Types of cement

There are many types of cement in the market to suit every need. Some of them which are included in the revised IS : 456-2000 are as follows: • Ordinary Portland Cement 33, 43, 53 grade (OPC), 53-S (Sleeper Cement) • Portland Pozzolana Cement (PPC), both Fly Ash and Calcined Clay based • Rapid Hardening Portland Cement • Portland Slag Cement (PSC) • Sulphate Resisting Portland Cement (SRC) • Low Heat Portland Cement • Hydrophobic Cement Even though only Ordinary Portland Cement is graded according to strength, the other cements too have to gain a particular strength. 33, 43 and 53 grade in OPC indicates the compressive strength of cement after 28 days when tested as per IS: 4031-1988, eg, 33 Grade means that 28 days of compressive strength is not less than 33 N/mm2 (MPa) . Similarly for 43 grade and 53 grade the 28 days compressive strength should not be less than 43 and 53 MPa respectively. 43 and 53 grade are also being introduced in PPC and PSC shortly by the Bureau of Indian Standards (BIS)

Portland Pozzolana Cement (PPC)

PPC is manufactured by inter grinding OPC clinker with 15-35% of pozzolanic material. Pozzolanas are essentially siliceous or aluminous material, which in itself possesses no cementitious properties, which will be in finely divided form and in the presence of moisture react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties. The pozzolanic materials generally used are fly ash or calcined clay. PPC produces less heat of hydration and offers greater resistance to attack of aggressive environment, gives long-term strength and enhances the durability of structures.

Portland Slag Cement (PSC)

PSC is obtained by mixing blast furnace slag, cement clinker and gypsum and grinding them together to get intimately mixed cement. The quantity of slag varies from 30-70%. The gain of strength of PSC is somewhat slower than OPC. Both PPC and PSC will give more strength than that of OPC at the end of 12 months. PPC and PSC can be used in all situations where OPC is used, but are preferred in mass construction where lower heat of hydration is advantageous or in marine situations and structures near seacoast or in general for any structure where extra durability is desired. The other types of cement listed above are for special purpose and their nomenclature indicates the purpose for which they are preferred. The following table gives the codal requirements at a glance.

Important requirements met by various types of Birla UltraTech Cement

S no Type of cement IS Code Fineness

m2/kg (min)

Setting Time in minutes Soundness Compressive Strength in MPa

Initial (min.)

Final (max.)

Le Chatelier (mm)

Auto Clave (%) 3 days 7 days 28 days

1 OPC 33 269 : 1989 225 30 600 10 0.8 16 22 33

2 OPC 43 8112 : 1989 225 30 600 10 0.8 23 33 43

3 OPC 53 12269 : 1987 225 30 600 10 0.8 27 37 53

4 PPC (flyash based)

1489 : 1991 (Part

1) 300 30 600 10 0.8 16 22 33

5 PSC (slag) 455 : 2002 225 30 600 10 0.8 16 22 33

6 SRC 12330 : 1988 225 30 600 10 0.8 10 16 33

7 White Cement 8042 : 1989 225 30 600 10 0.8 19.8 29.7

8 RHC (Rapid Hardening Cement)

8041 : 1990 325 30 600 10 0.8 27 16 @ 1 day

Selection of cement

The type of cement selected should be appropriate for the intended use. Functional requirement of the structure, design parameters, speed of construction, durability characteristics, environmental condition are some of the major factors which affect selection of cement for appropriate application. The suggested uses of various cements are given in the following table.

S no Type of cement End Use 1 OPC 33 Used for general civil construction works under normal/mild environmental conditions. OPC 33 grade is

normally not used where high grade concrete is required due to limitations of its strength. Nowadays this variety is not generally produced

2 OPC 43 Nowadays 43 grade is being used widely for general construction work. However, 43 grade OPC is gradually being replaced by blended cements.

3 OPC 53 Used in RCC and pre-stressed concrete of higher grades, cement grouts, instant plugging mortars etc. where initial higher strength is the criteria

4 Portland Slag Cement (PSC)

PSC provides better protection against chloride and sulphate attack. PSC is preferred over OPC for usage in constructions where the structures are susceptible to sulphate and chloride attack. eg Marine structure, structures near the sea, sewage disposal treatment works, water treatment plants, etc

5 Portland Pozzolana Cement (PPC)

PPC makes concrete more impermeable, denser as compared to OPC. The long-term strength of PPC is higher compared to OPC. PPC produces less heat of hydration and offers greater resistance to the attack of aggressive waters than normal OPC. PPC can be used for all types of construction.

