Slag Cement: The Better Cement
Due to contributions of slag in concrete durability and other environmental features, popularity of slag cement is increasing day by day. Currently at 28 to 30 millions tonnes of slag cement is produced in India by various cement manufacturers which is at 10 per cent of the total cement production in India.
IS 456 recommends to use Portland slag cement confirming to IS 455 in aggressive condition with minimum 50 per cent of ground granulated blast furnace slag (GGBFS), where sulphate, alkalies coming from aggregates or polluted ground water/hard water.
Studies shows that slag with blaine from 350 to 450 sq m/kg can produce concrete from M25 to M60 grade of concrete, however finer slag having blaine from 450 to 1,100 sq m/kg can produce concrete up to M100 to M120 Mpa.
PSC is manufactured by intergrinding Portland cement clinker, gypsum and granulated slag from steel plants. It is also manufactured by blending OPC with GGBFS through mechanical blenders. Slag is a non-metallic product consisting essentially of glass-containing silicates and alumino-silicates of lime and other bases. Slag is dominated by a dense-glassy structure and is obtained by rapidly chilling or quenching the molten slag with water, steam, or air. The granulated slag used for manufacture of slag cement should conform to IS: 12089-1987 standards. The slag constituent shall not be less than 25 per cent and not more than 70 per cent in PSC as per the IS: 455-2015 standard.
Advantages of PSC
1.Maximum reduction in heat of hydration: The heat of hydration is considerably controlled by use of PSC and is a major factor affecting durability. It significantly reduces maximum temperature rise. A temperature reduction results in reduction in thermal strain and ultimately reduces early age thermal cracking.
2.Excellent resistance to sulphate attack: In IS 456:2000 stipulates 'When chloride is encountered along with sulphates in soil or ground water OPC with C3A content 5 to 8 per cent shall be desirable. Alternatively PSC conforming to IS 455 having more than 50 per cent slag may be used. PSC with Slag content more than 50 per cent offers an improved resistance to sulphate attack because of depletion of Ca(OH)2, lower C3A content as well as improved water tightness of the concrete.
3.Improves corrosion resistance: High chloride diffusion affects the reinforcement steel and leads to premature deterioration. Concrete made with PSC leads to increase in resistivity of concrete, and reduces diffusivity of chloride ions in concrete. It improves the corrosion resistance of steel appreciably. The table (based on NCB study) amplifies the improvement in permeability of concrete with use of PSC.
4.Low risk of shrinkage: The total shrinkage, drying plus autogenous of concrete made with PSC is 15 per cent to 20 per cent lower compared to 100 per cent Portland cement concrete.
5.Improved water tightness: Use of PSC improves mineralogy of cement hydration products due to which the concrete becomes denser and, hence, concrete becomes resistance to moisture.
6.Decrease in pore volume: Pores are formed in concrete due to evaporation of excess water used for workability. PSC improves micro-structure of concrete and reduces the pore volume, which results in more denser and impermeable concrete.
PSC is highly value added cement for improving performance and durability of concrete. It is especially useful for all major infrastructure projects like metro rail, highways, bridges, fly overs, aqueducts and other transportation projects. PSC has been widely used abroad and in India on projects like Euro-Tunnel, Bahrain Causeway, Bandra-sewage outfall, Sardar Sarovar Narmada Project, Delhi Metro, Naphta Jhakri Hydro-electric project, etc.
AUTHOR: AK Jain of Vadraj Cement Limited.