Cement

There are various kinds of cement in use.

Ordinary Portland Cement (OPC) is obtained by intimately mixing chalk, marl, etc with other materials containing silica, alumina or iron oxide, burning them at a high temperature, and grinding the resulting product, clinker with gypsum.


Grades

The grade indicates compressive strength of the cement in N/mm&sup2 at 28 days.

Low alkali Portland cement is used where aggregates contain a reactive form of silica, which can result in ‘alkali-aggregate reaction’, causing cracking, and deterioration of the concrete.

Portland pozzolona cement (PPC) is manufactured by grinding Portland cement clinker and a pozzolona like burnt clay, or blending Portland cement and fine pozzolona such as fly ash. Pozzolona used varies between 10 and 25 percent by weight of cement.

Portland blast furnace slag (PBSC) cement is a mixture of Portland cement and ground granulated blast furnace slag. The granulated slag is a non-metallic product obtained from the blast furnace of a steel plant. The slag constituent is no more than 65 percent.

Sulfate-resisting Portland cement (SRPC) counters sulfate attack. Soluble sulfate salts when present in groundwater or soil penetrates the pores of the hardened concrete and chemically reacts.

Low heat Portland cement: The heat liberated due to hydration during hardening is reduced. It is suitable for massive structures, such as dams, bridge abutments, and retaining walls.

Ultra-rapid hardening cement (URHC) develops high strength in a matter of hours that ordinary Portland cements develops in 28 days. URHC is suitable for patch repairs and emergency jobs.

Adding water-repellent chemicals to OPC makes cement hydrophobic. This makes it amenable to storage under humid and damp conditions for prolonged periods without deteriorating or forming lumps.

Oil well cement (OWC) is used while drilling oil wells to fill the space between the steel castings and the wall of the well. It has to perform at large depths under high temperature and pressure.

White cement is used for decorative purposes and also fulfils the requirements of OPC 30. It can be used as a replacement for ordinary Portland cement in structural work. A variety of colours can be obtained by the addition of pigments.

Acid resisting cement has high resistance to most acids, corrosive organic chemicals, mineral oils and grease. It is used for jointing acid-proof bricks and tiles, lining of acid-storage vats and tanks, acid towers, fume stacks, and industrial floorings. Manufacturer’s instructions should be followed when this cement is used. This is not a hydraulic setting variety.

Test for adulteration

A sample of doubtful cement should be burnt for about 20 minutes on a steel plate on a stove. An adulterated sample will change in colour.

Small pats of about 50 x 50 x 20 mm size should be made. If adulterated, the pats can be broken easily with the pressure of fingers the next day.

It is always advisable to send a sample to a laboratory for analysis and tests.