7.1. General concepts
Inorganic binders are called powdery materials that, when mixed with water, form a viscous-plastic dough that can spontaneously or under certain conditions pass from a pasty to a stone-like state (solidify). These abilities determine the possibility of mixing the dough with grains of sand, gravel, crushed stone and their subsequent bonding between them (homogenization into a single solid body) during the solidification of the dough. This is the basis of the principle of obtaining the most common building materials - concrete, mortar and many other artificial materials.
The preparation of a solid with any mineral binders is based on the phenomenon of crystallization. When hydrated hardening as a result of hydration, the molecules of salt hydrates form nuclei of crystals, which begin to grow. When they fill all the free volume in the system, the dough begins to lose plasticity - this is the beginning of the setting. After that, the crystals begin to intertwine, grow together, until the dough finally loses its plasticity - the end of the setting. Why is the setting time so important? Before setting, the mixture (dough) can be moved, transported, laid. As soon as the setting has begun, interference with the structure can greatly damage, leading to a decrease in quality. The end of the setting indicates that the product has gained some structural strength, which means that it can already be dismantled.
Inorganic binders are divided into three groups: air, hydraulic, and hydrothermal synthesis binders (autoclave).
7.2. Air binders
Air binders are characterized by the fact that, when mixed with water, they harden spontaneously and for a long time retain strength only in air. With the systematic moistening of concrete, products and structures on air binders relatively quickly lose their strength and collapse. For air binders include gypsum binders, air lime, magnesia binders and liquid glass.
Gypsum binders
Depending on the heat treatment temperature, gypsum binders are divided into two groups: low-firing (construction plaster, technical (high-strength) plaster, molding plaster) and high-firing (anhydrite cement and estrih-plaster).
Building gypsum is a powdered material of white or grayish color, consisting mainly of β- modification crystals of CaSO4 · 0.5H2O, also contains a certain amount of anhydride (CaSO4) and particles of undecomposed raw material CaSO4 · 2H2O.
Construction gypsum is made from natural two-water gypsum, clay-gypsum, as well as some industrial wastes: phosphogypsum (waste in the production of superphosphates), borogypsa (waste from the production of boric acid), etc. Raw materials are burned in cooking boilers or furnaces at 130 ... 150 ° C : in the first case, the gypsum stone is first ground, and then in the form of powder, heated in boilers; in the second, gypsum pieces of various sizes are burned in shaft, rotating, chamber, etc. furnaces, and then crushed into powder. There are industrial plants in which the operations of drying, grinding and firing are combined. At this temperature, two-water gypsum is dehydrated: CaSO4 · 2H2O = CaSO4 · 0.5H2O + 1.5H2O.
Seizure and hardening of semi-aquatic gypsum due to its transition when interacting with water in dihydrate according to the scheme
CaSO4 · 0.5H2O + 1.5H2O = CaSO4 · 2H2O.
At the same time, 133.14 kJ / kg of heat is released. Molecules CaSO4 · 2Н2О provide the phenomenon of crystallization.
After the set in the structure is the process of curing. For gypsum, this set (hardening) ends after 2 hours. As a result, a solid body with high porosity, reaching 40 ... 60% or more, and compressive strength from 2 to 12 MPa (high-strength gypsum - up to 25 MPa) is formed. The tensile strength of stone from semi-aquatic gypsum is 6 ... 9 times less than compressive strength. The strength of hardened gypsum depends on its moisture content (softening coefficient ranges from 0.3 ... 0.45), which can be explained by dissolving two-water gypsum at the points of contact of crystalline intergrowths in the structure of gypsum stone, causing a decrease in its strength.
When hardening, construction gypsum expands by 0.05 ... 0.15%, which favors the production of architectural details by casting and dense filling of cracks, seams, sinks, etc. when finishing surfaces.
