Sustainable concrete using rice husk ash, pet, and tire rubber as fine aggregate substitutes

The current environmental crisis is not only linked to climate change, but also to inadequate waste management and the intensive consumption of natural resources. In 2023, approximately 2.3 billion tonnes of municipal solid waste were generated, and this figure is projected to reach 3.8 billion tonnes by 2050. A significant portion of this waste ends up in landfills or is incinerated, releasing greenhouse gases. In the case of PET (polyethylene terephthalate), 35% is incinerated, contributing to approximately 534 million tonnes of CO₂ equivalent per year.

At the same time, the construction industry is responsible for 40–50% of global CO₂ emissions, consumes nearly 40% of the world’s energy, and extracts up to 60% of the planet’s raw materials. Particularly concerning is the demand for sand, which reaches 50 billion tonnes annually—twice the amount that nature can replenish within the same period.

In this context, the incorporation of recycled materials as partial substitutes for fine aggregates emerges as a practical strategy to reduce emissions, decrease resource extraction, and move toward more sustainable construction practices.

From Waste to Resource: Rethinking Fine Aggregate

This study evaluated the behavior of concrete when fine aggregate (sand) was partially replaced by three recycled materials with high regional availability:

• Rice husk ash (RHA) 

• Crushed PET plastic 

• Rubber derived from end-of-life tires 

These materials were selected due to their abundance as solid waste in urban and agro-industrial sectors, as well as their potential to be directly incorporated into concrete mixtures without the need for complex chemical treatments.

Replacement levels of 2.5%, 5%, and 10% of the fine aggregate, by weight, were analyzed, and each mixture was compared with a conventional reference concrete. The objective was to assess how these substitutions influence key properties such as:

• Slump 

• Density 

• Air content 

• Compressive strength 

• Splitting tensile strength 

• Modulus of elasticity 

In addition, microstructural studies were conducted using Scanning Electron Microscopy (SEM), and thermal analyses were performed through Thermogravimetric Analysis (TGA) to better understand how these materials interact with the cementitious matrix.

¿Which Material Performed Best?

The results indicate that incorporating recycled materials into concrete is feasible, although certain technical limitations must be considered.

• Rice husk ash (RHA) 

RHA exhibited the best overall performance. At a 2.5% replacement level, the reduction in compressive strength was only 1.6% at 28 days, a value that is practically negligible from a structural standpoint. This behavior is attributed to its high silica content and pozzolanic nature, which promote chemical reactions that densify the concrete matrix. SEM analyses confirmed improved integration within the interfacial transition zone (ITZ) between the aggregate and the cement paste.

However, RHA presented a significant challenge: high water absorption, which required adjustments to the water-to-cement ratio to maintain workability. Nevertheless, up to a 5% replacement level can be considered technically viable, provided that proper mix control is ensured.

• Recycled PET 

PET showed intermediate performance. At a 2.5% replacement level, compressive strength decreased by approximately 10%, while tensile strength was reduced by around 18%.

As a hydrophobic and non-reactive material, PET does not form chemical bonds with the cementitious matrix. However, it maintained good workability and did not require adjustments to the water-to-cement ratio. Statistical analyses indicated that, in some cases, variations in tensile strength were not statistically significant.

Up to a 5% replacement level, the mechanical losses can be considered moderate and acceptable for non-structural or low-demand applications.

• Tire rubber 

Recycled rubber showed the greatest mechanical reductions, particularly at higher replacement levels. Although it provides advantages such as lower density (which may reduce seismic loads) and increased ductility, its hydrophobic nature and smooth surface lead to a weak interfacial transition zone with the cement paste.

SEM images revealed a more discontinuous interface compared to RHA. Nevertheless, up to a 5% replacement level, the performance losses remain technically manageable.

¿What Happens When the Replacement Level Increases?

A key finding was that the progressive increase in replacement levels leads to a corresponding decrease in mechanical properties. Up to 5%, reductions in compressive and tensile strength generally remained below 25%. At 10%, the reductions became more pronounced, particularly in the modulus of elasticity, with decreases reaching up to 34% in some cases.

Statistical analysis (ANOVA and Tukey’s test) confirmed that both the type of material and the level of substitution significantly influence mechanical performance. From a technical standpoint, a 5% replacement level represents the optimal balance between sustainability and structural performance.

Beyond Strength: Thermal Behavior and Microstructure

Thermogravimetric analysis (TGA) showed that thermal behavior does not follow a linear trend with increasing replacement levels. RHA exhibited more stable thermal performance, attributed to its chemical compatibility with cement. In contrast, PET and rubber showed greater mass losses at intermediate levels, associated with their polymeric nature.

From a microstructural perspective, the quality of the interfacial transition zone (ITZ) was a determining factor. Improved integration within this zone translated into lower strength losses.

Conclusions

The results indicate that:

• It is technically feasible to incorporate PET, rubber, and RHA as partial substitutes for fine aggregate. 

• RHA showed the best structural performance, particularly at low replacement levels. 

• A 5% replacement level represents the optimal balance between sustainability and mechanical performance. 

• A 10% replacement level leads to significant reductions, limiting its use in demanding structural applications. 

Beyond a simple modification of the mix design, this approach represents a paradigm shift in construction: moving from an extractive model to a circular one, where waste is transformed into resources.

The transition toward more sustainable materials does not require sacrificing safety or performance, but rather innovation, technical control, and willingness to change. Integrating valorized waste into concrete is a concrete step toward more resilient, responsible cities aligned with the environmental challenges of the 21st century.