Rock Solid with Slag
Slag is the glass-like by-product left over after a desired metal has been separated (i.e., smelted) from its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide. However, slag can contain metal sulphides and elemental metals. Slag is generally used to remove waste in metal smelting.
In nature, iron, copper, lead, nickel and other metals are found in impure states called ores, often oxidised and mixed with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. Slag is the collection of compounds that are removed.
In many smelting processes, oxides are introduced to control the slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. Quicklime and magnesite are introduced in molten steel ore for refractory protection, neutralising the alumina and silica separated from the metal, and assist in the removal of sulphur and phosphorus from the steel.
Slag from steel mills in ferrous smelting is designed to minimise iron loss and mainly contains oxides of calcium, silicon, magnesium and aluminium. Any sandy component or quartz component of the original ore automatically goes through the smelting process as silicon dioxide.
As the slag is channelled out of the furnace, water is poured over it for rapid cooling, often from a temperature of around 1,430 °C. The product so obtained is called granulated slag. This process causes several chemical reactions to take place within the slag, and gives the material its cementious properties.
The water carries the slag in its slurry format to a large agitation tank, from where it is pumped along a piping system into a number of gravel-based filter beds. The filter beds then retain the slag granules, while the water drains away and is returned to the system.
When the filtering process is complete, the remaining slag granules, which now give the appearance of coarse beach sand, can be scooped out of the filter bed and transferred to the grinding facility where they are ground into particles that are finer than Portland cement.
Slag as a material cannot be used as cement, but in the presence of lime, slag can become rock hard like cement. This property of slag is of interest to cement producers. Today slag as a replacement material in normal cement is as high as 50 per cent, which is the highest in any class of blended cement. There are many advantages of slag cement like excellent resistance to chemical attack, especially to chloride and sulphates. Therefore slag cement is the appropriate product to be used in marine atmospheres.
The advantages of slag cement concrete are many over ordinary Portland cement concrete. Today all over the world, coastal jobs are carried out only with slag cement concrete, especially the foundation jobs.
But slag cement has a few disadvantages like longer setting time and slow development of strength. Because of these reasons, cement users are sometime reluctant to use slag cement in routine jobs. However, through proper guidance from industry experts, the use of slag cement is now increasing.
Slag as a material also has many other applications other than usage in cement. Slag can also be used as aggregate in concrete.
There are two ways to make to make slag cement from slag, one is to grind clinker with slag and the other to grind slag and cement separately and then blend them together to get cement. A minimum of 50 per cent of slag is blended with clinker to make slag cement.
In the recent past, separate grinding of slag and cement is becoming more popular for the simple reason that energy required to grind slag is much higher than that of clinker. The wear properties of hardware are also higher for slag than that of clinker. A vertical roller mill is the most preferred equipment to grind either slag or slag cement. Brands like Loesche, Pfeiffer and FlSmidth-Atox have mastered the art of grinding slag in vertical roller mills.
Grinding-related parameters of blast furnace slag cements (BFC), such as bond grindability, specific rate of breakage and breakage distributions are determined by employing separate and inter-grinding modes. Strength tests are performed on mortar specimens made by BFC prepared by these modes of grinding to the same fineness. Overall results favour the use of separate grinding mode in view of lower specific energy consumption, ease of manufacture, higher addition of slag and more flexible product quality arrangement.
The best example of use of slag in concrete has been the Bandra-Worli Sea Link in Mumbai where near about 70 per cent of slag has been used to produce M60 grade of concrete. Similarly, worldwide, there have been many structures where slag has been a major component of concrete, apart from cement.
Today majority of slag is used domestically, barring some quantity that is exported to neighbouring countries. In the last decade or so, there have been attempts to use slag produced in other metal industries like copper, lead and zinc. A committee was formed by the Bureau of Indian Standards to look into the possibility of use of slag other than steel slag in cement.
The appointed committee studied materials like copper slag, lead-zinc slag (ISF Slag), LD slag from the iron & steel industry, Kimberlite tailings from diamond mines, steel slag from electric arc furnaces, ferro alloy slag, ferro chrome slag and spent fluidised catalytic cracking from Indian Oil Corporation. The task before the committee was to examine suitability of the materials for use as performance improver in OPC (up to 5 per cent maximum) and for production of PSC.
The committee examined suitability of all materials mentioned above. After detailed examination, only three materials having latent hydraulic properties were considered for replacing clinker either as performance improver in OPC or production of PSC.
The findings of the committee were submitted to BIS, and based on the recommendations, the bureau has permitted use of other type of slag (other than steel slag) to the extent of 5 per cent as performance improver. The three types of slag approved by the committee are:
i)Lead zinc slag (ISF slag) produced by Hindustan Zinc at Udaipur (the main producer in India);
ii)LD slag produced by Tata Steel, Bokaro Steel, Rourkela Steel, Durgapur Steel, Bhilai Steel and RINL Visakhapatnam (six main producers);
iii)Copper slag produced by Hindustan Copper, Hindalco Industries and Sterlite Industries (three main producers in India).
Tata Steel makes two types of steel slag, depending upon the production process of steel making: LD slag (Linz- Donawitz Converter) or BOF (Basic Oxygen Furnace) and EAF slag (Electric Arc Furnace). The marketable LD slag makes up about 10-15 per cent of the steel output. Primary steel manufacturers like SAIL, Tata Steel and RINL produce only LD/BOF slag while secondary producers produce EAF slag.
EAF slag was not found suitable by the BIS committee either as performance improver or for production of PSC. LD slag was found suitable only for use as performance improver in OPC.
In short, ISF slag, LD slag and copper slag have met all requirements for use as performance improvers in OPC (up to 5 per cent). Later on, BIS may take an appropriate view for formulating a new code. ISF slag is recommended for use as performance improver in OPC by the committee.
As detailed earlier, slag cement has a slow setting time, and this creates a problem in construction during winter. This problem can be addressed by adding some alkali activators in a limited dosage, but this process slightly increases production cost and makes the manufacturing a little cumbersome. In the journey towards 'green cement', slag as a material has tremendous potential, which will be exploited more in coming years as we understand materials better.