A microscopic look reveals how the unique ‘handshake’ between rubber and cement could be key to durable, sustainable construction.

India is the world’s third-largest automobile market and, as a result, produces 2.5 million metric tonnes (MT) of tyres annually. The growth also implies an increasing amount of waste tyres, which make up 1% of our solid municipal waste, with 2 million MT of tyres discarded as scrap annually. An additional 0.8 million MT of scrap tyres is imported into India annually from countries such as the United Kingdom, Australia and UAE, where recycling tyres is prohibited. India is the largest tyre recycler in the world, home to 800 registered tyre recyclers and accounting for up to 70% of all tyre recyclers worldwide. However, despite the government setting strict recycling rules, many recyclers continue to illegally burn the tyres or discard them in landfills. These mountains of discarded tyres are a persistent environmental hazard, piling up in landfills or releasing toxic fumes when burned.

One way to address the piling rubber waste is to reuse the rubber as a reinforcement agent in concrete. Researchers around the globe have been exploring ways to replace some of the natural aggregates in concrete, like sand and gravel, with shredded waste rubber (25-30 mm in size), dubbed WasRub, creating a material known as RubCrete. The method finds a use for problematic waste while reducing the need to quarry natural stone and sand, which are generally otherwise used in concrete. However, getting the right mix involves understanding precisely how the ingredients interact, especially at the microscopic level. This is exactly what researchers from the Indian Institute of Technology (IIT) Bombay have done in their recent study, under the guidance of Prof D. N. Singh from the Department of Civil Engineering.

Conventional concrete combines cement (the binder) and aggregates, like sand and gravel. The strength of the concrete heavily depends on the bond between the cement paste and these aggregates. This crucial microscopic connection zone is called the Interfacial Transition Zone, or ITZ. It's a microscopic region, thinner than the width of a human hair, where the properties are slightly different from the main bulk of the cement or the aggregate itself. In Rubcrete, some natural stone and sand are swapped with rubber pieces, which behave very differently from stone. Because of the rubber pieces, Rubcrete is flexible, less dense, and crucially hydrophobic or repels water.

Researchers from IIT Bombay embarked on a detailed investigation to understand the ITZ in RubCrete, zooming in on the microscopic handshake between rubber particles and the surrounding cement. They suspected that rubber's water-repelling nature and mere physical presence would significantly alter this zone compared to traditional concrete. The hydrophobic nature of the rubber means that the cement paste never touches the rubber particles, essentially forming a ‘wall’ around the rubber. “The wall effect disrupts the even distribution of cement grains near rubber particles, creating a porous zone around them,” explains Prithvendra Singh, a PhD scholar at IIT Bombay and the lead author of the new study.

The rubber also pushes away the water on its surface, which the cement needs to hydrate, react, and harden. This accumulation of water on the rubber’s surface ‘dilutes’ the reaction and is thus called the ‘dilution effect’. The IIT Bombay team hypothesised that the Wall and Dilution Effect (WDE) together predict a more porous and potentially weaker ITZ around the rubber particles, causing RubCrete to be softer than its natural concrete counterpart. This is where the challenge and the opportunity lie for RubCrete.

The researchers employed a suite of tools to probe the RubCrete samples to test this effect. They extracted powder samples from three specific regions in the RubCrete sample: right next to the rubber bits, near the natural aggregates, and from the main cement paste body. They identified the specific minerals and chemical bonds formed during the cement hydration process using imaging techniques like X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). They found the expected concrete hydration products like calcium silicate hydrate (C-S-H), the primary glue in concrete, and calcium hydroxide (C-H). However, the distribution and presence of other minerals like Anorthite differed in samples obtained near the rubber.

The researchers used Thermogravimetric Analysis (TGA), which measures an object's weight change with changing temperatures, to reveal how much water was trapped within the structure. They then turned to Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS) for a visual and compositional map of the ITZ, allowing them to see the microstructure, including the tiny pores, the unreacted cement grains, and the hydration products.

The team also recorded the hardness and stiffness of the Rubcrete using a process called Nanoindentation. This involved indenting the material's surface at various points across the ITZ with a diamond-tipped needle and recording its force and displacement. Next, they used X-ray micro-Computed Tomography (micro-CT), creating a high-resolution 3D X-ray image of the concrete's internal structure. Using the technique, they could measure the ITZ thickness and found that it ranged from 35 to 120 micrometres, wider than usually seen in conventional concrete. The method also allowed for the quantification of porosity within this zone for a selected region of interest, finding it to be around 5.8%. Critically, the 3D view suggested these pores were somewhat disconnected.

Their tests confirmed that the WDE creates a distinct ITZ in RubCrete. This zone is wider, more porous, and mechanically weaker at the micro-level compared to the bond with natural aggregates. This explains why RubCrete generally has lower compressive strength than conventional concrete.

However, this perceived weakness holds a surprising advantage. The increased porosity, especially the network of somewhat disconnected pores, acts like a more complex, tortuous pathway for aggressively corrosive substances like salt water. “The most surprising finding is that while rubber weakens the immediate bond with cement, it forms a disconnected pore network in the ITZ, which slows down mass flux (mass movement) of chlorides, moisture etc. This trade-off benefits applications where chemical resistance outweighs strength, such as in extreme climatic and marine environments,” remarks Prithvendra.

Furthermore, the inherent flexibility of the rubber particles and the slightly weaker ITZ contribute to RubCrete's known benefits: it's less brittle, can absorb more energy from impacts, and has better resistance to fatigue cracking. According to Prithvendra, “RubCrete’s strength lies in its damping and flexibility, as well as its resistance to environmental stresses (thermal and chemical). The most immediately beneficial applications would be road barriers and pavements exposed to temperature swings, coastal and marine structures where chloride ingress is a concern, and shock-absorbing systems like railway buffers or earthquake-prone infrastructure.”

This study advances our understanding of RubCrete and its unique properties by integrating multiple sophisticated techniques to paint a detailed, multi-scale picture. Although previous work on RubCrete identified its lower strength and higher ductility, the new research provides a more precise, microscopic explanation of the properties. It also identifies a surprising benefit arising from the disconnected pores formed due to the rubber.

The team behind the study are now looking to test RubCrete in real-world applications.“So far, we have focused more on microstructural behaviour than on direct exposure to environmental cycles. Real-world, long-term testing needs to be performed by placing RubCrete in marine zones, salt spray chambers, and freeze-thaw environments for long durations,” remarks Prithvendra, talking about the limitations and future directions of the research.

The study reinforces that RubCrete is not just a way to recycle old tyres. It's a potential pathway to creating smarter, more sustainable construction materials uniquely suited for the challenges of extreme conditions, helping us build greener and tougher structures in the face of environmental adversity.

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