Portland cement is often called simply cement - it is the most important mineral binder. A powdered material containing artificial minerals, most of which do not occur in nature or are extremely rare. These minerals are highly chemically active and are able to interact with water. Portland cement is a hydraulic binder obtained by finely grinding Portland cement clinker with gypsum and other special additives. Clinker is produced by firing a finely dispersed homogeneous raw material mixture consisting of limestone, clay, and silica until sintering. Gypsum is introduced for the purpose of regulation, setting speed and some other properties. Clinker powder without gypsum, when mixed with water, quickly sets and hardens into cement stone with reduced strength properties. According to GOST 1581-96, up to 15% of active mineral additives are allowed to be added to Portland cement during grinding. At the same time, the name of the cement does not change. The properties of Portland cement are determined primarily by the quality of the clinker.
It is believed that Portland cement was invented in England by mason Joseph Aspdin, who received a patent in 1824 for the production of a binder from a mixture of lime and clay by firing it until carbon dioxide is completely removed. He called this binder Portland cement. However, in Russia, Portland cement was obtained somewhat earlier, in 1817, by the head of the military work team E. G. Cheliev. In 1825, he published a book on the production of a binder similar in composition to the Portland cement currently used.
Historical reference
Portland cement clinker and its chemical composition
Portland cement clinker is usually obtained in the form of sintered small and larger granules and pieces up to 10-20 or 50-60 mm in size, depending on the type of kiln. In terms of microstructure, clinker produced by sintering is a complex fine-grained mixture of crystalline phases and a small amount of glassy phase. The chemical composition of clinker varies widely. The main oxides of cement clinker are calcium oxide CaO, silicon dioxide SiO 2 , aluminum oxide Al 2 O 3 and iron oxide Fe 2 O 3 , the total content of which usually reaches 95-97%. In addition to them, there are impurities of magnesium oxide MgO, sulfuric anhydrite SO 3 , titanium dioxide TiO 2 , chromium oxide Cr 2 O 3 , manganese oxide Mn 2 O 3 , alkalis Na 2 O and K 2 O, phosphorus anhydrite P 2 O 5 , etc. The approximate chemical composition of Portland cement is as follows: CaO 63-66%; SiO 2 21-24%; Al 2 O 3 4-8%; Fe 2 O 3 2-4%; MgO 0.5-5%; SO 3 0.3-1%; Na 2 O and K 2 O 0.4-1%; TiO 2 and Cr 2 O 0.2-0.5%; P 2 O 5 0.1-0.3%.
Mineralogical composition of Portland cement clinker
The clinker formed as a result of firing the raw material mixture has a rather complex mineralogical composition.
Four minerals play the main role in it. Tricalcium silicate Ca 3 SiO 5 or 3CaO•SiO 2 (C 3 S) . The tricalcium silicate formed in Portland cement clinker contains a certain amount of impurities MgO, Al 2 O 3 , Fe 2 O 3 , Cr 2 O 3 , which affect its structure and properties. This variety is called alite and is designated C 3 S. The alite content in clinker is the highest and amounts to 40-55%. When considering the processes of hydration of cements, impurities included in tricalcium silicate are, as a rule, neglected, and all calculations are carried out for the pure 3CaO•SiO 2 .
In Portland cement, alite ensures the accuracy of the stone in the early stages of hardening (from several days to 3 months). Tricalcium silicate is obtained in laboratory conditions from chemically pure components. Alite crystals usually have a hexagonal or rectangular shape, which is clearly visible in clinker sections in reflected light. Dicalcium silicate Ca 2 SiO 4 or 2CaO•SiO 2 (C 2 S) . In Portland cement clinker it is present in the beta modification called belite. Its amount in clinker is 20 - 30%. Belite has less hydraulic activity compared to alite and ensures an increase in the strength of cement stone in the later stages of hardening. Belite, like alite, is a solid solution of beta - dicalcium silicate (beta - 2CaO•SiO 2 ) and a small amount (1-3) of impurities such as Al 2 O 3 , Fe 2 O 3 , Cr 2 O 3 , etc. Hydraulic The activity of belite also depends on the structure of the crystals.
