Alite content

Conversely, coarsely ground clinkers with lower alite content are preferred for situations where cement emplacement is slow or temperatures can be high, as in the lining of oil wells. In such circumstances, organic... 

All the effects described above indicate that rapid cooling is desirable the aluminate phase reacts more slowly with water when finely grained and intimately mixed with ferrite, making it easier to control the setting rate (S24), decrease in alite content either from reactions involving the interstitial material or from decomposition is avoided, a higher MgO content can be tolerated, and the clinker is easier to grind. 

Minor or trace components derived from raw materials, fuel, refractories or other plant materials, or added deliberately, can affect the reactions of clinker formation, or the properties of the product, or both. Their effects can be beneficial or harmful. Beneficial effects include acceleration of the clinkering reactions or lowering of the temperature at which they occur, or increase in the reactivity of the product leading to more rapid strength development. Harmful effects include decrease in alite content, volatilization in the kiln with consequent formation of kiln rings or other deposits, decrease in the durability of concrete made with the cement, or the introduction of poisonous elements. Some elements have beneficial effects at low concentrations and harmful ones at higher concentrations. Bucchi (B33,B28) reviewed some of the effects on the manufacturing process. 

Several early studies using QXDA gave results in fair agreement with j those of the Bogue calculation, but in the light of the above and other recent evidence are of doubtful validity. Odier et al. (09) found that alite contents ... 

Parrott and co-workers (P30,P32,P35,P33) described a more sophisticated method for modelling the hydration process. The fraction of the total water porosity that was below 4nm was calculated by multiplying the volume fraction of C-S- H by an appropriate factor, which depended on whether the C-S-H was formed from alite or belite, the temperature and the amount of space available. The constants assumed were based on experimental data obtained using a procedure based on methanol sorption (Section 8.3.4). The effect of drying was allowed for (P35) by introducing a factor of 0.7 - -1.2(RH — 0.5) for 0.5 < RH < 1, or of 0.7 for RH 0.5. These refinements allow some deviation from the Powers-Brownyard postulate of a fixed volume ratio of gel porosity to product. Typical results for the volume fractions of pores larger than 4 nm in mature pastes of a cement with an alite content of 56% were approximately 0.26, 0.16 and 0.07 for w/c ratios of 0.65, 0.50 and 0.35, respectively (P32). For the two higher w/c ratios, these results are near the capillary porosities of Powers and Brownyard, but for w/c 0.35 the latter value is zero. 

In addition to the formation of alite, a-belite is transformed to P form and recrystalization and crystal growth proceeds for already formed components. If the clinker is underburned, it will contain an excessive quantity of CaO. The clinker, as a low alite content, is poor quality. If the clinker is overburned, the clinker results in large crystals. Table 31.5 shows the main characteristics and performances of the different classes of cement rotary kilns. 

The sulphate attack with C-S-H gel decomposition and thaumasite formation is particularly harmful for the durability of concrete, because it occurs with the destraction of the most important binding constituent of concrete and leads to the collapse of this material. Because of the relatively low temperatures of thaumasite formation, the concrete foundations, and elements of the underground sewage systems, as well as the concrete road elements are especially susceptible for this type of destructioa In order to prevent thaumasite formation the classic approach should be applied, first of all the permeability of concrete should be reduced, and the use of cement with mineral additions should be considered. Bensted [273] suggested the lowering the C3A and alite content in cement. 

Behte cements are not a technological novelty too. Portland cements produced in thirties, in XXc, had a considerably lower hme saturation factor and highbelite content. It was linked with much slower strength development and therefore it caused the gradual changes of cement phase composition, with the increase of alite content. 

Inorease in alite content and orystal size, increasing difficulty in burning, reduction in cement strength Relative increase in lime saturation factor (Long, 1982b)... 

High liter weight High alite content(Brown, 1948) increased time of burning at high temperature (Hawkins, 1979)... 

Increase in alite content, decrease in belite and free lime Increased nucleation of alite by adding 0.5% C3S (less than 20 pm) to raw mix (Strunge, Kndfel, and Dreizier, 1985)... 

Pronounced decrease in alite content Slow cooling under reducing conditions (Sylla, 1981)... 

Owing to the lower reactivity of belite, the overall rate of hydration, and along with it the strength development up to about 90-180 days, is slowed down with increasing belite and decreasing alite contents in the cement (Bei and Ludwig, 1990). At the same time the final strength of a Portland cement with an elevated C2S content may exceed that of an ordinary Portland cement, because more C-S-H and less portlandite is formed in the hydration of dicalcium than of tricalcium silicate. 

The main difference in the composition of cement pastes made from cements with increased belite and reduced alite contents is their lower calcium hydroxide content. This may affect positively the resistance of such hardened pastes to chemical corrosion. At the same time the depth of carbonation increases with declining C3S content in cement (Kelham and Moir, 1992). 

More recently, systems are being introduced that include an X-ray diffraction capability for the determination free lime content, and in some cases the C3S (alite) content, of the clinker. 

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