Advantages: gypsum binders differ from all known mineral binders by fast hardening (usually the beginning of the setting of the gypsum dough comes after 4 ... 5 minutes, and the end of the setting - 10 ... 15 minutes after mixing with water), good molding properties and the greatest economy. This allows relatively simple and in a short time to produce on their basis various types of products with low density (800 ... 1500 kg / m3), sufficient strength, good acoustic and thermal properties, high durability when serving them in an air-dry environment. They are easily machined (sawing, drilling, etc.). However, due to the low water and frost resistance, gypsum products can be used at a relative air humidity of not more than 65% (hydrophobization is necessary for operation in rooms of greater humidity) . In addition, it should be noted that gypsum accelerates metal corrosion.
Regulatory requirements for gypsum binders are made according to the following indicators: grinding fineness, setting time, compressive strength.
Apply construction plaster for the manufacture of: plasterboard sheets (dry plaster); plaster plates for partitions; gypsum concrete panels; gypsum sheets; gypsum blocks (stones); sound-absorbing plates; decorative plates; sanitary cabins; ventilation units; floor base panels. Also, construction gypsum is introduced into the composition of dry construction mixtures intended for plastering walls and ceilings in order to prepare grounds for painting works inside premises with normal humidity; for the production of gypsum cement lump binders (CPP).
High- firing (anhydrite) gypsum binders are obtained by firing at high temperatures (600 ... 900 ° C); anhydrous gypsum (anhydrite CaSO4) is predominantly present in them; they differ in slow hardening. High burners are:
a) anhydrite binder (anhydrite cement) - 600 ... 750 ° C;
b) high-calcined gypsum (estrikh-gypsum) - 800 ... 1200 ° С.
High-calcined gypsum slowly sets and hardens, but its water resistance and compressive strength are higher than plaster - 10 ... 20 MPa. Therefore, it is used in the device of seamless floors, in plaster and masonry solutions, for the manufacture of "artificial marble".
Air lime
Lime - the most ancient knitting substance. It was used several thousand years before our era. Air lime consists mainly of oxides of calcium and magnesium (CaO + MgO). This is a product of roasting at a temperature of 900 ... 1200 ° C of calcium-magnesium carbonate rocks (chalk, limestone, dolomitic limestone, dolomite - CaCO3 and MgCO3 · CaCO3) with their decomposition into CaO and CO2 by the reaction:
CaCO3 = CaO + CO2 ↑ - 425.2 kcal / kg (1777 kJ / kg ).
As a result of roasting in furnaces of various types (mine, rotating, in a fluidized bed, on sintering grids, etc.), lump quicklime is obtained in the form of pieces of various sizes (up to 60 mm). The true density of quicklime fluctuates within 3.1 ... 3.3 g / cm3 and depends mainly on the firing temperature, the presence of impurities, underburning and burnout. The average density of lump quicklime in a piece largely depends on the burning temperature and increases from 1.6 g / cm3 (lime, calcined at 800 ° C) to 2.9 g / cm3 (long-firing at 1300 ° C).
For fine grinding of lump lime, quick burnt ground (boil) lime is obtained. The bulk density in the loose state is 900 ... 1100, in the compacted 1100 ... 1300 kg / m3 . Ground quicklime is usually used immediately after grinding, as it loses its binding properties due to the absorption of moisture from the air.
In the interaction of quicklime with water (this process is called slaking) by the reaction
CaO + H2O = Ca (OH) 2 + 277 kcal / kg (1158 or, according to other data, 950 kJ / kg ) get hydrated lime (fluff) Ca (OH) 2 + Mg (OH) 2 as highly dispersed
dry powder. In the process of quenching, spontaneous decomposition of pieces of lime into fine particles with a size of no more than 5 ... 20 microns (thinner than that of cement) takes place and a fine powder is formed with a specific surface area of up to 10,000 cm2 / g and a bulk density of 400 ... 450 kg / m3. Air lime is the only binder that turns into a finely divided state by chemical dispersion. At the same time, lime increases in volume by 2 ... 3.5 times. The true density of hydrated lime depends on the degree of its crystallization and is equal for Ca (OH) 2, crystallized in the form of hexagonal plates - 2.23, amorphous - 2.08 g / cm3. Slaking lime powder is carried out in special damping devices (hydrators) of periodic and continuous action.
There are two types of lime hardening. When hydrated lime is used, carbonate hardening occurs due to the formation of calcium carbonate by the reaction of Ca (OH) 2 + CO2 + nH2O = CaCO3 + (n + 1) H2O.
