Portland cement and its manufacturing

Portland cement and its manufacturing

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  Portland Cement Portland cement,bindingmaterial in the form of a finely ground powder, usually gray, that is manufactured by burning and grinding a mixture of limestone and clay or limestone and shale. History Portland cement, arguably one of mankind’s most important manufactured materials, was invented and patented by Joseph Aspdin from Leeds in 1824. Aspdin produced cement by heating powdered limestone mixed with clay in a furnace,and grinding the resulting clinker to a powder. He called the product “Portland Cement” because of its resemblance, when set, to Portland stone, a type of stone quarried on the Isle of Portland. Aspdin’s cement was improved in 1843 by his son William, by vigorous heating and using better grinding equipment to handle the hard clinker. To this day, Portland cement is still the most commonly used cement around the globe Chemical composition Portland cement is made up of four main compounds: tricalcium silicate (3CaO · SiO2), dicalcium silicate (2CaO · SiO2), tricalcium aluminate (3CaO · Al2O3), and a tetra-calcium aluminoferrite (4CaO · Al2O3Fe2O3). In an abbreviated notation differing from the normal atomic symbols, these compounds are designated as C3S, C2S, C3A, and C4AF, where C stands for calcium oxide (lime), S for silica , A for alumina , and F for iron oxide. Small amounts of uncombined lime and magnesia also are present, along with alkalies and minor amounts of other elements Hydration The most important hydraulic constituents are the calcium silicates, C2S and C3S. Upon mixing with water the calcium silicates react with water molecules to form calcium silicate hydrate (3CaO · 2SiO2 · 3H2O) and calcium hydroxide (Ca[OH]2). These compounds are given the shorthand notations C–S–H (represented by the average formula C3S2H3) and CH, and the hydration reaction can be crudely represented by the following reactions:2C3S + 6H = C3S2H3 + 3CH2C2S + 4H = C3S2H3 + CH During the initial stage of hydration, the parent compounds dissolve,and the dissolution of their chemicalbonds generates a significant amount of heat. Then, for reasons that are not fully understood, hydration comes to a stop. This quiescent, or dormant, period is extremely important in the placement of concrete Without a dormant period there would be no cement trucks; pouring would have to be done immediately upon mixing. Following the dormant period (which can last severalhours), the cement begins to harden, as CH and C–S–H are produced. This is the cementitious material that binds cement and concrete together. As hydration proceeds, water and cement are continuously consumed. Fortunately, the C–S–Hand CH products occupy almost the same volume as the original cement and water; volume is approximately conserved,and shrinkage is manageable.
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  Although the formulasabove treat C–S–H as a specific stoichiometry, with the formula C3S2H3, it does not at allform an ordered structure of uniform composition. C–S–H is actually an amorphous gel with a highly variable stoichiometry. The ratio of C to S, for example, can range from 1:1 to 2:1, depending on mix design and curing conditions. Manufacture of Portland Cement Cement is usually manufactured by two processes:  Wet Process  Dry Process The two processes are fundamentally similar, except for the fact that in the wet process the raw materials are ground with water before they are fed into the kiln. All though there was little difference in efficiency between the two processes,the wet process had the disadvantages of CO2 emission and more fuel consumption to evaporate the water in the slurry. This made most cement manufactures prefer the dry process to wet process. Dry process 1.Mixing of raw material The first step in the manufacture of portland cement is to combine a variety of raw ingredients so that the resulting cement will have the desired chemical composition. These ingredients are ground into small particles to make them more reactive,blended together, and then the resulting raw mix is fed into a cement kiln which heats them to extremely high temperatures. Since the final composition and properties of portland cement are specified within rather strict bounds, it might be supposed that the requirements for the raw mix would be similarly strict. As it turns out, this is not the case. While it is important to have the correct proportions of calcium, silicon, aluminum, and iron, the overall chemical composition and structure of the individual raw ingredients can vary considerably. The reason for this is that at the very high temperatures in the kiln, many chemical components in the raw ingredients are burned off and replaced with oxygen from the air. Table 3.3 lists just some of the many possible raw ingredients that can be used to provide each of the main cement elements. Calcium Silicon Aluminium Iron Limestone Clay Clay Clay Marl Marl Shale Iron ore Calcite Sand Fly ash Mill scale Aragonite Shale Aluminum ore refuse Shale Shale Fly ash Blast furnace dust Sea Shells Rice hull ash
