Investigating the influence of curing regimes on the microstructural and mechanical evolution of one-part slag-based alkali-activated binders self-activated with biomass ashes

Abstract

Despite the growing interest in one-part alkali-activated materials, the current reliance on high-carbon footprint commercial solid activators (e.g., synthetic alkali hydroxides) remains a critical environmental barrier. To address this fundamental research gap, this study investigates the feasibility of developing a zero-commercial-chemical one-part alkali-activated matrix. In this study, the “one-part” activation potential of waste-derived biomass ashes—pine ash (PA), oak ash (OA), and hazelnut shell ash (HA)—in ground granulated blast furnace slag (GGBFS)-based systems, along with the influence of curing regimes on microstructural integrity, was comprehensively investigated. Biomass ashes were incorporated at 15%, 30%, and 45% as partial replacements and evaluated under standard air, thermally assisted air, and thermally assisted water curing conditions. Experimental results demonstrated that biomass ashes act not merely as inert fillers but as active components directly involved in matrix formation. Mechanical testing revealed that mixtures without external synthetic activators, particularly S/HA45 and S/PA15, outperformed KOH-activated reference samples, achieving compressive strengths of up to 11.9 MPa under ambient curing. The highest strength (15.5 MPa) was obtained from the S/OA45 specimen subjected to combined ambient, thermal, and water curing. Physical analyses further supported this densification; the S/HA45 mixture exhibited a notable reduction in apparent porosity to 17.31% and an improved ultrasonic pulse velocity (UPV) of 3619 m/s, significantly surpassing the conventional KOH-activated reference (18.18% and 3250 m/s, respectively). TGA/DTG and SEM analyses confirmed that PA promotes early-age formation of fibrous C-(A)-S-H networks, while HA, due to its high alkali content, triggers a strong self-activation mechanism. This was conclusively validated by XRD and FT-IR analyses. Curing analysis indicated that although thermal treatment at 90 °C accelerates reaction kinetics, air-curing without moisture results in drying shrinkage and microcracking. In contrast, water curing mitigates these effects, resulting in a more compact, monolithic matrix. Overall, the findings provide clear microstructural evidence that biomass ashes can be used to develop high-performance mechanically viable, potentially low-carbon binder systems suitable for non-structural or moderate-strength applications without the need for external chemical activators. Overall, the novelty of this study lies in demonstrating that high-alkali and high-calcium agricultural and forestry waste ashes can completely replace corrosive, commercial alkaline chemicals. The practical significance of these findings highlights a potentially sustainable, cost-effective, and industrially viable pathway for producing moderate-strength, "zero-cement" construction materials with a significantly reduced carbon footprint.