Which raw materials are usually used to produce portland cement? What are the main ingredients provided by various raw materials?

What material is cement made of? Simply put, it is calcium carbonate (limestone).

A ground material, after adding a proper amount of water, becomes a plastic slurry, which can be hardened not only in air, but also in water, and can firmly bond sand, stone and other materials together to form a solid stone-like hydraulic cementing material. Cement is one of the most widely used building materials and engineering materials among inorganic non-metallic materials, which are widely used in construction, water conservancy, roads, petroleum, chemical industry and

military affairs

In the project. In recent years, due to the demand, the cement output of developed countries has basically reached saturation, and the annual cement output tends to be stable or declining, while the cement output of developing countries such as China has increased rapidly. For example, in 1983, the world cement output was 900Mt, while that of China was100 mt.

Evolution About 2000 years ago, Greece and ancient Rome used a mixture of lime and volcanic ash in construction projects. This mixture slowly reacted in water to form a hard solid, which was the earliest cement used. /kloc-At the beginning of the 9th century, Britain, France and other countries sintered clay lime (or marl) into hydraulic materials. When the sum of alumina and silica content reaches 20% ~ 35%, it is called natural cement. This kind of cement has low firing temperature and does not control its composition. 1824, British J. asp Ding fired a hydraulic cementing material with artificial mixture of limestone and clay. After setting and hardening, its color and appearance are similar to the high-quality portland stone used in British architecture at that time, so it is called Portland cement. He obtained a patent for this and set up a factory in Britain on 1825. But the product of asp. Due to low firing temperature, the quality is not good enough. Portland cement, which is very similar to present cement, was made by Englishman Johnson in 1850. Since then, Portland cement industry has started. Over the past 100 years, the production technology and performance of portland cement have been continuously improved, and a large number of new varieties have been developed, which have been developed to 100 kinds of cement.

China established the first cement plant in Tangshan in 1889, and Qi Xin Lime Co., Ltd. (see Qi Xin Cement Plant) was established in Tangshan in 1906, which opened a precedent for the cement industry in China. The cement output of 1949 is 660kt, and 1984 has reached 120Mt, and the variety of cement has also developed from a single portland cement to more than 60 varieties.

Cement can be classified in many ways:

1. According to the properties of raw materials, it can be divided into natural cement, clinker cement (the product obtained by calcining limestone and clay at a higher temperature is called clinker) and clinker-free cement (the cement obtained by grinding industrial wastes such as fly ash and blast furnace slag or natural volcanic ash with alkaline activators such as lime, water glass and gypsum in proportion without calcination).

2. According to the properties of cement, it can be divided into quick-hardening cement (early-strength cement), low-heat cement, expansive cement, acid-resistant cement and refractory cement.

3. According to the purpose, it can be divided into oil well cement, dam cement, jet cement and marine cement.

4. According to the main chemical composition of cement, it can be divided into portland cement, aluminate cement (high alumina cement) and phosphate cement. The latter is rarely used. Although there are many kinds of cement, more than 95% of them belong to portland cement, and only the chemical composition is changed according to the requirements of the project, or some substances that adjust the performance are added when using.

Portland cement is a kind of hydraulic cement with high alkaline silicate as the main compound (commonly known as Portland cement in western countries). It is a mixture of calcareous (limestone, etc.). ) and aluminosilicate (clay, etc. ) raw materials are ground and calcined in a cement kiln at medium and high temperature (about 1720K) according to a certain proportion to obtain cement clinker, which is then ground with a proper amount of gypsum powder to a certain fineness.

The relative density of high performance portland cement is 3. 1 ~ 3.2. Cement will give off heat when it meets water, and it will coagulate after a certain time (different types of cement have different coagulation time). In order to ensure the proper setting time of cement, appropriate amount of gypsum is often added, and phosphogypsum and fluorgypsum, by-products of chemical fertilizer industry, can also be used as substitutes. The addition amount of gypsum mainly depends on the aluminate content in cement clinker, and the addition amount should not exceed 3.5% in terms of sulfur trioxide. Cement should have good volume stability. The solidified cement hardens rapidly in air and water and has mechanical strength (compressive strength and bending strength). According to Ji Xian. Com, the compressive strength and flexural strength of mortar samples with cement: sand = 1:2.5 in water for 3 days, 7 days and 28 days all meet the national standards as the strength index of cement, and the compressive strength value of 28 days is called the label of cement. The commonly used numbers of Portland cement are 325, 425, 525 and 625. The label of some high-strength and ultra-high-strength cements can even reach more than 1000. When minerals in cement clinker contact with water, hydration reaction (hydration) occurs, and calcium hydroxide, hydrated calcium silicate gel, hydrated calcium aluminate and hydrated calcium ferrite are generated at the same time. When gypsum exists, the latter two hydrates generate hydrated calcium sulphoaluminate and hydrated calcium sulphferrite respectively. Cement slurry will shrink under dry conditions and expand under wet conditions. Calcium hydroxide and hydrated calcium aluminate will be corroded by sulfate in seawater, so the content of calcium aluminate in harbor cement should be limited. If the alkali content in cement is too high, and the aggregate made of concrete contains active silica, alkali-aggregate reaction will occur, and the volume will expand, destroying cement stone and concrete. Excessive free calcium oxide and periclase in clinker will also cause volume expansion during hydration reaction, resulting in poor volume stability of cement, which is unqualified.