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  Cement kiln dustSlag The ingredients listed above include both naturally occurring materials such as limestone and clay, and industrial byproduct materials such as slag and fly ash. From Table 3.3 it may seem as if just about any material that contains one of the main cement elements can be tossed into the kiln, but this is not quite true. Materials that contain more than minor (or in some cases trace) amounts of metallic elements such as magnesium, sodium, potassium, strontium, and various heavy metals cannot be used, as these will not burn off in the kiln and will negatively affect the cement. Another consideration is the reactivity, which is a function of both the chemical structure and the fineness. Clays are ideal because they are made of fine particles already and thus need little processing prior to use, and are the most common source of silica and alumina. Calcium is most often obtained from quarried rock, particularly limestone (calcium carbonate) which must be crushed and ground before entering the kiln. The most readily abundant source of silica is quartz, but pure quartz is very unreactive even at the maximum kiln temperature and cannot be used Grinding and blending prior to entering the kiln can be performed with the raw ingredients in the form of a slurry (the wet process) or in dry form (the dry process). The addition of water facilitates grinding. However,the water must then be removed by evaporation as the first step in the burning process,which requires additional energy. The wet process,which was once standard, has now been rendered obsolete by the development of efficient dry grinding equipment, and all modern cement plants use the dry process. When it is ready to enter the kiln, the dry raw mix has 85% of the particles less than 90 £gm in size. 2.Burning The next step in the process is to heat the blended mixture of raw ingredients (the raw mix) to convert it into a granular material called cement clinker. This requires maximum temperatures that are high enough to partially melt the raw mix. Because the raw ingredients are not completely melted, the mix must be agitated to ensure that the clinker forms with a uniform composition. This is accomplished by using a long cylindrical kiln that slopes downward and rotates slowly This description refers to a standard dry-process kiln as illustrated in Figure 3-2. Such a kiln is typically about 180 m long and 6 m in diameter, has a downward slope of 3-4%, and rotates at 1-2 revolutions per minute. The raw mix enters at the upper end of the kiln and slowly works its way downward to the hottest area at the bottom over a period of 60-90 minutes, undergoing severaldifferent reactions as the temperature increases. It is important that the mix move slowly enough to allow each reaction to be completed at the appropriate temperature. Because the initial reactions are endothermic (energy absorbing), it is difficult to heat the mix up to a higher temperature until a given reaction is complete. Dehydration zone (up to ~ 450˚C): This is simply the evaporation and removal of the free water. Even in the “dry process” there is some adsorbed moisture in the raw mix. Although the temperatures required to do this are not high, this requires significant time and energy. In the wet process,the dehydration zone would require up to half the length of the kiln, while the dry process requires a somewhat shorter distance. Calcination zone (450˚C – 900˚C): The term calcination refers to the process of decomposing a solid material so that one of its constituents is driven off as a gas. At about 600˚C the bound water is driven out of the clays, and by 900˚C the calcium carbonate is decomposed, releasing carbon dioxide. By the end of the calcination zone, the mix consists
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  of oxides ofthe four main elements which are ready to undergo further reaction into cement minerals. Because calcination does not involve melting, the mix is still a free-flowing powder at this point. Solid-state reaction zone (900˚ - 1300˚C): This zone slightly overlaps, and is sometimes included with, the calcination zone. As the temperature continues to increase above ~ 900˚C there is still no melting, but solid-state reactions begin to occur. CaO and reactive silica combine to form small crystals of C2S (dicalcium silicate), one of the four main cement minerals. In addition, intermediate calcium aluminates and calcium ferrite compounds form. These play an important role in the clinkering process as fluxing agents, in that they melt at a relatively low temperature of ~ 1300˚C, allowing a significant increase in the rate of reaction. Without these fluxing agents,the formation of the calcium silicate cement minerals would be slow and difficult. In fact,the formation of fluxing agents is the primary reason that portland (calcium silicate) cements contain aluminum and iron at all. The final aluminum- and iron-containing cement minerals (C3A and C4AF) in a portland cement contribute little to the final properties. As the mix passes through solid-state reaction zone it becomes “sticky” due to the tendency for adjacent particles to fuse together. Clinkering zone (1300˚C – 1550˚C): This is the hottest zone where the formation of the most important cement mineral, C3S (alite), occurs. The zone begins as soon as the intermediate calcium aluminate and ferrite phases melt. The presence of the melt phase causes the mix to agglomerate into relatively large nodules about the size of marbles consisting of many small solid particles bound together by a thin layer of liquid (see Figure 3-3). Inside the liquid phase, C3S forms by reaction between C2S crystals and CaO. Crystals of solid C3S grow within the liquid, while crystals of belite formed earlier decrease in number but grow in size. The clinkering process is complete when all of silica is in the C3S and C2S crystals and the amount of free lime (CaO) is reduced to a minimal level (<1%). Cooling zone: As the clinker moves past the bottom of the kiln the temperature drops rapidly and the liquid phase solidifies, forming the other two cement minerals C3A (aluminate) and C4AF (ferrite). In addition, alkalis (primarily K) and sulfate dissolved in the liquid combine to form K2SO4 and Na2SO4. The nodules formed in the clinkering zone are now hard, and the resulting product is called cement clinker. The rate of cooling from the maximum temperature down to about 1100˚C is important, with