Sri Ganesh Kumar Mohan Kumar

The increasing environmental burden associated with Portland cement production has encouraged the use of waste-derived materials as partial cement replacements; however, their environmental benefits must be considered alongside changes in concrete performance. This study comparatively investigates burned wood ash (WA) and waste marble powder (WMP) as independent partial replacements for Portland cement (PC) at 20%, 40%, and 60% by mass. The chemical composition of WA indicates the presence of silica-bearing constituents that may contribute to cementitious or pozzolanic reactions; however, its pozzolanic activity was not independently quantified in this study. WMP was considered primarily as a calcium-rich mineral powder whose contribution may include physical filler and particle-packing effects rather than being assumed to possess independently verified cementitious activity. A 100% PC mixture was used as the control. Concrete performance was evaluated through slump, density, compressive strength at 7, 14, and 28 days, splitting tensile strength at 28 days, and water absorption. Microstructural characteristics were examined using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), while embodied energy and carbon emissions were estimated comparatively to assess the environmental implications of cement replacement. The control achieved the highest 28-day compressive strength of 31.6 MPa. At 20% replacement, WMP and WA mixtures achieved 21.1 and 19.5 MPa, respectively, with further strength reductions at higher replacement levels. Workability decreased with increasing replacement, although WMP mixtures retained greater slump than corresponding WA mixtures. Cube water absorption generally increased at higher replacement levels, whereas cylindrical specimens exhibited greater variability. SEM-EDX revealed differences in matrix morphology and local elemental composition associated with replacement material and proportion. Increasing cement replacement reduced the calculated embodied energy and carbon emissions, but these environmental improvements were accompanied by substantial engineering-performance penalties at higher replacement levels. Among the replacement mixtures investigated, 20% WMP provided the most favourable balance between engineering performance and reduced Portland cement consumption. The findings demonstrate that the replacement level should be selected by considering engineering performance together with environmental benefit rather than by maximizing cement replacement alone.

The transition toward low-carbon and resource-efficient construction materials has intensified interest in geopolymer binders incorporating industrial and post-consumer wastes. Waste glass powder (WGP), a silica-rich component of the global glass waste stream, has emerged as a promising circular-economy precursor in alkali-activated systems; however, reported durability trends remain inconsistent and are often interpreted without mechanistic integration. This review synthesises current knowledge of WGP reactivity, gel chemistry, and long-term performance through an explicit reaction–transport–ageing (R–T–A) framework that links dissolution behaviour and phase assemblage development to pore connectivity, ion ingress, and time-dependent degradation. Under alkaline activation, the amorphous structure of WGP promotes silica release, modifying Si/Al ratios and governing the formation of N-A-S-H or hybrid N-A-S-H/C-(A)-S-H gels. These reaction products determine transport characteristics and ageing evolution, which collectively control chemical resistance, chloride ingress, alkali–silica reaction-type instability, and dimensional stability. Variability across studies is shown to arise from imbalances in particle fineness, replacement level, precursor chemistry, and activator design rather than intrinsic inconsistency in WGP behaviour. The R–T–A framework clarifies how reaction completeness, pore network architecture, and long-term phase stability interact to produce system-dependent durability outcomes. WGP demonstrates strong potential as a circular-economy precursor in alkali-activated binders; however, reliable structural application requires durability-informed mix design grounded in coupled reaction–transport–ageing mechanisms and supported by extended exposure testing under realistic service conditions.

Geopolymers are an environmentally sustainable class of low-calcium alkali-activated materials (AAMs), distinct from high-calcium C–A–S–H gel systems. Synthesized from aluminosilicate-rich precursors such as fly ash, metakaolin, slag, waste glass, and coal gasification fly ash (CGFA), geopolymers offer a significantly lower carbon footprint, valorize industrial by-products, and demonstrate superior durability in aggressive environments compared to Ordinary Portland Cement (OPC). Recent advances in thermodynamic modeling and phase chemistry, particularly in CaO–SiO₂–Al₂O₃ systems, are improving precursor selection and mix design optimization, while Artificial Neural Network (ANN) and hybrid ML-thermodynamic approaches show promise for predictive performance assessment. This review critically evaluates geopolymer chemistry and composition, emphasizing precursor reactivity, Si/Al and other molar ratios, activator chemistry, curing regimes, and reaction mechanisms in relation to microstructure and performance. Comparative insights into alkali aluminosilicate (AAS) and aluminosilicate phosphate (ASP) systems, supported by SEM and XRD evidence, are discussed alongside durability challenges, including alkali–silica reaction (ASR) and shrinkage. Emerging applications ranging from advanced pavements and offshore scour protection to slow-release fertilizers and biomedical implants are reviewed within the framework of the United Nations Sustainable Development Goals (SDGs). Identified knowledge gaps include standardization of mix design, LCA-based evaluation of novel precursors, and variability management. Aligning geopolymer technology with circular economy principles, this review consolidates recent progress to guide sustainable construction, waste valorization, and infrastructure resilience.