Optimization of Na2O and Activator modulus to produce sustainable ground pond ash and GGBS-based geopolymer concrete

Abstract

Construction industry is progressively seeking sustainable approaches to reduce its environmental footprint. Due to the large volume of concrete consumption, there is extensive focus on enhancing its engineering properties without neglecting the sustainability concerns. In the present work, an attempt has been made to produce geopolymer concrete with industrial by-products. Grinding treatment was carried out to improve the reactivity of the as-received/unprocessed pond ash (UPA), and the output was ground pond ash (GPA). A combination of GPA and ground granulated blast furnace slag (GGBS) in the ratio of 70:30 was used as binders in the production of geopolymer concrete (GPC). Furthermore, due to the sustainability factor, UPA was incorporated as a complete replacement for fine aggregate. Sodium hydroxide (NH) and sodium silicate (NS) were used as alkaline activators. In the GPC binder, Na₂O% and activator modulus (AM), i.e., SiO₂/Na₂O, play a vital role in deciding the degree of polymerization. Thus, two binder contents, namely 500 kg/m³ and 550 kg/m³, were used. Two variations of Na₂O content, namely 5% and 7.5% of binder, have been included. Three AM values-1, 1.5, and 2-were used to prepare concrete samples. Compressive strength and flexural strength of GPC were evaluated. Microstructural properties using scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared radiation (FTIR), thermo gravimetric analysis (TGA), and differential thermal analysis (DTA) have been explored in the present study. Cost and embodied energy analysis were also carried out. Strength results indicated that AM of 2 yielded high early strength due to the rapid setting caused by the reaction between GGBS and the readily available higher proportion of SiO₂ in NS. In later stages, it was observed that the AM of 1.5 yielded higher strength because of the synergy in the reaction between GPA and GGBS in combination with alkaline solutions. Furthermore, increased Na₂O from 5% to 7.5% enhanced the strength by improving the dissolution of aluminosilicates from GPA and GGBS. Among all the combinations, 550, 7.5, 1 mix yielded the highest compressive strength of 42.91 MPa at 28 days. Also, the same combination 550, 7.5, 1, consumed the lowest cost of 205 INR for unit compressive strength production. The combination 500, 5, 1.5 consumed the lowest embodied energy of 40 MJ per unit compressive strength production. However, the results of the mechanical properties, cost analysis, embodied energy analysis, and microstructural studies substantiate that mix with the combination of 550, 7.5, 1 serves as an optimized blend of binder content, Na₂O, and AM for sustainable production of GPA and GGBS-based GPC.

Keywords: Activator modulus; Cost analysis of concrete; Embodied energy of concrete; Geopolymerization; Pond ash; Sustainable concrete.