The compressive strength of mortars and concretes on hydrated lime when cured under normal conditions (10 ... 20 ° С) during the month reaches small values of the order of 0.5 ... 1 MPa. With the help of artificial carbonization it is possible to produce concretes with a strength of up to 30 ... 40 MPa. With prolonged hardening for many tens (and sometimes hundreds) of years due to the interaction of sand with calcium hydroxide, the strength increases to 6 ... 7 MPa.
When using ground quicklime, hydrated hardening occurs. The hardening effect is due to the mutual adhesion and accretion of lime formed by the interaction of water with properly selected water-lime ratio (0.9 ... 1.5) of submicroscopic calcium hydroxide CaO · H2O particles that grow together and quickly form a solid crystalline joint. Solutions on ground quicklime set in 15 ... 60 minutes after mixing. The self-heating of the material (mortar or concrete) at the same time contributes to the acceleration of hardening and increase the strength of the solution, which is especially important during winter work (masonry, plaster, etc.). After 28 days of hardening in air, the strength of the solutions under compression reaches 2 ... 3 MPa, i.e. three times more than slaked . Over time, hydrated hardening turns into carbonate.
In wet conditions, lime mortar and concrete gradually lose their strength and collapse. The destruction at the same time comes especially quickly, if the concrete is then frozen, then thawed. The more active in mortars and concretes are lime carbonization processes, the more waterproof and frost resistant they are. This is convincingly demonstrated by the long-term preservation of many facades of buildings plastered with lime mortar.
The most important operational and technical indicators of air lime:
- content of active CaO + МgO (lime activity) : according to the activity, lime is divided into 3 grades: 1st grade - not less than 90% CaO + MgO; 2nd - 80% and 3rd - 70%;
- according to the MgO content , calcium (or low-magnesia - no more than 5%), magnesian (5 ... 20%) and dolomitic (20 ... 40%) lime are distinguished;
- temperature and time of extinguishing lime : quick- extinguishing - no more than 8 minutes, average-extinguishing - no more than 25 minutes, slow- extinguishing - more than 25 minutes
- content of non-quenched grains : when slaking lime it is necessary to strive for the complete conversion of calcium and magnesium oxides to hydroxides, since non-quenched particles can subsequently negatively affect the quality of hardened mortars and concretes (
ki);
- the residue on sieve № 02 powdered lime should be no more than 1.5%; No. 008 - no more than 15%.
- uniform change in volume , which is defined as for cement, but with some changes.
The widespread use of lime in construction is due to the fact that it is a local binder. Raw materials and fuel for its production are almost everywhere, and the organization of production is associated with relatively low capital investments. Construction lime is manufactured from:
1) mortars intended for land masonry of parts of buildings and plasters working in air-dry conditions.
2) low-grade concretes used in structures operating in air-dry conditions;
3) lime paint compositions ;
4) mixed hydraulic binders (lime-slag and lime-cocoa-pizza cements);
5) autoclaved binders .
Magnesian astringent
Magnesia binders - caustic magnesite and caustic dolomite - fine powders of magnesium oxide MgO. Magnesia binder is produced by moderate roasting (at a temperature of 750-850 ° C) of magnesite (less often dolomite): MgCO3 = MgO + CO2 ↑.
Magnesia binder is most often closed with an aqueous solution of magnesium chloride MgCl2 (or other magnesian salts). This accelerates the hardening and significantly increases the strength, since along with the hydration of magnesium oxide by the reaction MgO + H2O = Mg (OH) 2, the formation of magnesium hydrochloride 3MgO · MgCl2 · 6H2O occurs. When mixed with water, magnesium oxide hydrates very slowly.
