Hydrated aluminate phase

Copeland et al. (C38) observed that the XRD patterns of many pastes included peaks of a hydrogarnet in addition to those of other hydrated aluminate phases, and obtained evidence that this had formed by hydrationr... 

The loss above 550°C is due partly to CO2 and partly to the final stages of dehydration of C-S-H and the hydrated aluminate phases. It is not practicable to distinguish the contributions from TG evidence alone, and, unless evolved gas analysis is used, a separate determination of COj should be made. As with calcium silicate pastes, serious errors arise if TG determinations are carried out on material that has been treated with an organic liquid, e.g. to stop hydration. Losses above 550°C of more than about 3%, referred to the ignited weight, indicate serious carbonation either from this or other causes. 

The loss below the CH step is due to decomposition of C-S-H and the hydrated aluminate phases. Although the TG curves of pure AFm phases are markedly stepped in this region (Fig. 6.2), those of cement pastes normally show only slight indications of steps. Weak peaks can, however, sometimes be seen on DTG curves. The absence of steps is probably due to a combination of low crystallinity, the presence of other phases and the presence of AFm phases of different compositions in mixture or solid solution or both. For typical experimental conditions with a 50 mg sample, heating rate of 10 deg C min and Nj flow rate of 15 ml min , the volatiles retained at about 150°C, after correction for COj, correspond to the non-evaporable water, and those retained at about 100"C to the bound or 11% RH water, but this last temperature, in particular, is very dependent on experimental conditions (T5). 

Opinions have differed as to the possibility of determining the hydrated) aluminate phases by thermal or X-ray methods. The determination of ettringite was discussed in Section 6.2.2. Bensted (B99), who used DTAJ found that for ordinary Portland cements the ettringite content increase with time during the first 2 h to maximum values of 2.2-2.8%, and that th quantity of ettringite formed at any given time increased with the w/c ratio ... 

As with the slag cement discussed in Section 9.2.7, the calculated water contents for different humidity states are lower, and the porosities higher than for comparable Portland cement pastes. The water contents are lower because replacement of CH by C-S-H or hydrated aluminate phases causes relatively little change in the HjO/Ca ratio, so that the water content tends to fall towards the value which would be given by the Portland cement constituent alone. Diamond and Lopez-Flores (D39) found that, for two 90-day-old pastes similar to that under discussion (30% pfa, w/s = 0.5), the non-evaporable water contents were l2.5 /o and 13.0%, while that of a Portland cement paste was l5.4 /o. The porosities are discussed in Section 9.7. 

DTA (K32,025) and XRD (A29) studies indicate that cement pastes cured at temperatures of or approaching lOO C contain little or no detectable hydrated aluminate phases, the observation of hydrogarnet noted above thus being unusual. Kalousek and Adams (K32) considered that the Al , 864 and other ions were incorporated into a Phase X , which possibly included all the oxide components of the cement. In pastes cured normally, much of the AFm phase is probably poorly crystalline and intimately mixed with the C-S-H (Seetion 7.3.1). This tendency may be increased if hydration takes place more rapidly. 

Solutions of alkali hydroxides generally do not exhibit a corrosive action towards pastes of hardened Portland cement and related binders. Only at very high concentrations may a moderate corrosion become apparent, probably owing to degradation of the hydrated aluminate phases. Also highly resistant to alkaline solution are alkali-activated slag binders (see section 8.5). In contrast, hydrated calcinm aluminate cement may be attacked by high-pH solutions. 

C4AF. Aluminate phases and their hydration products therefore play an important role in the early hydration... 

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],... 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

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. 

The fact tliat both conventional water reducers and superplasticizers are more efTective if added some time after mixing provides a strong indication that adsorption probably occurs at least in part on the hydrated phases, as the anhydrous surfaces have by that time become covered with hydration products. Chiocehio cl al. (C57) found that the optimum time for addition was at the start of the induction period. More of the admixture seems to be taken up by the early hydration products, especially of the aluminate phase, if it is added before the early reaction has subsided. 

Carbon dioxide dissolves in the pore solution of cement paste, producing CO3", which reacts with Ca to produce CaCOj. The OH and Ca ions required by these reactions are obtained by the dissolution of CH and decomposition of the hydrated silicate and aluminate phases. C4AH. is quickly converted into C ACH, and ultimately into CaCOj and hydrous alumina monosulphate and ettringite yield CaCOj, hydrous alumina and gypsum. C-S-H is decalcified, initially by lowering of its Ca/Si ratio, and ultimately by conversion into a highly porous, hydrous form of silica. The... 

Leaching of concrete by percolating or flowing water has sometimes caused severe damage, e.g. in dams, pipes or conduits, and is potentially important for the long-term storage of nuclear wastes. Pure water may be expected to remove alkali hydroxides, dissolve CH and decompose the hydrated silicate and aluminate phases. Reference to the equilibria discussed in Chapters 5 and 6 indicates that, for practical purposes, the ultimate residue will consist... 

Some organic compounds, such as sugar solutions, attack concrete, presumably through complexing of Ca and consequent dissolution of CH and the hydrated silicate and aluminate phases. 

The calcium aluminates, especially CaAl204 (CA) and Ca3Al20e (C3A) react readily with water, contributing to the hydraulic activity of the cement. The Al in the hydrated phases is exclusively in six-fold coordination, making Al NMR a convenient method for monitoring the progress of hydration of both the pure aluminate phases and alumina cements (Figure 5.32A). This technique has been used to study the... 

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