A new study shows that blending waste tire rubber with pumice in geopolymer concrete enhances impact resistance and sustainability, offering a promising path toward lighter, greener, and more resilient building materials.
Study: Mechanical and impact behavior of lightweight geopolymer concrete produced by pumice and waste rubber. Image Credit: Terelyuk/Shutterstock.com
A paper recently published in Scientific Reports investigated the impact and mechanical behavior of lightweight geopolymer concrete (GPC) made using waste rubber and pumice.
Pumice and Waste Rubber in GPC
Recent research has explored the use of pumice as a lightweight aggregate in GPC, thanks to its beneficial properties. Various studies have investigated how different mix parameters influence the fresh and hardened behavior of geopolymer mortars containing pumice, particularly in terms of rheology, workability, and mechanical performance. The results highlight the importance of optimizing pumice content to achieve a balance between acceptable compressive strength (CS) and desirable lightweight characteristics. In one study using fly ash (FA)-based GPC with basaltic pumice, increasing the pumice proportion led to a noticeable reduction in strength.
In parallel, researchers have examined the incorporation of waste tire rubber (WTR) as a partial or full replacement for conventional aggregates in GPC. Variations in rubber content, binder formulation, and pre-treatment methods have shown that rubberized GPC demonstrates shifts in ductility, density, elastic modulus, splitting tensile strength (STS), and water absorption—all closely linked to CS. Notably, substituting fine and coarse aggregates with crumb rubber has been shown to significantly enhance the impact resistance of GPC, as reflected in energy absorption and crack propagation behavior.
While these findings provide valuable insight, most studies to date have looked at pumice and rubber separately. There remains a substantial gap in understanding how these two materials interact when combined, especially under impact loading.
The authors of recent study identified this as a key limitation in existing literature and aimed to address it through targeted mechanical and impact testing. Closing this gap is essential for advancing high-performance, sustainable, and impact-resistant geopolymer composites that make effective use of environmentally challenging waste materials.
The Study
This study examined the combined effect of incorporating WTR and pumice as aggregate replacements in GPC. The researchers tested different replacement levels and combinations to address the gap in existing literature regarding their joint influence. Specifically, coarse aggregate was partially replaced with pumice in varying proportions by weight, while WTR was added by volume.
Pumice was used to replace coarse aggregate at 0 %, 5 %, 10 %, and 15 % by weight, and WTR was introduced at 0 %, 5 %, 10 %, and 15 % by volume of the coarse aggregate. The WTR was produced by shredding waste tires and resizing the material to meet coarse aggregate specifications. Pumice particles used in the mixes had diameters ranging from 6–11 mm.
A total of 12 different GPC mixes were developed based on these variable ratios, from which 144 specimens were cast and tested. The experimental program included CS, flexural strength (FS), STS, and dynamic impact resistance via drop-weight testing. Additionally, setting time and workability were evaluated to understand fresh mix behavior.
Mix Design and Materials
Lightweight GPC was produced using a fly ash (FA)-based binder, with pumice and WTR serving as lightweight aggregate alternatives. The binder system was activated with a combination of sodium silicate and sodium hydroxide, maintaining a binder-to-activator ratio of 0.57 and a sodium silicate-to-sodium hydroxide ratio of 2.34. Curing was conducted in an oven at 85?°C for 24 hours, followed by 28 days of conditioning at 21?°C and 35 % relative humidity. No additional water was added, as the required workability was achieved using the activator solution alone. The binder-to-aggregate ratio remained constant across all mixtures.
To achieve lightweight properties, pumice was used to replace part of the coarse aggregate by weight, and WTR was added by volume. In mixes containing both pumice and WTR, a further reduction in unit weight was observed. This was attributed to pumice’s high porosity and low density, and WTR’s contribution to energy absorption and improved impact resistance. Slump tests showed that increasing pumice content significantly reduced workability, with slump values dropping from around 100 mm to as low as 35 mm. In contrast, WTR led to more moderate reductions of about 5–15 %.
Mix Notation and Testing Procedures
Each of the 12 GPC mixtures was identified using a specific notation. For example, GPC_P0_R0 refers to the control mix with no pumice or rubber, GPC1_P5_R0 includes 5 % pumice and 0 % WTR, and GPC5_P10_R5 includes 10 % pumice and 5 % WTR. Workability was monitored during mixing and placement, and no significant handling issues were reported.
For testing, prism samples (40×40×160 mm) were cast for FS following ASTM C349, while smaller 40×40×40 mm cubes were used for CS to conserve material. Cylindrical specimens (100×200 mm) were used for STS in accordance with ASTM C496/C496M. A laboratory concrete mixer was used to ensure uniformity, and molds were filled with a combination of compression and vibration. The dynamic drop-weight impact test involved dropping a 16.235 kg mass from a height of 314 mm, generating approximately 50 J of impact energy.
Results
The study successfully explored the combined effects of pumice as a partial coarse aggregate replacement and WTR as an additive in GPC. Mechanical properties such as CS, STS, and FS decreased with increasing replacement levels. However, optimal performance was observed at 5 % pumice and 10 % WTR. Pumice reduced density and improved sustainability but weakened strength due to its porosity, while WTR enhanced impact resistance and energy absorption, making mixes suitable for non-load-bearing applications.
CS decreased by up to 18.17 % and STS by 37.13 % with 15 % pumice. Increasing WTR from 0 % to 15 % further reduced FS by 20–26 %. The best impact performance was achieved in the mix containing 5 % pumice and 10 % WTR, which showed the highest peak impact force (≈ 41 kN) and minimal displacement.
An empirical relationship between compressive and splitting tensile strength was also developed using linear, quadratic, and cubic regression models, achieving an R2 value of up to 0.84. This model provides predictive insight for future GPC design using similar materials.
Scanning electron microscopy, energy dispersive X-ray spectroscopy, and thermogravimetric analysis confirmed strength reductions due to voids and unreacted particles, though the matrix showed good thermal stability. TGA revealed a total mass loss of around 6 % up to 1000 °C, indicating robust thermal resistance.
Conclusion
The findings highlight the potential of GPC incorporating pumice and waste tire rubber as a sustainable alternative to traditional concrete. While mechanical strength decreases with higher replacement levels, optimized combinations—particularly 5 % pumice and 10 % WTR—offer improved impact resistance, reduced density, and strong environmental benefits.
Further optimization of curing regimes and aggregate ratios could enhance the balance between strength, weight, and resilience for future lightweight construction applications.
Journal Reference
Çelik, A. I. et al. (2025). Mechanical and impact behavior of lightweight geopolymer concrete produced by pumice and waste rubber. Scientific Reports, 15(1), 1-23. DOI: 10.1038/s41598-025-19202-4, https://www.nature.com/articles/s41598-025-19202-4
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.