9.7. Varieties of cements based on Portland clinker
The different exposure conditions to which concretes and mortars are subjected in service in various environmental and constructional situations has necessitated the production of a wide assortment of cement types based on Portland cement clinker. Greater parts of the volume output of Portland cement in the world contain mineral additives. The use of different mineral additives results in the economy of the most expensive and power-consuming intermediate product – Portland clinker and utilization of different wastes. These cements are more water and corrosion-resistant, than the cement without the additives. Portland cement without additives is used for the production of ultra-strength, frost-resisting concretes and in a number of other cement types.
Rapid hardening cement and high-strength cements. The clinker of rapid-hardening cement contains an enhanced amount (60-65%) of the most active minerals - the tricalcium silicate (C3S) and tricalcium aluminate (C3A). This type of cement is milled to a much higher fineness, and higher specific surface area (3500 cm2/g). Increasing the C3S content of the cement clinker to 60-65% and specific surface area to 4000 cm2/g or more produces cements with high early-compressive strength of 25-30 MPa at 1 days and 80-90 MPa at 28 days. However, their production involves a sharp growth of the power inputs, whilst the productivity of equipment become reduced.
It is rational to use high-early-strength cement and high strength cements for the production of precast concrete, which provides rapid growth of the strength of products and declining of binder content per 1 m3 of concrete.
For rapid-hardening Portland cement (Fig 9.8.) an additional requirement is achievement in 2 days of hardening of the compressive strength not less than 15-25 MPa.
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number of new methods of hardening acceleration and increase of
cement activity has been developed within the last few years.
The technology has also been developed for extra-rapid hardening cement that contains fluorine. Limestone, slags of the meltback of aluminium, calcium fluoride and special additive for retaining fluorine in a clinker are used as raw materials. Cement, modified by fluorine, achieves compressive strength of 5-8 MPa after 6 hours of normal hardening. The use of fluoride cement allows the production of concrete products having the required strength after about 1 hour of thermal treatment.
The production of sulphoaluminate clinkers by adding over 5% of sulfates to the raw meals is the future direction for the production of rapid-hardening binders. The calcium sulphoaluminates produced, gives the clinker a capacity for rapid hardening, and high hydraulicity. The phosphogypsum and other gypsum wastes can be the source of sulfates for the production of sulphoaluminate clinker.
Extra-rapid-hardening cement is produced by the milling of a clinker with special composition (sulphoaluminate belite) and gypsum. A strength of 7-15 MPa may be reached in about 6 hours after tempering at a temperature of 20°C. This enables its use for producing precast concretes and structures without thermal- moisture treatment. This type of cement is characterized by a short setting time( – initial set is not earlier than 5 minutes, and final set of not later than 1 hour. Concrete made with this type of cement needs to be applied immediately after mixing.
Technology of binders of low water demand is based on fine milling of Portland cement with addition of mineral additives or without them and enhanced dosages of dry superplasticizer. Depending on the content of mineral additives these binders are divided into different types. The use of these binders allow to produce concretes with the strength to 100 MPa and higher. At the same time the production of binders of low water demand is related with certain difficulties, including the necessity of the dry a superplasticizer using, etc. Method, based on making of fine multi-component cements with different mineral additives and specific surface area 4500-5000 cm2/g and adding of liquid superplasticizers at production of concrete mixes, is more simple.
High-early-strength and sulfate-resistant blastfurnace cements are also produced along with the ordinary one. Clinker, which contains not more than 8% C3A and slag which contains not more than 8% Al2O3 is used for its manufacture.
The quality of blastfurnace slag is characterized, mainly by the value of the coefficient of quality which represents the ratio between the total percentage of oxides CaO, MgO, Al2O3 and the content of SiO2 and Fe2O3.
Blast furnace cements, has a higher firmness in soft and mineralized waters, and enhanced heat-resistance compared to Portland cement. This can be explained by the denser microstructure of the former due to reactions involving the silica (and/or alumina) and calcium hydroxide to form extra hydration products; and through thermo-gravimetric analysis, by the insignificant content of free hydroxide of calcium in the cement stone. It hardens more intensively at thermal-moisture treatment, more slowly at a reduced temperature and has a lower freeze-thaw resistance. The compressive strength of blast-furnace cement is usually 30-50 MPa at 28 days.
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Blast furnace cements are economically effective types of cements. The Power consumption at blast-furnace cement production on average to 25% is lower than energy consumption at production of Portland cement.
Sulfate-resistant cements. The sulfate-resistant Portland cement and Portland cement with mineral additives, pozzolanic Portland cement and sulfate-resistant blastfurnace cement belong to this group of cements.
Sulfate-resistant Portland cement differs from the ordinary Portland cement because it has an enhanced resistance to sulfate aggression in the conditions of systematic alternate freezing and thawing or moistening and drying. The main feature of sulfate-resistant Portland cement is that it has a mineralogical composition with low content of tricalcium aluminate (C3A). The mineralogical composition of such clinker must comply with three conditions: 1) content of C3A is not more than 5%; 2) content of C3S is not more than 50%; 3) sum of C3A+C4AF is not more than 22%.
Next to sulfate-resistant Portland cement, to which it is not allowed to add mineral additives, industry produces sulfate-resistant Portland cement with mineral additives. These may be granulated blast furnace and electrothermophosphoric slags at content of Al2O3 not more than 8 %.
The content of C3S in the clinker of sulfate-resistant Portland cement with mineral additives is not specified. The compressive strength of this cement at 28 days is 40-50 MPa. The sulfate-resistance of cements, as well as a range of other properties, are improved by addition of surface active admixtures.
Pozzolanic Portland cement is a hydraulic binding agent, which is produced by inter-grinding cement clinker with acid active mineral (hydraulic) additive and gypsum.
