In the preceding section, it was shown that the heat of hydration of cement is a simple additive function of the compound composition of ce- ment. It would seem, therefore, that the various hydrates retain their identity in the cement gel, which can be considered thus to be a fine physical mixture or to consist of copolymers of the hy- drates. A further corroboration of this supposition is obtained from the measurement of specific sur- face of hydrated cements containing different amounts of C3S and C2S: the results agree with the specific surface areas of hydrated neat C3S and C2S. Likewise, the water of hydration agrees with the additivity of the individual compounds.
This argument does not, however, extend to all properties of hardened cement paste, notably to shrinkage, creep, and strength; nevertheless, the compound composition gives some indication of the properties to be expected. In particular, the
composition controls the rate of evolution of the heat of hydration and the resistance of cement to sulfate attack, so that limiting values of oxide or compound composition of different types of ce- ment are prescribed by some specifications. The limitations of ASTM C 150-09 are less restrictive than they used to be (seeTable 1.9).
Table 1.9. Compound Composition Limits for Cements of ASTM C 150-09
The difference in the early rates of hydration of C3S and C2S – the two silicates primarily re- sponsible for the strength of hydrated cement paste – has been mentioned earlier. A convenient approximate rule assumes that C3S contributes most to the strength development during the first
four weeks and C2S influences the gain in strength from 4 weeks onwards.1.35 At the age of about one year, the two compounds, mass for mass, contribute approximately equally to the ul- timate strength.1.36Pure C3S and C2S have been found to have a strength of about 70 MPa (10 000 psi) at the age of 18 months, but at the age of 7 days C2S had no strength while the strength of C3S was about 40 MPa (6000 psi). The usually accepted development of strength of pure com- pounds is shown inFig. 1.18.
Fig. 1.18. Development of strength of pure compounds according to Bogue1.2 However, these relative values of the contri- bution to strength of the individual compounds in Portland cement have been challenged.1.87Tests, using particles with the same size distribution and at a fixed water/solid ratio of 0.45, have shown that, up to at least the age of 1 year, C2S exhibits a
lower strength than C3S. Nevertheless, both silic- ates are much stronger than C3A and C4AF, al- though the latter compound exhibits a significant strength while C3A has a negligible strength1.87 (seeFig. 1.19).
Fig. 1.19. Development of strength of pure compounds according to Beaudoin and Ramachandran (reprinted from ref.1.87by kind permission of Elsevier Science Ltd, Kid-
As mentioned on p. 14, the calcium silicates appear in commercial cements in ‘impure’ form. These impurities may strongly affect the rate of reaction and of strength development of the hy- drates. For instance, the addition of 1 per cent of A12O3 to pure C3S increases the early strength of the hydrated paste, as shown inFig. 1.20. Ac- cording to Verbeck,1.55 this increase in strength probably results from activation of the silicate crystal lattice due to introduction of the alumina (or magnesia) into the crystal lattice with result- ant activating structural distortions.
Fig. 1.20. Development of strength of pure C3S and C3S with 1 per cent of Al2O31.55
The rate of hydration of C2S is also acceler- ated by the presence of other compounds in ce- ment but, within the usual range of the C2S con- tent in modern Portland cements (up to 30 per cent) the effect is not large.
The influence of the other major compounds on the strength development of cement has been established less clearly. C3A contributes to the
strength of the cement paste at one to three days, and possibly longer, but causes retrogression at an advanced age, particularly in cements with a high C3A or (C3A + C4AF) content. The role of C3A is still controversial, but is not important with respect to strength in practice.
The role of C4AF in the development of strength of cement is also debatable, but there certainly is no appreciable positive contribution. It is likely that colloidal hydrated CaO.Fe2O3 is deposited on the cement grains, thus delaying the progress of hydration of other compounds.1.7
From the knowledge of the contribution to strength of the individual compounds present, it might be thought possible to predict the strength of cement on the basis of its compound compos- ition. This would be in the form of an expression of the type:
strength = a(C3S) + b(C2S) + c(C3A) +
where the symbols in brackets represent the per- centage by mass of the compound, and a, b, etc. are constants representing the contribution of 1 per cent of the corresponding compound to the strength of the hydrated cement paste.
