A recent study published in Engineering offers new insights into the potential of non-traditional and natural pozzolans (NNPs) as sustainable precursors for alkali-activated binders. The research, conducted by a team from Clarkson University, Purdue University, and Penn State University, evaluates the reactivity, phase assemblage, and composition of various non-traditional aluminosilicate materials, including ground bottom ashes (GBAs), low-purity calcined clays (CCs), volcanic ashes (VAs), and fluidized bed combustion ashes (FBCAs).

Alkali-activated materials (AAMs) have long been recognized as eco-friendly alternatives to ordinary Portland cement (OPC) due to their lower CO₂ emissions and minimal natural resource usage. However, the adoption of AAMs in engineering applications has been limited by the availability of conventional precursors like fly ash and slag. This study explores alternative aluminosilicate sources to address these limitations while supporting global sustainability goals.

The researchers activated the four groups of non-traditional pozzolans using a mixture of sodium silicate and sodium hydroxide solution under optimized conditions. They analyzed the reaction behavior, pore solution changes, phase assemblage, and composition using various material characterization tools, including isothermal calorimetry, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and energy-dispersive X-ray spectroscopy (EDS).

The results indicate that the reaction rate and total heat release of these non-traditional precursors are similar to those of conventional fly ash, with total heat release 38%–82% lower than that of OPC. Pore solution analyses revealed the formation of typical alkali-activated gel phases, including calcium aluminosilicate hydrates (C-A-S-H), sodium aluminosilicate hydrates (N-A-S-H), and calcium sodium aluminosilicate hydrates (C-N-A-S-H), with aluminum/silicon (Al/Si) ratios ranging from 0.07 to 0.36.

One of the key innovations of this study is the development of an Al-NMR-based reactivity index, which provides a quantitative framework for evaluating precursor performance. The findings suggest that CCs and GBAs exhibited superior performance compared to VAs and FBCAs, making them promising candidates for sustainable alkali-activated binders.

The study also highlights the role of calcium in modifying the activation mechanisms and the dynamic interplay between dissolution and polycondensation processes. The researchers found that high-calcium materials, such as GBA3 and FBCA2, formed more C-A-S-H gels, while low-calcium materials produced more N-A-S-H and C-N-A-S-H gels. The Al/Si ratios of these gels varied significantly, reflecting the complex chemistry of the reaction products.

The practical application of these findings could have significant implications for the construction industry. Alkali-activated concretes (AACs) have demonstrated broad applicability in various sectors, including residential construction, hydraulic engineering, and transportation infrastructure. Despite the higher cost of NNP concrete, its significant reduction in CO₂ emissions positions it as a more sustainable option. The study emphasizes the potential of tailored precursor combinations to enhance performance and expand the applicability of AAMs.

Future research should focus on long-term durability assessments and large-scale field validation to predict phase stability across diverse environmental conditions. The expansion of the Al-NMR reactivity index to encompass a broader range of precursor chemistries will be essential for standardizing alternative binder formulations and accelerating the transition toward structural applications of concrete containing these precursors.

This study not only advances the fundamental understanding of reaction mechanisms in non-traditional precursors for AAMs but also provides a foundation for optimizing alkali-activated binder formulations. By promoting waste beneficiation and reducing carbon emissions, this research supports a circular economy and enhances both environmental and economic feasibility in the construction industry.

The paper “Non-traditional and Natural Pozzolans as Precursors for Sustainable Alkali-Activated Binders: Reactivity, Phase Assemblage, and Composition Analysis,” is authored by Roshan Muththa Arachchige, Shubham Mishra, Jan Olek, Farshad Rajabipour, Sulapha Peethamparan. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.05.003. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.