IIT Madras uses Solar Thermal Energy to Recycle Waste concrete

Researchers at the Indian Institute of Technology Madras have developed a treatment process using solar thermal energy to recycle construction and demolition debris. Waste concrete from demolition was heated using solar radiation to produce recycled concrete aggregate (RCA); it was of higher quality compared to that obtained from mechanical crushing, and met the requirements for typical structural applications.


The demonstration was done at the India One Solar Thermal Power Plant located in Shantivan - headquarters of the Brahma Kumaris organization in Rajasthan. The plant 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 has low maintenance. Two of the concentrators were used in the full-scale trials for treating the waste concrete.

By using concentrated solar energy for 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 trial were published in the reputed and peer-reviewed journal ‘Materials and Structures’ (https://doi.org/10.1617/s11527-022-02065-w) which was co-authored by Rohit Prajapati, Surender Singh, 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 intention of the present study was to develop 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, resulting in a circular economy.”

The objectives addressed in this study can be divided into three parts:

• Use of concentrated solar energy in the thermomechanical beneficiation of concrete waste
• Production of high-quality RCA from the waste
• Assessing the performance of RCA in concrete to establish the fact that waste can be recycled.

Conclusions from the study can be summarised as follows:

• It was observed that the required temperature of about 500 °C could be achieved and maintained for a long duration with the reflector-receiver set-up used.
• 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.
• Preliminary results on concrete made with the RCA indicate its suitability for typical concrete applications.

Concrete is the most common construction material used universally with an annual production estimated to be 10–30 billion tons. The global consumption of construction aggregates, including that needed for making concrete, is projected to reach 63 billion tons 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 tons per annum. Some developed countries recycle up to 90% of the construction and demolition (C&D) waste, while others still resort to dumping of large quantities in landfills. A rational way 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.


Summary

• The study attempts to mitigate the limitations of conventional thermomechanical techniques with regard to harmful emissions through utilization of concentrated solar energy.
• The methodology was demonstrated at the India One Solar Thermal Power plant in Rajasthan and is expected to save 250 kWh of electricity per ton of concrete waste.
• Heating is part of the treatment done on the waste to extract components that can be reused in new concrete.
• Treatment can be deployed in large-scale construction projects and for demolition waste processing plants to make the construction industry more sustainable.