rapid cooling giving a more reactive cement. This occurs because in this temperature range the C3S can decompose back into C2S and CaO,among other reasons. It is thus typical to blow air or spray water onto the clinker to cool it more rapidly as it exits the kiln. 3.Grinding of clinker Once the nodules of cement clinker have cooled, they are ground back into a fine powder in a large grinding mill. At the same time, a small amount of calcium sulfate such as gypsum (calcium sulfate dihydrate) is blended into the cement. The calcium sulfate is added to control the rate of early reaction of the cement,as will be discussed in Section 5.3. At this point the manufacturing process is complete and the cement is ready to be bagged or transported in bulk away from the plant. However,the cement is normally stored in large silos at the cement plant for a while so that various batches of cement can be blended together to even out small variations in composition that occur over time. Cement manufacturers go to considerable lengths to maintain consistent behavior in their cements over time, with the most important parameters being the time to set,the early strength development, and the workability at a given water content.
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  4.Storageand packaging The grindedcement is stored in silos, from which it is marketed either in container load or 50kg bags. Wet Process The raw materials are firstly crushed and made into powdered form and stored in silos. The clay is then washed in washing mills to remove adhering organic matters found in clay. The powdered limestone and water washed clay are sent to flow in the channels and transfer to grinding mills where they are completely mixed and the paste is formed, i.e., known as slurry. The grinding process can be done in ball or tube mill or even both. Then the slurry is led into collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln. Variations of Portland Cement
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  Portland cement thatis used for most construction purposes. Other modified varieties of Portland cement are also manufactured in order to meet various specific construction purposes/requirements. Sulphate-Resisting Portland Cement: The presence of heavy sulphate content in ground water causes damages such as cracking, scaling and expansion to concretes. The use of sulphate-resisting Portland cement prevents the sulphate from entering the concrete thereby enhancing its longevity. White Portland Cement: This type of cement is intended for use in architectural purposes where white coloured concrete is desired. This differs from the grey cement primarily in colour. The white colour is attained by controlling the use of ferrite, that which gives cement its characteristic Grey colour. Rapid-Hardening Portland Cement:This is similar to ordinary Portland cement,except that its particles are more finely ground to facilitate quick reaction with water. It is used when fast strength growth of forms or structures is required. LowHeat ofHydration Portland Cement: This type of cement develops strength at a slower rate than other cement types. This cement is used in huge concrete structures such as dams. These are produced upon specific requests for large projects. Types of Portland cement The ASTM has designated five types of Portland cement, designated Types I-V. Physically and chemically, these cement types differ primarily in their content of C3A and in their fineness. In terms of performance,they differ primarily in the rate of early hydration and in their ability to resist sulfate attack. The generalcharacteristics of these types are listed in Table Type Classification Characteristic Applications Type I General purpose Fairly high C3S content for good early strength development General construction (most buildings, bridges, pavements, precast units, etc) Type II Moderate sulfate resistance Low C3A content (<8%) Structures exposed to soil or water containing sulfate ions Type III High early strength Ground more finely, may have slightly more C3S Rapid construction, cold weather concreting Type IV Low heat of hydration (slow reacting) Low content of C3S (<50%) and C3A Massive structures such as dams. Now rare Type V High sulfate resistance Very low C3A content (<5%) Structures exposed to high levels of sulfate ions White White color No C4AF, low MgO Decorative (otherwise has properties similar to Type I)
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  The differences betweenthese cement types are rather subtle. All five types contain about 75 wt% calcium silicate minerals, and the properties of mature concretes made with all five are quite similar. Thus these five types are often described by the term “ordinary portland cement”,or OPC. Types II and V OPC are designed to be resistant to sulfate attack. Sulfate attack is an important phenomenon that can cause severe damage to concrete structures. It is a chemical reaction between the hydration products of C3A and sulfate ions that enter the concrete from the outside environment. The products generated by this reaction have a larger volume than the reactants,and this creates stresses which force the concrete to expand and crack. Although hydration products of C4AF are similar to those of C3A,they are less vulnerable to expansion, so the designations for Type II and Type V cement focus on keeping the C3A content low. There is actually little difference between a Type I and Type II cement, and it is common to see cements meeting both designations labeled as “Type I/II”. The phenomenon of sulfate attack will be discussed in much more detail in Sections 5.3 and 12.3, but it should be noted here that the most effective way to prevent sulfate attack is to keep the sulfate ions from entering the concrete in the first place. This can be done