The manufacturing process for Portland cement causes high levels of greenhouse gas emissions. However, environmental impacts can be reduced by using more energy-efficient kilns and replacing fossil energy with alternative fuels. Although carbon capture and new cements with less CO2 emission are still in the experimental phase, all these innovations can help develop a cleaner cement industry.
Hydraulic cement, chiefly Portland cement or similar cement having Portland cement as a base, is the binding agent in concrete and most mortars, and is thus a key component of construction activity worldwide (van Oss and Padovani, 2002). Hydraulic cements derive their strength through the hydration (chemical combination with water) of their component cement compounds or minerals. World output of cement in 2009 of about 3 Gt was sufficient for about 24 Gt of concrete, or about 3.5 metric tons (t) of concrete annually per person on the planet. Worldwide, concrete is thus the most abundantly manufactured material. Most of the environmental issues surrounding cement production concern the manufacture of clinker, the dominant issue of global concern being emissions of carbon dioxide (CO2), an important greenhouse gas (GHG).
Manufacturing Portland cement involves converting limestone and a variety of other raw materials into clinker, and then grinding this with about 5% of calcium sulphate and other additives into a fine powder. The composition of clinker does not vary much worldwide, and most production involves rotary kilns that share a similar technology. Thus, the manufacturing process for cement, and the associated environmental issues, are common worldwide.
Calcination and heating requirements, highly CO2-emissive
Clinker is composed mainly of four oxides: about 65% calcium oxide or lime (CaO), 22% silicon dioxide (SiO2), 6% aluminum oxide (Al2O3), and 3% ferric (iron) oxide (Fe2O3). The remaining 4% is made up of minor amounts of oxides of magnesium (MgO), usually less than 2%, and various alkalis. In ‘straight’ Portland cement, the major oxides are combined within four hydraulically reactive cement minerals in the clinker component – tricalcium silicate or ‘alite’ (C3S, typically 50-55%), dicalcium silicate or ‘belite’ (C2S, 19-24%), tricalcium aluminate (C3A, 6-10%), and tetracalcium aluminoferrite (C4AF, about 7-11%), to which about 5% gypsum is added.
Because of the predominance of calcium oxide (C), raw materials must include an abundant and inexpensive supply of it for clinker manufacture to be practicable. Traditionally, calcium oxide has been supplied by limestone or similar rocks. Limestone is mainly composed of calcite, which is calcium carbonate (CaCO3). This reliance on limestone is the root of most of the environmental problems associated with cement manufacture.
Calcium carbonate in the raw material mix is thermally decomposed in the kiln to make its calcium oxide available, via a reaction called calcination. Because calcium carbonate is composed of 56% CaO and 44% CO2, calcination releases a great deal of this GHG. If calcium carbonate is the only source of CaO available to the kiln, the plant will need to calcine 1.16 metric tons (t) of CaCO3 to yield 1 t of clinker of 65% CaO content, and this calcination will release 0.51 t of CO2.
The energy required for calcination is enormous. Because most limestones are not pure calcite, the mass ratio of limestone to clinker will actually be more like 1.5 instead of 1.16, and along with some other materials such as clay and silica sand, it takes about 1.7 t of total raw materials to make 1 t of clinker. Calcination of the raw material takes place at 750-1,000°C, with the heat being supplied by the combustion of fossil fuels, chiefly coal and petroleum coke, which releases more CO2. Once calcination is complete, the subsequent formation of C3S, C2S, C3A, and C4AF requires only a little extra heat, even though the reactions occur at higher temperatures (1,000-1,450°C). This is in part because some of their formational reactions, especially that to form C3S (via the reaction C2S + C à C3S), actually release heat (are exothermic).
Overall, to make 1 t of clinker, about 3.9 billion joules (GJ) of heat are required. This is for a dry kiln, which takes its raw materials in a dry state. Some older plants, however, operate wet kilns, which take their raw material mix in the form of a slurry containing about 35-40% water. For wet kilns, evaporation of this water ahead of the preheating step will require an additional 1.6-1.8 GJ/t of clinker.
Kiln technology determines fuel needs
These heat requirements are notional; in reality, it is usually higher because of varying amounts of heat loss from equipment (particularly the enormous kiln tubes). But there are opportunities for saving heat, particularly relating to the combustion air/exhaust and the air used to cool the clinker. On the latter, clinker emerges from the kiln red hot and must be cooled in a dedicated apparatus to 100-200°C before it can be ground into cement. This superheated air can be rerouted to the kiln burner or be used for preheating raw material, saving fuel.
