A new study published in Scientific Reports shows that recycled concrete powder (RCP), clay brick powder (CBP), and volcanic pumice powder (VPP) can improve the strength and durability of geopolymer concrete (GPC) when used under water curing conditions—pointing to a promising path for more sustainable construction materials.
Study: Valorization of recycled concrete powder, clay brick powder, and volcanic pumice powder in sustainable geopolymer concrete. Image Credit: Denis Torkhov/Shutterstock.com
Why It Matters
Concrete remains one of the most widely used materials in infrastructure, prized for its strength, low porosity, and long lifespan. But its environmental cost—mainly from Portland cement (PC) production—continues to raise concerns.
Geopolymer concrete (GPC) addresses this challenge by using industrial by-products like fly ash (FA) and ground granulated blast furnace slag (GGBS) as binders instead of PC. This approach not only cuts emissions but can also enhance performance. However, the effects of substituting a large portion of FA and GGBS (up to 75 %) with other waste materials—like RCP, CBP, and VPP—have been less thoroughly explored, especially under water curing conditions, which are commonly used in real-world applications.
While prior studies have looked at these additives individually, their combined impact on the fresh and hardened properties of GPC has remained largely untested—until now.
How the Study Was Conducted
The researchers sourced RCP from crushed waste concrete cubes, CBP from demolished construction waste, and VPP from naturally occurring pumice in Hurghada, Egypt. All materials were finely ground before being used in the mixes.
Ten different concrete mixes were prepared by replacing 25 %, 50 %, and 75 % of FA and GGBS with RCP, CBP, or VPP. These were compared against a control mix containing only FA, GGBS, and silica fume (SF). To ensure consistency, water was added gradually to each mix to achieve a slump of 9.0 cm—maintaining similar workability across the board.
The team evaluated a range of mechanical properties—compressive strength (CS), flexural strength, and splitting tensile strength—as well as key durability metrics such as water absorption and permeability. Additional tests included resistance to chemical attack and heat exposure.
For durability, samples were submerged in 5 % magnesium sulfate (MgSO4) and 5 % sulfuric acid (H2SO4) solutions for 28 days. Researchers tracked changes in weight and strength. For thermal testing, specimens were heated to 200 °C, 400 °C, and 600 °C for 1.5 hours each, then assessed for mass loss and remaining strength.
Microstructural characteristics were analyzed using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD), giving insight into how these materials behave at a deeper level.
What They Found
Water demand varied across the three materials: CBP required the most due to its fine, porous structure and high reactivity, followed by RCP. VPP needed the least, thanks to its lower specific surface area.
At a 25 % replacement level, both RCP and CBP enhanced compressive strength. RCP-25 % increased CS by 14.88 %, while CBP-25 % delivered a 21.12 % boost—making it the highest-performing blend at that level. VPP-25 %, however, resulted in an 8.68 % decrease in strength, likely due to its porous nature and the voids it introduced in the concrete matrix.
Higher replacement levels (50 % and 75 %) saw a drop in compressive strength across all mixes. This decline was attributed to dilution of the binder, increased porosity, and the presence of unreacted particles and microcracks.
Durability tests showed that the control mix had the best resistance to sulfate and acid attack. Among the alternatives, CBP-25 % outperformed the others in acid resistance, while VPP-25 % exhibited the most degradation. The control mix lost only 1.6 % of its weight under acid exposure, compared to 2.2 % for RCP-25 %, 1.8 % for CBP-25 %, and 4.5 % for VPP-25 %.
Thermal resistance tests at 600 °C revealed that all mixes containing recycled materials experienced more mass loss than the control. Still, RCP-based mixes retained relatively strong residual strength, showing promise in high-temperature conditions.
SEM images showed denser microstructures in RCP and CBP mixes compared to VPP, with fewer cracks and voids—explaining their superior mechanical and durability performance. XRD analysis supported these observations, revealing the formation of beneficial crystalline phases in RCP and CBP blends.
Key Takeaways
This study highlights the potential for using RCP, CBP, and VPP as partial replacements for FA and GGBS in geopolymer concrete. At 25 % substitution levels, RCP and CBP significantly improved compressive strength by 14.88 % and 21.12 %, respectively, over the control. VPP, while less effective mechanically, may still offer niche benefits due to its thermal properties.
Although the control mix remained the top performer overall, the recycled material blends, especially those incorporating CBP, demonstrated encouraging durability and strength under water curing, chemical exposure, and heat.
Journal Reference
Tahwia, A. M., Abdellatief, M., Salah, A., & Youssf, O. (2025). Valorization of recycled concrete powder, clay brick powder, and volcanic pumice powder in sustainable geopolymer concrete. Scientific Reports, 15(1). DOI: 10.1038/s41598-025-93598-x, https://www.nature.com/articles/s41598-025-93598-x
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