by using mix designs that give a low permeability (mainly by keeping the w/c ratio low) and, if practical, by putting physical barriers such as sheets of plastic between the concrete and the soil. Type III cement is designed to develop early strength more quickly than a Type I cement. This is useful for maintaining a rapid pace of construction, since it allows cast-in-place concrete to bear loads sooner and it reduces the time that precast concrete elements must remain in their forms. These advantages are particularly important in cold weather,which significantly reduces the rate of hydration (and thus strength gain) of all portland cements. The downsides of rapid-reacting cements are a shorter period of workability, greater heat of hydration, and a slightly lower ultimate strength .Type IV cement is designed to release heat more slowly than a Type I cement, meaning of course that it also gains strength more slowly. A slower rate of heat release limits the increase in the core temperature of a concrete element. The maximum temperature scales with the size of the structure, and Type III concrete was developed because of the problem of excessive temperature rise in the interior of very large concrete structures such as dams. Type IV cement is rarely used today, because similar properties can be obtained by using a blended cement. White portland cement (WPC) is made with raw ingredients that are low in iron and magnesium, the elements that give cement its grey color. These elements contribute essentially nothing to the properties of cement paste, so white portland cement actually has quite good properties. It tends to be significantly more expensive than OPC,however, so it is typically confined to architectural applications. WPC is sometimes used for basic cements research because the lack of iron improves the resolution of nuclear magnetic resonance (NMR) measurements. Advancement in Portland Cement While cement production has traditionally been focused on OPC,composite and blast furnace slag cements have been developed and are now a central part of the cement-type portfolio of producers. At the same time Portland limestone and Portland pozzolanic cements have gained importance, especially in regions where slag or fly-ash are not available. In the global context of cost reduction and CO2 constraints, cement producers strive to lower the clinker content in their cements. Limits are given by cement performance, so that product quality of the final
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  concrete may notbe impaired. shows the different cements types and their calcium oxide, silicon dioxide and aluminium/iron oxide content. The reduction of clinker levels in cement predominantly takes into account well-tried and tested main constituents. While the global availability of latent hydraulic and pozzolanic materials of industrial origin is certainly limited, a special focus is on cements with high limestone content.This is basically an extension of the current cement standards as they have been developed worldwide and certainly provide opportunities for the future. As an example, research is performed in the context of the European standard with the main focus on strength development and durability of the concrete produced.shows the range of current cement types standardised in Europe today and the extension that is currently under research. In any case, the production of cements with extended use of well tried and tested constituents certainly requires excellent quality assurance mechanisms as they have been successfully implemented in the cement industry. In addition, the inherent characteristics of cement production guarantee large volume flows and good homogenisation resulting in constant product quality. 1.Dry kilns With multistage pre-heater and Pre-calcination Multistage preheaters and pre-calciners make use of the waste heat from the kiln and clinker cooler to pre-heat and pre-process the kiln feed, and thereby allow for considerable energy savings. Whenever economically feasible a wet process kiln can be converted to a state-of-the art dry process production facility that includes either a multi-stage preheater,or a multi-stage pre-heater and a pre-calciner. Such transformations are usually feasible for new plants and major upgrades. Kiln systems with five cyclone preheater stages and precalciner are considered standard technology for ordinary new plants. 2.Replacing Vertical shaft kilns For vertical shaft kilns (VSK), which are commonly used in China, switching over to a new and more efficient suspension preheater/precalciner kiln system offers the main energy efficiency opportunity. Such replacements may only be feasible when coupled with capacity expansions. 3. Kiln shell Heat Loss Reduction The use of better insulating refractories can reduce heat losses. Refractory choice is the function of insulating qualities of the brick and the ability to develop and maintain a coating. The coating helps to reduce heat losses and to protect the burning zone refractory bricks. The use of improved kiln-refractories may also lead to improved reliability of the kiln and reduced downtime, reducing production costs considerably, and reducing energy needs during start-ups. Structural considerations may limit the use of new insulation materials. 4.Proper sealing and Seal Replacement Seals are used at the kiln in- and out-let in order to reduce unintended air entry to, or escape from, the kiln. Leakage at the kiln outlet can both lower the combustion efficiency of the burner and increase heat losses and are thefore of particular importanceAir leakage can be controlled by employing an appropriate seal - common types include pneumatic, lamella-, and spring-type seals. Although seals can last up to 10,000 to 20,000 hours, regular inspection may be needed to reduce leaks..