The presence of active mineral additives in pozzolanic cement as well as in Portland blast-furnace cement promotes water- and sulfate-resistance. The limitation of tricalcium aluminate (C3A < 8%) is also desirable in pozzolanic Portland cement. The low density of pozzolanic Portland cement (2.7-2.9 g/cm3) results in an enhanced volume of cement paste from it. This provides a more compact microstructure of the cement stone and ensures higher water impermeability of concretes, based on pozzolanic Portland cement.
Pozzolanic Portland cement at ordinary and especially at low temperatures hardens slowly. The strength development of this cement is especially slowed during hardening in air conditions. Adding some mineral additives (diatomites, tripoli powders and others like that) with high specific surface area and fineness to cement, increases the water demand and accordingly the cementitious content of concrete mixes. Some concretes and mortars, based on pozzolanic Portland cement, are not frost-resistant. Large shrinkage and low resistance to alternate moistening and drying are possible imperfections also observed in such cements.
It is most rational to use pozzolanic Portland cement for underground and underwater structures on account of their enhanced water impermeability and resistance both in soft and in mineralized waters. It is possible to use this type of cement for ordinary structures, which are exposed to conditions of high humidity, but not subject to freezing and thawing.
Masonry cements are made by inter-grinding Portland clinker, active mineral additives and filling agents. The content of clinker must be not less than 20% of the weight of the binder. Ash, quartz sand, limestone and others like that are used as fine milled additives. Such cements have a compressive strength of 5-20 MPa at 28 days. Due to the slow hardening characteristics, these cements are used at air temperatures not below 10°C for making of masonry mortars and plasters, and also for concrete of low strength, to which frost-resistance is not a requirement.
Cements with mixtures of surface-active agents. Surface active agents can be added in an amount not more than 0.3% by weight in different types of Portland cements at grinding. During the process of cement grinding, surface active agents are adsorbed on its grains and give to it a number of new properties depending on whether the surface agent added is a hydrophilic or hydrophobic agents.
Hydrophilic agents are absorbed on the surface of cement grains and form hydrophilic films which are instrumental in the better moistening of the particles with water, diminishing of their tripping and increasing the plasticity of cement paste. Plasticized Portland cement is produced by adding hydrophilic agents, in an amount 0.15-0.3% during the process of grinding.
Plasticity of cements is determined by the degree of cone spread of a cement-sandy mortar on a shaking table. The mortar from a mix, based on plastificised portland cement with standard sand composition of 1:3 (by weight) and a water-cement ratio (W/C) of 0.4 after a 30 shakings on a shaking table has a cone spread of not less than 135 mm. This index is 106-115 mm for not plasticized Portland cement
The use of plasticized Portland cement allows the production of concrete mix of better plasticity and facilitates its compaction and placing without a corresponding increase of water content. It can also be used to reduce the cement content by 8-12%, with unchanging plasticity and water-cement ratio. By keeping the cement content and required plasticity constant, the water-to-cement ratio of concrete mix diminishes, which results in the increase of strength, frost-resistance and water impermeability of concrete.
Hydrophobic Portland cement is produced by adding hydrophbic agents to the cement composition during the process of grinding. These agents form the thinnest adsorption layers - unwettable membranes on grains of cement such that a drop of water on the surface of hydrophobic cements must not be sucked for 5 min.
Hydrophobic property of cement stipulates has as its main advantage – a capacity to protracted storage. Hydrophobic cements do not reduce activity during 1-2 years in unfavorable conditions; compared to ordinary cements which may lose up to 30% of their initial strength within 3-6 months. The hydrophobic films are destroyed during the mixing with water of these cements that provides the normal flowing of processes of hydration and hardening.
The cement hydrophobization is instrumental in the increase of density and improvement of cement stone structure, which allows attainment of greater frost-resistance and water impermeability of concretes, their firmness in corrosive environments, to decrease the leaching in the plasters and finishing mortars and others like that. The adsorption action of hydrophobic admixtures is instrumental in the increase of plasticity of concretes and mortars.
Surface active agents can slightly slow down the strength growth in the first periods of hardening.
Extended and straining cements. The shrinkage which causes stretching tensions in a concrete even to formation of cracks is characteristic of ordinary cements during hardening in air conditions. Special cements which increase their volume during hardening are called extended cements. Expansion of cements during early age hardening, compensates the negative influence of shrinking deformations in the future. Extended cements as a rule are composite and consist of basic binding agent and additive which cause expansion. Expansion of cement stone takes place, as a rule, due to the chemical interaction of components of additive and formation of calcium hydrosulphoaluminates and also Ca(OH)2 and Mg(OH)2.
Straining cements are the variety of extended cements. They have energy of expansion, sufficient for armature tensioning in concrete constructions. They are divided into cements with small (1-2 MPa), middle (4 MPa) and high (6 MPa) energy of expansion. Cements which are strained are produced both for the conditions of thermal treatment and for the normal hardening. Application of these cements allows to produce effective prestressed structures. Their use allows lower expenses on reinforcement by up to 30-50%.
Self-stressing Portland cement is produced by fine milling of 65-70% Portland clinker, 16-20% high-aluminous slag and 14-16% of the gypsum. Relative linear expansion equals 3-4%.
Technology of straining cement is developed on the basis of sulphoaluminate clinker which contains the sulphoaluminate calcium as a basic mineral. Such clinker is produced by the burning of kaolin or ash mixed with limestone and gypsum.
White Portland cement is produced by compatible milling of white low ferrous clinker, mineral additives and gypsum.
Coloured Portland cements are produced by the adding of coloring agents in the white cements.
Apart from the cements considered above on the basis of Portland clinker, there are a range of other varieties: oil-well, air-entraining cements etc, not covered here.