Cements in which belit is represented by rounded dense crystals with jagged edges with an average size of 20-50 microns are characterized by increased strength. The splitting of crystals helps to increase its hydraulic activity. The intermediate substance located between the alite belite crystals includes aluminoferrite and aluminate phases. Calcium aluminates are usually found in clinker in the form of tricalcium aluminate C 3 Al 2 O 6 or 3CaO•Al 2 O 3 (C 3 A) . C 3 A crystallizes in the cubic system in the form of very small hexagons and rectangles. Contained in cement clinker in amounts up to 15%. This is the most chemically active mineral in clinker and it is its hydration that determines the setting time of cement mortars. Its presence in large quantities accelerates the setting and hardening of Portland cement mortar at low temperatures. With an increased content of tricalcium aluminate, the stability of cement stone in environments containing sulfates and hydrogen sulfide is weakened. The aluminoferrite phase is a solid solution of calcium aluminoferrites of different composition, which in turn depends on the composition of the raw material mixtures, firing conditions, etc. In this case, the formation of a series of solid solutions between C 6 A 2 F, C 4 AF, C 6 AF 2 and C 2 F is possible. In clinker, the aluminoferrite phase is close in composition to tetracalcium aluminoferrite.
Tetracalcium aluminum ferrite Ca 4 •Al 2 O 5 •Fe 2 O 5 or 4CaO•AI 2 O 3 •Fe 2 O 3 (C 4 AF) (brownmillerite) is an iron-containing mineral that has a fairly high hydration rate and ensures an increase in the strength of the system in the first hours hardening. In Portland cements its amount is in the range of 10-20%. The rates of hydration processes are approximately equal.
In addition to the indicated minerals, the clinker composition includes a glassy phase containing non-crystallized ferrites, aluminates, magnesium oxide, alkaline compounds, etc. When the cement clinker is cooled sharply, the glass phase, covering the surface of the minerals, prevents phase transitions. Magnesium oxide is found in clinker in the form of: a) periclase mineral; b) solid solution in the aluminoferite phase or in tricalcium silicate; c) in clinker glass. The harmful effect of MgO at a content of more than 5% on the uniformity of changes in the volume of cement is manifested in the case when it is present in the form of periclase crystals, which slowly react with water in already hardened cement and give Mg(OH) 2 , characterized by an increased specific volume. Alkalis: sodium and potassium are present in clinker in the form of sulfates, and also occur in the aluminate and aluminoferrite phases.
To regulate the setting time of cement when grinding clinker, 3-5% gypsum dihydrate is added. In addition, Portland cement can contain up to 15% silica-containing components, which can be ground sand, slag, and ash from burning solid fuels. By introducing additives, two advantages are achieved: firstly, cement is cheaper because Portland cement clinker is more expensive than any additive; secondly, additives can be used to regulate the properties of mortar and stone. To impart special properties to cement during its grinding, water repellents, plasticizers and other substances are introduced.
Hardening of Portland cement
When mixing cement with water at the initial stages of hardening, calcium aluminates and aluminoferrites intensively enter into the hydration reaction, due to a higher dissolution rate constant compared to alite and belite. The solution becomes supersaturated in relation to the final product and from it needle-shaped crystals of calcium hydroaluminates and hydroferrites of various compositions are formed on the surface of the clinker grains and in the volume of the solution. In general, their composition can be denoted as xCaO•yAI 2 O 3 •mH 2 O and xCaO•yFe 2 O 3 •mH 2 O. The values of the coefficients x, y, m vary in different proportions and depend mainly on thermodynamic conditions hydration processes. After some time (3-6 hours), quite a lot of crystalline hydrates accumulate in the system and “constrained” conditions are formed, leading to the formation of a coagulation structure, which, as hydroaluminates accumulate, turns into a crystallization structure. After 6 - 10 hours, the entire volume between the gradually decreasing cement grains is filled with a skeleton of needle-shaped crystals - products of hydration of the aluminate components of clinker. This structure is sometimes called aluminate. The cement mortar, which was previously plastic, begins to lose mobility and gain strength.
In the remaining volume, simultaneously with the aluminate one, but at a much lower rate, hydration products of the silicate clinker minerals alite and belite appear. The latter form an extremely finely porous pile of very small crystals, the so-called silicate structure. The influence of this structure on the strength of hardening cement stone increases more and more over time. It is already the carrier of the strength of the cement stone and after approximately 1 day it begins to prevail over the aluminate one. By one month, almost only a silicate structure is found in the cement stone. By this time, the hydration process does not end and in some cases can continue for years due to the unused clinker stock of cement.
