Sunita Mishra
Nowadays the worldwide consumption of sand as fine aggregate in concrete production is very high. Crushed rock, sand, and gravel account for the largest volume of solid material resources extracted from systems globally (Peduzzi, 2014; Beiser, 2018). An estimated 40-50 billion metric tonnes is extracted from quarries, pits, rivers, coastlines and the marine environment each year. Aggregate extraction in rivers has led to pollution and changes in pH levels (Saviour, 2012), instability of river banks leading to increased flood frequency and intensity (Sreebha & Padmalal, 2011), lowering of water aquifers (Myers et al., 2000) exacerbating drought occurrence and severity (John, 2009). Although more aggregates are extracted from nature than any other material after water, reliable data on their extraction is only available in certain developed countries. Concrete is a major construction material that is used worldwide, because of its considerable durability than other construction materials. Ever-expanding paved surfaces accompany ever-growing cities. When rain falls on impervious paving, that water flows across the surface of the land. With large volumes of rainfall, urban floods become more likely and more severe. Even moderate rainfalls can result in significant erosion and water pollution as any fuel, oil or chemical substances are washed into rivers, lakes, and coastal waters. Society can make more efficient use of sourced aggregates through land use planning, and pursuing alternative infrastructure and building design and construction methods. The goal of this strategy is reducing unnecessary construction. The second is avoided the use of cement and concrete where possible so that demand for natural sand is reduced to responsible levels.
Permeable pavement also referred to as porous pavement, is one example of green infrastructure that replaces traditional concrete and asphalt to allow for absorption and infiltration of rainwater and snowmelt in urban environments. It is used in cities around the world, particularly in new cities projects in China and India to reduce surface water runoff volumes and rates by allowing water to infiltrate soil rapidly, helping to reduce flooding while replenishing groundwater reserves.
In many cases, permeable roadways, pedestrian walkways, playgrounds, parking zones can also act as water retention structures, reducing or eliminating the need for traditional stormwater management systems. Vegetated surfaces in cities are also allowing water absorption, store Carbon Dioxide and have esthetical value. Additional proven benefits include improved water quality, reduced pollutant runoff into local water bodies, reduced urban heat island effects (great advantage for adaptation to climate change), lower cost of road salting (in cold environments), among others. Less noted is the indirect contribution to reduced demand for natural sand both in constructing these permeable surfaces and in reducing the need for built drainage systems.
Most permeable pavement designs – porous (or pervious) concrete, interlocking pavement slabs, crushed rock, and gravels, or clay, amongst other materials – do not use fine aggregates (sand). For example, pervious concrete is 15- 25% porous space made by mixing coarse aggregate materials, cement or cement-like additives (i.e. bottom ash or fly ash) and water with little or no sand, depending upon the strength of concrete required. Recent experimentation has shown that introducing end-of-life tyre aggregates can increase the flexibility of rigid permeable pavement systems and with that their capacity to cope with ground movement or tree root systems. New compositions are being developed to improve strength and allow for heavier weight loads. And as each design issue is resolved, the opportunities for scaling increase.
Where construction or traditional cement cannot be avoided, reduction of natural sand use can be achieved through some tried and tested, as well as new emerging technologies and materials. Experimentation has led to a wide variety of “green concrete” forms such as bottom ash or fly ash concrete, ultrahigh performance concrete, geopolymer concrete, lightweight concrete (Liew et al., 2017).
Waste foundry sand (WFS), a by-product of ferrous and non-ferrous metal casting industries is one such promising material, which can be used as an alternative to natural sand in concrete. Foundry sand is high quality silica sand that is a by-product from the production of both ferrous and non-ferrous metal casting industries. It is used for the centuries as a moulding casting material because of its high thermal conductivity. For various foundry operations, raw sand is used and several binder sand additives are added into it to enhance its properties. The WFS is collected at Hindustan Foundries near Nava India, Coimbatore. The sand, which is used for an experiment, is no longer being useful for the foundry industries. Experiment leads to the study of strength parameters in the form of a beam.
Steel slag aggregate is a byproduct of the production of steel in an electric arc furnace. The high iron oxide content of the aggregate results in an aggregate that is very hard and very dense. Steel slag is 20-30% heavier than naturally occurring aggregates such as basalt and granite. It also contains a high content of calcium and magnesium oxides, which make it expand when it comes into contact with moisture. It is very angular and porous. In India, some road construction works have been completed by using slag. However, some concerns due to the high concentration of hexavalent chromium are still under study.
in India there are cases of used non-toxic municipal waste as a replacement for aggregates in road-building, as well as the use of waste foundry sand used (Siddique et al. 2004, 2015), waste rubber (Gupta et al., 2014), waste tiles (Singha & Singla, 2014) to produce concrete. IL&FS Environment has set up India’s first operational large-scale Construction & Demolition Waste Recycling facility for North Delhi Municipal Corporation, on a PPP framework. The plant at Burari will help ease the pressure of the 5000 tons of C&D waste that Delhi generates per day, by recycling it into construction-grade aggregates.
Bottom line is the primary limitation to responsible sand extraction is not technical; it is an awareness and governance issue. A paradigm of infinite sand resources still dominates. Challenging this is the difficult task ahead as we aim for a rapid yet smooth transition to more sustainable sourcing, while reducing consumption and demand in parallel. To do this, we must first acknowledge the scale of the issue as one of the major sustainability challenges of the century. Most large rivers of the world have lost between half and 95% of their natural sand and gravel delivery to ocean. The damming of rivers for hydro-electricity production or irrigation is reducing the amount of sediment flowing downstream. This broken replenishment system exacerbates pressures on beaches already threatened by sea level rise and intensity of storm-waves induced by climate change, as well as coastal developments. Imbibing innovative technologies, institutionalizing appropriate standards and right implementation of regulatory measures could bring positive changes in the environment.
(This article has references from Sand and Sustainability, 2019, UN Environment)
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