activators. The activators involved in alkali activation process are concentrated aqueous solutions, which are viscous, corrosive and caustic. In addition, complexity in transportation and impracticalities in site such as, difficulty in handling, not user friendly, and hard to use for mass production. This study reports on development of a novel ‘one-part’ or ‘just-add water’ geopolymer binder produced by dry blending the solid aluminosilicate precursors, solid alkali source and then adding free water to the blended dry mix to produce a binder as similar to OPC. One-part geopolymers (OPG) have immense potential in large-scale structures owing to their improved safety and convenience of handling over the conventional geopolymer mixing procedure. This study aims to optimize the mixes by understanding, assessing the influence of binder content, activator dosage and water to geopolymer solids (W/GS) ratio on the fresh and hardened properties of one-part geopolymer mixes (namely pastes, mortars and concretes). Various fly ash and slag-based OPG mixes have been developed and studied. The GGBS substitution was chosen as 25, 50, and 75% by volume of fly ash. The activator dosage was taken as 8, 12, and 16% by mass of total binder content and at varied W/GS ratios of 0.35, 0.40, and 0.45. The test results were utilized to develop models which can predict the desired properties of mixes and optimize the mix proportions of OPG mixes using the response surface method (RSM). The microstructural characterization adopting techniques like Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) was carried out to study microstructural changes, mineral phases, thermal mass loss and molecular bonding of OPG mixes. The elevated temperature studies, ecological and cost analysis studies were also performed. x Based on the material characterization observations, the change in GGBS addition, activator dosage, and W/GS ratio were observed to have a considerable impact on both the fresh and hardened properties. The optimum mix proportion of OPG paste obtained was 51.4% GGBS substitution, 12.4% activator content, and 0.32 W/GS ratio with 191 mm flow, 68.6 MPa of compressive strength, 59 and 191 mins of initial and final setting times, respectively. The optimum mix proportion of OPG mortar obtained consists of 49.8% GGBS, 13.6% activator dosage, and 0.37 W/GS ratio. This mix achieved 170.4 mm flow, 57.8 MPa and 5.9 MPa compressive and flexural strengths, respectively and also 1626 microstrain of 180 days drying shrinkage. The optimum mix composition of OPG concrete for achieving a 125 mm slump while maximizing strengths comprises of 75% GGBS, activator dosage of 13.8%, and W/GS ratio of 0.34. This optimized mix achieved compressive, flexural, and split tensile strengths of 73 MPa, 6.2 MPa and 3.9 MPa, respectively. The verification of experimental values of proposed optimized mix are within the absolute deviation of 10% of predicted values, indicating the accuracy of the models and effectiveness of RSM in designing the optimum mix proportions of OPG mixes. Elevated temperature endurance of OPGC mixes increases with both GGBS content and activator dosage. Embodied CO2eq (ECO2eq) and embodied energy (EEeq) increases with increase in activator dosage. The ECO2eq and EEeq of OPG concrete mixes are lower compared to OPC based concrete mixes. Hence the OPG mixes can be considered as more eco-friendly and sustainable materials, as against conventional OPC based mixes.