What is cement and how it is made?
Cement is a substance which is applied to the surface of solid bodies to make them adhere firmly or more specifically a powdered substance which is made plastic with water is used in a soft and pasty stage. On hardening or drying it binds together with bricks, stones, etc., in construction. Portland cement is a calcined material comprising lime and silicates, which is mixed with sand and stone and upon hydration forms a plastic material which then sets and hardens to a rock like material called concrete.
Portland cement is manufactured in a series of process. Limestone (Calcium Carbonate) and other material containing appropriate proportions of Calcium, silicon, aluminium and iron oxides are crushed and milled to a fine flour like stuff called “Raw Meal“. This is preheated in a pre heater or in A rotary kiln to dis-associate calcium carbonate to active calcium oxide with The evolution of carbon dioxide and then to react calcium oxides with Other component in a Rotary Kiln to form calcium silicates and aluminates Which partially fuse at material burning temp up to 1400oC. The Reaction Product leave the kiln as a dark grayish nodules called “ Clinker “ The Clinker is finally inter ground with a small proportion of Gypsum to control The rate of hydration and the fine product is cement. |
Why is Gypsum added with Clinker in the production of cement?
Gypsum is added to the cement production process mainly for the purpose of regulating its setting time. It prevents flash setting and makes the concrete workable for hours. In addition, it also influences other cement properties like grindability, sensitivity to storage, volume stability and development of strength. |
What is setting time?
It is the time required to change the stage of cement paste from fluid to a rigid one. |
What is the false set of cement paste?
False set is used to describe the premature stiffening of the cement paste which at times occur within 10-15 minutes of mixing with water. This is caused by dehydration of Gypsum due to high temp during the cement grinding process. However, the plasticity of the paste is regained on remixing. |
What is flash set of cement paste?
This refers to rigidity of the cement paste which can occur within 5-10 minutes of mixing with water and is referred to as irreversible rigidity. This is caused by the formation of aluminate hydrate due to high content of
C3A and / or a small amount of easily soluble calcium sulphate. Cement produced with this behavior is unsuitable to use. |
What is consistency of cement paste?
Consistency of cement paste is the amount of water required for preparing a workable cement paste. |
What is Heat of Hydration in cement paste?
When cement is mixed with water, calcium silicates react with the cement producing a gel like calcium silicates hydrate and Calcium hydroxide is called “Hydration of cement paste". This hydration Reaction is an exothermic reaction which means heat is liberated. This is called heat of hydration in cement paste. The quantity of heat liberated in quite Appreciable and due consideration to be taken while making mass concrete construction. During cooling of the concrete after hydration reaction, cracks can develop due to thermal gradients if enough design consideration are not taken care. |
What is Soundness in cement?
Soundness provides an index of potential delayed expansion caused by Hydration of Calcium Oxide, or Magnesium Oxide, or both when present in Portland Cement. This, in turn, provides information on the volume stability of concrete when Portland Cement is used. |
What is Shrinkage in cement?
The term ‘drying shrinkage’ can be defined as the decrease in length of the cement test specimen, where the decrease is caused by any factor other than externally applied forces under stated environmental conditions. The terms include the net effect of a variety of phenomena, including hydration of the cement, tending to bring about both increase and decrease in length during the period in which the cement test specimens are stored under stated environmental conditions. |
What is Air Content of cement?
This indicates the volume of air (and other gases, if any) in a freshly mixed cement mortar, expressed as a percentage of total volume of the mortar. The test for air content is essential to assess the cement for its requirement with respect to air-entraining requirement. |
What is Compressive strength?
Compressive strength is the usual primary requirement of good concrete in it’s hardened state and this is an overall measure for the quality of cement and concrete. In order to ascertain the compressive strength of cement and concrete, a predetermined ratio of cement, aggregate and water are thoroughly mixed and specimens are cast. These specimens are cured and stored in specified environmental conditions prior to testing it’s compressive strength at various ages like three days, seven days, 28 days, etc., Many of the desirable properties of concrete like durability, impermeability, abrasion resistance, etc., are highly influenced by compressive strength. Water cement ratio, workability and maximum size of aggregate also affects the compressive strength of the concrete. |
What is Tensile strength?
The term is used to define the maximum tensile stress measured in force per unit cross sectional area that a cement mortar specimen can resist before it ruptures. Concrete, a mixture of loosely bonded sand and gravel, has a low tensile strength because of it’s coarse structure and hence must be reinforced to withstand the tensile stresses encountered in structure. |
What is Flexural Strength?
Generally compressive strength is used as an overall measure for the quality of cement and concrete. However, in specific applications like roads and runways flexural strength is more important. The flexural strength of cement mortar / concrete is determined by subjecting a plain prison / beam to flexure under transverse loads. Flexural strength is more sensitive to inadequate curing and type of aggregate since the effects of non-uniform shrinkage adversely affects flexural strength. |
What is Impermeability?