Most kilns built in recent years are modern preheater-precalciner kilns. The majority of new cement plants in recent years have been built in developing countries, leading to developing countries typically having more modern cement plants than many developed countries. However, because of ongoing plant upgrades, installed kiln technology varies worldwide.
Abandoning wet kilns in favour of dry kilns, and upgrading or replacing older dry kilns with more modern dry technology, improve fuel efficiency. Data from the U.S. illustrates this: in 2007 wet kilns required an average of 6.5 GJ/t of clinker; long, dry kilns averaged 5.3 GJ/t; preheater kilns averaged 4.1 GJ/t, and preheater-precalciner kilns averaged 3.6 GJ/t. Preheater kilns use a separate apparatus for preheating rather than the less efficient kiln tube, and the heating is from hot ‘waste’ air. Preheater-precalciner kilns also use an independently fueled calciner apparatus that is far more efficient than a kiln tube for the calcination task; the kiln tube only has to perform the final stage of clinker mineral formation. Efficiencies of scale were also evident: larger plants tended to be more fuel efficient.
Additionally, most plants offer opportunities to achieve smaller improvements in efficiency through upgrades or ‘tweakings’ of existing systems, especially for electricity savings. The results can be cumulatively significant.
Reducing fuel consumption or using alternative fuels
CO2 emissions from fuel combustion are typically around 0.40-0.45 t CO2 per 1 t of clinker, and with calcination emissions added, the total becomes about 0.91-0.96 t of CO2, unless lower-carbon fuels and/or non-carbonate sources of calcium oxide are used. Some hydraulic cements have a lower clinker content than Portland cement; still, it is estimated that at current manufacturing rates, the world cement industry releases about 2.2-2.6 Gt of CO2 annually.
To save fuel , the cement industry has long been on a trend of lowering its per-unit (per ton of product) energy consumption, by installing modern technology. Ongoing modernisation is part of the industry’s strategy to further reduce CO2 emissions (U.S. Environmental Protection Agency, 2010).
Cement plants can also burn a wide variety of alternative fuels (AF), including a variety of industrial wastes, some hazardous. Many of these have a lower carbon content than conventional fuels. In carbon-emissions-reporting protocols, deductions may be allowed for AF use. Likewise, deductions may be allowed for biofuels (including the natural rubber content of used tyres). Biofuels are generally considered to be carbon-neutral in climate change modelling. Constraints on the use of AFs include environmental permission (especially for hazardous waste fuels); availability in sufficient quantity; cost of procurement, storage, and blending; and quality, as they are commonly more variable in their heat and moisture contents than conventional fuels.
Three other current practices are also part of the emission reduction strategy; two of these reduce overall plant emissions, and all three reduce emissions on a per-ton of cement basis.
Towards a greener cement mix
Cement plants can make use of a wide variety of alternative raw materials (ARM), in addition to traditionally used materials, such as limestone. Among the ARMs in common use are the industrial ‘wastes’, such as coal ashes from power plants, iron and steel slags, and industrial residues. Of particular interest are the slags and coal ashes, many of which have a similar composition to clinker. Most importantly, certain ARMs (but especially ferrous slags) can be significant non-carbonate sources of CaO, reducing limestone consumption and attendant calcination emissions of CO2 during clinker production. These ARMs require less heat for combustion, reducing fuel consumption and fuel emissions of CO2.
Limits on ARM use revolve around availability and cost (especially for transport), environmental permission for their use, and their oxide balances. Within these limits, by consuming ARMs, the U.S. industry in recent years has reduced its calcination emissions of CO2 by about 0.7-1.3 million tons per year (or about 2.4-3%); reductions at the ARM-using plants themselves have been in the range of 2-10% or so. It is harder to gauge the reduction of fuel-related emissions, but ARM-consuming plants typically have energy consumption levels 3-30% lower than U.S. industry averages for the kiln technologies concerned.
The clinker content of finished cement can be reduced by incorporating supplementary cementitious materials (SCM), such as fly ash, ground granulated blast furnace slag, silica fume, metakaolin, and pozzolanic volcanic ash, to make blended cements. These have many of the same uses in concrete manufacture as Portland cement. The use of SCM reduces the carbon ‘footprint’ attributable to the cement industry, but most SCMs derive from industries that also emit CO2. SCMs develop their cementitious properties by reacting with the CaO released during the hydration of Portland cement. Concrete producers can also directly introduce SCMs into the concrete mix to reduce the Portland cement (hence clinker) content. In either case, the use of SCMs commonly improves the quality of the concrete. Typical SCM contents in blended cements worldwide, and substitution ratios for Portland cement in concrete, are in the range of 5-50%, but can be higher for some applications. Limitations on the use of SCMs mainly revolve around availability and whether local building codes allow their use.
