CHENNAI : Indian Institute of Technology Madras (IIT Madras) researchers have developed a treatment process involving solar thermal energy to recycle construction and demolition debris.

The waste concrete from demolition was heated using solar radiation to produce recycled concrete aggregate (RCA) that was higher in quality when compared to those obtained from mechanical crushing. The concrete made using this technology met the requirements for typical structural applications.

The demonstration was done at the ‘India One Solar Thermal Power Plant,’ located in ‘Shantivan,’ the headquarters of the Brahma Kumaris organization in Rajasthan. It has 770 solar concentrators to produce electricity using steam generated at high pressure. The plant has been operational since 2017 and provides power to a community of about 25,000 people at a reasonable cost and low maintenance. Two of the concentrators was used in the full-scale trials for treating the waste concrete.

By using concentrated solar energy for the heating, the thermo-mechanical beneficiation of the concrete waste results in high-quality recyclable materials, which can substitute stone (blue metal) aggregates and sand in concrete. In this pioneering study, concrete from a demolition site was heated using solar radiation concentrated through large reflectors and cast iron receivers to more than 550 °C and subsequently scrubbed mechanically to yield coarse and fine RCA, with properties similar to those of pristine aggregates.

The findings of this story were published recently in the reputed, peer-reviewed journal Materials and Structures (https://doi.org/10.1617/s11527-022-02065-w). The paper was co-authored by Mr. Rohit Prajapati, Mr. Surender Singh, Mr. BK Jayasimha Rathod and Prof. Ravindra Gettu.

Elaborating on this study, Prof. Ravindra Gettu, VS Raju Chair Professor, Department of Civil Engineering, IIT Madras, said, “The main intention of the present study was to develop the proof-of-concept that solar radiation could be used in the thermomechanical beneficiation of concrete waste to produce good quality recyclable material for new concrete. This study presents strong evidence for the use of concentrated solar energy for recycling waste concrete, with promise for large-scale waste concrete recycling. This would reduce the energy footprint of Construction and Demolition waste processing significantly, and lead to savings in raw material and electricity, towards circular economy.”

The objectives addressed in this study can be divided into three parts — the use of concentrated solar energy in the thermomechanical beneficiation of concrete waste, the production of high-quality RCA from the waste, and assessing the performance of the RCA in concrete to establish the fact that waste can be recycled.

The main conclusions from this study can be summarised as follows:

Ø  It was observed that the required temperature of about 500 °C could be achieved and maintained for a long duration with the reflector-receiver setup used.

Ø  The properties of the aggregates produced were found to be comparable with those of RCA produced in an electric furnace, with the total yield of recycled products being [90% of the feed concrete).

Ø  Preliminary results on concrete made with the RCA indicate its suitability for typical concrete applications.

Concrete is the most common material used in construction universally with an annual production estimated to be 10–30 billion tonnes. The global consumption of construction aggregates, including that needed for making concrete, is projected to reach 63 billion tonnes in 2024, according to studies.

Practically all aggregate demand is currently met by extensive quarrying and mining, leading to the depletion of primary mineral resources. Moreover, there is a severe shortage of fine aggregate in many countries due to bans on the mining of river sand to avoid serious environmental damage.

On the other hand, construction activities generate considerable waste, estimated to be about 3 billion tonnes per annum. Some developed countries recycle up to 90% of the construction and demolition (C&D) waste whereas others still resort to the dumping of large quantities in landfills.

A rational way is to provide an alternative supply of aggregates is through recycling of C&D waste, which would curtail mining for aggregates and free up space used as landfills. The present study attempts to mitigate the limitations of conventional thermomechanical techniques with regard to harmful emissions through the utilization of concentrated solar energy.