Magnesia binder has a high strength, reaching 60100 MPa under compression, it adheres well to wood, so it can be used for the manufacture of fiberboard and magnesian-sawdust (xylolite) floors - monolithic and tiled. Liquid glass
Liquid glass is a colloidal aqueous solution of sodium silicate Na2O · (2.5 ... 3) SiO2 or potassium silicate K2O · (3 ... 4) SiO2 , produced by cooking glass from silica sand SiO2 and soda Na2CO3 in glass melting furnaces, as usual glass, with the subsequent dissolution of the solidified melt at elevated temperature and pressure of 0.6-0.7 MPa. Liquid glass has a yellow or brown color, density of 1.3-1.5 g / cm3 with a water content of 50-70%. Sodium and potassium silicates in water undergo hydrolysis: Na2SiO3 + 3H2O = 2NaOH + SiO2 · 2H2O.
The secreted silica gel is SiO2 · 2H2O and has astringent properties. To accelerate the hardening of liquid glass, sodium fluoride sodium Na2SiF6 is added to it, which accelerates the precipitation of silica gel and the hydrolysis of liquid glass.
Sodium liquid glass is used for the manufacture of acid-resistant and heat-resistant concretes, for compaction of soils. Potassium glass, more expensive, is used mainly in silicate paints.
Acid-resistant quartz cement is a powdery material obtained by the joint grinding of pure quartz sand and sodium fluorosilicate Na2SiF6 (it is possible to mix separately crushed components). Acid-resistant cement shut with an aqueous solution of liquid glass, which is a binder. The powder itself has no astringent properties. Acid-resistant cement is used for the manufacture of acid-resistant solutions and concrete, putty. At the same time take acid-resistant fillers:
quartz sand, granite, andesite, etc.
7.3. Hydraulic binders
Hydraulic binders harden and for a long time retain strength (or even increase it) not only in air, but also in water. They can be divided into three main groups: hydraulic lime and romance cement, silicate cements (portland cement and its varieties) and aluminate cements (aluminous cement and its varieties).
Hydraulic lime and romance cement
Hydraulic lime is obtained by firing in shaft kilns not sintering (900 ... 1100 ° C) of marl limestone with a clay content of 6 ... 20%. The resulting lime is ground and used in the form of powder or quenched in fluff. In the process of calcining marl limestones after the decomposition of calcium carbonate (900 ° C), part of the formed CaO remains in a free state, and a part combines with the oxides SiO2, A12O3, and Fe2O3, which are part of the clay materials. In this case, low-base silicates (2CaO · SiO2) aluminates (CaO · Al2O3) and ferrites (CaO · Fe2O3) calcium are formed, which give lime hydraulic properties. Compressive strength after 28 days of combined (7 days in humid air and 21 days in water) storage of samples from a 1: 3 solution by weight 2 ... 5 MPa and higher. Hydraulic lime is used for the manufacture of mortars, low grade concrete and concrete stones.
Romance cement is a hydraulic binder obtained by fine grinding of calcined limestone or magnesian marls, which were not sintered before sintering (900 ° C), containing more than 20% clay. Calcium silicates and calcium aluminates formed during roasting give the romance the ability to harden and retain its strength in water. Romancement produce three brands: M25, M50 and M100. It must withstand the test of uniform change in volume. It is used for the manufacture of mortars, concrete, concrete stones.
Hydraulic lime and romance cement were previously widely used, but now these binders have given way to a more advanced hydraulic binder and above all to Portland cement.
Getting lime-slag binders основано на способности тонкоизмельченных гранулированных доменных шлаков твердеть при добавке извести. Обычно шлак размалывают совместно с воздушной известью, содержание которой в вяжущем составляет 20…30 %. При помоле добавляют до 3…5 % гипса для улучшения процессов твердения. Известь, реагируя с низкоосновными алюминатами и силикатами шлака, способствует образованию высокоосновных гидроалюминатов и гидросиликатов кальция. Добавляемый гипс реагирует в водном растворе с алюминатами кальция, образуя гидросульфоалюминат кальция. В результате обоих этих процессов возрастает прочность вяжущего. Известково-шлаковые вяжущие схватываются и твердеют медленно, но при тепловлажностной обработке твердение ускоряется. Они стойки в пресной воде, но имеют низкую морозостойкость. Известково-шлаковые вяжущие применяют в бетонах невысоких марок и в строительных растворах.
