Construction and Demolition Waste: The Elephant in the room!
Updated: Nov 13
Solid waste generation will peak in the 21st century with population growth and rapid urbanisation as waste management technologies are trying to play catchup. The construction and demolition waste constitutes more than 50% of the total solid waste generated on a mass basis currently in India. This waste consists of concrete and masonry as retrievable materials after demolition like bricks, wood, metal, tiles are recycled through existing supply chains in India. The construction industry alone generates 10 million tons of this waste annually as India is bullish with its infrastructure development. India plans to spend US$ 1.4 trillion on infrastructure during 2019-23 to have a sustainable development of the country. While, this is a welcome move, it must be planned thoroughly from the waste valorisation perspective as a knee jerk reaction to the construction and demolition waste generation through legislature can result in a fiasco for infrastructure development. The supreme court of India mandates that the waste generator must take care of the proper disposal of the construction and demolition waste. There are typically two avenues for managing this waste currently. It is either used as filler for embankments, or dumped in designated areas such as those used in reclamation of land for further infrastructure development. India has been laggard so far in utilizing the construction and demolition waste owing to lack of standardization in recycling techniques, recycled materials, techniques not being listed in the Indian Standard Codes and/or Schedule of Rates, and a lack of awareness.
Construction and demolition waste is bulky, heavy and is unsuitable for disposal by incineration or composting. Concrete is the main problem in this waste and the second most used material after water by human beings. We have 3 tons of concrete per human being on the planet now. While it has shaped our skylines, it is also responsible for raising the average global temperature beyond 1° C. Concrete alone is responsible for 8% of total greenhouse gases generated on the planet. Concrete consists of gravel (aggregates), sand, water and cement (which holds everything together). Cement is chemically known as calcium oxide (CaO) and is obtained by heating calcium carbonate (CaCO3) found in limestone. Every calcium carbonate (CaCO3) molecule breaks down into one cement molecule (CaO) and one carbon dioxide molecule (CO2) on heating. Thus, every ton of cement manufactured generates 0.8 ton of carbon dioxide which is a greenhouse gas. Further, the heating of calcium carbonate (CaCO3) requires energy that is obtained from fossil fuels. One option to stop the detrimental environmental impact of construction and demolition waste disposal is to reuse the concrete as a substitute for aggregates in fresh construction thereby reducing the need of energy intensive operations for generation of virgin construction materials. Apart from the environmental benefits, the recycling of concrete offers benefits in terms of reduced transportation costs, reduction in capital investment on raw material stockpiling and provides the means for higher profits in infrastructure development work.
Humans have always dreamed of creating long lasting structures that will create a legacy. Romans first developed concrete like material by mixing volcanic ash with sea water. The technology has matured since then and in modern times uses concrete (which is a mixture of sand, gravel (called aggregates), water and cement) to create grand structures. Since the concrete was designed to be durable, recycling concrete was considered difficult until the last decade. However, the technology of recycling the concrete and masonry waste has now matured and even standards have been established for safe and unabated usage of recycled concrete aggregates. Thermal, chemical and mechanical treatments are used to remove the porous residual cement mortar attached to concrete aggregates that can cause lower compression strength of recycled concrete aggregates compared to naturally sourced aggregates. The thermal techniques heat the recycled concrete to a temperature high enough to dehydrate the cement hydration products and weaken the attached mortar that can then be separated from aggregates by mechanical rubbing. The drawback of thermal treatment is the usage of fossil fuels for heating thereby producing the greenhouse gases and nullifying the environmental advantage of recycling the concrete. The chemical techniques on the other hand use low concentration acids instead of heat to dissolve the residual mortar from aggregates before the mechanical rubbing. However, a large quantity of water is required for washing out the acids after treatment. Considering the drawback of only thermal or chemical techniques, new hybrid techniques using CO2 as a reactant have recently developed that offer the benefit of CO2 sequestration along with lower energy and water input to the concrete recycling process. The techniques using CO2 as reactant yield marketable calcium carbonate and calcium acetate by-products along with recycling the concrete as aggregates. With the advent of these recycling technologies, it’s becoming feasible to reduce the environmental impact of infrastructure development and truly leave behind a legacy for our future generations.
Dr. Saurabh C. Patankar &
Mr. Kunal K. Godambe