Hydration of Clinker Minerals

Popular models of hydration of clinker minerals can be divided into the delayed nucleation model and the protective layer model. 

During the setting of a cement, a large amount of added water will be used to form hydrate phases such as C-S-H, portlandite, and AFt and AFm phases. Yet, even after complete hydration some free water will remain confined within the porous C-S-H gel and the capillary pore spaces. Notably, this occurs when surplus water is added so that the water/cement (W/C) ratio exceeds 0.6. A lack of water (W/C raho <0.2) results in an incomplete hydration of clinker minerals. The optimum W/C raho for Portland cement, leading to maximum strength, is 0.3 < W/C < 0.5. 

In the hydration of cement, Mg ions present in the crystalline lattices of clinker minerals are incorporated into the stmcture of the formed hydrate phases. Free MgO, present as periclase, also hydrates, yielding hexagonal magnesium hydroxide [Mg(OH)2], called brucite ... 

Alkali metals are present in clinker preferentially in the form of sulfates. The rest are incorporated into the crystalline lattices of clinker minerals. Sodium tends to be preferentially taken up by the calcium aluminate phase, whereas potassium is mainly incorporated into the crystalline lattice of dicalcium silicate, which becomes stabilized in its form. In either case, in the cotrrse of hydration Na and iorts enter the liquid phase together with the balancing aniorts, which eventrrally become incorporated into the hydration products, being replaced with eqirivalent quantities of OH ions ... 

Finally, microscopical examination alone may not provide sufficient answers to the questions of clinker microstructure or a cement s inferior performance. Cement particle size distribution, variations in crystal chemistry, mineral and chemical admixtures, as well as the effectiveness of the set-controlling material (normally gypsum or similar minerals), may have stronger effects on cement hydration than the clinker production problems inferred by routine microscopy. Some clinker and cement problems, however, are simple and easily solved others require the analysis of a tangled set of multiple causes and effects. Microscopy should be one of the first steps in that analysis. 

Morphology changes taking place during hydration reactions of Portland cement clinker mineral - tricalcium silicate 3 CaO Si02 and various cements were monitored by DSA [35,36]. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

Scharf, H., and Odler, 1. (1992) Intrinsic bond properties of hydrates formed in the hydration of pnre clinker minerals, in Proceedings 9th ICCC, New Delhi, Vol. 4, pp. 265-270. 

Table 19.1 Heat of hydration of pure clinker minerals.

Figure 5.9 summarizes the processes that occur during the hydration of anhydrous clinker minerals (left-hand column) to form the products shown in the right-hand column. The rectangular areas occupied by the individual phases correspond approximately to their volumetric proportions in Portland cement. 

Tamas, F., The Hydration of Portland Cement and Clinker Minerals Investigated by Thermal Methods, Proc. 6 Conf. Silicate Industry, pp. 425-436, Budapest (1961)... 

Skalny, J., and Odler, L, The Effect of Chlorides Upon the Hydration of Portland Cement and Upon Clinker Minerals, Mag. Concr. Res., 19 203-210(1967)... 

Portland cement is manufactured by burning until partial melting of an intimate mixture of substances that mainly contain calcium Ca, silicon Si, aluminium A1 and iron Fe. During burning, so-called clinkers are formed. When ground, these clinkers are able to react with water - to hydrate - and to deposit hydrates with binder properties. A typical clinker mineral composition of Portland cement is ... 

The four clinker minerals mentioned have characteristic properties with regard to rate of heat development and rate of strength development during hydration subsequent concrete durability and colour. Therefore, the useful quahties of Portland cement depend, at least in part, on the quantitative proportions of these clinker minerals in the cement. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Portland cement clinker potential phase composition is presented in Table 4. It could be seen that the C3A content in the clinker was 9.46% which is important for the cement hydration rate and cement sulfate resistance. Common Portland cement is not resistant to the sulfate influence because of the significant C3A content, whose hydrates react with sulfate ions resulting in expansive compounds. Portland cement with the higher resistance to sulfates must have low C3A content. Moderate to high content of mineral alite - C3S (54.72%) is usual for the Serbian cement plants and enables the addition of higher quantities of mineral admixtures without influencing the quality of final cement. 

By mixing Portland cement with water, the hydration process begins. The hydration process is the combination of physical and chemical changes during which, the cement previously mixed with water becomes a porous sohd. During this process a number of exothermic reactions give rise to cement matrix. In the case of Portland cement, the main reactions are based on interaction between two mineral components present in higher proportion clinker and water. These reactions can be written in simplified form thus [30, 65, 66] ... 

FIG. 4—The mineral alite (A) is found exclusively in clinkered cement products. Note how the relict grains appear to stand out above the surrounding matrix. This is a function of the slower light velocity through the mineral as opposed to surrounding materials. This relief helps identify alite. The arrow indicates a colorless halo around the grain that represents a hydration rim produced during the reaction of alite with added mix water. 

---