The clinker proportion of cement can be further reduced, where allowed, by incorporating relatively inert bulking agents or extenders, the most common of which is (uncalcined) ground limestone. Incorporation can be as high as 20% or more in some Portland-limestone cements, but is typically less than 10%. Between inert extenders and SCMs, the world average clinker content of hydraulic cement is currently around 75-80%, compared with about 95% for traditional ‘straight’ Portland cements. Importantly, although using SCM or other extenders in cement and concrete does not reduce the cement industry’s emissions of CO2, overall, it reduces unit emissions and thus allows more cement (and concrete) to be made from the same amount of clinker.
Carbon sequestration at experimental stage
Because they are large stationary emitters of CO2, cement plants are considered good candidates for the future incorporation of carbon sequestration technology, especially if the CO2 content of the exhaust stream can be concentrated through using oxygen, rather than air, for combustion. A concentrated CO2 stream reduces the overall volume of gas to be processed, and may reduce the size of the sequestration facility needed as well as the consumption of any absorptive reagents. Proposed sequestration methods include producing a CO2 gas or liquid stream for use elsewhere or for permanent underground injection, absorption by some reagent, which would need to be disposed of, and the formation of a marketable product such as sodium bicarbonate. Overall, carbon sequestration technologies for cement plants are presently perceived as being largely experimental, costly, and, for some proposed systems, requiring a facility of similar size to the cement plant itself. Very few plants have as yet installed carbon sequestration technology and, indeed, it may be unaffordable for many smaller or older plants. Also, with few exceptions, cement plants are situated next to limestone quarries, and these locations may not be conducive to future CO2 transport piping infrastructure.
Cement plants have been cited as potential consumers of the calcium carbonate formed by some new CO2 sequestration technologies or by so-called calcium-looping circuits proposed for thermal power plants. The use of such calcium carbonate by cement plants would, of course, return the CO2 to the atmosphere, but would at least reduce the need for the plant to burn its own limestone.
New cements to be considered in the medium to long term
Although made today in tiny quantities, a number of new cements have been developed in recent years that could be suitable for at least some forms of construction. Among these are geopolymer cements and several MgO-based binders. Advantages claimed for these cements include a lower energy (heat) required for manufacture and hence lower CO2 emissions, and for MgO binders, that they actually absorb CO2 from the air and may thus be CO2-neutral or even net-negative. MgO binders develop strength through ‘carbonation’.
Apart from issues in getting any new cement accepted into local and national building codes, there are constraints on the widespread use of binders (CaO or MgO) that work via carbonation. Carbonation (hence strength-development) requires sustained exposure to the atmosphere, and although suitable for some high surface area applications (such as stuccos, thin slabs, and small blocks), this may not occur sufficiently rapidly in bulk concrete applications where CO2 permeability could be problematic, and raw materials of sufficient purity for MgO binder manufacture may be more limited in this regard than for Portland cement.
Even where shown to have suitable strength, durability, and applicability, to significantly reduce CO2 emissions, billions of tons of these new cements will have to be manufactured annually. The new cements will have to compete against an established output from thousands of Portland cement plants worldwide, representing billions of dollars in investment. And the few new cements are several hundred dollars per ton more expensive than Portland cement. Yet the cost of Portland cement has been increasing over the years, largely because of fuel cost increases, and is likely to continue to increase over the long term. If some of the new cements could be manufactured in large quantities, economies of scale would be realised in their production costs, and within the next 30-50 years, some may become cost-competitive with Portland cement. During that time, many existing Portland cement plants may have exhausted their local limestone reserves, or their equipment may be in need of replacement, and the original cost of the plants will have been fully amortised. At that time, the world may enter a post-Portland cement age.
References / U.S. Environmental Protection Agency, 2010. Available and emerging technologies for reducing GHG emissions from the Portland cement industry, rapport, October. / U.S. Environmental Protection Agency, 2011. Inventory of U.S. greenhouse gas emissions and sinks-1990-2009, report April 15. / van Oss, H.G., 2011. Cement: chapter in the U.S. Geological Survey Minerals Yearbook / van Oss, H.G. et Padovani A.C., 2002, Cement and the environment-Part 1, Chemistry and technology, Journal of Industrial Ecology, volume 6, n°1, January, 89-105. / van Oss, H.G., et Padovani A.C., 2003, Cement and the environment-Part 2, Environmental challenges and opportunities, Journal of Industrial Ecology, volume 7, n°1, January, 93-126.