Valorization of Fly Ash and GGBS Through Orthophosphoric Acid Activation for Geotechnical Applications

Abstract

The limited reactivity of fly ash (FA) under ambient conditions and its low early-age strength hinder its direct use in construction and soil stabilization. This study examines a blended binder system in which fly ash is partially replaced with ground granulated blast furnace slag (GGBS) and activated using orthophosphoric acid (H₃PO₄). GGBS was incorporated at 0%, 5%, 10%, and 15% replacement levels, with H₃PO₄ molarities of 0 M, 2 M, 4 M, and 6 M. Unconfined compressive strength (UCS) and split tensile strength (STS) were assessed on specimens compacted to standard Proctor density. Increasing GGBS content raised the maximum dry density (MDD), while the optimum moisture content (OMC) decreased to 5% GGBS before increasing again. Strength properties improved consistently with higher GGBS content, greater compactive effort, and increasing acid concentration, reaching a peak UCS value of 11.66 MPa and STS value of 580 kPa at 15% GGBS and 6 M acid after 28 days. Microstructural analyses (SEM, XRD, FTIR) revealed that these gains were associated with enhanced phase formation, densification, and refined pore structure. Integrating GGBS and H₃PO₄ activation significantly improves early strength, durability, and microstructural integrity of fly ash-based binders. This sustainable binder system not only demonstrates strong potential for use in subgrade, pavement layer, and foundation applications, particularly in aggressive environments, but also offers a promising solution for waste valorization and reduced reliance on conventional cementitious materials.