Additives as Performance Enhancers
When we talk of sustainability in the case of cement manufacturing, the focus is always on the clinker factor. As the clinker factor goes on reducing, more and more replacement materials play a dominant role in cement hydration. In order to maintain the properties of cement to the same level as that ordinary Portland cement, the use of additives is becoming essential.
Cement additives are a mixture of chemicals, either powder or liquid, but generally liquids are being used on a regular basis in cement grinding for improving cement mill productivity. These additives act as a grinding aid, and the other components in combination alter the properties of cement, targeting higher early/later age compressive strengths of cement mortars, and in some cases also improve the concrete performance of cements (PPC/PSC).
There are a number of additive suppliers, like Fosroc, Sika, BASF (MBT), Grace and Chryso, etc., having different chemical compounds designed for desired effects of cement mill and cement properties, as indicated above. Most of these cement additives are aqueous solutions with different percentage of water and dissolved solids (water making up 40 to 70 per cent). Very rarely are cement additive formulations in powder form. These are added at the entry point of ball mill or VRM, depending on the location.
The ingredients of cement additives commonly are organic and inorganic compounds like amines, poly carboxylate ether, polynapthalene, lignosulfonate, acetic acid, sodium salts, etc. The cement additives should be non-chloride based. As the presence of chlorides will lead to the initiation of corrosion of re-bars in concrete, the content of chloride is restricted in all the codeal provisions.
Once a plant decides on using additives, it will have to invest in providing feeding arrangements and dispenser pumps. A plant operator will have to carry out a detailed study on the benefits of the use of additives, especially in terms of quality improvement, reduced clinker factor, and enhanced throughput of mill, etc.
Main Objectives of using additive:
1. To increase fly ash/slag absorption
2. To improve cement mill throughput
3. To improve strength profile of PPC/ PSC
4. To improve concrete properties
5. To have Improved flowability of cement and concrete
6. Use of CPP (captive power plant) fly ash
7. Use of phosphor-gypsum in the range of 30 to 50 per cent
The use of cement additives affects both the physical nature of a cement, i.e., its particle size distribution and also the chemical reactions that occur during the hydration reaction. These effects are often overlapping, and understanding these interactions can lead to the formulation of more effective customised cement additives.
Additive Development for Portland Slag Cement
In the case of Portland Slag Cement (PSC), the story has been slightly different. The blast furnace slag is very difficult to grind as compared to clinker, apart from this, the blast furnace slags have a latent hydraulic nature, necessitating finer grinding of the slag. Generally the slag is ground finer than fly ash. Thus particle size distribution of blast furnace slag is a governing factor for strength development in PSC. Normally the received slag at plant has very high moisture, necessitating a slag dryer in case of ball mill inter-grinding mode and hot air for separately grinding the slag in VRM. The temperature of hot air is the range of 350 to 550 deg C. However, the additives used for improving properties of PSC are mostly organic in nature, and are steam volatile when used either in direct grinding mode or in blending mode, resulting in a negating effect of the grinding aids and performance enhancer. There is significant improvement in properties of PSC, resulting in lowering of clinker factor. Considering all the above aspects, studies for development of additive combinations for use in inter-grinding mode, as well as separate grinding mode, are in progress.
The use of additives involves not only capital cost but also recurring cost. You are required to procure and stock the materials required. The dependency of plant on one additional material makes it a little complex. You also need to have quality control in place to check the effect of additive and also to check the quality of additive being used. It has been observed that the cost per tonne of cement works out to be a maximum of Rs 30. Considering the benefits we get by using additives, the cost works out to be very negligible.