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For ages, man used mud to bond stones and to plaster wooden walls of huts. Lime is first cementitous material used in construction of ancient structures. Romans knew using lime and pozzolanic materials to derive cementing products.
Modern Cement is made with limestone or chalk and silica or clay and other ingredients. They are burned into clinker and ground fine after adding a few more ingredients. The main compounds found in cement in dry state are the following:
Tricalcium Silicate C3S
Dicalcium Silicate C2S
Tricalcium Aluminate C3A
Tetracalcium Aluminoferrite C4AF
In the above, the C, S, A and F are short for Calcium, Silicate, Aluminate and Ferrite respectively. These compounds are also called as Alite, Belite, Celite and Felite respectively. First two of these compounds of cement are primarily responsible for the binding action. When these chemicals react with water, they get cementitous property and bind readily with aggregates and reinforcement. This process is called as hydration of cement.
Tricalcium Silicate and Dicalcium Silicate together constitute 70 to 80 percent of cement.
Tricalcium Aluminate and Tetracalcium Aluminoferrite form most of the rest and some other impurities are also present in minor proportions.
The quality of concrete or mortar depends on the quality, stability, quantity, continuity and rate of formation of hydration products. The products of hydration have very low solubility and hence precipitate as what we see as hardened cement.
This process of hydration is faster in early stages and declines there after indefinitely. The hydrates being impervious, gives a coat on cement grain, reducing further contact of inner parts with water and further hydration of grain core. This leaves an unhydrated core within a shell of hydrate product. Complete hydration is not seen even after a year or so, unless the cement is ground extremely fine and reground again in paste form to open up unhydrated grains for further hydration.
The depth of hydrated silica layer is found to be in the order of 4 microns, while complete hydration under normal conditions is expected only for the grains less than 50 microns in size. As the hydrated silica coats the unhydrated core well, it is seen not to decrease strength. The cores might be behaving like finer aggregates in the paste!
So much so that with a water-cement ratio as low as 0.08 and a very thorough compaction, a strength of 280 MPa was obtained, when only surface hydration of silicates is expected from low water ratio.
When calcium silicates react with water they form calcium hydroxide [Ca (OH)2] among others. This is soluble in water and leaches out in hydraulic structures making them porous. Further, this oxide reacts with sulphates in the presence of water and forms byproducts which further react with C3A and destroys concrete. When compared to C2S, C3S produces more Ca(OH)2 and less silicate hydrates. So it is better to have a higher percentage of C2S.
But the C3S hydrates fast and gives more heat making it suitable for cold weather concreting. Whereas, the C3S gives quick hydrates of inferior quality, the C2S produced good quality hydrates slowly. This is because it reacts slowly but develops hydrates with much larger specific area, hence results in dense, well bonding mass.
Crystalline Ca(OH)2 is responsible for less durability as it do not have any binding or strength properties. To reduce its action, pozzolanic materials are added during cement manufacture. Further, C3A present in cement hydrates is very quick to form an extremely stable calcium aluminate – C3AH6. This flash set is rather quick that it is undesirable. It is prevented by adding gypsum at the time of grinding. Gypsum dissolves before C3A and arrests its solubility by forming other compounds including one called Ettringite. Ettringite is the first solid to form in hydrated cement.
The density of mass of hydrated concrete also depends on water to cement ratio. Any thing in excess of 0.7 ratios would result in formation of excess capillaries and weakens the concrete.
Hence, though the basic chemistry of cement hydration is known, being a complex process, it still needed more research and understanding.
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