Slag Cement for Wide Range of Applications
Portland Slag Cement (PSC) is manufactured by inter-grinding Portland cement clinker, gypsum and granulated slag obtained from steel and ferro alloys industries. Alternatively, it is also manufactured by blending OPC with granulated blast furnace slag (GGBS) through mechanical blenders. The latter of the two is more common. As per the IS:455-1989 (as amended) standards, the slag constituent in PSC should not be less than 25 per cent and not more than 70 per cent.
The production of PSC dates back to 1892 in Germany. Its manufacturing in India, however, started only in the second half of the 1960s. With a widespread usage across Europe and in Asia, particularly in Japan and Singapore, today more than 15 countries in the world produce PSC in large tonnage. Production of slag cement in India is 18-25 mtpa. The compressive strength of PSC is generally lower than Ordinary Portland Cement (OPC), however, slag cements of strength equivalent to "43" and "54" grade OPC are available in the market.
The use of slag cement is very popular in the eastern parts of the country due to easy availability of slag from steel plants. Since major brands like UltraTech Cement, Dalmia Cement, Star Cement and others manufacture the product in this region, it has been well promoted and manages to get a premium as compared to its counterpart blended cement, i.e., Portland Pozzolana Cement (PPC). On the other hand, in western and southern India (longest coastline), due to lower availability of slag and greater availability of fly ash, the situation is exactly the opposite. Despite the fact that use of PSC is more suitable for coastal areas, companies find it difficult to position the product on par with PPC. JSW Cement, Vadraj Cement, ACC Cement are amongst the few manufacturers in this part of the country.
PSC can be used for all plain and reinforced concrete general construction as well as wherever OPC or PPC is used. Concrete is essentially a composite construction material composed of fine and coarse aggregate bonded together with a fluid cement (paste). Few of the chemical reactions between the aggregates containing reactive constituents and the alkalis and hydroxyl ions released during the hydration of cement, can have a disastrous effect on concrete. The reactions being expansion and cracking thereby leading to loss of strength, elasticity and durability. The key USP of PSC is its superiority in concrete durability, in turn extending the life span of buildings from 50 to 100 years. Additionally, use of PSC in concrete reduces pore volume and improves its microstructure. Simply put, PSC usage makes the concrete dense, which has the following advantages:
Improved resistance to sulphate attack by sea water due to low permeability
Improved water tightness preventing entry of moisture
Improved corrosion resistance resulting in increased life of reinforcement in concrete
PSC has a positive effect on compressive and flexural strength of concrete after 28 days, until then, it lags OPC. Curing period for slag cement is longer compared to OPC, a minimum of 14 days as recommended by IS 456:2000.
The use of slag cement in concrete production reduces the rate of heat generation and improves workability. The reduction is directly proportional to that of slag content in the cement used. These properties make slag cement concrete most suitable for concrete pavements, airport runways, concrete bridges, as well as water retaining and storage structures.
Slag cement can be used to build mass concrete works like large foundations, dams, floating structures, sewerage and underground structures, pipelines and mines, port-and-harbor structures such as jetties, break-waters, wharfs. It is also suitable for shore protection works such as blocks, tetrapods, machine foundations, piling, caissons, piers, wells, effluent and sewerage treatment plants, buildings, industrial structures, cooling towers, silos and storage structures.
Some of the key projects in India where PSC has been used is Kolkata Metro, Thane Creek Bridge in Maharashtra, Power Station at Trombay, Tata Centre in Kolkata, Karjan Dam in Gujarat and Hindustan Zinc Ltd. Currently it is being extensively used in Mumbai Underground Metro, HPCL Barmer Refinery & Petrochemical Complex in Rajasthan, Naval Dockyard at Bombay Port amongst a few major projects.
Jharkhand, Chattisgarh, Karnataka, Andhra Pradesh, West Bengal and Odisha have a number of cement plants that have absorbed the slag potential within these states. Nevertheless, signs of revival in the Indian steel industry reflect the increase in potential of PSC production owing to future availability of slag.
The sale of blended cement, however, is plagued by the following challenges:
1)Distant location of steel plant (iron ore deposits) which is the source of slag from the market resulting in increased input logistics cost. Supply chain costs, both inward and outward, have become one of the keys parameters governing the viability of a cement plant in today's time.
2)Mindset of OPC being a superior product, which has led to the use of OPC against blended cement even at small sites (site mix concrete). This is due to the belief that faster setting time implies better cement.
3)Rapid conversion of the market from site mix to ready mix concrete (RMC) due to the scale of projects. This is being witnessed in infrastructure projects as well as building construction projects in metros, tier 1 and tier 2 cities. Mixing OPC directly with fly ash or GGBS in order to make concrete is much more cost effective than direct purchase of blended cement.
My organization has been marketing PSC for almost two decades for formerly Indorama - Heidelberg cement plant, which is now JSW cement in Dolvi (Maharashtra) as well as Vadraj Cement in Hazira (Gujarat). Recently, we have concluded the construction of a 75,000 sq ft industrial building in Vasai (outskirts of Mumbai) using PSC in majority.
In order to promote a sustainable future, cement companies should pass on cost benefits and the government must provide incentives along with compulsory use of blended cement in all government projects. Since blended cement uses factory waste, a by-product, as its raw material, it is important for environmental reasons that we maximise the use of PSC and move towards a greener path.
AUTHOR: Mohit Nawany, Director, ASL Sales & Services.