Ordinary Portland cement is one of the most widely used type of Cement. 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. In this article we discuss about manufacture,constituents, properties, types , uses,advantages, disadvantages of Ordinary Portland cement.
Constituents and Properties of Ordinary Portland Cement
The constituents of ordinary portland cement are ,
Calcium oxide, CaO – 61–67%
Silicon dioxide, SiO2 – 19–23%
Aluminium oxide, Al2O3 – 2.5–6%
Ferric oxide, Fe2O3 – 0–6%
Sulfur (VI) oxide, SO3 – 1.5–4.5%
The properties of ordinary portland cement are,
Specific Gravity 3.12
Normal Consistency 29%
Initial Setting time 65min
Final Setting time 275 min
Fineness 330 kg/m2
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 or Advantages
- 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
- 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.
- 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.