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g out proper curing requires maintaining the appropriate moisture content necessary for the hydration reactions during the setting and the hardening processes if hydraulic cements dry out during the curing phase the resulting product can be insufficiently hydrated and significantly weakened a minimum temperature of 5 c is recommended and no more than 30 c 51 the concrete at young age must be protected against water evaporation due to direct insolation elevated temperature low relative humidity and wind the interfacial transition zone itz is a region of the cement paste around the aggregate particles in concrete in the zone a gradual transition in the microstructural features occurs 52 this zone can be up to 35 micrometer wide 53 351 other studies have shown that the width can be up to 50 micrometer the average content of unreacted clinker phase decreases and porosity decreases towards the aggregate surface similarly the content of ettringite increases in itz 53 352 safety issues edit bags of cement routinely have health and safety warnings printed on them because not only is cement highly alkaline but the setting process is exothermic as a result wet cement is strongly caustic ph 13 5 and can easily cause severe skin burns if not promptly washed off with water similarly dry cement powder in contact with mucous membranes can cause severe eye or respiratory irritation some trace elements such as chromium from impurities naturally present in the raw materials used to produce cement may cause allergic dermatitis 54 reducing agents such as ferrous sulfate feso 4 are often added to cement to convert the carcinogenic hexavalent chromate cro 4 2 into trivalent chromium cr 3 a less toxic chemical species cement users need also to wear appropriate gloves and protective clothing 55 cement industry in the world edit interactive cement production visualization global cement capacity in 2022 this section needs to be updated please help update this article to reflect recent events or newly available information august 2026 see also list of countries by past cement production and cement industry in the united states in 2025 the world production of cement was 3 800 megatonnes 4 200 10 6 short tons the top three producers were china with 1 700 india with 470 and vietnam with 100 million tonnes for a total of over half the world total 56 for the world capacity to produce cement in 2025 the situation was similar with the top three states china india and the us accounting for over half the world total capacity 56 china representing an increasing share of world cement consumption remains the main engine of global growth by 2012 chinese demand was recorded at 2160 mt representing 58 of world consumption annual growth rates which reached 16 in 2010 appear to have softened slowing to 5 6 over 2011 and 2012 as china s economy targets a more sustainable growth rate outside of china worldwide consumption climbed by 4 4 to 1462 mt in 2010 5 to 1535 mt in 2011 and finally 2 7 to 1576 mt in 2012 iran is now the 3rd largest cement producer in the world and has increased its output by over 10 from 2008 to 2011 57 because of climbing energy costs in pakistan and other major cement producing countries iran has held position as a trading partner utilizing its own surplus petroleum to power clinker plants now a top producer in the middle east iran is further increasing its dominant position in local markets and abroad 58 the performance in north america and europe over the 2010 12 period contrasted strikingly with that of china as the 2008 financial crisis evolved into a sovereign debt crisis for many economies in this region clarification needed and recession cement consumption levels for this region fell by 1 9 in 2010 to 445 mt recovered by 4 9 in 2011 then dipped again by 1 1 in 2012 the performance in the rest of the world which includes many emerging economies in asia africa and latin america and representing some 1020 mt cement demand in 2010 was positive and more than offset the declines in north america and europe annual consumption growth was recorded at 7 4 in 2010 moderating to 5 1 and 4 3 in 2011 and 2012 respectively as at year end 2012 the global cement industry consisted of 5673 cement production facilities including both integrated and grinding of which 3900 were located in china and 1773 in the rest of the world total cement capacity worldwide was recorded at 5245 mt in 2012 with 2950 mt located in china and 2295 mt in the rest of the world 5 china edit main article cement industry in china china is the world s largest producer and user of cement in 2006 it was estimated that china manufactured 1 235 billion tonnes of cement which was 44 of the world total cement production 59 demand for cement in china is expected to advance 5 4 annually and exceed 1 billion tonnes in 2008 driven by slowing but healthy growth in construction expenditures cement consumed in china will amount to 44 of global demand and china will remain the world s largest national consumer of cement by a large margin 60 in 2010 3 3 billion tonnes of cement was consumed globally of this china accounted for 1 8 billion tonnes 61 environmental impacts edit further information environmental impact of concrete 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 co 2 emissions edit global carbon emission by type to 2018 carbon concentration in cement spans from 5 in cement structures to 8 in the case of roads in cement 62 cement manufacturing releases co 2 in the atmosphere both directly when calcium carbonate is heated producing lime and carbon dioxide 63 64 and also indirectly through the use of energy if its production involves the emission of co 2 the cement industry produces about 10 of global human made co 2 emissions of which 60 is from the chemical process and 40 from burning fuel 65 a chatham house study from 2018 estimates that the 4 billion tonnes of cement produced annually account for 8 of worldwide co 2 emissions 4 nearly 900 kg of co 2 are emitted for every 1000 kg of portland cement produced in the european union the specific energy consumption for the production of cement clinker has been reduced by approximately 30 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 co 2 emissions this accounts for approximately 5 of anthropogenic co 2 66 the majority of carbon dioxide emissions in the manufacture of portland cement approximately 60 are produced from the chemical decomposition