CEMENT

CEMENT

  Cement is a binding material which can form a paste when react with water and when hardened, it bind to fine and coarse aggregates particles to form a concrete. The cement and water hardened through a hydration chemical process. It has adhesive and cohesive properties. The most common chemical content in the cement are calcium oxide, silica, alumina and iron.

      

 Mix proportions of concrete

              The proportions of the constituent materials, by volume, in freshly mixed concrete:

              6 -16% cement

              12-20% water

              20-30% fine aggregate

              40-55% coarse aggregate

                 The properties of concrete are affected by the amounts of the constituents of the mix proportion. The mix proportions are usually expressed as the weight of each material required in a unit volume of the concrete production usually in kg/m³.

                     The mix proportion by weight is:

                  Cement                              150-600kg/m³

                  Water                          110-250kg/m³

                  Aggregates (coarse+fine)   1600-2000kg/m³

             

            The water/cement ratio is an important factor influencing many of the concrete           properties. The proper ratio of water/cement ratio will give adequate strength to the concrete itself.

The production of cement in wet process:

1.     Chalk and clay reduced to particles size less than or equal to 75 micrometer and then mixed with the required proportion

2.     Mixture mixed with water to form slurry.

3.     After that, slurry mixes with water and fed into higher end of inclined rotary kiln

4.     The mixture will heat when it passes down the kiln under combined action of rotation and gravity

5.     The mixture under high temperature will be undergoes 4 physical and chemical change which are Drying, Preheating, Calcining, Clinkering

The production of cement in dry process:

1.     Chalk and clay reduced to particle less than or equal to 75 micrometer and then mixed in the required proportion

2.     Mixture mixed with water to form slurry as a dried powder form

3.     After that, the dried powder preheated and some calcining which start to break in the heat exchanger.

4.     Mixture fed into higher end of inclined rotary kiln and under increasing temperature

5.     The mixture will be undergoes 2 chemical changes which are calcining and clinkering.

After cooling, a small amount of gypsum which is calcium sulfate dehydrate is added to the clinker before the mixture is ground to fine powder. The usage of gypsum act is to retard the curing process so that the cement paste cannot stiffen immediately during hydration.

The types of Portland cement

1)    Ordinary Portland cement

  Ordinary Portland cement more often used in the construction due to it is cheaper. It is suitable for normal construction of building.

                 

2)    Rapid-hardening Portland cement

  It can be achieved by having higher tricalcium silicate and tricalcium aluminate in the cement content. Rapid-hardening cement has grater surface area for hydration process which can develop the strength more rapidly. It used in the early stripping of  formwork and early loading of requires structures.

                              

3)    Ultra-high early strength Portland cement

  It is mostly used in the cold country which it can achieve strength by hardening very rapidly. Even higher early strength can also be achieved by steam curing process.

                                         

4)    Low-heat Portland cement

  This cement is required for thick concrete work. Because the heat generated by this ordinary cement will be excessive and will causes cracking of the concrete. It can be manufactured by either less fine grinding of cement clinkers or lower tricalcium silicate and tricalcium aluminate in the cement content.

                                         

5)    Sulphate resisting Portland cement.

 This cement is used in the presence of external source which is sulphates in those areas.  Sulphates react chemically with the hydration products of calcium aluminates and causing cracking.

                                             

6)    White Portland cement.

  The grey color if Portland cement is due to presence of ferrite in the limestone. The white cement is made from non-ferrite containing material which is more expensive compare to normal Portland cement, because of the higher raw materials costs and greater care during manufacturing.

                                      
                                           

7)    Portland blast-furnace cement

  This manufactured from adding 30-35% of weight blast furnace slag to ordinary Portland cement clinker before grinding. This cement is not suitable for use at low curing temperature due to low rate of hardening. However, the strength of mature concrete is the same with concrete made from ordinary Portland cement. It can be used in the construction in the sea area due to having good resistance to dilute acids and sulphates.

                                    

8)    Pozzolanic cement

This cement is made from pulverized fuel ash and opc. This cement is low-heat cement which initially slower in hardening, however, it will be attain its strength after 3 month. Pozzolanic cement also resistance to sea water and sulphates.

                                      

The types of non-Portland cement

1)    High alumina cement

High alumina cement is grey-black in color which different from Portland cement. This cement can resistant to sugar, oils, fertilizers, beer, acids, and sulphates. However, it is low in protection from alkalis sodium and potassium hydroxide.

                           

2)    Supersulphated cement

This cement has resistance to high acid content and unlike high alumina cement and also resistant to caustic alkalis. It is low-heat cement which suitable for mass concrete and perform work in hot climates. Initially, the development of strength is low, however at later ages; strengths are at least equal to those of Portland cement.

                                  

ADMIXTURES FOR PORTLAND CEMENT

Admixture

  Admixture is a chemical that are added to the concrete before or during mixing. They are used to change its fresh, early age or hardened state of the concrete. There are 5 distinct types of admixtures which are plasticizers, superplasticizers, accelerators, retarders and air entraining agent.

1)      Plasticizer

   These are workability aids for the increasing fluidity of a concrete cement paste. Plasticizers are the polymers which are absorbed onto the surface of cement grains with an ionic group pointing outward. As a result, the negatively charged surface cause the mutual repulsion of the cement particles, thereby dispersing the particles and increasing the fluidity due to released of the entrapped water.

 

  Side-effects of plasticizers will causing  the delaying of set , decreasing early strength gain, entrain air bubbles inside the cement and contain impurities affecting strength of hardened concrete.

2)    Superplasticizers

It also knows as high-range water reducers which used to achieved increases in fluidity and greater workability than those obtainable with plasticizers. They are manufactures to higher standards of purity and without any side-effects.

3)    Accelerator

  The accelerator more prefer used in cold weather due to increase high rate of setting and heat evolution. It is used to accelerate the setting and the rate of hardening of cement paste. It is enhancing early strength gain, reducing the curing time for concrete placed in cold weather and also reducing the setting time.  One example of accelerator is Calcium chloride. A number of alternative chloride –free accelerators are calcium formate, sodium aluminate or triethanolamine.

4)    Retarder

It is used in delay the setting time and hardening of cement mix. Their usage include counteracting the accelerating effect of hot climate, controlling the set in the large pours where concreting may take several hours and delaying the set where the concrete have to transported over long distance from one place to another.

Sugars, starches, critic acid, zinc oxide are used to retard the setting of cement.

5)    Air entraining agent

  They are added to entrain air which in the form of very small disconnected air bubbles in cement paste. The primary effects of entrained air provide resistance to frost which will lead to progressive deterioration of the concrete and cement. Besides that, it also increase workability of mix which the air bubbles now act like small ball-bearing but increase the porosity. As a result, the strength will be loss but improvement in workability which means the loss can be partially offset by reducing the water/cement ratio. Air entrainment can be achieved by adding vinsol resins, alkysulfonates and alkysulfates.

Damp-proofing mixtures

It prevent water movement by capillary action.