Известково-пуццолановые вяжущие изготовляют путем совместного помола трепелов, диатомитов и других активных минеральных добавок с известью. При твердении во влажных условиях или в воде образуются низкоосновные гидросиликаты кальция. На воздухе в сухих условиях гидросиликаты способны дегидратироваться, при этом прочность изделия может сильно снижаться. Прочность этих вяжущих невысока, и они применяются там же, где и известково-шлаковые вяжущие.
Портландцемент
Портландцемент – порошкообразный продукт темно-серого или зеленовато-серого цвета, получаемый тонким измельчением клинкера с добавкой 3…5 % двуводного гипса (гипсового камня). Самое распространенное сегодня вяжущее. Ежегодно в мире выпускается около 3 миллиардов тонн цемента.
Готовый клинкер представляет собой зернистый материал в виде спекшихся гранул размером 10…40 мм ("горошек") темно-серого или зеленовато-серого цвета. Получение портландцементного клинкера включает подготовку (измельчение) сырьевых материалов: известняков (мел, плотный известняк и др.) и глинистых пород (глины, глинистые сланцы), их смешивание в соотношении 3:1 (т.е. берется около 75 % известняка и 25 % глины) по сухому или мокрому способу и обжиг смеси во вращающихся печах при 1450 °С до спекания.
В среднем на 1 т цемента расходуется около 1,5 т минерального сырья. В сырьевую смесь вводят добавки, корректирующие химический состав, регулирующие температуру спекания смеси и кристаллизацию минералов клинкера.
В процессе обжига сырье комкуется при перекатывании, дегидратируется, декарбонизируется, распадается на оксиды, которые вступают в химическое взаимодействие друг с другом с образованием силикатов, алюминатов и алюмоферритов кальция в виде минералов кристаллической структуры, а некоторая часть их входит в стекловидную фазу.
Помол клинкера в тонкий порошок производится преимущественно в трубных (шаровых) мельницах. Природный гипс в портландцемент добавляют для регулирования скорости схватывания и некоторых других свойств. Клинкерный порошок без гипса при смешении с водой быстро схватывается и затвердевает в цементный камень, который характеризуется пониженными техническими свойствами.
Химический состав клинкера :
СаО – 63…66 %,
SiO2 – 21…24 %,
Аl2O3 – 4…8 %
Fe2О3 – 2…4 %,
The mineralogical composition of clinker :
Alit 3CaO · SiO2 (С3S) - 45 ... 60%
Belit 2CaO · SiO2 (С2S) - 20 ... 30%
Three-calcium aluminate 3CaO · Al2O3 (С3А) - 4 ... 12%
Four- calcium alumoferrite 4CaO · Al2O3 · Fe2O3 (C4AF) - 10 ... 20% Clinker glass - 5 ... 15%. Alkalis (Na2O, K2O) up to 0.6%.
The true density of Portland cement (without mineral additives) is 3.05 ... 3.15. Its bulk density depends on compaction and in loose cement it is 1100 kg / m3, in highly compacted it is up to 1600 kg / m3, on average it is 1300 kg / m3.
After mixing cement with water, the cement dough for 1 ... 3 hours is plastic and easily molded. At this time, the clinker minerals listed by us (the reaction of the anhydrous salts with water, accompanied by the release of heat) hydrate with the formation of hydrosilicates, hydroaluminates and calcium hydroferrites, as well as calcium hydroxide.
These compounds precipitate and form crystals that begin to grow in the bulk of the cement paste. This period is called grasping, at which time the cement paste thickens, losing mobility, but its mechanical strength is still small. The setting time depends on the mineral composition of the clinker and on the fineness of cement grinding. Tricalcium aluminate hydrates most quickly, therefore, the higher its content in clinker, the faster the setting ends. Two-water gypsum (or gypsum stone) serves just to slow down the hydration of aluminate.
In terms of setting the cement is divided into:
- slow-setting - with a normalized start-setting time of more than 2 hours;
- normal setting - with a normalized start time setting from 45 minutes to 2 hours;
- quick-setting - with a normalized start-setting time of less than 45 minutes.