Structure of cement stone
For complete hydration of cement grain, the presence of 0.4 times the amount of water from its mass is necessary. In this case, only 60% of it (i.e. 0.25 by weight of cement) is chemically bound, the rest (40% of the original water) remains in the pores of the cement gel in a weakly bound state. The size of the gel pores is about 3•10 -8 cm. They are inevitable and cause the finely porous structure of the gel mass. When chemically bound, water undergoes volumetric contraction, which is about 1/4 of its original volume. Therefore, the dense volume of the gel (without pores) is the same amount less than the sum of the volumes of the original components of cement and water. This process is called shrinkage, and the volume released in the cement stone is called shrinkage volume. When cement stone hardens in an aqueous environment or at high humidity, the considered pore volume is filled with water. Thus, with complete hydration of the cement, a gel is obtained, the volume of which consists of approximately 30% pores.
The considered case is ideal and almost never occurs in practice. If the amount of water is less than 0.4 by weight of cement, then it will not be enough to completely hydrate the cement grains, and unreacted cement grains will remain in the cement stone. If there is an excess amount of water, part of it does not participate in the hydration process and forms capillary pores in the stone with a diameter of about 10 -4 cm, which are several orders of magnitude larger than the gel pores. The voids that arise as a result of the already mentioned shrinkage reach approximately the same size. Thus, the water-cement ratio (W/C) largely determines the structure of the cement stone and its physical and mechanical properties. The total porosity of the stone increases with increasing W/C.
Portland cement production
The production of Portland cement can be divided into two sets of operations. The first of them involves the production of clinker, the second - the production of Portland cement by grinding clinker together with gypsum, active minerals and other additives. Producing clinker is the most complex and energy-intensive process, consisting of extracting raw materials, mixing and firing them. Currently, two main methods are used for preparing the raw material mixture from the initial components: “wet”, in which the grinding and mixing of raw materials is carried out in an aqueous environment, and “dry”, when the materials are crushed and mixed in dry form. Each of these methods has its positive and negative sides. In an aqueous environment, grinding of materials is facilitated; when grinding them together, a high homogeneity of the mixture is quickly achieved, but the fuel consumption for firing is 1.5-2 times more than when dry. The dry method, despite its technical and economic advantages over the wet method, has found limited use for a long time due to the reduced quality of the resulting clinker; however, advances in the technology of fine grinding and homogenization of dry mixtures have made it possible to obtain high-quality Portland cement using the dry method. The third, so-called combined method is also used. Its essence lies in the fact that the preparation of the raw material mixture is carried out using the wet method, then the sludge is dewatered in special installations and sent to the furnace. According to some data, the combined method reduces fuel consumption by almost 20-30% compared to the wet method, but at the same time the labor intensity of production and energy consumption increases.
The raw material mixture is fired in rotary kilns. The length of modern rotary kilns reaches 150-185 m or more, and the diameter is 4-7 m. The kiln rotation speed is 0.5-1.2 rpm. Sludge, passing through the furnace and exposed to gases of increasingly high temperatures, undergoes a number of physical and physicochemical transformations. At temperatures of 1300-1500 °C the material is sintered, and clinker grains up to 15-20 mm in size and larger are formed. Having passed through the high temperature zone, the clinker begins to be cooled by flows of colder air. It leaves the oven at a temperature of 1000-1100 °C and is sent to a grate refrigerator, where it is cooled to 30-50 °C. The cooled clinker arrives at the warehouse. During the movement of sludge through the furnace, the following physical and chemical processes occur. In that part of the furnace where the temperature is 300-600 °C, vigorous evaporation of water begins, which is accompanied by a gradual thickening of the sludge. Large lumps form. Then, at three temperatures of 400-500 °C, organic impurities burn out from the material; dehydration of kaolinite and other clay minerals begins with the formation, in particular; kaolinite anhydride. Removal of hydration water from clay is accompanied by a loss of plasticity and binding properties, which leads to the disintegration of previously formed lumps of material into a mobile powder. The area of the kiln where the water evaporates and the material dries is called the drying zone . The next zone, where clay is dehydrated and further heated to 700-800 °C, is called the heating zone . These two zones occupy up to 50 - 55% of the furnace length. At temperatures of 750-800 °C and above, reactions in the material begin in the solid state between its components. At first they are barely noticeable, but