Mechanical and durability analysis of geopolymer concrete made with recycled silicate activator for low carbon breakwaters

Abstract

This study investigates the feasibility of using recycled sodium silicate (RSiA) extracted from waste glasses as an alternative to commercial sodium silicate (CSiA) in the production of alkali-activated geopolymer concrete for breakwater applications. The influence of RSiA on the chemical characteristics, mechanical performance, and environmental impact of geopolymer mortars and concrete was evaluated. Elemental composition and FTIR analyses were conducted to characterise the silicate activators, revealing comparable chemical profiles with a notable difference in the SiO₂/Na₂O molar ratio (Ms: CSiA = 2.81; RSiA = 1.53). Mortar mixes with varying fly ash (FA) and ground granulated blast furnace slag (GGBS) ratios (e.g., FA:GGBS = 40:60, 80:20) and different Na₂SiO₃:NaOH ratios (1:2, 1:1, and 2:1) were tested for compressive strength under various curing regimes, including air, water, and partial hot water curing. Microstructural analysis via SEM and XRD confirmed the formation of dense geopolymer gels and crystalline phases. CO₂ emissions and cost analysis indicated that a 2:1 Na₂SiO₃:NaOH ratio minimised both environmental and economic impact. The optimised mix (C40–2111) achieved a compressive strength of ∼47 MPa under partial hot water curing, ∼35 MPa under air curing and ∼33 MPa under water curing after 28 days. Compared to conventional concrete used in coastal defence structures, the RSiA-based geopolymer blocks met or exceeded performance standards in terms of compressive strength, suggesting their suitability as sustainable alternatives to traditional breakwater materials.