CEMENT - A SMALL GUIDE

TYPES OF CEMENT

There are various types of cement in use and as per IS 456:2000, Indian Standard code of practice for plain and reinforced concrete permits the use of 10 different types of cement.

 PORTLAND CEMENT

Portland cement is a hydraulic binder and a finely ground inorganic material. When mixed with water, it forms a paste which sets and hardens by means of hydraulic reactions.

ORDINARY PORTLAND CEMENT

Ordinary Portland cement is a product obtained by intimately mixing together calcareous (limestone, chalk, etc) and argillaceous (clay, shale,etc) materials, with or without other materials containing silica, alumina, or iron oxide, burning them at a high temperature, and grinding the resulting intermediate product, clinker with gypsum. After burning, no material other than gypsum is added.

Grades of Ordinary Portland Cement (OPC)

The Bureau of Indian Standards has classified OPC into three grades for producing different grades of concrete to meet the demands of the construction industry. The classification is made on the basis of Compressive strength at 28 days as:

33 grade – Ordinary Portland Cement

43 grade – Ordinary Portland Cement

53 grade – Ordinary Portland Cement

The grade indicates compressive strength of the cement in N/mm2 at 28 days. Since higher grades of concrete necessitate the use of higher strength of cement at 28 days, use of 33 grade cement has dropped during the last decade.

Both 43 grade and 53 grade cement can be used for producing higher grades of concrete.

Physical and Chemical properties of various grades of Ordinary Portland cement

BLENDED CEMENTS

Blended cements or composite cements are those cements in which a mineral additive has been added to Portland cement. Blended cement is a hydraulic cementitious product, similar to ordinary Portland cement, but due to the addition of blending material it has certain improved properties compared to OPC.

Portland Pozzolana Cement (PPC)

Portland Pozzolana Cement (PPC) is manufactured either by grinding intimately together Portland cement clinker, gypsum and a pozzolana such as fly ash, or by intimately and uniformly blending Portland cement and fine pozzolana. The BIS has differentiated PPC based on the pozzolana added to the mix. Thus IS 1489 (part 1) is Portland pozzolana cement (fly ash based).According to the latest amendment in July 2000, the proportion of fly ash as a pozzolana used can vary between 15 and 35 percent by weight of cement, as stipulated by IS 1489 (part 1) 1991.

Increased impermeability, lower heat of hydration, lower plastic shrinkage, reduced alkali-aggregate expansion and improved resistance to aggressive chemical agents and corrosion are some of the major benefits to be derived from the use of PPC. The use of PPC is, thus desirable for enhancing durability in different construction jobs, specially for structures subjected to aggressive environments. In mass concrete construction, PPC concretes have shown rather

better behavior in respect of cracking than OPC concretes because of lower heat of hydration.

Portland Blast furnace Slag Cement (PBSC)

Portland blast furnace slag cement is an intimately ground mixture of Portland cement clinker and granulated blast furnace slag, either inter ground or ground separately and blended together. The granulated blast furnace slag is a non-metallic product obtained by rapidly chilling or quenching in water the molten tapped from the blast furnace of a steel plant. As per the latest amendment to IS 455 in May 2000, the slag constituent should not be less than 25 percent nor more than 70 percent of the Portland cement. PBSC generally has higher fineness, lower heat of

hydration, lower permeability and better resistance to chemical attack and corrosion than OPC.

Portland slag Cement can be used for all construction jobs in place of ordinary Portland cement, but its special properties render its adoption highly desirable fro marine structures involving large masses of concrete such as dams, retaining walls, and bridge abutments, and for structures exposed to sulphate bearing soils such as foundations and roads.

Benefits of Blended Cements

The use of blended cement improves the properties of both, fresh and hardened concrete. These can be as a result of the extended hydration of the cement-pozzolana mixture, reduced water demand, and due to the improved cohesion of the paste. Another important benefit is the improvement in durability resulting from the lower permeability and improved microstructure of the concrete. This arises from the reduction in pore size of the cement paste and the refinement of pore structure of the cement paste as well as improvements in the properties of the

‘interfacial zone’ between he cement paste and the aggregate/inerts.

Physical and Chemical properties of blended cement (PPC and PBSC)

# - x + (4.0(100-x)/x) where x is the declared percentage of pozzolana in PPC.

4. STORAGE OF CEMENT

Since cement is a very finely ground hygroscopic material: i.e. it readily absorbs moisture every precaution should be taken to ensure that the cement is kept free from contact with moisture in any form. The storage shed should have a pucca floor raised at least 150mm above ground level, and it should be provided with air tight doors and windows. It is a good practice that cement is moved in and out of the godowns in the “first-in-first-out” method. The drainage systems on the roof and around the godown should be well maintained, specially during the monsoon months.

At site, the cement bags should be kept on a raised platform and covered with a tarpaulin.

Cement stored for a long time tends to deteriorate and an indicative rate of its deterioration is given.

Possible reduction in strength of concrete made with stored cement

PERIOD OF STORAGE OF CEMENT        -         MINIMUM EXPECTED REDUCTIONS IN STRENGTH

                                                                                                          AT 28 DAYS (%)

                 Fresh                                                -                                            0

                3 months                                        -                                           20

                6 months                                        -                                           30

                1 year                                               -                                           40

                2 years                                             -                                           50

TEST FOR ADULTERATION

A sample of doubtful cement should be burnt for about 20 minutes on a steel plate heated on a stove. An adulterated sample will change in colour; unadulterated cement, on the other hand, will remain unchanged.

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

It is, however, always advisable to send a sample to a laboratory for analysis and tests whenever there is doubt regarding the quality of cement.