There are different kinds of portland cement according to the change of mineral composition and the different mixed materials added. Cement made by adding gypsum to clinker is called pure portland cement. If mixed materials such as blast furnace slag, fly ash or volcanic ash are mixed into clinker, they are called slag portland cement, fly ash portland cement and volcanic ash portland cement respectively. The addition of the above-mentioned mixed materials does not exceed 15%, which is called ordinary portland cement (ordinary cement for short).

The above five kinds of cement have the largest output and are often called five kinds of cement. According to the engineering requirements, the chemical composition of cement clinker can be changed in the production process, so that the contents of various minerals generated are different. For example, for high temperature oil well cement, it is required to increase the content of dicalcium silicate; Cement for dam construction requires low content of calcium aluminate and tricalcium silicate; White Portland cement requires iron oxide content less than 0.5%; Fast hardening cement should make tricalcium silicate content high. In order to save energy and develop resources, gypsum and fluorite are sometimes added to the components of portland cement to increase the content of iron oxide and decrease the content of calcium oxide, resulting in cement clinker with dicalcium silicate, calcium ferric aluminate and anhydrous calcium sulphoaluminate as the main minerals. Some cements also contain a small amount of calcium fluoaluminate.

The main chemical components in raw material Portland cement clinker are calcium oxide, silicon oxide, aluminum oxide and iron oxide. Calcium oxide mainly comes from calcareous raw materials, such as limestone, chalk and marl. Alumina and silica are derived from substances containing aluminum silicate, such as clay, blast furnace slag and fly ash. Iron oxide utilizes pyrite slag in sulfuric acid production. The content of calcium oxide in calcareous raw materials used to produce portland cement is generally about 52%; The content of silica in clay raw materials is about 57%, while the content of alumina is less than 20%. In order to reduce the calcination temperature and produce a part of melt in the calcination process, a small amount of iron oxide raw materials are often added. The content of alkali and magnesium oxide in raw materials should also be controlled, that is, the content of magnesium oxide in cement clinker should be less than 5%, the total alkali content (Na2O+K2O) should be less than 1.2% for ordinary cement and less than 0.6% for low alkali cement.

Production technology Portland cement production technology can be divided into raw material preparation, clinker calcination, cement preparation (grinding) and packaging.

1. Raw meal preparation includes the process from raw material crushing to batching and then to qualified raw meal. There are two methods to prepare raw materials: dry method and wet method. In the process of dry preparation, large hard raw materials such as limestone are crushed into blocks with the size of about 100mm by traditional technology, or into blocks with the size less than 25mm by secondary technology (block technology with the size less than 25mm has been developed in recent years). Water-bearing raw materials such as clay should be dried, sent to a mill in proportion with limestone and iron ore, ground into fine raw meal powder, then sent to a mixing bin, stirred with compressed air, and the ingredients adjusted to qualified raw meal powder. The main difference between wet method and dry method is that clay is washed into slurry with water first, and then ground with limestone and iron ore until the water content is about 35%. The main advantage of preparing raw meal by dry method is that the heat consumption when calcining cement clinker is lower than that by wet method. The heat consumption per kilogram of clinker is only 3.6 ~ 4.6 MJ, while the wet process needs 5.2 ~ 6.3 MJ. However, the ingredients of raw meal prepared by wet method are easy to be uniform. In recent years, some advanced dry cement plants have adopted measures such as pre-homogenization of raw meal and automatic control of raw meal composition to ensure the uniformity of raw meal powder composition.

The grinding of raw materials is carried out in different types of mills, mainly including ball mill, tube mill, vertical mill and intermediate discharge mill, while drying and grinding. In order to save electricity in the grinding process and improve the efficiency of the mill, closed-circuit (circulating) grinding is often used in production, that is, the materials coming out of the mill first pass through a particle classifier, and the fine particles are selected as products, and the coarse particles return to the mill to continue grinding. Compared with open-circuit grinding (classification without classifier), the output of closed-circuit grinding system can be increased by about 15% ~ 25%, and the over-grinding phenomenon is reduced. Disadvantages are large equipment investment and complicated operation and management. In recent years, a new type of vertical roller mill with classifier has been adopted, which can crush, grind, dry and classify in the same equipment. At present, the maximum vertical mill output can reach 400 tons per hour.

2. Clinker calcination The prepared raw meal is calcined into cement clinker in different types of kilns. Usually, raw meal powder or slurry is calcined in a rotary kiln. Most small cement plants in China use vertical kiln for calcination. When the shaft kiln is used for calcination, the raw meal powder is mixed with the required pulverized coal, and an appropriate amount of water is added to make raw meal balls with a diameter of10 ~ 30 mm. The quality of cement clinker calcined in the shaft kiln is slightly poor, but the calcination temperature is low and the coal consumption is low. In order to save energy and improve the production capacity of rotary kiln, since 1970s, the out-of-kiln decomposition technology with preheater and calciner has been developed.