as the temperature of the material increases to 1000 °C or more, their intensity increases sharply. First, aluminum and iron oxides react. They add calcium oxide to themselves and monocalcium aluminate and monocalcium ferrite are formed. In their pure form, these compounds do not exist, but form a solid solution and dissolve in each other. The amount of attached calcium oxide increases with increasing temperature. At 900-1000 °C, the decomposition of calcium carbonate sharply increases with the formation of free calcium oxide and carbon dioxide CO 2 . This section of the furnace is called the calcination zone . In this zone, due to the fact that the decomposition of CaCO 3 occurs with the absorption of heat, the consumption of the latter is greatest. In the section of the furnace where the temperature of the material reaches 1000-1100 °C and where the bulk of CaCO 3 has already turned into free calcium oxide, the intensity of reactions in the solid state increases sharply. A solution of calcium aluminate and ferrite binds an increasing amount of calcium oxide and dicalcium aluminate and calcium ferrite are already formed. This solid solution contains equal amounts of aluminum oxide and calcium oxide. This solution has the composition 4CaO•AI 2 O 3 •Fe 2 O 3 . The raw material mixture contains more aluminum oxide than iron oxide, so the remaining calcium aluminate continues to bind calcium oxide to form tricalcium aluminate. Its formation ends at a temperature of 1200 °C. The addition of calcium oxide to silicon oxide begins at 600 °C, but occurs quite quickly at a temperature of 900-1100 °C. The formation reactions of silicates, aluminates and calcium ferrites are exothermic, which leads to an intense increase in the temperature of the material by 150-200 ° C in a short furnace section of several meters. This section of the furnace is called the exothermic zone. By the end of the exothermic zone, the temperature of the material reaches approximately 1300 °C. At a temperature of 1250 °C, the formation of dicalcium silicate ends. Since the raw mixture contains more calcium oxide than is needed for the formation of C 2 S, C 4 AF, C 3 A, the remaining amount of CaO is used for the formation of tricalcium silicate. At a temperature of 1300 °C, sintering of the material begins due to the formation of a melt in it in an amount of 20-30% of the volume of the mass that has begun to sinter. At the initial moment of sintering, C 3 A, C 4 AF, and CaO pass into the melt, and later dicalcium silicate C 2 S begins to dissolve in it. At the same time, favorable conditions are created in the liquid phase for the formation of the main mineral of Portland cement - tricalcium silicate C 3 S from C 2 S and CaO. This compound is poorly soluble in the melt, as a result of which it is released from it in the form of small crystals, which can subsequently increase significantly in size. The release of C 3 S from the melt is accompanied by a decrease in the concentration of C 2 S and calcium oxide in it, which leads to the transition into the melt of new portions of these substances remaining in the solid state in the total mass of the material. This, in turn, determines the further course of the process of formation in the melt and release of C 3 S from it until almost complete binding of free calcium oxide with C 2 S. Tricalcium silicate is released from the melt along with small amounts of Al 2 O 3 and MgO, forming a solid with them a solution called alite. The area of the furnace where the material is sintered and alite is formed is called the sintering zone . Here the materials are heated from approximately 1300 to 1450 °C, which promotes faster absorption of calcium oxide by dicalcium silicate and the formation of alite. After the sintering zone, the fired material passes into the cooling zone. Up to a temperature of approximately 1300 °C, the liquid phase is still present in it and the reaction of assimilation of calcium oxide and the formation of C 3 S continues. Then the liquid phase solidifies and sintering ends. The last section of the kiln, where the resulting clinker is cooled by air from 1300 °C to the temperature at which it leaves the kiln (1000-1100 °C) is called the cooling zone . Usually, when clinker is cooled from 1450 to 1300 °C and below, the liquid phase in it partially solidifies in the form of glass, and partially crystallizes C 3 A, C 4 AF, and MgO from the melt. The degree of crystallization of the melt depends on the cooling rate of the material after it leaves the sintering zone. Cooled clinker mainly consists of crystals of minerals - silicates (alite and belite) and an intermediate substance, which includes glass, smelting minerals (C 4 AF, C 3 A, C 3 A 3 ), as well as calcium and magnesium oxide (in the form crystals).
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Lectures by FHPTKTs
Thank you for the lecture notes on the discipline Physico-chemical processes of hardening and corrosion of cement stone (PHPTCC), I read it in my spare time. It perfectly covers the issues of shrinkage, strength and permeability of cement stone. The process of regulating the hardening of cement mortar did not stand aside either.
I believe that adding such lectures and notes will be very useful for the community. I’ll look for my student records and post the most interesting ones.
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