Hydration of the clinker phases

In the system Portland cement-fly ash-water the initial hydration of the clinker phases, especially those of tricalcium silicate and tricalcium aluminate, progresses much faster than that of the ash. Just as in the hydration of pure Portland cement, a C-S-H phase, ettringite, and calcium hydroxide are formed as the first hydration products. Any alkali sulfate salts, commonly present in the clinker, also dissolve and convert to alkaU hydroxides and calcium sulfate in a reaction with calcium hydroxide. The alkalinity of the liquid phase increases, and may exceed pH=13. 

The microstracture of the hardened paste of Portland-fly ash cements does not differ significantly from that of ptrre Portland cement. Amorphous or poorly crystalline phases formed in the hydration of the clinker phases and the pozzolanic reaction accoimt for the main mass of the hardened cementitiorrs matrix. The amormt of portlandite is redirced at later stages of hydration, especially in mixes with high ash contents. The ash particles... 

Under these conditions only a small fraction of the ash present participates in the hardening process, as the amount of calcium hydroxide liberated in the hydration of the clinker phases and needed for a pozzolanic reaction is very limited. The products of the pozzolanic reaction concentrate preferentially in the vicinity of the residual ash particles (Zhang, 1995). The ash present in the fresh concrete mix affects its rheology positively, and acts as a filler in the hardened concrete. 

The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20°C and 100% RH, Patel el al. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, 1.00 and 0.51. If the RH was lowered to 80%, the corresponding values were 0.77, 0.19, 0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of RH on the hydration rate (da/d/) of each of the phases could be represented by a factor (RH — 0.55)/0.45. ... 

Uchikawa (UI7) reviewed the hydration chemistry of pfa and other composite cements. Pfa cements differ from pure Portland cements notably in (i) the hydration rates of the clinker phases, (ii) CH contents, which are lowered both by the dilution of the clinker by pfa and by the pozzolanic reaction, (iii) the compositions of the clinker hydration products and (iv) formation of hydration products from the pfa. The two last aspects cannot be wholly separated. 

In contact with an aqueous medium (water or POW) the cement content of both c/p dispersions undergoes hydration. This is manifested in the decrease of the clinker phases as well as the increase of the amorphous content. This is due to a preferential precipitation of hydration products in the form of small, needle-like crystals - the so called C-S-H gel - which are too small for proper XRD detection. The decrease of the clinker phases especially of the C3S phase is fairly rapid in the first 24 hours, but faster in POW than in water. In addition, less portlandite is formed in water than in POW. The higher portlandite content after exposure to POW can be attributed to the high content of hydroxide ions (pH=13.4) in the solution forcing the precipitation of calcium ions. 

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. 

The amount of calcium hydroxide in Portland-fly ash cement pastes undergoing hydration increases in the initial stage of the process, as the rate of the liberation of this phase in the hydration of the C3S and C2S is higher than the rate of its consumption in a reaction with the glass phase of the ash. However, after reaching a maximum, the amount of free calcium hydroxide starts to dechne, as the hydration of C3S slows down and a more intensive hydration of the glass phase gets under way. Table 9.4 shows the free calcium hydroxide in a series of pastes made with cements that contained 0%, 20%, and 50% of fly ashes with different CaO contents. Cements in which quartz was used to replace clinker, instead of fly ash, served as controls. 

Some indication of the respectively contributions of the clinker phases to the strength development of cement is given in Fig. 9. However, these results obtained for individual phases cannot be directly applied to the conditions actually occurring in cement paste, as is apparent also from the heat of hydration values given in Table 10. Fig. 17 schematically shows the sequence of formation of the hydrate phases and the structure development in the setting and hardening of Portland cement. 

The alkali in clinker is combined as a solid solution with the C3A phase. The crystalline stmcture changes from cubic to orthorhombic or monoclininc structure, depending on the content of Na in the C3A phase. Shin and Han studied the effect of different forms of tricalcium aluminate on the hydration of tricalcium silicate by applying DTA, TG, and conduction calorimetry. It was concluded that the hydration of tricalcium silicate is accelerated when orthorhombic, monoclinic, or melt C3A was present in the mixture. The cubic form of tricalcium aluminate was least effective for accelerating the hydration of the silicate phase. 

