4 PRIMARY CEMENT COMPOUNDS & THEIR ROLES

cement compound
cement compound

About 90-95% of Portland cement is made up of four main cement minerals, which are C3s, C2s, C3a, And C4a f, the remainder is comprised of calcium sulfate, alkali sulfate, unreacted (free) CaO, MgO, and other minor components, which are left over from the clinkering and grinding steps. The four cement minerals play very different roles in the hydration process that converts dried cement into a hardened cement paste.

C3s and c2Contributes almost all of its beneficial properties by generating the main hydration product, CSH Gel. although C3S hydrates much faster than C2s and is thus responsible for the initial power development,


C3a and c4AF minerals also hydrate, but the products they form contribute little to the properties of the cement paste. These minerals exist because pure calcium silicate cement would be nearly impossible to produce economically.

The crystal structures of cement minerals are quite complex, and since these structures do not play a significant role in the properties of cement paste and concrete, we will present only the most important features here.

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1. Tricalcium silicate (C)3s)

C3S is the most abundant mineral in Portland cement, occupying 40–70 wt% of cement, and is also the most important. C. hydration of3S gives cement paste most of its strength, especially in the initial setting.

pure c3S can form with three different crystal structures. The equilibrium structure at temperatures below 980˚C is triclinic, At a temperature between 980˚C – 1070˚C, the structure is monoclinic, and above 1070˚C it is mainly rava, Furthermore, triclinic and monoclinic structures each have three polymorphs, so there are a total of seven possible structures. However, all these structures are similar and there are no significant differences in reactivity. The most striking feature of the structure is a strange and asymmetric packing of calcium and oxygen ions that leave large “holes” in the crystal lattice. Essentially, the ions don’t fit together very well, leading to a high internal energy in the crystal structure. As a result, c3S is highly reactive.

C3The S formed in cement clinker consists of CaO and SiO. Contains about 3-4% oxide in addition to2, Strictly speaking, this mineral should be called alite instead of Si.3S. C in a specific clinker3Each MgO in S, Al. will be approximately 1 wt% of2hey3, and fee2hey3, Na. with a very small amount of2Ok2o, p2hey5, therefore3, These quantities can vary greatly with the composition of the raw materials used to make cement.

One effect of the impurities is to “stabilize” the monoclinic structure, meaning that the structural change from monoclinic to triclinic that would normally occur upon cooling is prevented. Thus in most cements C. is a monoclinic polymorph of3s.

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2. dicalcium silicate (Si2s)

as with C3scheduled caste2S can form with a variety of different structures. have a high temperature α Structure with three polymorphisms, a β In that the structure is in equilibrium at intermediate temperatures, and at low temperatures I structure.

C. an important aspect of2s he is GC2S has a very stable crystal structure that is completely inert in water. Fortunately, the B structure is easily stabilized by the other oxide components of the clinker and thus the G form is never present in Portland cement. bc. crystal structure of2S is irregular, but C is . much less than3s, and this c. is responsible for the low reactivity of2s. C2S in Cement C. The level of impurities is slightly higher than3s. According to Taylor, the overall substitution of the oxide is 4–6%, with significant amounts of Al. Is2hey3MILF Fay2hey3, and of2O

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3. Tricalcium Aluminate (C)3a)

Tricalcium Aluminate (C)3a) Contains anywhere from 0 to 14% of Portland cement. like c3S, it is highly reactive, releasing a significant amount of exothermic heat during the initial hydration period. Unfortunately, C. hydration products made from3Little contribution to the strength or other engineering properties of cement paste. Under certain environmental conditions (ie, the presence of sulfate ions), C3A and its products can actually damage concrete by participating in extensive reactions that cause stress and cracking.

pure c3A is formed only with cubic crystal structure. Structure Ca. is characterized by+2 Six AlO . K atoms and rings4 as with quaternary C3S, bonds are distorted from their equilibrium position, leading to a higher internal energy and thus a higher reactivity. CaO and Al. significant amount of2hey3 in c3One structure can be replaced by another oxide, and at high levels of substitution it can give rise to other crystal structures. C3Portland cement clinker, which typically has about 13% oxide substitution, is predominantly cubic, with small amounts of orthorhombic C.3a c3a and c4AF minerals are formed by simultaneous precipitation as the liquid phase formed during the clinkering process cools, and thus they are closely interconnected. This makes it difficult to ascertain the exact compositions of the two phases. The cubic form usually has a 4% substitution of SiO2, Fe. 5% replacement of2hey3, and Na. about 1% of each2Ok2O, and MgO. The orthorhombic form has similar levels, but K. With replacement of more (~5%) of2O

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4. Tetracalcium aluminoferrite (C)4a f)

C. a stable compound with any association between2a and c2F can be made, and the cement mineral can be converted to C. is called4AF is a conjecture that represents the midpoint of this compositional chain. The crystal structure is complex, and is believed to be related to the mineral perovskite. C . actual structure of4The AF in cement clinker is generally higher in aluminum than in iron, and SiO. has a lot of replacement2 and MGO. Taylor Ca. reports a specific structure (in common chemical notation) to be2AlFe0.6mg0.2Si0.15ti0.5hey5, However, the composition will vary somewhat depending on the overall composition of the cement clinker.

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Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.