CEMENT

WHAT IS CEMENT

v  Powdered materials which develop strong adhesive qualities when combined with water;

v  A Binder, a substance used in construction that sets and hardens and can bind other materials together

v  A fine grayish powder which, when mixed with water, forms a thick paste.  When this paste is mixed with sand and gravel and allowed to dry it is called concrete.

v  Used as a component in the production of Mortar and of Concrete - which is a combination of cement and an Aggregate to form a strong building material.

Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to set in the presence of water 

v  Hydraulic cements are materials which set and harden after combining with water, as a result of chemical reactions with the mixing water and, after hardening, retain strength and stability even under water.

v  Most construction cements today are hydraulic, and most of these are based upon Portland cement

v  Non-hydraulic cements include such materials as (non-hydraulic) lime and gypsum plasters, which must be kept dry in order to gain strength, and oxychloride cements, which have liquid components. Lime mortars, for example, "set" only by drying out, and gain strength only very slowly by absorption of carbon dioxide from the atmosphere to re-form calcium carbonate through carbonatation.

TYPE OF CEMENT

1.      Portland cement

·           The most common type of cement in general use around the world,

·           Has medium rate of strength development and heat generation,

·           Has Adequate resistance to dry shrinkage and crack but less resistance to chemical attack,

·           Made by heating limestone (calcium carbonate) with other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination

·           Molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix to form calcium silicates and other cementitious compounds. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement'

Uses:

Ø   In general concrete construction where there is no contact to sulphates,

Ø   In small structure where heat of hydration will not cause any defect.

2.      Modified Portland cement

·           On setting, develop less heat of hydration then OPC.

Uses

Ø   Use in hot climate

Ø   For construction of heavy abutments, large piers, retaining wall etc; where sulphate attack is not high.

3.      Rapid Hardening Portland Cement

·           Strength develop is so fast,

·           Contains large amount of lime and others constitutions,

·           Lighter than OPC,

·           Curing period is short,

·           Strength developed at the age of 3 days is of same order as the 7 days strength of OPC with the same W/C ratio.

Uses

Ø   Where rapid strength development is required.

4.      Low Heat Portland cement

·           Hydrate slowly,

·           Contains less lime than OPC; other materials are same as OPC,

·           Less compressive strength.

Uses

Ø   For mass concrete works

5.      Sulphate Resisting Portland Cement

·           C₃A is kept below 5% and results in the increase in resisting power against sulphates.

Uses

Ø   Used at placed where sulphate action is servre.

6.       High Alumina Cement

·           Rapid hardening cement of chocolate colour,

·           Is manufactured from bauxite and limestone in special reverberatory furnaces.

·           Contains nearly 35% of alumina and ratio of alumina to lime lies between 0.55 to 1.3

·           Resists the action of acid and high temperature and does not expand on setting.

·           It sets quickly and attains higher ultimate strength in short period,

·           Completely resistance to the action of sulphates and excellent resistance to fire,

·           Much costlier than OPC,

Uses

Ø   In chemical plants and furnaces,

Ø   In colder regions having temperatures 18º C or below and during war time emergency.

Ø   Under water construction.

7.      Quick Setting Cement

·           Contains less percentage of gypsum and is ground much finer than OPC

·           Setting action is accelerated by adding a small amount of aluminium sulphate, during grinding,

·           Initial and final setting time are 5 minutes and 30 minutes respectively.

·           Costlier than OPC

Uses

Ø   Under water and running water

8.      Blast furnace salg cement

·           Contains up to 95% ground granulated blast furnace slag with the rest Portland clinker and a little gypsum.

·           All compositions produce high ultimate strength,

·           As slag content is increased, early strength is reduced,

·           While sulfate resistance increases and heat evolution diminishes.

·           Used as an economic alternative to Portland sulfate-resisting and low-heat cements

·           Cheaper than OPC

Uses

Ø   Used in mass concreting structures

Ø   Sea water construction

9.      White Cement

·           Made from raw materials containing very little iron oxide and manganese oxide,

·           Dries quickly,

·           Possesses high strength,

·           Has superior aesthetic values,

Uses

Ø   Floor finish plaster work ornamental works,

Ø   As mortar for marbles and tiles,

Ø   Moulding sculptures and statues, painting garden furnace,

Ø   For ready mixed concrete and precast blocks.

10.  Portland pozzolana cement

·           An Interground blended mixture of cement and pozzolana

·           About 25% of pozzolanic material is added to the OPC clinkers and mix thoroughly ground,

·           Possesses higher tensile strength,

·           Evolves less heat during setting,

·           Offers great resistance to expansion

·           Imparts higher degree of water tightness,

·           Imparts plasticity and workability to mortar and concrete,

·           Offers higher resistance to chemical attack and to the action of sea water,

·           Not costly

Uses

Ø   Widely used for hydraulic structure (mass concrete works)

Ø   Used in sewage works and for laying concrete under water.

                           ©Ishu Mainali