Effect of Activated Carbon and Waste Glass Powder on the Compressive and Flexural Strength of Fly-Ash-Based Geopolymer Concrete

Abstract

Portland cement (OPC) remains the default binder for concrete almost everywhere, yet its manufacture is one of the more carbon-intensive steps in the construction supply chain, and alkali-activated geopolymer binders are among the more mature alternatives on offer. Geopolymers are formed by activating an aluminosilicate precursor - fly ash, slag, metakaolin, and several natural pozzolans have all been used - with a strongly alkaline solution, and the resulting matrix can match or exceed OPC concrete on several counts, including early strength gain and resistance to elevated temperature. A large amount of that work has focused on fibers and on nano- or micro-silica as strength-enhancing additives; comparatively little has looked at two additives that are, in effect, waste streams in their own right: activated carbon and finely ground waste glass. We tested both, separately, as partial substitutes for the fly ash in an alkali-activated (NaOH plus sodium silicate) class-F fly-ash geopolymer concrete. Activated carbon replaced 1, 2, and 3 wt% of the fly ash; waste glass powder (400 mesh) replaced 5, 10, and 15 wt%. Cube and prism specimens were cast, cured at 90 °C for 24 h, and tested at 7 and 28 days for compressive strength (BS EN 12390-3) and flexural strength (ASTM C348). Both additives raised strength relative to the unmodified control across most of the ranges tested, though not monotonically: activated carbon peaked at 2% replacement, where 28-day compressive strength rose by roughly 8% and flexural strength by roughly 22%; waste glass powder peaked at 10%, with corresponding gains of about 18% and 29%. These optimum replacement levels are specific to the mixture design, materials, and curing regime investigated here and should not be assumed to generalize without further testing. In both series, the improvement was consistently larger for flexural than for compressive strength, which points toward a microstructural mechanism - pore-filling and interfacial densification - rather than a purely pozzolanic one, although the two are not mutually exclusive and are discussed together. Beyond the mechanical case, using two waste-derived materials as fly-ash substitutes is a reasonably direct way to divert them from landfill and reduce demand on virgin fly ash.