Enhanced ASR Mitigation and Carbon Reduction Potential of Local Natural Pozzolans as Alternatives to Fly Ash in Cement-Based Systems.

The declining availability of fly ash has intensified the need to identify alternative supplementary cementitious materials (SCMs) capable of maintaining engineering performance while improving durability and reducing greenhouse gas (GHG) emissions. This study evaluates three locally sourced natural pozzolans (NPs) as potential regional alternatives to fly ash (FA), with particular emphasis on alkali-silica reaction (ASR) mitigation performance, hydration behavior, mechanical properties, setting characteristics, and embodied carbon reduction. Physical and chemical characterization revealed high silica contents (65-71%) and relatively fine particle size distributions (d50 between 10 and 14 μm). Accelerated mortar bar testing demonstrated that the natural pozzolans provided substantially greater ASR mitigation than FA. While the FA mixtures continued to exhibit noticeable expansion growth during the testing period, NP1 and NP3 maintained expansion values near or below the commonly used 0.10% mitigation threshold and exhibited significantly reduced visible surface cracking, indicating superior resistance to ASR-related deterioration. Isothermal calorimetry indicated slightly lower early-age heat release for the NP systems compared with OPC and FA, reflecting reduced clinker content and moderate pozzolanic reactivity. Although the natural pozzolans generally exhibited lower early-age strength and stiffness than FA, all NP systems demonstrated continuous long-term mechanical development. At 91 days, compressive strength and dynamic modulus reached up to 83% and 92% of OPC, respectively, with NP1 showing the closest overall mechanical performance to FA. In contrast to FA, the natural pozzolans accelerated both initial and final setting times. A cradle-to-gate life cycle assessment further showed that SCM incorporation significantly reduced embodied carbon emissions, with natural pozzolans achieving greater carbon reduction than FA at equivalent replacement levels. Overall, the results demonstrate that locally available natural pozzolans, particularly NP1 and NP3, can serve as promising alternatives to fly ash by combining superior ASR mitigation performance, meaningful long-term mechanical properties, and substantial carbon reduction potential.