PORTLAND CEMENT

Portland cement was patented by Joseph Aspdin in 1824 and was named after lime stone cliffs on the Isle of Portland in England. 


Manufacture of Portland Cement
How to make cement?
With this video can help you more understand the general making process of cement. 

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Video 4 : From Discovery 

    Production of portland cement is produce from calcium carbonate in the form of crushed limestone or chalk and an argillaceous material such as clay, marl or shale. 
    Minor constituents such as iron oxide, or sand may be added depending upon the composition of the raw materials and the exact product required then are passed through a grinding mill, using either wet process or dry process. 




Wet Process

Figure 1 : Wet Process

The wet process, which was the precursor to the developments, is still used in some areas for processing chalk and marl clay. Clay is mixed with water to form slurry, when any excess sand is removed by settlement. Equivalent slurry is prepared from the chalk, which is then blended with the clay slurry, screened to remove any coarse material, and stored in fed into the top of large, slowly rotating kilns. The kilns, which are refractory brick-lined steel cylinders up to 200m long, are fired to approximately 1450^’oC, usually with pulverized coal. The slurry is dried, calcined and finally sintered to hard grey/ black lumps of cement clinker.

Dry Process

Figure 2: Dry Process

In the dry process limestone, shale and sand are milled to fine powders, and then blended to produce the dry meal, which is stored in soils. The meal is passed through a series of cyclones, initially using recovered kiln gases to preheat it to 750^oC, then with added fuel to precalcine at 900^oC, prior to passage into a fast-rotating 60m kiln for clinkering 1450^oC. In all processes an intimately mixed feedstock to the kiln is essential for maintaining quality control of the product. Most plants operate primarily with powdered coal, waste type chips, smokeless fuel plant residues, or reclaimed spoil- heap coal are used when available. Oil, natural gas and landfill gas have also been used when economically viable. The grey/ black clinker manufactured by all process is cooled with full heat recovery and ground up with 5% added gypsum retarder to prevent excessively rapid flash setting of the cement.

Hydration of Portland Cement
Hydration is the process of chemical combination between the cement and water. After that will result first in setting. and then hardening. The process are require the presences of water. The hydration rate depend on the surface area of clinker exposed. The heat is given off during the setting and early hardening cement paste.

Initial period after mixing	Between 2 – 4 hrs after mixing	Between 3-10  hrs after mixing	Between 7-28 days after mixing	28 days till many months
fluidity of cement and water paste constant	initial set – paste starts to stiffen but no strength	final set – paste starts to harden and gain strength	rate of strength gain rapid	hardened cement continues to gain strength but at a slower rate

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Only a small proportion of the added water is actually required for the chemical hydration of the cementitious constituents to hydrated calcium silicates. The additional water is needed to ensure the work-ability of the mix when aggregates are added, so that concrete, for example, can be successfully placed within formwork containing steel reinforcement. Water in excess of that required for hydration will ultimately evaporate leaving capillary pores in the concrete and mortar products. 
For future understanding about hydration can refer to the following video:

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Water Cement Ratio

Cement only need a small quantity of water to hydrate. Additional water will evaporate and leaves voids which can reduce the strength and durability of harded cement.

Types of Portland Cement.

There have different types of portland cement are produce to meet the different requirement. Which are Ordinary portland cement, Rapid-hardening portland cement, Ultra -high early strength Portland cement, Low-heat portland cement, Sulphate resisting portland cement, White portland cement, Portland blast-furnance cement and Pozzolanic cement.   

Types of Portland Cement	Description
Ordinary Portland Cement	·      Widely used and least expensive ·      Suitable for normal purpose
Rapid-Hardening Portland Cement	Achieve by having: ·      Higher C₃S and lower C₂S content in the cement ·      Grinding the cement clinkers to finer particles
Ultra-high early strength Portland Cement	·      Cement clinker are ground to very fine particles 1.       In 16 hours after mixing can achieve 3 days strength 2.       24 hours can achieve 7 days strength
Low-heat Portland Cement	·      Required for thick concrete work where the heat generated by ordinary cement would be excessive and lead to serious cracking.
Sulphate resisting Portland Cement	·      Tricalcium aluminate content is reduced 1.       Better to resist chemical attack arising in wet condition from sulphate industry waste 2.       good in dense concrete 3.       good quality in ground water which have the high concentration of sulphure trioxide 4.       suitable use in sewage system, industrial waste
White Portland Cement	·     Manufacture from white china clay and white limestone ·     More expensive compare to normal Portland cement
Portland blast-furnace Cement	·     The first 28 day is the hardening process of this cement ·      So less heat is involved ·      Not suitable for low curing temperatures ·      Strength of mature concrete same as the ordinary Portland cement
Pozzolanic Cement	·      Made from pulverized fuel ash ordinary Portland cement ·      Slower in hardening ·      Good resistance to sea water