of limestone to lime an ingredient in portland cement clinker these emissions may be reduced by lowering the clinker content of cement they can also be reduced by alternative fabrication methods such as the intergrinding cement with sand or with slag or other pozzolan type minerals to a very fine powder 67 to reduce the transport of heavier raw materials and to minimize the associated costs it is more economical to build cement plants closer to the limestone quarries rather than to the consumer centers 68 as of 2025 update carbon capture and storage is emerging as a way to decarbonise cement production french company air liquide was granted eu funding for two ccs projects in kujawy poland and the k6 program aimed at producing the first carbon neutral cement in europe in lumbres france the projects are expected to start operation between by 2028 and capture 18 1 mtco2 emissions over a decade 69 co 2 absorption edit hydrated products of portland cement such as concrete and mortars slowly reabsorb atmospheric co 2 gas which has been released during calcination in a kiln this natural process reversed to calcination is called carbonation 70 as it depends on co 2 diffusion into the bulk of concrete its rate depends on many parameters such as environmental conditions and surface area exposed to the atmosphere 71 72 carbonation is particularly significant at the latter stages of the concrete life after demolition and crushing of the debris it is estimated that nearly 30 of atmospheric co 2 generated by cement production is reabsorbed during the life cycle of cement products 72 carbonation process is considered as a mechanism of concrete degradation it reduces ph of concrete that promotes reinforcement steel corrosion 70 however as the product of ca oh 2 carbonation caco 3 occupies a greater volume porosity of concrete reduces this increases strength and hardness of concrete 73 there are proposals to reduce carbon footprint of hydraulic cement by adopting non hydraulic cement lime mortar for certain applications it reabsorbs some of the co 2 during hardening and has a lower energy requirement in production than portland cement 74 a few other attempts to increase absorption of carbon dioxide include cements based on magnesium sorel cement 75 76 77 heavy metal emissions in the air edit 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 e g thallium 78 cadmium and mercury are the most toxic heavy metals tl cd hg and also selenium are often found as trace elements in common metal sulfides pyrite fes 2 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 as of 2011 in the united states cement kilns are legally allowed to pump more toxins into the air than are hazardous waste incinerators 79 heavy metals present in the clinker edit 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 porewater 12 5 ph 13 5 limits the mobility of many heavy metals by decreasing their solubility and increasing their sorption onto the cement mineral phases nickel zinc and lead are commonly found in cement in non negligible concentrations chromium may also directly arise as natural impurity from the raw materials or as secondary contamination from the abrasion of hard chromium steel alloys used in the ball mills when the clinker is ground as chromate cro 4 2 is toxic and may cause severe skin allergies at trace concentration it is sometimes reduced into trivalent cr iii by addition of ferrous sulfate feso 4 use of alternative fuels and by products materials edit 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 80 scrap automobile and truck tires are useful in cement manufacturing as they have high calorific value and the iron embedded in tires is useful as a feed stock 81 p 27 clinker is manufactured by heating raw materials inside the main burner of a kiln to a temperature of 1 450 c the flame reaches temperatures of 1 800 c the material remains at 1 200 c for 12 15 seconds at 1 800 c or sometimes for 5 8 seconds also referred to as residence time these characteristics of a clinker kiln offer numerous benefits and they ensure a complete destruction of organic compounds a total neutralization of acid gases sulphur oxides and hydrogen chloride furthermore heavy metal traces are embedded in the clinker structure and no by products such as ash or residues are produced 82 the eu cement industry already uses more than 40 fuels derived from waste and biomass in supplying the thermal energy to the grey clinker making process although the choice for this so called alternative fuels af is typically cost driven other factors are becoming more important use of alternative fuels provides benefits for both society and the company co 2 emissions are lower than with fossil fuels waste can be co processed in an efficient and sustainable manner and the demand for certain virgin materials can be reduced yet there are large differences in the share of alternative fuels used between the european union eu member states the societal benefits could be improved if more member states increase their alternative fuels share the ecofys study 83 assessed the barriers and opportunities for further uptake of alternative fuels in 14 eu member states the ecofys study found that local factors constrain the market potential to a much larger extent than the technical and economic feasibility of the cement industry itself reduced footprint cement edit growing environmental concerns and the increasing cost of fossil fuels have resulted in many countries in a sharp reduction of the resources needed to produce cement as well as effluents dust and exhaust gases 84 reduced footprint cement is a cementitious material that meets or exceeds the functional performance capabilities of portland cement various techniques are under development one is geopolymer cement which incorporates recycled materials thereby reducing consumption of raw materials water and energy another approach is to reduce or eliminate the production and release of damaging pollutants and greenhouse gasses particularly co 2 85 recycling old cement in electric arc furnaces is another approach 86 a team at the university of edinburgh has developed the dupe process based on the microbial activity of sporosarcina pasteurii a bacterium precipitating calcium carbonate which when mixed with sand and urine can produce mortar blocks with a compressive strength 70 of that of concrete 87 see also edit asphalt concrete calcium aluminate cements cement chemist notation cement render cenocell energeticall...