6 Sulphate Resisting Portland Cement (SRC)

IIn SRC, amount of C3A is restricted to lower than 5% and 2C3A + C4AF lower than 25%. SRC can be used for Foundation, Piles, Basements, Underground structures, sewage and water treatment plants and coastal works, where Sulphate attack due to water or soil is anticipated

7 Low Heat Portland Cement

Low heat Portland cement is particularly suitable for making mass concrete for dams and many other types of water retaining structures, bridge abutments, massive retaining walls, piers and slabs, etc

8 Rapid Hardening Cement (RHC)

The Rapid hardening cement is used for repair and rehabilitation works and where speed of construction and early completion is required due to limitations of time, space or other reasons

9 Hydrophobic Portland Cement

It is manufactured under special requirement for high rainfall areas to improve the shelf life of cement. The cement particles are given a chemical coating during manufacturing, which provides water-repelling property, and the cement is not affected due to high humidity and can be stored for longer period. Due to abundant availability of cement in all parts of the country, this cement is very rarely produced these days.

10 White Cement White Portland Cement is made from raw materials containing very little iron oxide and manganese oxide. Burning in the kiln is done with furnace oil or gas instead of coal. Limited quantities of certain chemicals, which improve whiteness of cement, are added during manufacturing. This type of cement is generally meant for non-structural works. It is used for architectural purposes such as mosaic tiles, wall paintings, GRC and special effects.

Tests to be done on cement

The usual tests carried out for cement are for chemical and physical requirements. They are given in relevant Indian Standards. IS 4031 (different parts) and IS 4032. The chemical standards gives permissible limits for insoluble residue, loss of ignition and other compounds and impurities like Magnesium Oxide, Sulphate, etc. The physical requirements are for fineness, soundness, setting time and compressive strength. These are mostly conducted at a laboratory.

A field engineer is advised to purchase a reputed brand of cement from an authorized dealer, and ask for the test certificate. He can conduct a few field tests at the site itself for ascertaining the quality of cement.

• Check at random that the packing bags are fresh and that the bags are machine stitched • Check that the bag displays Indian Standards mark and number to which it conforms. • Check the manufacturing week and the year on the bag • Open the bag and take a good look at cement, there should be no lumps • Check that the colour of cement is uniform • Check for setting. Take some quantity of cement, make it to a paste, place it on a plate and give it a square shape. Immerse

the same slowly in to a bucket of water. Check that it has not lost shape and that it hardens after 24hrs

Storage of cement The binding property and strength of cement depends upon its capacity for chemical reaction, which can take place in the presence of water. Cement tends to absorb moisture, and react with it chemically. Basic requirement is that cement should not come in contact with water or moisture till it is put to use. If it comes in contact, it will react and form lumps. Then it loses its reactivity and this in turn will result in less strength. The site engineer should bear in mind that even the atmospheric moisture is sufficient for deterioration of cement. Hence cement should be stored in such a manner that no moisture or dampness is allowed to reach cement either from the ground or from the environment. Cement loses strength as it ages. Approximate reduction in strength with age (in standard water tight storage condition)

S no

Age in months Loss of strength in %

1 3 5 to 10

2 6 20-30

3 12 30-40

Note: BIS prescribes that cement should be used within 90 days of its production. In case it is used at a later date then it should be tested before use.

Good axioms of proper inventory management are:

• Buy only the quantity required for the week or for the fortnight • Please see that cement bag is not torn • No windows/ventilators should be allowed in godown, if unavoidable, it should be kept closed all the time • Do not allow workers to use hooks or sharp tools to lift the bags • Do not allow workers to use hooks or sharp tools to lift the bags • The cement should be stored in a godown built with minimum 300mm plinth height • Check whether the roof of the shed leaks, if so, repair it • Always store cement away from the wall • Do not store cement directly on the floor. Use wooden planks/pallets or polythene sheets • Place cement bags one layer lengthwise and one layer widthwise (header and stretcher style) for better stability of the pile • Always adopt first in first out approach (FIFO system). • Sometimes, when the cement bag is brought down, it may appear hard (pack lumps) because of the load of the bags above it.

Roll the bag two to three times before opening it.

Classification Grade Applications

Ordinary

M10 PCC (Plain Cement Concrete) e.g. Levelling course, bedding for footing, concrete roads, etc.

M15 PCC e.g. Levelling course, bedding for footing, concrete roads, etc.

M20 RCC (Reinforced Cement Concrete) e.g. Slabs, beams, columns, footings, etc. (for mild exposure)

Standard

M25 RCC (Reinforced Cement Concrete) e.g. Slabs, beams, columns, footings, etc.

M30 RCC e.g. Slabs, beams, columns, footings, etc.

M35 RCC e.g. Slabs, beams, columns, footings, etc.

M40 RCC e.g. Pre-stressed concrete, slabs, beams, columns, footings, etc.

