ICR charts out the evolution of gypsum and the role it plays in manufacturing in a bid to understand the economics of sustainability in cement production.
The word gypsum is derived from the Greek word ‘gypsos’ meaning ‘plaster.’ The quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. The ability to harden or set when added with water makes it a very useful mineral for construction. In the mid-18th century, Gypsum was found to have great capabilities as a fertiliser. It is this connection as a fertiliser that today the world over phospho gypsum is now available aplenty as a by-product from fertiliser plants, and which can be gainfully used as an additive in the cement making process, replacing mineral gypsum.
The production of phosphate fertilisers requires breaking down calcium-containing phosphate rock with acid, producing calcium sulphate waste known as phospho-gypsum (PG). Similar is the case with the desulphurisation process of flue gas (to take out the SOx from the emissions) from power plants when natural limestone is used for this process resulting in FGD gypsum as the bi-product. This product is pure enough to replace natural gypsum in a wide variety of fields including drywalls, water treatment and cement set retarder.
As a sustainability initiative, replacing natural gypsum scores better, but first let us understand the role of gypsum in the cement to concrete process.
The main purpose of adding gypsum in the cement is to slow down the hydration process of cement once it is mixed with water. The process involved in hydration of cement is that, when the water is added into cement, it starts reacting with the C3A (tricalcium aluminate, which is the main component of Portland cement) and hardens. The time taken in this process is very less, which doesn’t allow time for transporting, mixing and placing. When gypsum is added into the cement and water is added to it, reaction with C3A particles takes place to form ettringite. This ettringite is initially formed as very fine-grained crystals, which form a coating on the surface of the C3A particles. These crystals are too small to bridge the gaps between the particles of cement. The cement mix therefore remains plastic and workable. The time allowed for mixing, transporting and placing plays an important role in strength, composition and workability of concrete. As gypsum retards the process of hydration, it is termed as retarding agent of cement.
The role of gypsum in concrete making can be summarised as follows:
- Gypsum prevents flash setting of cement during manufacturing.
- It retards the setting time of cement.
- Allows a longer working time for mixing, transporting and placing.
- When water is mixed to cement aluminates and sulphates react and evolve some heat but gypsum acts as coolant and brings down the heat of hydration.
- Gypsum cements possess considerably greater strength and hardness as compared to non-gypsum cement.
- Water required in gypsum based cement for the hydration process is less.
The use of gypsum as an additive in cement ranges from 2.5 to 5 per cent.
In its natural form, gypsum can be found as thick layers in shale and as attractive crystals. No gypsum deposits are 100 per cent pure. It is usually found with deposits of a combination of the following: limestone, sand, shale, anhydrite and sometimes rock salt. To be a commercial deposit, gypsum content should be at least 75 per cent. But as mines get old the percentage of gypsum could be as low as 45 per cent in many of the natural deposits.
Gypsum mines or deposits can be found all over the world, but Spain, Thailand, United States, Turkey, Russia, UAE, Oman and Chile are the leading producers. India has deposits mainly in Rajasthan and that makes the logistics cost play an important role in the use of gypsum in cement and
in India. There are two components to be seen, the percentage of gypsum in the mineral (purity) that one is transporting and therefore total cost of moving it when compared with other forms of gypsum, which could be non-mineral, from synthetic or anhydrous to simply the spent acid or other forms of industrial or chemical waste.
The desulphurisation process itself now being made mandatory for all coal fired power plants creates an enormous opportunity for non-mineral gypsum to be used in cement. But the economics could be very tricky. Let us see the cost dynamics in some details as this could be the most sustainable way for producing gypsum for cement and concrete.
It is calculated that a 500 MW power plant would need 40,000T of limestone annually to take care of the SOx emissions through the desulphurisation process. This would amount to about 12 million tonne of limestone consumption (less than 3 per cent of the total limestone use per year) for the entire power generation of India. But the economics would lie in transportation. Even if limestone is available free of cost, the transportation cost including handling and royalty beyond 250 km could rise to Rs 1000/T as the landed cost at the power plant. The FGD gypsum after production would need to be transported to the cement grinding unit, which if more than 250 km would again cost the same. Thus the FGD gypsum would then compete with phospho gypsum, which is available aplenty in fertiliser or phosphate plants.
As these options compete with each other,use of natural gypsum would subside as the
enormous logistics cost of either importing it or transporting it across India would not be sustainable in the future.
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