The use of such an expression would make it easy to forecast, at the time of manufacture, the strength of cement and would reduce the need for conventional testing. Such a relation does indeed exist in laboratory tests using cements prepared from the pure four main compounds. In practice, however, the contribution of different compounds is not simply additive and has been found to de- pend on age and on the curing conditions.
All that can be said is that, in general terms, an increase in the C3S content increases strength up to 28 days;1.56 Figure 1.21 shows the 7-day strength of standard mortars made with cements of different composition and obtained from dif- ferent plants.1.37The C2S content has a positive influence on strength at 5 and 10 years only, and C3A a positive influence up to 7 or 28 days but a
negative influence later on.1.56,1.57The influence of the alkalis is considered on p.46. Prediction of the effects of compounds other than silicates on strength is unreliable According to Lea,1.38these discrepancies may be due to the presence of glass in clinker, discussed more fully in the succeeding section.
Fig. 1.21. Relation between 7-day strength of cement paste and the C3S content in ce-
ment.1.37Each mark represents cement from one plant
An extensive review by Odler1.79 has shown, moreover, that a generally applicable strength prediction equation for commercial cements is not possible for several reasons. These are: the in-
teraction between the compounds; the influence of the alkalis and of gypsum; and the influence of the particle size distribution of the cement. The presence of glass, which does not contain all the compounds in the same proportions as the rest of the clinker, but which affects reactivity, as well as the amount of free lime, are also factors varying between cements with nominally the same com- position of the four main compounds.
Attempts1.93 have been made to generate strength prediction equations for mortar on the basis of parameters which include, in addition to the main compound composition, terms for SO3, CaO, MgO and the water/cement ratio, but the re- liability of prediction is marginal.
From the foregoing, we can conclude that the relations between strength and compound com- position of Portland cements in general which have been observed are stochastic in nature. De- viations from these relations arise from the fact that they ignore some of the variables in- volved.1.14 It can be argued, in any case, that all
constituents of hydrated Portland cement contrib- ute in some measure to strength in so far as all products of hydration fill space and thus reduce porosity.
Furthermore, there are some indications that the additive behaviour cannot be fully realized. In particular, Powers1.22suggested that the same products are formed at all stages of hydration of the cement paste; this follows from the fact that, for a given cement, the surface area of hydrated cement is proportional to the amount of water of hydration, whatever the water/cement ratio and age. Thus the fractional rates of hydration of all compounds in a given cement would be the same. This is probably the case only after the rate of diffusion through the gel coating has become the rate-determining factor, but not at early ages,1.65 say up to 7 days.1.49 Confirmation of equal frac- tional rate of hydration was obtained by Khalil and Ward,1.70but we now accept that early hydra- tion of the different compounds proceeds at dif- ferent rates; later on, the rates become equal.
There is another factor influencing the rate of hydration: the fact that the composition is not the same at different points in space. This arises from the fact that, for diffusion to take place from the face of the still unhydrated part of the cement grain to the space outside (see p. 13), there must be a difference in ion concentration: the space outside is saturated but that inside is supersatur- ated. This diffusion varies the rate of hydration.
It is likely, therefore, that neither the sugges- tion of equal fractional rates of hydration, nor the assumption that each compound hydrates at a rate independent of other compounds, is valid. Indeed, we have to admit that our understanding of the hydration rates is still unsatisfactory.
For instance, the amount of heat of hydration per unit mass of hydrated material has been found to be constant at all ages1.34(see Fig. 1.22), thus suggesting that the nature of the products of hy- dration does not vary with time. It is therefore reasonable to use the assumption of equal frac- tional rates of hydration within the limited range
of composition of ordinary and rapid-hardening Portland cements. However, other cements which have a higher C2S content than ordinary cement or rapid-hardening cement do not conform to this behaviour. Measurements of heat of hydration in- dicate that C3S hydrates earlier, and some C2S is left to hydrate later.
