India ranks among the foremost global producers and consumers of cement, and the cement industry contributes significantly to carbon emissions. Alkali-activated materials have gained significant attention as a sustainable alternative to Portland cement, offering the potential to mitigate carbon dioxide emissions and promote effective recycling of waste materials. Fly ash (FA) and Ground granulated blast furnace slag (GGBS) are preferred raw materials for Alkali-activated concrete (AAC) owing to their effective repurposing of waste, widespread accessibility, advantageous chemical composition, and performance attributes. This review provides a comprehensive analysis of the current state-of-the-art on the durability aspects of fly ash/slag-based AAC. The paper explores the unique characteristics of FA/GGBS-based AAC, emphasizing their potential to enhance the durability of concrete structures. Insights into the material behaviour under various environmental exposures, including aggressive chemical environments and freeze–thaw cycles, are presented. Furthermore, the article addresses both the obstacles and prospects associated with implementing fly ash/slag-based AAC as a potential construction material suitable for large-scale infrastructure projects. This overview is designed to direct future research efforts and provide practitioners with insights into the potential of FA/GGBS-based AAC for ensuring the prolonged durability of concrete structures.

Although alkali-activated concrete (AAC) has gained significant attention as a sustainable alternative to traditional Portland cement-based concrete due to its reduced carbon emissions and improved durability, optimizing AAC mix design still remains a challenging task as it involves complex interactions between various factors, constituent materials, and their proportions. This study presents the performance based multi-response optimization of alkali-activated concrete mix using Taguchi-integrated Grey Relational Analysis aiming for improved workability, mechanical and permeability properties. This study employs Taguchi’s L₉ orthogonal array to reduce the experimental trials, thereby saving time and resources. The Grey Relational Analysis optimizes factors like binder ratio, solution to binder (Al/b) ratio, and molarity of NaOH and Na₂SiO₃/NaOH ratio, encompassing alkali-activated concrete’s fresh, mechanical, and durability characteristics with structural grade properties. The results show that binder ratio (FA:GGBS) of 90:10, Al/b ratio of 0.45, 10 M NaOH solution with Na₂SiO₃/NaOH ratio of 1 has produced the optimum alkali-activated concrete mix of structural (M40) grade.

The study reports the properties of slag–flyash–microsilica ternary blended one-part alkali-activated binder system. Microsilica, characterised by high surface area, helps in reducing porosity and also the presence of active silica contributes enhancing the reactivity of binders, while flyash and GGBS offer aluminosilicates to support alkali activation. Taguchi’s design of experiment, integrated with Taguchi grey relational analysis (GRA) is employed to determine the optimal precursor blends, water content and activator dosages to achieve setting times, flow characteristics and compressive strength comparable to conventional OPC. The optimised mix exhibited flowability of 120%, an IST of 46 min, FST of 95 min, and compressive strengths of 54.65 MPa and 69.75 MPa at 7 and 28 days, respectively. The predicted results of the proposed regression model were experimentally validated, with deviations not exceeding 5%. Furthermore, microstructural analyses were performed using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) to examine changes in morphology, mineral phases, and molecular bonding of the binder mixes, respectively. The microanalysis has confirmed formation of dense C/N-A-S-H resembling gels as product of alkali-activation.. Carbon footprint, eco-efficiency and cost analysis were performed and found that the optimised mix is an eco-friendly alternative to OPC-based binders. Graphical Abstract

High-temperature exposures of concrete lead to serious damage in concrete structures, resulting in the significant decay of mechanical properties and spalling of concrete. Alkali-activated concretes (AAC) of blended aluminosilicate precursors and activators have been proven to have higher thermal endurance than conventional portland cement concrete. Incorporation of gypsum (GY) in alkali-activated systems has proven to positively impact the mechanical properties when adopted in controlled amounts. GY releases SO42- to the binder system, which helps in the formation of ettringites, along with Ca2+, which leads to the formation of hydrates. This causes a reduction in porosity and improves strength gain. Incorporation of GY into the fly ash-slag-based alkali-activated system further improves thermal endurance by retaining considerable residual strengths even after 800°C exposure. In the present study, the influence of GY on the residual mechanical properties of fly ash-slag-based AAC is investigated to explore the thermal endurance of the ternary mix at elevated temperatures. The mechanical properties of fly ash (FA), Ground Granulated Blast Furnace Slag (GGBS), and gypsum (GY) ternary blended AAC subjected to elevated temperatures are studied in comparison with conventional portland cement concrete (control mix). AAC design mixes with varying proportions of GY as a replacement to FA-GGBS precursor are tested for mechanical properties to obtain the optimum mix. The residual mechanical properties of the FA-GGBS-GY optimum ternary AAC mix are obtained after exposure to elevated temperatures up to 800°C. The morphology and microstructural characteristics of AAC are studied by Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) analyses to investigate the influence of gypsum on the thermal endurance of concrete when exposed to elevated temperatures. Improved thermal endurance is observed for AAC when FA-GGBS precursors are replaced with 5% of GY as compared to the thermal endurance of conventional portland cement concrete (PCC) of the same compressive strength. Doi: 10.28991/CEJ-2024-010-03-017 Full Text: PDF