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  5.Combustion systemimrovement Improved combustionsystems aim to optimize the shape of the flame, the mixing of combustion air and fuel and reducing the use of excess air. Different techniques have been developed to this effect. For rotary kilns, one of such technique, the Gyro-Therm technology improves gas flame quality while reducing NOx emissions. The technology can be applied to gas or gas/coal dual fuel burners. The Gyro-Therm burner uses a patented "precessing jet" technology. The nozzle design produces a gas jet leaving the burner in a gyroscopic-like processing motion. This stirring action produces rapid large scale mixing in which pockets of air are engulfed within the fuel envelope without using high velocity gas or air jets. Besides reducing energy consumption, the technology is reported to help reduce NOx emissions by 30 to 70%, and increase productivity by more than 5%. 5.Cement with Pozzolana Pozzolana are defined as substances of siliceous or silico-aluminous composition. Finely ground pozzolanic materials react in the presence of water at normal ambient temperature with dissolved calcium hydroxide (Ca(OH)2) to form strength-developing calcium silicate and calcium aluminate compounds. In general, the use of a pozzolana as a main constituent of cement is possible if the content of reactive silicon dioxide amounts not less than 25% by mass. Often a differentiation is made between natural pozzolana and natural calcined
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  pozzolana. The productionof cements with pozzolana as a main constituent involves the pre-treatment of the pozzolana like crushing, drying and grinding and the intergrinding or mixing of the cement clinker with the pozzolanic material. The use of pozzolana as a main constituent can lead to better workability of the concrete due to a better grain size distribution as well as to higher long term strength and improved chemical resistance. 6.High Efficiency Separator Separators and classifiers send larger particles so that they can be sent back to mill for further grinding. Separators or classifiers with higher efficiency separate larger particles more accurately and thereby reduce over-grinding and decrease mill energy consumption. By replacing the external separator to ball mills with higher efficiency external separators,energy consumption in the mill can be reduced and mill capacity can be increased. 7.Vertical Roller Mills For Finishing Grinding Ball mills, that are commonly used for finish grinding, have high energy demands, consuming up to 30-42 kWh/t clinker depending on the fineness of the cement. Complete replacement of ball mills by vertical roller mills (VRMs) with an integral separator – as opposed to the use of VRMs as pre-grinding to ball mills – is regarded as a breakthrough. Use of VRM in finish grinding combines grinding and high efficiency classification and improves both energy efficiency and productivity.According to MIIT of China, instability of material bed, vibration in the mill mill, serious wear of grinding roller and grinding disc, and product quality issues can be encountered when VRMs are used for finish grinding, and further improvements in these areas are necessary. This technology is considered to be suitable for new installations as well as for those undertaking major upgrades. The penetration rate of this technology in Chinese market is reported to be 5% in 2012. This figure is expected to reach 30% during the twelfth five-year development period. Plants interested in this technology are advised to carefully consider logistical aspects of maintenance and parts replacement by technology providers. 8.Emerging Grinding Technologies There are emerging grinding technologies such as ultrasonic comminution and plasma comminution. Ultrasonic comminution transfers the energy needed for crushing to the material by acoustic pulses. This approach was introduced in 2003. Two counter-rotating disks with special aerodynamic surfaces generate ultrasonic pulses, which due to their small pulse duration exert pressure waves on the particles which are pulverized very efficiently. The results from first tests with granulated blast furnace slag of different origins were promising, but future research remains necessary. The scaling up to industrial dimensions in particular is an open question. The system has been tested for slag grinding in model scale only. Plasma comminution is performed in a liquid by using shock waves. The application is tested on semiconductor materials. 9.High Efficiency Fans For Preheaters Older generation, low-efficiency, high energy-consuming pre-heater fans can be replaced with a high efficiency fan resulting in electricity saving. 10.High Pressure Press Rollers
