Feature

Why does it make sense to replace some cement in construction?

December 2018
Studies on the chemistry of fly-ash-cement reactions show that adding 50 per cent of fly-ash to the cement mix can achieve similar compressive strength as that of pure Ordinary Portland Cement (OPC) by the 28th day and a higher strength beyond day 56. By using admixtures and controlling the water/cement material ratio, fly-ash blended concrete surpasses the strengths of OPC-concrete from the third day onwards. FaL-G is a ground blend of fly-ash, lime and gypsum in proportions which, upon hydration, yields strengths in the range of 20-40 MPa, rendering a highly water impervious hard matrix, explains N Kalidas, Director, Institute for Solid Waste Research & Ecological Balance, one of the product developers.

'Upon hydration, FaL-G exhibits properties similar to those of Ordinary Portland Cement (OPC), so it can be effectively used as a cement replacement (or cementitious media) in the production of bricks and blocks, and as a value-add material in concrete. When added to cement, fly-ash enhances the durability and strength of concrete by chemically reacting with the excess lime released during hydration. It is this surplus lime which reacts with deleterious chemicals in the atmosphere or ground, causing concrete to deteriorate.'

The lime for FaL-G was traditionally sourced from the acetylene industry and later from mineral sources. FaL-G, made of lime procured from the acetylene industry, costs about Rs 2,500 per tonne as against Rs 6,000-7,000 per tonne of OPC, which shows how much the cost of construction can be reduced by FaL-G concrete. Since FaL-G has become more popular and procuring lime has become cumbersome and expensive, an alternative FaL-G has been developed, wherein, cement is the source of lime. 'FaL-G in the cement route is more popular nowadays as standard quality cement is more widely and readily available than mineral lime,' adds Kalidas.

                                                                                             - CHARU BAHRI

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