Fig. 1.22. Relation between the heat of hydra- tion and the amount of non-evaporable water
for ordinary Portland cement1.22
Furthermore, the initial framework of the paste established at the time of setting affects to a large degree the subsequent structure of the products of hydration. This framework influences
especially the shrinkage and development of strength.1.14 It is not surprising, therefore, that there is a definite relation between the degree of hydration and strength. Figure 1.23 shows, for instance, an experimental relation between the compressive strength of concrete and the com- bined water in a cement paste with a water/ce- ment ratio of 0.25.1.39 These data agree with Powers’ observations on the gel/space ratio, ac- cording to which the increase in strength of a ce- ment paste is a function of the increase in the relative volume of gel, regardless of age, water/ cement ratio, or compound composition of ce- ment. However, the total surface area of the solid phase is related to the compound composition, which does affect the actual value of the ultimate strength.1.22
Fig. 1.23. Relation between compressive strength and combined water content1.1
Effects of alkalis
The effects of the minor compounds on the strength of cement paste are complex and not yet fully established. Tests1.3 on the influence of al- kalis have shown that the increase in strength beyond the age of 28 days is strongly affected by the alkali content: the greater the amount of al- kali present the lower the gain in strength. This has been confirmed by two statistical evaluations of strength of several hundred commercial ce- ments.1.56,1.57The poor gain in strength between 3 and 28 days can be attributed more specifically to water-soluble K2O present in the cement.1.58 On the other hand, in the total absence of alkalis, the early strength of cement paste can be abnor- mally low.1.58 Accelerated strength tests (see p. 623) have shown that, up to 0.4 per cent of Na2O, strength increases with an increase in the alkali content1.75(Fig. 1.24).
Fig. 1.24. Effect of alkali content on acceler- ated strength1.75
The influence of the alkalis on strength is complicated by the fact that they may be incor- porated into the calcium silicate hydrates or may exist as soluble sulfates; their action in the two cases is not the same. K2O is believed to replace one molecule of CaO in C2S with a consequent rise in the C3S content above that calculated.1.6 However, we can say that, generally, the alkalis increase the early strength development and re- duce the long-term strength.1.79 Osbæck1.95con-
firmed that a higher alkali content in Portland ce- ment increases the early strength and decreases the long-term strength.
The alkalis are known to react with the so- called alkali-reactive aggregates (see p.144), and cements used under such circumstances often have their alkali content limited to 0.6 per cent (measured as equivalent Na2O). Such cements are referred to as low-alkali cements.
One other consequence of the presence of al- kalis in cement should be mentioned. Fresh Port- land cement paste has a very high alkalinity (pH above 12.5) but, in a cement with a high alkali content, the pH is even higher. In consequence, human skin is attacked and dermatitis or burns may result; eyes can also be injured. For this reas- on, the use of protective clothing is essential.
We can see then that the alkalis are an im- portant constituent of cement, but full informa- tion on their role is yet to be obtained. It may be noted that the use of pre-heaters in modern dry- process cement plants has led to an increase in
the alkali content of cement made from given raw materials. The alkali content, therefore, has to be controlled, but limiting the alkali content too severely results in an increased energy consump- tion.1,76A more efficient dust collection also in- creases the alkali content of the cement when the dust is re-incorporated into the cement because the dust contains a large amount of alkalis; this may be as high as 15 per cent, in which case the dust, or some of it, has to be discarded.
Effects of glass in clinker
It may be recalled that, during the formation of cement clinker in the kiln, some 20 to 30 per cent of the material becomes liquid; on subsequent cooling, crystallization takes place but there is al- ways some material which undercools to glass. In fact, the rate of cooling of clinker greatly affects the properties of cement: if cooling were so slow that full crystallization could be achieved (e.g. in a laboratory), β-C2S might become converted to γ-C2S, this conversion being accompanied by ex-