A series of physical and chemical changes have taken place in the heating process of cement raw meal in kiln, such as evaporation of free water, removal of crystal water in clay, decomposition of calcium carbonate into calcium oxide and so on. The latter reacts with silica, alumina and iron ore in clay to form compounds, which mainly exist in four forms, namely tricalcium silicate (3CaO SiO2, abbreviated as C3S), dicalcium silicate (2CaO SiO2, abbreviated as C2S), tricalcium aluminate (3 Cao al2o 3, abbreviated as C2A) and tetracalcium aluminate (4cao Al2O3). There is also a small amount of uncombined calcium oxide and periclase (MgO). Sometimes there are sulfates, titanates and so on. , but the number is small. Because clinker also contains other oxides, the above compounds do not exist in pure state, and often there are other oxides in solid solution. Therefore, they are named according to the mineral phase (i.e. crystalline phase). For example, tricalcium silicate is called park jung-su, which accounts for more than 50% in clinker. Dicalcium silicate is called belite, and its content is about 25%. Tricalcium aluminate is aluminate; Iron, aluminum and tetracalcium are called park jung-su. From the structure of cement clinker observed under the reflective microscope, it can be seen that the hexagonal crystal is Park Jung Su and the round crystal is Belit. The substance between crystals is formed by melting and cooling about 30% of the substance at the temperature of about 1450℃, which is called mesophase. The bright part is Park Yung Su, also called white mesophase (i.e. amorphous amorphous phase), and the dark part is aluminate, also called black mesophase. The chemical composition (%) of cement clinker has certain requirements, including calcium oxide 62-67, silicon oxide 20-24, aluminum oxide 4-7 and iron oxide 3-5.

3. Cement is manufactured and packaged. After cooling, appropriate amount of gypsum is added to the cement clinker discharged from the kiln (SO3 in cement is controlled to be less than or equal to 3.5%), and it is ground in a mill to make portland cement. The fineness of cement grinding has a great influence on cement quality. Increasing fineness can improve the strength of cement, but the corresponding power consumption also increases. Generally, the fineness is controlled on a 0.08mm square hole screen, and the screen allowance is not more than 10%, or the specific surface area is about 3000 cm2/g ... The dust in the cement grinding process is relatively large, so dust removal equipment, such as settling chamber, cyclone and bag filter, should be installed at the equipment import and export, transportation and packaging. Some advanced factories are equipped with electrostatic precipitator. In China, clay or potash feldspar with high K2O content is also used to replace clay raw materials, so that oxides are volatilized into dust during calcination, and the dust with high K2O content is collected and can be used as potash fertilizer. Cement powder is usually packed in paper bags, but in recent years it has been transported by bulk carriers and bulk trucks, which improves the transportation efficiency and reduces the cost.

Widely used in civil and industrial construction projects, such as oil and gas field cementing, dams in water conservancy projects, military emergency repair projects, etc. It can also be used as acid-resistant and refractory materials and sprayed in tunnels instead of pit wood. Cement can also be used instead of wood and steel in many occasions, such as telephone poles, railway sleepers, oil and steam pipelines, crude oil storage and gas tanks.

High alumina cement, also known as bauxite cement, is a hydraulic material with low alkali aluminate as the main mineral. The raw materials for producing high alumina cement are limestone and bauxite, including sintering method and melting method. China is mainly produced by sintering method. Its main chemical components (%): calcium oxide 32-34, aluminum oxide 50-60, silicon oxide 4-8, iron oxide 1-3, ferrous oxide 0- 1, titanium oxide 1-3. The fired minerals are: monocalcium aluminate (CAo Al2O3, abbreviated as Ca), monocalcium aluminate (Cao 2al2o3, abbreviated as CA2), dodecacalcium heptaaluminate (12cao 7al2o3, abbreviated as C 12A7), dicalcium silicate and dicalcium aluminosilicate (2cao). The main mineral components (%) of high alumina cement produced in China are: CA40 ~ 50, CA220 ~ 35 and C2As20 ~ 30. CA is a kind of mineral with high hydraulic property, and its setting speed is not fast, but it hardens quickly and its early strength is high. CA2 has low hydraulic property, while crystalline C2AS has no hydraulic property.

The early strength of high-alumina cement develops rapidly, reaching 80% of the ultimate strength in 24 hours. The 3-day compressive strength value is taken as its label (including 1 day and 3-day compressive strength and flexural strength), and the labels are 425, 525, 625 and 725. High alumina cement has good sulfate resistance and high temperature resistance, and can maintain 70% and 53% of its original strength at 900℃ and 1300℃ respectively. It is mainly used for emergency repair projects, projects with high early strength requirements, sulfate corrosion resistance and frost resistance. It is suitable for winter construction, but it cannot be used as a permanent structural material. It can also be combined with refractory aggregate to make refractory concrete, which can be used as the lining of industrial furnaces. High alumina cement can also be made into different varieties, such as aluminate self-stressing cement, aluminate expansive cement, heat-resistant concrete with fire resistance not lower than 1580℃, etc. China began to produce high alumina cement in 1955, and it was first produced by rotary kiln sintering method.