Impermeability plays a primary role on the durability of concrete, which in turn depends on the water cement ratio. In the air entrained concrete, minute air bubbles are trapped inside, which improves the workability of the mix and also function as minute safety values by providing room for the free water in hardened concrete to escape. |
Why there are so many types of special cements?
Special cements are usually developed and produced to meet performance and durability requirement in particular.
- Improved strength development.
- Increased resistance to chemical attack.
- Improved compatiability with reactive – aggregates.
- Suitability for use at elevated temperature and pressures.
- Suitability for use in special application
- Applicability in architectural purposes.
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What is Sulphate Resisting Cements?
Sulphate can be found in natural and industrial wastes as well as in soils. Soluble sulphates can react with lime and aluminates present in the hardened Concrete and form respectively gypsum and ettringite. Both reaction involves expansion and consequent concrete deterioration. Sulphate Resisting Cement are characterised by a low C3A content to minimise the risk of ettringite formation. |
What is Low Heat Cements?
Hydration reaction of cement develop heat. When cement is used in the production of mass concrete, temperature gradients generate between the core and the surface of the mass, causing strains and may eventually lead to cracking. Low heat cements have a reduced heat of hydration obtained by altering the chemical composition to have low C3S and low C3A contents. This type of cement is recommended for use for mass concrete production or for large structural sections. |
What is Leaching Resistant Cements?
Water with low salinity or a high content of carbon dioxide are capable to dissolve hydration lime presence in concrete and can damage the structure. Leaching Resistant cement are produced with low C3S content which has a low development of calcium hydroxide or lime. Their use is suggested in hydraulic works like basins, river barriers and sides etc. |
What is Low Alkali Cement?
Some aggregates may contain forms of reactive amorphous silica. In the presence of water, this can react with the soluble alkali content of cement and form locally an expansive gel that can deteriorate the concrete. In such case cements are produced to have lower content of K2O and Na2O or to add some special pozzolan or slag material which can immediately react with alkali and prevent subsequent reaction with aggregates in the concrete. |
What is Seawater-Resisting Cement?
Deterioration of the concrete which is in contact with sea water takes place due to chemical attack by MgSO4, mechanical stresses by tidal waves, Crystallization pressure due to deposition of salts in the wind and water line. To absorb all these factors, sea water resisting cements are produced with moderate C3A content or as blended cements. |
What is Blended Cement?
In blended cement production, some portion of the clinker component is replaced with a suitable pozzolan or a slag or limestone or fly ash etc. The use of these mineral will improve the durability and the pore structure of the hardened concrete by lowering its lime content and porosity as a consequence of the enhanced development of calcium silicate hydrates. |
What is High Early Strength and Rapid Hardening Cement?
The requirement of these types of cement arise when a rapid strength development is required like if form work has to be removed or reused after a short time or when sufficient strength is required for further construction in slip forming. In all such cases the cement used should develop the required strength as soon as possible. These types of cement are made by grinding of clinker with more fines or by altering the quality of clinker with high C3S and C3A content. Since rapid strength gain is usually associated to high rate of heat development, this kind of cement should not be used in mass concrete or in large structural sections. |
What is Oil Well Cement?
Oil Well cement are developed for use in oil and gas wells and are designed to set at high temperatures and pressures in well grouting. They can also be used for sealing water wells, waste disposal pits and geo-thermal wells. In order to meet the above requirements, oil well cements are made with special composition or additives enabling then have low permeability, good bond between rock and casing, to withstand and set under high temperature and pressure and to protect the casing against corrosion and collapse. |
What is White Cement?
As the name suggests the color of this cement is white and the mechanical properties are comparable to those of gray cement. It is produced with suitable selection of raw materials in which coloring elements like Iron, Chromium and manganese must be kept at the lowest possible level. In order to ease the clinker burning, floride is used as a mineraliser. This kind of cement is used mainly for architectural purposes in white or colored concrete. |
What is Masonry Cement?
The Masonry Cements are those, which have low early and late strength, low shrinkage, limited water permeability, high water retention, excellent plasticity and cohesiveness in the fresh stage and better workability. This cement is produced by a low to medium clinker content and the use of limestone or other mineral additions and use of air entraining agents. |
What is Free Lime in Cement?
During clinker formation, all the uncombined lime released from the limestone gets combined with silica, iron and alumina to form silicates, ferrite and aluminate phases. The lime left uncombined is known as free lime and it is responsible for unsoundness in cement if present in excess. |
Quality of Cement
The quality of cement is judged by its performance in combination with water as a binder in a material and it includes properties like setting behavior, strength development, heat development, volume stability and durability. It is very essential to ensure that the contribution to these properties of the cement is kept at a certain level and with variations as small as possible to meet the demand expressed in the standard specifications and to comply with the wishes and needs of the market.
These factors are associated with the characteristics of the materials being used for the cement viz. Clinker, Gypsum and Additives, and how they are altered during grinding and storage. |