Calcined gypsum is mixed with water and forms a widely used binder. It is usual to grind the calcined substance with additions of lime or dolomite, which act as catalysts for setting. The calcined gypsum can even be used as plaster of Paris. Plaster products are flame retardant since the gypsum calcinates with extreme heat, releasing water.
Cement is often complemented with additives, either initially or during on site mixing (Table 6.10). The first additives were used as early as 1920. During the 1960s and 1970s the amounts grew. In Denmark there are now additives in 60-70% of all concrete (Strunge et al., 1990). The amounts vary, but the additives seldom form more than 10% of the weight of the cement. Amongst the most important additives are:
• Accelerators, which increase the rate of setting, usually sodium silicate (waterglass) 1-3% by weight of the cement. Different amounts of aluminates, carbonates, formiates, thiocyanates can be used as well as triethanolamine in some products.
• Air-entraining agents, used to allow a controlled quantity of small, uniformly distributed air bubbles to be incorporated in the mix, thereby increasing the workability and improve frost resistance. Most used are soaps from natural resins, alkylpolyglycolether, alkylsul-phates and alkylsulphonates, usually mixed in with 0.05-1% of the weight of the cement.
• Retarders, which delay setting during transport in concrete mixer trucks. These contain sucrose, gluconates, phosphates, lignosul-phates and typically comprise 2% by the weight of the cement.
• Water reducing agents can be up to 5-10% by weight of the cement and reduce the surface tension of water. Admixtures are waterglass, sodium and soda. In industrialized products lignosulphonates, melamine sulphonates, naphtalene sulpho-nates and polycarboxylates are much used. In some products nonylphenol may be found. These are usually at the same time plasticizers.
• Water repellents, which make the substance more waterproof. Calcium stearate is much used, in proportions of 1-5%.
Energy consumption in production varies widely according to the type. Portland cement has a relatively high energy consumption, largely due to the high temperatures needed for production (up to 2000 °C in the firing zone). The cement industry is usually very centralized and the use of energy for transport is thus also high.
It would be a significant achievement to reduce energy consumption in both production and transport. Decentralizing of cement production could save a great deal of energy, not only in transport, but also because smaller plants can be as efficient as larger plants. Today rotary kilns are used, but smaller, more efficient, modern shaft kilns could reduce energy consumption by 10-40%. Rotary kilns are very specialized - shaft kilns have a greater variety of possibilities. They can be used for both calcination and sintering of most cement materials.
In the last decades, the pre-calcination technology has been introduced as an energy saving measure. Another energy saving measure is an increased number of pre-heaters in the cement factories. There are also many ways of utilizing the waste heat; for example, as district heating.
Another step in the right direction is developing cements where lower temperatures are required in the production process. Most important here are the geopolymeric cements and cements with high amounts of pozzolanas mixed in. Also increased use of limestone powder ground directly into the clinker has a positive effect on the energy demand (Jahren, 2003).
To produce Portland cement in rotary kilns requires energy sources such as coal, crude oil or natural gas. Emissions and pollution is therefore the same as for other uses of fossil fuels, including high emissions of the greenhouse gas carbon dioxide. The temperature in the firing zones is so high that nitrogen oxides are also emitted. This is not removed from the effluent today, though the technologies exist; for example, catalytic reduction. Shaft kilns can in principle be fired with wood, which is considered as climate neutral.
The raw materials used in cements and limes also emit large amounts of carbon dioxide and sulphur dioxide. The extremely high temperatures used in the production of Portland cement also suggest that heavy metals are emitted. Sulphur dioxide can, in principle, be cleaned by adding lime to the flue gases.
Part of the carbon dioxide emitted will be slowly reabsorbed in a process called carbonatation. This is greatest for the pure lime products (Fossdal, 2006). The process is:
Carbonatation can in theory be up to 90% of the carbon dioxide chemically emitted from the calcination over a 50-year period in the case of an exposed lime plaster - which also leads to an increase in weight of around one kilogram per square metre. For Portland cement based plasters carbonatation can attain up to 70%; a lower figure partly because the material is less porous, and part of the calcium compounds are retained within the silicious constituents. For thicker, typical concrete constructions, carbonatation is unlikely to surpass 10 to 15% in practice. Carbonatation will continue after demolition of cement-based structures, and will be more rapid if these are broken up into small pieces and exposed to the air.
Methods are being investigated to add carbon dioxide during the setting process. The most effective step towards reducing the climate impact of cements however lies in the increased use of pozzolana mixtures in both hydraulic lime and Portland cements. In this way the amount of lime can be reduced. Here the use of aplite cement and calcium sulfoaluminate cement are also interesting options. Geopoly-meric cements for uses requiring somewhat lower strength could also lead to significant reductions.
The problem of dust has previously received much attention in connection with cement production. Today the dust problem is often much reduced as a result of better, closed systems for handling clinker, more efficient dust filters, etc.
Dust can still be a major problem on building sites and wet Portland cement can cause skin allergies. During the construction phase itself, cement products are relatively free of problems, though if setting is not effective chemical reactions can occur between it and neighbouring materials, such as PVC floor coverings.
At the stage of waste, cements are relatively inert as long as they are free of additives. There is not yet much knowledge as to the behaviour or leakage into the environment of typical concrete additives once deposited as waste. Sulphur compounds can however be leached out from gypsum-based products.
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