Ordinary Portland cement is one of the most widely used type of Cement. Types, properties, constituents, manufacture, uses and advantages of Ordinary Portland Cement is discussed.
In 1824 Joseph Aspdin gave the name as Portland cement as it has similarity in colour and quality found in Portland stone, which is a white grey limestone in island of Portland, Dorset.
Constituents of Ordinary Portland Cement
The principal raw materials used in the manufacture of Ordinary Portland Cement are:
- Argillaceous or silicates of alumina in the form of clays and shales.
- Calcareous or calcium carbonate, in the form of limestone, chalk and marl which is a mixture of clay and calcium carbonate.
The ingredients are mixed in the proportion of about two parts of calcareous materials to one part of argillaceous materials and then crushed and ground in ball mills in a dry state or mixed in wet state.
The dry powder or the wet slurry is then burnt in a rotary kiln at a temperature between 1400 degree C to 1500 degree C. the clinker obtained from the kiln is first cooled and then passed on to ball mills where gypsum is added and it is ground to the requisite fineness according to the class of product.
The chief chemical constituents of Portland cement are as follows:
|Lime (CaO)||60 to 67%|
|Silica (SiO2)||17 to 25%|
|Alumina (Al2O3)||3 to 8%|
|Iron oxide (Fe2O3)||0.5 to 6%|
|Magnesia (MgO)||0.1 to 4%|
|Sulphur trioxide (SO3)||1 to 3%|
|Soda and/or Potash (Na2O+K2O)||0.5 to 1.3%|
The above constituents forming the raw materials undergo chemical reactions during burning and fusion, and combine to form the following compounds called BOGUE COMPOUNDS.
|Tricalcium silicate (3CaO.SiO2)||C3S|
|Dicalcium silicate (2CaO.SiO2)||C2S|
|Tricalcium aluminate (3CaO.Al2O3)||C3A|
|Tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3)||C4AF|
The proportions of the above four compounds vary in the various Portland cements. Tricalcium silicate and dicalcium silicates contribute most to the eventual strength. Initial setting of Portland cement is due to tricalcium aluminate. Tricalcium silicate hydrates quickly and contributes more to the early strength.
The contribution of dicalcium silicate takes place after 7 days and may continue for up to 1 year. Tricalcium aluminate hydrates quickly, generates much heat and makes only a small contribution to the strength within the first 24 hours. Tetracalcium alumino-ferrite is comparatively inactive.
All the four compounds generate heat when mixed with water, the aluminate generating the maximum heat and the dicalcium silicate generating the minimum. Due to this, tricalcium aluminate is responsible for the most of the undesirable properties of concrete.
Cement having less C3A will have higher ultimate strength, less generation of heat and less cracking. Table below gives the composition and percentage of found compounds for normal and rapid hardening and low heat Portland cement.
Composition and compound content of Portland Cement:
|Portland Cement||Normal||Rapid hardening||Low heat|
|(a) Composition: Percent|
|(b) Compound: Percent|
Properties of Ordinary Portland Cement
Table 2 : Properties of OPC cement
|Initial Setting time||65min|
|Final Setting time||275 min|
|Bulk Density||830-1650 kg/m3|
Manufacture of OPC cement
Majorly there are 5 steps involved in the manufacture of OPC cement,
1. Crushing and grinding of raw material
In the first step of the manufacture of cement, the raw materials are crushed and grinded into small suitable size particles. There are 3 type of manufacture process of cement
- Dry Process
- Wet Process
- Semi wet Process
Crushing and grinding process varies depending upon the type of manufacturing process. For dry process the raw materials are dried up before crushing.
2. Mixing or Blending
In this step, the grinded raw material (lime stone) is blended or mixed with clay in the desired proportion ( limestone :75%, clay : 25%) and mixed well by the help of compressed air to get a homogeneous mix. In the dry process these mixes are stored in silos; slurry tanks are used in the wet process. The resulting material is known as slurry having 35-40% water.
This is the main important step in the manufacture of OPC cement, the product obtained from the mixing is passed into the Kiln by the help of conveyor belts.
Firstly the mix is preheated to 550C ,where all the moisture content is evaporated and clay is broken into silica, aluminium oxide, iron oxide.
In the next zone the temperature is rised to 1500 degree Celsius where the oxides form respective silicate, aluminates & ferrite.
In the final step the product is cooled down to 200C , where the end product obtained in the kiln is known as cement Clinkers, which is in the form of greenish black or grey colored balls.
In this step the cement clinkers and Required amount of gypsum is mixed and grinded into very fine particles which are stored in the silos and later packed in cement bags and distributed.
The Expiry date of OPC cement is normally 3 months.
Types of Ordinary Portland Cement
The differentiation of OPC cement is based on the different codes of different countries.
1. AS per ASTM 150 (American Standards)
- Type I Portland cement is known as common or general-purpose cement. It is generally assumed unless another type is specified.
- Type II provides moderate sulfate resistance, and gives off less heat during hydration.
- Type III has relatively high early strength. This cement is similar to type I, but ground finer.
- Type IV Portland cement is generally known for its low heat of hydration.
- Type V is used where sulfate resistance is important. This cement has a very low (C3A) composition which accounts for its high sulfate resistance.
2. As per EN 197 norm ( European norm)
- CEM I Comprising Portland cement and up to 5% of minor additional constituents.
- CEM II Portland cement and up to 35% of other single constituents
- CEM III Portland cement and higher percentages of blastfurnace slag
- CEM IV Portland cement and up to 55% of pozzolanic constituents
- CEM V Portland cement, blastfurnace slag or fly ash and pozzolana
3. As per CSA A3000-08 ( Canadian standards)
- GU, GUL > General use cement
- MS > Moderate sulphate resistant cement
- MH, MHL > Moderate heat cement
- HE, HEL > High early strength cement
- LH, LHL > Low heat cement
- HS > High sulphate resistant; generally develops strength less rapidly than the other types.
Uses of Ordinary Portland Cement
- It is used for general construction purposes where special properties are not required such as reinforced concrete buildings, bridges, pavements, and where soil conditions are normal.
- Used for most of concrete masonry units
Advantages of Ordinary Portland Cement
- It has great resistance to cracking and shrinkage but has less resistance to chemical attacks.
- Initial setting time of OPC is faster than PPC so it is recommended in projects where props are to be removed early.
- Curing period of OPC is less than PPC and curing cost reduces. Hence recommended where curing cost prohibitive.Disadvantages
Disadvantages of Ordinary Portland Cement
- It cannot be used for mass concreting as it has high heat of hydration as compared to PPC.
- The durability of concrete made using OPC is less than that of the concrete made using PPC.
- It produces comparatively less cohesive concrete than PPC, hence concrete pumping becomes a little difficult.
- OPC has lower fineness, hence has higher permeability and as a result it has lower durability.
- OPC is costlier than PPC.