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  In a high-pressureroller press, two rollers pressurize the material up to 3,500 bar, improving the grinding efficiency dramatically and significantly reducing energy consumption. High-pressure roller presses are most often used to expand the capacity of existing grinding mills, and are found especially in countries with high electricity costs or with poor power supply.This technology is considered to be suitable for both for raw materials grinding and for finish grinding of cement, as well as ultrafine grinding of blast furnace slag. In China,the penetration rate of this technology was expected to reach 80% during the eleventh five-year development period – requiring an investment of 1 billion RMB and holding a potential to reduce energy consumption by 800 GWh (NDRC,2008. p.49). 11.Cement suspension preheater calcining technology with Highsolid-gas Ratio A High solid-gas ratio preheater system provides for two parallel twin series of air (laden with the solid materials) streams,making solid-gas ratio per unit doubled or more for each preheater,so that the heat consumption of clinker production tends to be close to the theoretical thermal consumption. The Preheater system is set as a combined multi-level series of five preheater cyclones in the framework of 2-2-2-2-1, in which gas from each of the five cyclones flows evenly through each stage of parallel dual series preheater,all the powder alternately step-wise fed into preheater cyclones from one series to another series,and solid-gas ratio of the preheater cyclone increased to about 2.0, so that it significantly improves the heat exchange efficiency of the total preheater system and reduces the exhaust gas temperature at the preheater exit. An “External Circulation Calciner” system of high solid-gas ratio adopts a "bypass outside" approach, in which a certain amount of solid materials exit from the calciner and return to the calcining furnace, so that large particles of insufficient calcining rate continue to calcine repeatedly through the high furnace temperature and the more intense heat release area by the second, third or even fourth times, so that it can increase the calcination rate of material exiting from the Calciner, reduce the thermal load of the rotary kiln, greatly improve the kiln production of per unit volume, simultaneously strengthening the thermal stability of precalciner, and finally reducing harmful gas emissions According to NDRC,the main technical indicators of this technology include the following  exhaust gas temperature from Kiln inlet is less than 260 ℃;  heat energy consumption for clinker making is below 2.85 GJ/t-clinker (680Mcal/t-clinker);  power consumption is less than 56kHh/t-clinker;  operating rate of kiln system is greater than 90%;  NOx and SO2 contents in exhaust gas are less than 200ppm and 50ppm, respectively (NDRC,2011. p.36) Necessities for new technologies All cements have to fulfil the requirements on durability, strength development, early strength development, workability, cost and environment. Depending on the cement composition, these criteria can be fulfilled to different degrees. It is in the hands of the producer to optimise the different cement types with respect to these categories. The consumer will choose the
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  appropriate cement typefor the dedicated construction. Especially for new cements to be developed in the future, durability is one of the essential requirements. The question of carbonation resistance,resistance against chloride penetration, only to give a few examples, must be complied with. In temperate climates durability aspects to a high degree are also determined by frost-thaw resistance. Among constituents that might not have developed to their full potential as cement constituents, calcined clays could play an important role. It is known that these materials exhibit pozzolanic properties. However,the calcination process is pretty much determined by the origin and the composition of these clays. Typically, calcination temperatures are in the range of 700-850°C. Availability on a global scale is good, although in some countries clays are not available at all. Future cements In the literature, quite a few reports are given with respect to new types of cements on a research scale. Celitement, for example, is based on calcium silicate hybrid phases. Production is foreseen by hydro-thermal synthesis and by reactive milling of lime in a silicon component. The Ca/Si ratio is lower than OPC clinker, consequently CO2 emissions and energy requirements might be lower. However,it is currently much too early to give any estimation about the future potential of this binder with respect to durability, production cost or even the technical potential for relevant substitution of current cements. Novacem has reported a cement based on magnesium oxide and hydrated magnesium carbonates. According to Novacem,the raw material is based on magnesium silicates which are digested and subsequently carbonated at elevated temperature and pressure. While magnesia-based cements have been known for a long time, it is an open question whether in the end Novacem will provide sufficient durability to substitute relevant amounts of today's cement. Novacem indicates that significant research has to be done, but has made significant progress to date. References . Jonas, What carbon capture and storage (CCS) is expected to? Describing
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