M45 RCC e.g. Runways,Concrete Roads (PQC), Prestressed Concrete Girders, RCC Columns, Prestressed beams

M50 RCC e.g. Runways,Concrete Roads (PQC), Prestressed Concrete Girders, RCC Columns, Prestressed beams

M55 RCC e.g..Prestressed Concrete Girders and Piers

High Strength M60 - M80

RCC work Where high compressive strength is required such as high rise buildings, long span bridges, ultra-thin white topping etc and constructions in aggressive environment e.g. Spillways of dams, coastal construction

Frequently Asked Questions

1. What will happen if shape of aggregates used in concrete is not rounded/cubical?

If the shape of aggregates used in concrete is not rounded (or) cubical the interlocking of aggregate in concrete will not be proper and strength of concrete will decrease. If the aggregates are elongated (or) flaky the strength of concrete will be seriously affected and concrete will become porous..In UltraTech Concrete the 20 mm & 10 mm aggregates are properly checked for elongation or flakiness and their quantity is restricted within the limits of BIS standards, to get cohesive & durable concrete.

2. What is workability of concrete?

The ease of placing, compacting and finishing of concrete in desired manner is called workability. Normally the workability is measured through slump. More the slump, higher the workability. Low slump leads to difficulties in placing of concrete. Therefore it is required to decide the workability based on experience and site conditions.

3. How will you ensure that the ingredients of concrete are proper?

The right proportion of aggregate, sand, cement, water in concrete are ensured by experience in volumetric proportion. However, in UltraTech Concrete, the mix proportions are designed in a scientific method and proportions are fixed by weighment. Mixing is generally done through high efficiency pan mixers(machine mixing). The number of rotations of the shaft is very important in proper mixing and should be monitored strictly. The material may segregate in case the raw mix is over mixed. Improper mixing may lead to reduced strength. UltraTech Concrete is equipped with computerized batching and mixing plants, to strictly monitor the quality of concrete. The recipe of raw mix design (Cement: Sand: Coarse aggregate: water: admixture) is stored in the computer and the quantities of raw materials are weighed automatically as per the design mix. The water-cement ratio for a particular mix is fixed as per the design, which is kept constant throughout and hence consistency in quality is maintained.

4. What is the importance of water-cement (w/c) ratio?

W/c ratio is very important in making concrete to satisfy the strength & durability criteria of concrete. If w/c ratio is more, the strength of concrete will be reduced due to void formations in concrete. In UltraTech Concrete, w/c ratio is pre-designed through scientific mix design as per BIS specification. The effect of w/c ratio on compressive strength of concrete is as shown in the table below:

Water Cement (w/c) 0.40 0.50 0.60 0.70 0.80

Probable Compressive Strength (%) 100 87 70 55 44

Importance of Water-Cement Ratio:

Factors Low w/c ratio High w/c ratio Compressive strength High Low Water permeability Low High Shrinkage Low High Water Bleeding Low High

5. What is the importance of mixing in concrete ?

The mixing of concrete may be compared with Dosa mix. If the ingredients of Dosa are measured & ground properly, the Dosa will be soft, thin, of good taste and non sticky. In the same way, if the ingredients of concrete are properly proportioned and mixed, the quality of concrete will be uniform, cohesive and consistent. In UltraTech Concrete, the mixing is done through turbo mixer, which ensures uniform and consistent quality of concrete.

6. What is the segregation of Concrete?

Segregation is the separation of coarse aggregate from cement paste. It mainly occurs due to excess water in concrete and non-cohesiveness of concrete. In UltraTech Concrete, the well-designed concrete is pumped from the delivery truck to the location where it is being placed through concrete pumps with horizontal and vertical pipelines. The flexible hose pipe attached at the end of pipe line makes the pouring convenient at the point of placing, thus reducing shoveling / dragging andavoiding segregation.

7. What is bleeding of concrete?

Bleeding is the emergence of water to the surface from freshly placed concrete which results in deposition of solids to the bottom during compaction. It can be avoided by proper control of w/c ratio and desired level of compaction.

8. Why concrete has to be cured?

Curing is unfortunately neglected at the construction site. In case of roof slabs, quick drying of surface will lead to plastic shrinkage cracks. Apart from this, if timely curing is not done, the top surface dries to such an extent that hydration process almost stops at the top surface making it weak and porous. Therefore, it is emphasized that curing is the essence of good concrete.

9. What is plastic shrinkage crack in concrete?

During the hydration of cement paste in concrete heat of hydration is produced. This heat dissipates from the surface of concrete leading to formation of cracks on surface of concrete. These cracks are called plastic (or) drying shrinkage cracks.These can be reduced to a great extent by early curing.

10. What is the importance of testing concrete?

The concrete has to be tested to know the strength of concrete at 28 days for each desired grade. Usually the concrete at site is not tested for its strength, workability, yield and other factors that ensure the performance of concrete over a period of time. However in UltraTech Concrete, the samples of concrete are tested for every batch to ensure consistent quality and supply of durable concrete.