Typically, portland cementing ends within 5 ... 10 hours after mixing. When the growing crystals fill the entire volume of cement paste, they begin to grow together, crystalline intergrowths are formed, ensuring the transition of the thickened cement paste to a solid state. This means the end of setting and the beginning of hardening (increase in strength due to the formation of spatial structure with fused crystals).
The estimated age of concrete on cement is 28 days - it is believed that by this time almost all particles of cement are hydrated. However, in fact, the largest particles (more than 80 microns) are hydrated only outside, and remain non-hydrated inside - this is the so-called “clinker foundation”, which provides the subsequent strength build-up for many years. It is also important that the temperature increase significantly speeds up the processes of hydration and hardening of the cement, reducing the time of hardening to 6 ... 10 hours. This effect underlies the heat and moisture treatment of concrete products - an essential attribute of the industrial production of concrete and reinforced concrete. However, the price paid for the reduction of hardening times and the accelerated production of products is a decrease in the strength of concrete due to the increased imperfection of its structure due to the temperature and humidity gradients encountered during heat treatment.
As a result of hardening cement stone is formed. Its solid phase consists of submicrocrystalline (gel) and crystalline products of cement hydration and numerous inclusions in the form of non-hydrated clinker grains (clinker foundation). Thanks to this foundation, hardening under favorable conditions lasts for years - up to the complete hydration of the cement.
In addition, capillary and gel pores are present in the structure of the cement stone.
Their content depends on the water demand of the cement, which is determined by the amount of water (in% by weight of cement) necessary to produce a cement paste of normal density . The water requirement of ordinary (normal) Portland cement in the range from 22 to 28%. With the introduction of active mineral additives of sedimentary origin (diatomite, tripoli, flask), the water requirement of cement increases and can reach 32 ... 37%. The setting time and water demand of cement depend on the fineness of its grinding - the residue on sieve No. 008 is no more than 10 ... 15%. The specific surface of the cement of such grinding is usually 2500 ... 3000 cm2 / g . The higher the fineness of grinding, the higher the water requirement of cement, the higher the porosity and the lower the strength of the cement stone. To reduce water demand, the cement is plasticized or water-repellent.
Important in the process of hardening is also the uniformity of change in the volume of cement, determined in the test of normal density.
The strength properties of Portland cement are assessed by class and activity . Cement class is the value from the unified series, determined by the average value of compressive strength (in MPa) of a cement-sand mortar of 1: 3 with W / C = 0.5, tested at the design age (2, 7 or 28 days) curing under water. By compressive strength at the age of 28 days, cements are divided into classes 22.5; 32.5; 42.5; 52.5. In regulatory documents on cements of specific types, additional strength classes may be established. For some special types of cements, taking into account their purpose, strength classes are not established. Activity is a specific value of strength, not reduced to a unified series.
By appointment, cements are divided into general construction and special.
By type of clinker, cements are divided on the basis of:
- Portland cement clinker;
- alumina (high-alumina) clinker; - sulfoaluminate (-ferritic) clinker.
According to the speed of hardening, general construction cements are divided into:
- normal hardening with rationing strength at the age of 2 (7) and 28 days;
- fast hardening with standardization of strength at the age of 2 days, increased compared with normal hardening, and 28 days.
According to the material composition, cements are divided into five types:
- CEM I - Portland cement;
- CEM II - Portland cement with mineral additives;
- CEM III - slag Portland cement; - CEM IV - pozzolan cement; - CEM V - composite cement.
The cement marking indicates its type, strength class, subclass of setting speed: H (normal-hardening) and B (fast-hardening) and the designation of the regulatory document on which the cement is supplied.
Varieties of Portland cement: slag Portland cement ; pozzolanic portland cement ; gypsum cement lump binders (GPCV); rapid hardening (BTT), especially high-strength hardening (OPTC) and ultra-fast hardening (SBTC) portland cement ; plasticized and hydrophobic portland cement ; astringent low water demand (VNV) ; sulphate-resistant portland cement ; white and colored portland cement ; grouting portland cement ; portland cement with moderate exotherm; magnesia portland cement and others
Corrosion of cement stone is caused by the influence of aggressive gases and liquids on the constituent parts of hardened Portland cement, mainly on Ca (OH) 2 and 3СаО · А12O3 · 6Н2O. There are dozens of substances that can affect the cement stone and be harmful to it. Despite the variety of aggressive substances, the main causes of corrosion can be divided into three types:
1) the decomposition of the components of the cement stone, the dissolution and leaching of calcium hydroxide (leaching);
2) the formation of soluble salts as a result of the interaction of calcium hydroxide and other components of the cement stone with aggressive substances and the leaching of these salts (acid, magnesia corrosion);
3) the formation in the pores of new compounds occupying a larger volume than the initial reaction products; This causes the appearance of internal stresses in the concrete and its cracking (sulfoaluminate corrosion).
