Notes on Calcium silicate Hydrates(C-H-S) and Calcium Aluminium Hydrates (C-A-H).

 Calcium silicate Hydrates(C-H-S)

              During the coarse  of reaction of C3S and C2S  of both side of water are evolved.

                                     2C3S +6H----------->C3S2H3+3Ca(OH)2

                                                 2C3S +4H----------->C3S2H3+Ca(OH)2

                    C3S as compared to C2S produces less quantity of calcium silicate Hydrate and more quantity of Ca(OH)2  in the hydration process.

                Ca(OH)2  is not desirable product in the concrete mass because it makes the concrete porous as C3S readily reacts in to water and produces more heat of hydration and it is responsible for early strength of concrete.

                 C3S Hydrates rather slowly and is responsible for later strength of concrete and produces less heat of hydration.The quality and density is an of calcium silicate hydrates produces C2S is better than that produces in the hydration of C2S.

Calcium Aluminium Hydrates (C-A-H)

            Due to hydration of C3 A a calcium aluminate system of the form CaO Al2O3H2O. The Cubic Compound C3 A H6  is only stable compound formed and remains stable up to 225 C .The Reaction of  C3 A with water is very fast and this may lead flash set.The hydrated Aluminate do not contribute any thing to strength  of the cement post and it hydrates very fast and it may contribute little early strength.

           On hydration of C4AF ,it is believed system of the form CaO Fe2O3 H2O and this hydrated calcium Ferrite and is compared to be more stable.This Product doesn't contribute any thing to the strength.

           The hydrates of C4AF shows a compartively higher resistance to sulphate attack then hydrates of C2HA.

Calcium Hydroxide:

          The other products of hydration of   C3S and  C2S is Ca(OH)2. The solution contributes 20-25% of the volume of the solids in the hydrate paste.The lack of durability of he concrete is on the account of Ca(OH)2 .It reacts with sulphates present in water or soil to form calcium sulphate which is further reacts with C3 A and cause detotration of concrete in which reaction with gypsum are

               C3 A+32H +3CaSO4---------------->C6AS3H2

                    C3 A+18H+CaSO4-------------------> C4ASH

Hydration of Cement

             Anhydrous cement compound when mix with water and react with each other to form hydrated compounds of very low solubility .The hydration of cement can be visualized into following ways

• Through Solution
• Second Possibility

Through Solution:

            The mechanism of through solution is cement compound dissolve to produce super saturated solution from which different hydrated products get precipitated.

Second Possibility:

             In this ,when the water  attacks cement compounds in the solid state converting the compund during coarse of reaction between cement and water.

Structure of Hydrated Cement:

             To understand the behaviour of cement it is necessary to study the structure of hydrated,hidden cement base .

If concrete is consider as two phase materials namely

• Paste Phase
• Aggregate Phase

Water Requirement for Hydration:

             C2s are required about 21% of water by weight of cement and C3S requires 24% of water for chemical reaction with Portland compound.
              Generally ,23% of water chemically combines with cement and therefore is called Bound Water. A certain quantity of water is imbibed  within gel spores .This water is knowns as Gel water.It can be said that bound water and gel water are complementary to each other.
              Generally,15% of water by weight is required for the complete chemical reaction and for Gel formation.Here it is assumed that hydration is taking place in the sealed container where the moisture to and from the paste does n't takes place.

Introduction of Concrete

                             Cement And Admixtures  

Manufacture of Portland Cement:

                 The Raw materials required for manufacture of Portland cement are calcareous materials like limestone or chalk and argillaceous materials like shale and clay.

                 The Process of manufacture of cement consists of grinding of raw materials ,mixing  them intimately in such certain proportions depending upon their purity ,composition and burning them in kiln at temperature of about 1300-1500 C.At this Temperature materials sinters  and particularly fuss to form modular shaped kilnker is cooled and grounded in to fine powder with addition of about 2-3% Gypsum .There are two process known as 

• Wet
• Dry Process.

Depending upon the weather,mixing and grinding of raw materials is done wet or dry in condition.

With the little change in the above process we have to do semi dry process also,where the raw materials are ground dry and hen mixed with about 10-14% of water and further burst kilnkering temperature.

Chemical Compostion of Cement:

            The Raw materials used for manufacturing of cement consists of mainly of lime,silica,aluminium and iron oxide.These oxides interact with one another in kiln  to form compler compounds.