Compounds of Cement

About 90-95% of a Portland cement is comprised of the four main cement minerals, which are C₃S, C₂S, C₃A, and C₄AF, with the remainder consisting of calcium sulphate, alkali sulphates, unreacted (free) CaO, MgO, and other minor constituents left over from the clinkering and grinding steps. The four cement minerals play very different roles in the hydration process that converts the dry cement into hardened cement paste. 

The C₃S and the C₂S contribute virtually all of the beneficial properties by generating the main hydration product, C-S-H gel.  However, the C₃S hydrates much more quickly than the C₂S and thus is responsible for the early strength development. 

The C₃A and C₄AF minerals also hydrate, but the products that are formed contribute little to the properties of the cement paste.  These minerals are present because pure calcium silicate cements would be virtually impossible to produce economically. 

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

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1. Tricalcium Silicate (C₃S)

     C₃S is the most abundant mineral in portland cement, occupying 40–70 wt% of the cement, and it is also the most important.  The hydration of C₃S gives cement paste most of its strength, particularly at early times. 

     Pure C₃S can form with three different crystal structures.  At temperatures below 980˚C the equilibrium structure is triclinic.  At temperatures between 980˚C – 1070˚C the structure is monoclinic, and above 1070˚C it is rhombohedral.  In addition, the triclinic and monoclinic structures each have three polymorphs, so there are a total of seven possible structures.  However, all of these structures are rather similar and there are no significant differences in the reactivity.  The most important feature of the structure is an awkward and asymmetric packing of the calcium and oxygen ions that leaves large “holes” in the crystal lattice.  Essentially, the ions do not fit together very well, causing the crystal structure to have a high internal energy.  As a result, C₃S is highly reactive. 

   The C₃S that forms in a cement clinker contains about 3-4% of oxides other than CaO and SiO₂.  Strictly speaking, this mineral should therefore be called alite rather than C₃S. In a typical clinker the C₃S would contain about 1 wt% each of MgO, Al₂O₃, and Fe₂O₃, along with much smaller amounts of Na₂O, K₂O, P₂O₅, and SO₃.  These amounts can vary considerably with the composition of the raw materials used to make the cement. 

One effect of the impurities is to “stabilize” the monoclinic structure, meaning that the structural transformation from monoclinic to triclinic that would normally occur on cooling is prevented.  Most cements thus contain one of the monoclinic polymorphs of C₃S. 

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2. Dicalcium Silicate (C₂S)

     As with C₃S, C₂S can form with a variety of different structures.  There is a high temperature α structure with three polymorphs, a β structure in that is in equilibrium at intermediate temperatures, and a low temperature γ structure. 

An important aspect of C₂S is that g-C₂S has a very stable crystal structure that is completely unreactive 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.  The crystal structure of b-C₂S is irregular, but considerably less so than that of C₃S, and this accounts for the lower reactivity of C₂S.  The C₂S in cement contains slightly higher levels of impurities than C₃S.  According to Taylor, the overall substitution of oxides is 4-6%, with significant amounts of Al₂O₃, Fe₂O₃, and K₂O. 

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3. Tricalcium Aluminate (C₃A)

     Tricalcium aluminate (C₃A) comprises anywhere from 0 to 14% of a portland cement.  Like C₃S, it is highly reactive, releasing a significant amount of exothermic heat during the early hydration period.  Unfortunately, the hydration products of formed from C₃A contribute little to the strength or other engineering properties of cement paste.  In certain environmental conditions (i.e., the presence of sulfate ions), C₃A and its products can actually harm the concrete by participating in expansive reactions that lead to stress and cracking. 

     Pure C₃A forms only with a cubic crystal structure.  The structure is characterized by Ca⁺² atoms and rings of six AlO₄ tetrahedra.  As with C₃S, the bonds are distorted from their equilibrium positions, leading to a high internal energy and thus a high reactivity.  Significant amounts of CaO and the Al₂O₃ in the C₃A structure can be replaced by other oxides, and at high levels of substitution this can lead to other crystal structures.  The C₃A in portland cement clinker, which typically contains about 13% oxide substitution, is primarily cubic, with smaller amounts of orthorhombic C₃A.  The C₃A and C₄AF minerals form by simultaneous precipitation as the liquid phase formed during the clinkering process cools, and thus they are closely intermixed.  This makes it difficult to ascertain the exact compositions of the two phases.  The cubic form generally contains 4% substitution of SiO₂, 5% substitution of Fe₂O₃, and about 1% each of Na₂O, K₂O, and MgO.  The orthorhombic form has similar levels, but with a greater (~5%) substitution of K₂O.   

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4. Tetracalcium Aluminoferrite (C₄AF)

     A stable compound with any composition between C₂A and C₂F can be formed, and the cement mineral termed C₄AF is an approximation that simply the represents the midpoint of this compositional series.  The crystal structure is complex, and is believed to be related to that of the mineral perovskite.  The actual composition of C₄AF in cement clinker is generally higher in aluminum than in iron, and there is considerable substitution of SiO₂ and MgO.  Taylor reports a typical composition (in normal chemical notation) to be Ca₂AlFe_0.6Mg_0.2Si_0.15Ti_0.5O₅.  However, the composition will vary somewhat depending on the overall composition of the cement clinker.

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