Aluminous and other types of cements
Alumina cement is a fast-hardening and high-strength hydraulic binder produced by finely ground clinker containing predominantly low-base calcium aluminates. Alumina cement has high strength only if it hardens at moderate temperatures, not exceeding 25 ° C. Therefore, alumina cement cannot be used for concreting massive structures due to the heating of concrete, as well as subjected to heat and moisture treatment.
Aluminous cement quickly hardens (for 3 days get the brand M400, M500 and M600) with normal setting time. Heat dissipation of aluminous cement during hardening is about 1.5 times more than that of Portland cement. Concrete on high alumina cement is more resistant than portland anti-leaching, as well as in solutions of calcium and magnesium sulphate (in particular, in seawater). However, hardened alumina cement is destroyed in solutions of acids and alkalis, therefore alumina cement can not be mixed with Portland cement and lime.
Taking into account the specific properties and high cost of alumina cement is intended to produce quick-hardening, as well as heat-resistant concrete and mortars. In addition, high alumina cement is used to produce expandable cements.
7.4. Autoclaving binder
Autoclaved binder is a substance capable of forming a dense, durable stone in the conditions of hydrothermal synthesis that occurs during autoclave processing. This group includes: lime-silica, lime-ash, lime-slag binder, nepheline cement, etc. In essence, they can also be attributed to hydraulic binder, since they are also able to maintain strength in wet conditions.
This is the product of the joint or separate grinding of lime and silica-containing component to the specific surface of the mixture of 2000 ... 4000 cm2 / g. Lime is usually used ground quicklime, as it provides more durable and durable products compared to slaked lime. As a silica component, mainly quartz sand is used, containing not less than 75% quartz, as well as other natural and artificial silica materials - tripoli, flask, marshalite, diatomite, slags, coal, peat, and slate ash containing 45% of SiO2 ; ore dressing products containing SiO2 not less than 60%.
On the basis of lime-sand binder silicate concretes are produced. At the same time, the binder itself is produced directly at the enterprises for the production of silicate-concrete products.
Hydrosilicate or synthetic hardening refers to the process of interaction of lime, silica SiO2 and water at a temperature of 174.5 ... 200 ° C with the formation of calcium hydrosilicates, providing high strength to the resulting material:
Ca (OH) 2 + SiO2 + (n-1) H2O → CaOSiO2nH2O
The presented reaction occurs under natural conditions, but very slowly, sometimes over the centuries. But at a temperature of 170 ºС and higher, the reaction takes only 6 ... 12 hours, since silica acquires chemical activity.
In order to heat the product to 170 ºС and not to lose moisture, an overpressure of 0.8 ... 1.2 MPa is necessary, therefore for the implementation of hydrothermal synthesis, steaming of products in autoclaves is used. The autoclave is a horizontal sealed steel cylinder with a diameter of 2 meters or more, up to 30 m long, closed on one or both sides with lids. In the autoclave on special trolleys load products, tightly close the lid and inject water vapor to a predetermined pressure. The role of pressure is reduced only to the preservation of liquid water in the material at high temperatures. At atmospheric pressure, the water would evaporate and completely stop the reactions associated with the formation of cementing substances.
The production of autoclave curing binders is much cheaper than the clinker ones, since the firing temperature and the quantity of the material being baked are much lower. At the same time, the quality of silicate concretes is in no way inferior to cement ones. The cost of silicate concrete products, including large reinforced ones, is usually 20 ... 30% less than the cost of concrete based on cement. Building their qualities are equivalent.
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