The world is sitting over a landfill of possible substitutes for sand
Everyone seems to be getting something constructed (directly or indirectly) nowadays. My government, my boss, my neighbour, my father, in fact my laundry guy is also getting his house made in his village in north Bihar. Irrespective of location, scale or type, concrete is the base of all the construction activity in the nation. In fact, concrete is the second most consumed material after water, with nearly three tonnes used annually for each person on the planet. India consumes an estimated 450 million cubic metre of concrete annually, which approximately translates to 1 tonne per Indian. We still have a long way to go by global consumption levels but do we have enough sand to make concrete?
My neighbour has already emptied his coffers and his construction is just half way through. He blames multiple factors, but the insane cost of sand in Delhi and erratic supply tops his list. “How can anything be built without sand?” he frequently inquires. He is not the only one. A few months back I got a call from Times of India asking me the same question. A developer friend of mine told me, “use glass façade to reduce the need of plaster and mortar; it reduces the application of concrete and, therefore, sand.” But then glass is nothing but molten sand.
Value of construction industry grew at staggering rate of 15.10 per cent annually even in the economic slowdown and has contributed to 7.5-8.5 per cent of the country’s GDP (at current prices) for the past eight years. Thus, it is becoming increasingly embarrassing for people like me who talk about greening the industry to have no practical answer to this very critical question. Bigger question is: does the scientific community have the answer? And the answer I found to my delight is: yes, they do.
Apparently we have been sitting over a landfill of possible substitutes for sand all while along. Industrial waste and by-products which have been raising hazardous concerns both for the environment and human health can have major use in construction. This can be useful for both the economy and the environment. Let’s see what my scientist friends say about different industrial waste and their ability to replace the much sought after natural river bed sand.
Presently, worldwide, about 33 million tonnes of copper slag is generated annually with India contributing 6-6.5 million tonnes. Khalifa S Al Jabri of Oman in his findings published in the internationally referred journals such as Cement and Concrete Composites and Construction and Building Materials in 2006 recommends that 50 per cent copper slag can be used as replacement of sand in order to obtain concrete with good strength and durability requirements. Back in India, a study carried out by the Central Road Research Institute (CRRI) has also shown that copper slag can be used as a partial replacement for sand as fine aggregate in concrete up to 40 per cent in pavement grade concrete without any loss of cohesiveness and the compressive and flexural strength of such concretes is about 20 per cent higher than that of conventional cement concrete of the same grade.
Granulated blast furnace slag
According to the report of the Working Group on Cement Industry for the 12th five year plan, around 10 million tonnes blast furnace slag is currently being generated in the country from iron and steel industry. M C Nataraja in his study published in the International Journal of Structure & Civil Engineering Research in May 2013, says that the data obtained from his research shows that the compressive strength of cement mortar increases as the replacement level of granulated blast furnace slag (GBFS) increases. He further concludes that from the test results it is clear that GBFS sand can be used as an alternative to natural sand from the point of view of strength. Use of GBFS up to 75 per cent can be recommended.
Adding to Nataraja’s work, Meenakshi Sudarvizhi of K L N College of Information Technology, Tamil Nadu, in her paper published in the International Journal of Civil and Structural Engineering in 2011 says that a mix of copper slag and ferrous slag can yield higher compressive strength of 46.18MPa (100 per cent replacement of sand) while corresponding strength for normal concrete was just 30.23MPa. Though she warns that with higher levels of replacements (100 per cent) there might be some bleeding issues and, therefore, she recommended that up to 80 per cent copper slag and ferrous slag can be used as replacement of sand.
India is currently producing in excess of 100 million tonnes of coal ash. Out of the total ash produced in any thermal power plant, approximately 15 –20 per cent is bottom ash and the rest is fly ash. Fly ash has found many takers but bottom ash still continues to pollute the environment with no safe disposal mechanism on offer. Mohd Syahrul Hisyam of Malaysia offers a solution. In his study published in the International Journal of Sustainable Construction Engineering & Technology in December 2010, he says that the mechanical properties of special concrete made with 30 per cent replacement of natural sand with washed bottom ash by weight has an optimum usage in concrete in order to get a favourable strength and good strength development pattern over the increment ages.
About 20 to 25 per cent of the total production in each crusher unit is left out as the waste material-quarry dust. Chandana Sukesh in his study published in the International Journal of Innovative Technology and Exploring Engineering in May 2013 says that the ideal percentage of the replacement of sand with the quarry dust is 55 per cent to 75 per cent in case of compressive strength. He further says that if combined with fly ash (another industrial waste), 100 per cent replacement of sand can be achieved. The use of fly ash in concrete is desirable because of benefits such as useful disposal of a byproduct, increased workability, reduction of cement consumption, increased sulfate resistance, increased resistance to alkali-silica reaction and decreased permeability. However, the use of fly ash leads to a reduction in early strength of concrete. Therefore, the concurrent use of quarry dust and fly ash in concrete will lead to the benefits of using such materials being added and some of the undesirable effects being negated.
India ranks fourth in terms of total foundry production (7.8 million tonnes) according to the 42nd Census of World Casting Production of 2007. Foundry sand which is very high in silica is regularly discarded by the metal industry. Currently, there is no mechanism for its disposal, but international studies say that up to 30 per cent foundry sand can be utilized for economical and sustainable development of concrete.
Construction and demolition waste
There is no documented quantification of amount of construction and demolition (C&D) waste being generated in India. Municipal Corporation of Delhi says it is collecting 4,000 tonnes of C&D waste daily from the city which amounts to almost 1.5 million tonnes of waste annually in the city of Delhi alone. Even if we discount all the waste which is illegally dump around the city, 1.5 million of C&D waste if recycled can significantly substitute demand for natural sand by Delhi. Recycled sand and aggregate from C&D waste is said to have 10-15 per cent lesser strength then normal concrete and can be safely used in non-structural applications like flooring and filling. Delhi already has a recycling unit in place and plans to open more to handle its disposal problem.
Indian standards lag
Today, the use of copper slag as a substitute for sand in the production of concrete is widespread in Singapore with the majority of the “ready mixed concrete” companies using it. The new application of copper slag as a partial substitute for sand also triggered the change of the Singapore's Aggregates Standards which allows both natural and non-natural aggregates, including recycled aggregates, in concrete, thereby, opening the door to the use of more recycled/waste materials in concrete production.
Similar approach is needed in India; Indian standards need to expand themselves to incorporate these new substitute. Right now only natural sand and aggregate is allowed by the standards to be used in construction. Effectively closing down all the options slated above, except the quarry dust which is a natural product. All the substitutes come with some riders, and they cannot be adopted blindly; there is a need for standards and checks to be put in place. Most of these are already accepted substitutes around the globe, with specific standards being designed to control the quality. It is high time Indian standards rise to the occasion.