Hydration of fly ash cement differs from pure cement in terms of the hydration rates of the clinker phases, amount of calcium hydroxide formed, composition of the clinker hydration products, and additional hydration products from the reaction of the fly ash.I l Lower amounts of lime are formed in the presence of fly ash because ofthe pozzolanic reaction between the fly ash and lime formed in cement hydration. Fly ash generally retards the reaction of alite in the early stages and accelerates the middle stage reaction. The accelerated reaction is attributed to the existence of nucleation sites on fly ash particles. The aluminate and ferrite phases hydrate more rapidly in the presence of fly ash, and also there is a significant difference in the hydration rate of the belite phase up to 28 days. Table 1 gives the relative hydration rates of cement compounds in the presence of fly ash as derived from conduction calorimetry. [" 1 It can be seen that the earlier rates of hydration are generally retarded, and the later stage hydration rates are accelerated. 

Concerning portland cement, the free lime is the phase most sensitive to moisture uptake (Dubina et al. 2011) see Table 1.1. As the water sorption starts at such a low relative humidity, the transformation of calcium oxide to portlandite cannot really be avoided in practise. Thus, the determination of free lime in clinkers and cements by quantitative XRD should always be verified by other methods, e.g. the extraction method of Franke (1941). From the cement clinker phases the orthorhombic C3A phase is the most sensitive to moisture. The prehydrated surfaces retard further hydration of the clinker grains (Dubina et al. 2011). Calcium silicates and calcium sulfates are involved as well in the prehydration process, as they may take up moisture already at quite low relative humidity. 

Figure 2.7 Comparison between measured and calculated heat flows of a portland cement hydrated at 23°C using a w/c of 0.50. Heat flow was calculated from quantitative XRD anaiyses using the dissolution enthalpies of the clinker phases and the precipitation enthalpies of the hydrate phases. (From Jansen, D. et al., Cement and Concrete Research, 42(1), 134-138, 2012a. With permission.)...

Filler effect Accelerating effect of fine, inert materials on the hydration kinetics of the clinker phases in blended cements because of the presence of additional nucleation sites and the presence of a higher effective water-to-cement ratio often wrongly assigned to (early) supplementary cementitious material reactivity. 

Initial reaction period (cement hydration) Occurs upon contact of the cement with water and is observed as an early exothermal signal in the calorimetry plot of cement hydration. Cement powder surface wetting and initial rapid dissolution of the clinker phases, CaO and alkali sulfate are considered to be the main contributors to the heat release. 

This chapter deals with chemical and physical properties other than ones for which the nature of the hydration products must be considered, which are treated in Chapters 5 to 8. In general, properties of the whole clinker or cement are alone considered, those of the constituent phases having been dealt with in Chapter 1, but factors affecting the reactivities of these phases are included as a link with the following chapters on hydration. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

By definition, the kinetic curve of a cement is the weighted sum of the curves for its constituent phases as they occur in that cement. The reactivities of individual clinker phases were considered in Section 4.5 and some effects of particle size distribution, which is a particularly important variable, in Section 4.1.4. Although many data relating particle size distribution directly to strength exist, much less is known about its relation to degrees of reaction. Parrott and Killoh (P30) presented data indicating that the rate of hydration, as represented by that of heat evolution, was proportional to the specific surface area during the period of hydration in which the rate was controlled by diffusion. 

The effects of the limestone are partly physical and partly chemical. As with many other finely divided admixtures, including pfa, the hydration of the alite and aluminate phases is accelerated. Because of its fineness the material also acts as a filler between the grains of clinker, though it is unlikely to be as effective in this respect as microsilica. Chemically, it reacts with the aluminate phase, producing C ACHjj, thus competing with the gypsum. 

A significant tx)le of the liquid phase has been found in many discussions dealing with the hydration process of particular cement clinker constituents. The changes of concentration, particularly those of SO and Al(OH)4, affect the rate of the process. Simultaneously, the easily ciystallized aluminate products, under the influence of the hquid phase, change into the other aluminates having another composition and morphology. For this reason, the liquid phase composition plays a key role in cement hydration. 

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