The manufacturing process for green cement succeeds in reducing and even eliminating, the production and release of damaging pollutants and greenhouse gasses, particularly CO2 writes Vinay Pathak.
Cement manufacture causes environmental impacts at all stages of the process. These include emissions of airborne pollution in the form of dust, gases, noise and vibration when operating machinery and during blasting in quarries, and damage to countryside from quarrying. Equipment to reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment to trap and separate exhaust gases are coming into increased use. Environmental protection also includes the re-integration of quarries into the countryside after they have been closed down by returning them to nature or re-cultivating them.
CO2 emissions Carbon concentration in cement spans from ÿ5 per cent in cement structures to ÿ8 per cent in the case of roads in cement. Cement manufacturing releases CO2 in the atmosphere both directly when calcium carbonate is heated, producing lime and carbon dioxide, and also indirectly through the use of energy if its production involves the emission of CO2. The cement industry is responsible for about 5 per cent of global man-made CO2 emissions, of which 50 per cent is from the chemical process, and 40 per cent from burning fuel. The amount of CO2 emitted by the cement industry is nearly 900 kgs of CO2 for every 1000 kgs of cement produced. In the European Union, the specific energy consumption for the production of cement clinker has been reduced by approximately 30 per cent since the 1970s. This reduction in primary energy requirements is equivalent to approximately 11 million tonnes of coal per year with corresponding benefits in reduction of CO2 emissions. The high proportion of carbon dioxide produced in the chemical reaction leads to large decrease in mass in the conversion from limestone to cement. So, to reduce the transport of heavier raw materials and to mimimise the associated costs, it is more economical for cement plants to be closer to the limestone quarries rather than to the consumer centers. In certain applications, lime mortar reabsorbs the same amount of CO2 as was released in its manufacture, and has a lower energy requirement in production than mainstream cement. Newly developed cement types from Novacem and Eco-cement can absorb carbon dioxide from ambient air during hardening. Use of the Kalina cycle during production can also increase energy efficiency.
Heavy metal emissions in the air
In some circumstances, mainly depending on the origin and the composition of the raw materials used, the high-temperature calcination process of limestone and clay minerals can release in the atmosphere gases and dust rich in volatile heavy metals such as thallium, cadmium and mercury, which are very toxic. Heavy metals (Tl, Cd, Hg,) are often found as trace elements in common metal sulfides (pyrite (FeS2), zinc blende (ZnS), galena (PbS) present as secondary minerals in most of the raw materials. Environmental regulations exist in many countries to limit these emissions. In the United States, cement kilns are ´legally allowed to pump more toxins into the air than are hazardous-waste incinerators.´
Heavy metals present in the clinker
The presence of heavy metals in the clinker arises both from the natural raw materials and from the use of recycled by-products or alternative fuels. The high pH prevailing in the cement pore water (12.5 < pH < 13.5) limits the mobility of many heavy metals by decreasing their solubility and increasing their absorption onto the cement mineral phases. Nickel, zinc and lead are commonly found in cement in non-negligible concentrations.
Use of alternative fuels and by-products materials
A cement plant consumes 3 to 6 GJ of fuel per tonne of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil. Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln (referred to as co-processing), replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications. Selected waste and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in the kiln, replacing raw materials such as clay, shale, and limestone.
Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value, and burns to give ash containing minerals useful in the clinker matrix.
Normal operation of cement kilns provides combustion conditions which are more than adequate for the destruction of even the most difficult to destroy organic substances. This is primarily due to the very high temperatures of the kiln gases (2000 ¦C in the combustion gas from the main burners and 1100 ¦C in the gas from the burners in the pre-calciner). The gas residence time at high temperature in the rotary kiln is of the order of 5-10 seconds and in the pre-calciner more than 3 seconds.
Due to bovine spongiform encephalopathy (BSE) crisis in the European beef industry, the use of animal-derived products to feed cattle is now severely restricted. Large quantities of waste animal meat and bone meal (MBM), also known as animal flour, have to be safely disposed of or transformed. The production of cement kilns, together with the incineration, is to date one of the two main ways to treat this solid effluent of the food industry.
Green cement
Green cement is a cementitious material that meets or exceeds the functional performance capabilities of ordinary Portland cement by incorporating and optimising recycled materials, thereby reducing consumption of natural raw materials, water, and energy, resulting in a more sustainable construction material. The manufacturing process for green cement succeeds in reducing, and even eliminating, the production and release of damaging pollutants and greenhouse gasses, particularly CO2. Growing environmental and increasing cost of fuels of fossil origin has resulted in many countries in sharp reduction of the resources needed to produce cement and effluents like dust and exhaust gases.
Use of the Kalina cycle during production can also increase energy efficiency.
Vinay Pathak,
Lab Head,
3M R&D Centre, Gurgaon.
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