Could Municipal Solid Waste Ash be a Green Substitute for Cement?

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 29 2024

A recent article published in Case Studies in Construction Materials explored the feasibility of using sorted municipal waste, specifically paper ash (PA) and food waste ash (FWA), as a partial cement substitute (up to 15 %) in paste and mortar.

Background

Concrete production has raised substantial environmental concerns due to CO₂ emissions from the extensive use of cement. Alternative materials and methods are being developed to mitigate the environmental impact associated with concrete production. One promising approach is the incorporation of waste materials as fillers or pozzolanic substances in concrete.

Moreover, as global populations and urbanization rates climb, municipal solid waste (MSW) generation is expected to rise markedly. Utilizing waste materials in concrete not only curtails cement usage but also helps manage waste disposal issues, reinforcing the principles of a circular economy.

A key component of contemporary waste management is the pre-incineration separation of MSW. Notably, food and paper waste, which comprise approximately 40-44.5 % and 8.5-14.5 % of MSW, respectively, are the primary combustible components. This study focused on incinerating these sorted waste streams to produce homogeneous ashes. The team then assessed the viability of these processed paper ash (PA) and food waste ash (FWA) as partial substitutes for cement in concrete paste and mortar.

Methods

In this study, researchers collected paper and food waste from a landfill near Selangor, Malaysia. The collected waste was dried and incinerated, and the resultant ash was finely ground. This ash was then mixed with ordinary Portland cement (OPC) to create various cement pastes. Additionally, mortar samples were formulated using a blend of paper ash (PA), food waste ash (FWA), OPC, and graded sand.

The chemical composition of PA, FWA, and OPC was analyzed using X-Ray fluorescence (XRF), while particle size distribution was examined through particle size analysis. The morphology of the PA and FWA particles was further studied using scanning electron microscopy (SEM).

For quality assurance, the newly formulated cement pastes underwent a series of tests adhering to European standards. These included a soundness test to check the volume stability and susceptibility to expansion or disintegration under heat and moisture and a setting time test to evaluate the workability and handling properties of the cement.

Mortar mixtures were then prepared with varying concentrations of PA and FWA (0 %, 5 %, 10 %, and 15 %) as partial substitutes for cement. The compressive strength of these mixtures was measured using 50 mm cubic specimens under a loading rate of 1.35 kN/second after curing periods of 3, 7, and 28 days. Additionally, the chloride content in these mixtures was analyzed to assess potential risks of reinforcement corrosion in concrete structures.

Lastly, a heavy metal leaching test was performed on a mortar mix containing 15 % PA and 15 % FWA. The leachate was scrutinized using inductively coupled plasma-mass spectrometry (ICP-MS) to detect and quantify the presence of harmful elements such as As, Mo, Zn, Pb, Cr, Ni, Cu, Ba, Cd, Sb, and Se. 

Results and Discussion

The results underscored both the potential benefits and the limitations of using PA and FWA as partial substitutes for cement. The XRF data confirmed that neither type of ash exhibited pozzolanic properties. Morphologically, the PA particles were irregular, averaging 12.9 μm in size, while FWA particles were spherical with an average diameter of 26.5 μm.

Introducing PA and FWA into the cement mix resulted in increased expansion of the blended cement paste, yet it remained within acceptable standards. Significantly, the high CaO content in PA markedly affected both the initial and final setting times of the cement paste, suggesting a faster setting process.

Regarding mechanical properties, substituting cement with 5-15 % PA reduced compressive strength by 19.6 % to 27.3 % after 28 days. In contrast, a 5 % substitution with FWA had a smaller impact, with only a 7.2 % decrease in 28-day strength, highlighting FWA's potential as a more viable substitute for maintaining structural integrity.

For environmental safety, the cement paste with 15 % PA and FWA substitution adhered to the standard chloride content limit of 0.1 %, mitigating concerns about reinforcement corrosion in concrete structures. Although PA and FWA contained higher levels of certain heavy metals like Zn and Cu—exceeding hazardous waste thresholds—these concentrations significantly decreased in the hardened mortar. At a 15 % substitution level, the blend met standard criteria for being classified as non-hazardous and even inert waste, emphasizing its environmental safety.

Conclusion

This study successfully demonstrated the potential of using sorted municipal solid waste-derived PA and FWA as partial substitutes for cement, ranging from 0 % to 15 %, in the construction industry. The cement pastes containing PA and FWA met European standards in terms of soundness and setting times. However, a notable decrease in the compressive strength of mortar was observed with increased levels of waste ash substitution.

Despite the reductions in compressive strength, PA and FWA present significant environmental benefits. They contribute to reducing the cement industry's carbon footprint and help manage waste more effectively by converting it into valuable construction materials. Additionally, the formulations complied with standards for chloride content and heavy metal leaching, crucial for ensuring the durability and safety of concrete structures.

Given these promising results, the researchers recommend further studies to refine the properties and performance of these materials. Optimizing the blend and treatment processes could enhance the mechanical properties and overall usability of PA and FWA in concrete, potentially broadening their application in sustainable construction practices.

Journal Reference

Lin, J. et al. (2024). Sorted municipal solid waste ash as cement substitute: A study on paper ash and food waste ash. Case Studies in Construction Materials, 20, e03329. https://doi.org/10.1016/j.cscm.2024.e03329, https://www.sciencedirect.com/science/article/pii/S2214509524004807

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