What are the C3s, C3A and C4AF?

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    The raw materials of cement also called clinker phases or minerals are C3S, C2S, C3A, C4AF. Each of these chemical compounds contributes and affects differently the cement.

    C3S: Also called Alite or tricalcium silicate, has a chemical composition of 3CaOSiO2. This compound is responsible for the early strength development. 70% of C3S will be reacted by 28 days.They constitute 40%-70% of cement.

    C2S: Also called Belite or dicalcium silicate, has a chemical composition of 2CaOSiO2. This compound is responsible for the late strength development. Only 30% of C2S will be reacted by 28 days. They constitute 20%-40% of cement.

    C3A: Also called Aluminate or tricalcium aluminate, has a chemical composition of 3CaOAl2O3. This compound contributes little to long term strength. It provides early high heat generation in hydration(rapid hydration) thus responsible for the setting time of cement.(10 F – 15 F for 100 lb cement). C3A has a weak resistance against sulfate.

    C4AF: Also called Ferrite or tetra calcium alumina ferrite, has a chemical composition of 4CaOAl2O3Fe2O3. Little to now contribution to strength. It gives the color of cement.

    It is clear that C3S and C2S are responsible for strength of cement due to the presence of silica. When mixed with water they produce CH and CSH.

    C3A can be responsible cracks (ex: shrinkage cracks) due to the high temperature release. As result a 4%-6% gypsum CaSO4.2H20 must be added to attack C3A and regulate the setting time of cement.When mixed with water C3A produce ettringite and mono sulfate hydrate.

    Different proportions of these chemical compounds give us five types of cement each can be used in different situation and case.

    The short-hand notation for these compounds used in cement industry is C3S, C2S,C3A, and C4AF respectively. C3S is responsible for early strength development, setting and hardening of concrete; C2S determines later strength development, C3Acontrols heat evolution during hydration, and C4AF is for sulfate resistance

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    I am not a doctor or any expert. But, I underwent the procedure of C3R for both of my eyes.

    So, what I can do is basically share my experience.

    I underwent C3R for my left in in the month of September, 2016. I was recommended this treatment as I was sufferimg from keratoconus.

    I was taken to the operation theatre and the process started with the doctor comforting me. What he basically did was use anaesthesia around my eye using a cotton ball. He then used a device(whatever they call it to stretch my eyes, it kept my eyelashes away). Then there was some stuff like plastic wrapped around my eyes. Then, I could see him scraping my eye and putting some eye drop sort of thing. The scraping was cleaned in the end.

    Then, he used some eye drop using a machine at regular intervals. The whole process took 45 minutes. He then put a bandage lens over it. This was my experience.

    However, to keep it short C3R is basically scraping first layer of cornea along with the use of some eye drop.

    My answer is not at all the exactly what the doctors do. But, I felt like sharing whatever little piece of information and experience I had.

    If you are going for C3R , all the very best. Be brave, and you will be fine, soon. 🙂

    Thank you for A2A :).

    In cement clinker, the C3A acts as a flux, as part of the liquid phase developed in the burning zone. C3A itself is quite refractory but with C4AF it forms most of the 22 to 28% clinker melt necessary for C3S and C2S formation. Ordinary Portland Clinker contains between 8 and 11% C3A.

    But your question is about the role of C3A in cement, not in the clinker. In cement, C3A’s role is related to the kinetics of cement hydration.

    Cement hydration occurs in 3 different steps:

    a) pre-induction period, where C3S and C3A quickly react with water releasing heat and forming the Ettringite gel.

    followed by

    b) the dormant period where the cement particles are temporarily shielded from water by a layer of gel.

    followed by

    c) the destabilization of the gel followed by a continuation of the hydration process.

    So, C3A is responsible for the rapid initial hydration of cement as soon as it contacts water. It is also the cement component with the highest heat of hydration. If not controlled, C3A will cause the quick setting of the cement paste, making it unworkable. And here comes the role of Gypsum in set control: it controls the hydration of C3A because the presence of Sulfates in the liquid slows down the hydration of C3A and C3S.

    As far as cement strength, C3A has but a moderate contribution.

    Sorry for this simplified response to your excellent question.

    Tetracalcium aluminoferrite (C4AF) is responsible for early setting of cement and It is also related to heat of hydration. This compound is formed within 24 hours. Generally high quantity of this compound is avoided as it leads to cracking.

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    Why is C3A low in type 2 and type 5 cement?

    I can only assume you are thinking of the ASTM C150 type categories. Note that the other major player in the cement specification league is EN 197 which uses the same codes, but (naturally) those codes have different meanings to the US intent.

    Types II and V in ASTM lingo are sulphate resistant cements. Having low C3A leads to a lower likelihood of ettringite (a calcium aluminium sulphate) formation, and hence it is more sulphate resistant.

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    In Type I Ordinary Portland Cement clinker the amount of C3A+C4AF rarely exceeds 18%. While C3A accelerates cement hydration, C4AF is more a consequence of clinker pyroprocessing than a necessary component. The C4AF acts as a powerful flux in the rotary kiln. Together with the C3A it forms a liquid phase at 1338C, necessary to the formation of C3S from C2S and free CaO. The C3A activity during hydration is controlled by the addition of gypsum to the finish mill. C4AF is responsible for the color of cement. In order to make white cement the C4AF has to be reduced to trace values.

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    I cannot answer that question.

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