Hydration of fly ash

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. 

Knudsen, T. (1985). On the possibility of following the hydration of fly-ash microsilica and fine aggregates by means of chemical shrinkage . Cement and Concrete Research 15(4) 720-722. 

The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

Options 5 and 6. Co-disposal with Fly Ash. Cost calculations for these two options were made on a basis similar to that for Option 4. The amount of fly ash to be co-disposed with coal-cleaning waste was equivalent to the addition of 25% of hydrated lime to the pile [2 ]. In addition to the extra capital and operating costs of earth-moving equipment at the disposal site, the costs of uploading, truck hauling for 15 miles, and unloading are included. A credit from the power plant is also included. 

Pozzolanic and blast furnace cements (or, alternatively, addition of fly ash or GGBS to Portland cement at the mixing plant) may be the most suitable choice for many stmctures that are critical from the durability point of view. In fact, they reduce the rate of development of heat of hydration, they lead to a lower content of alkalis and Ume in the cement paste, and they can produce a denser cement paste. They should be preferred, for instance, for massive structures (to reduce the rate of development of heat of hydration), or in sulfate-contaminated environments (Section 3.3), when there is risk of ASR (Section 3.4), or in chloride-contaminated environments (Section 12.5.1). 

In the Feret-Florentino test the dissolntion in HCl of the following fly ash samples is measured fly ash sample without treatment, second fly ash sample hydrated with lime for 7 days at temperatnre of 20 °C, the third one hydrated for 4 days at temperature of 50 °C. The difference between the solnbilities is the measure of fly ash pozzolanie activity. 

The observations under the scanning electron microscope revealed the occurrence of water film, 0.5 to 1 pm thick and free of hydrates, between the surface of fly ash grain and the layer of hydrates. This water film is filled up gradually with the hydration products however, this process is slow and not completed even after two years [29]. 

In Fig. 8.6 the hydration process of fly ash grain in the mixture with CjS is shown schematically, according to Takemoto and Uchikawa [23]. The calcium hydroxide from the liquid phase is adsorbed on the surface of fly ash grain negatively charged. 

The fly ash with low SO3 and alkali content and high calcium ion adsorption ability is promoting the hydration of C3A and accelerate the transition of ettringite into the monosulphoaluminate. On the other hand, fly ash, as well as natural pozzolana, rich in sodium and potassium sulphates lowers the rate of C3A hydration [18]. It is assumed that SO3 from fly ash hinders C3A hydration more intensively than gypsum, which is lirrked with other components of fly ash released to the solution at the same time. In this condition also the transformation of ettringite into the monosulphoalnminate is delayed [18]. 

As it can be concluded from aforementioned considerations, the quality of fly ash is determined by the alkali and sulphate content. These components have a great impact on the C3A and C3S hydration and consequently on setting and other properties of cement paste. 

Lime-activated fly ash binders consist of fly ash blended with hydrated or non-hydrated lime in amounts corresponding to 15-25 wt% of Ca(OH)2. 

The hydration of the clinker constituents is accelerated in the presence of fly ash, and it has been suggested that the fly ash particles act as nucleation sites for the C-S-H and CH precipitation (Uchikawa, 1986 Masazza, 1998). 

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. 

Fernandez, J. Renedo, J,. and Garea, A., Preparation and characterization of fly ash/hydrated lime sorbents for sulphur dioxide removal. Powder Technol., 94(2), 133-140 (1997),... 

The influence of fly ash and SF was tested by many researchers, and since the 1990s these microfillers have been considered as necessary constituents of high performance concretes (Malier 1992). As an example, tests carried out according to Spanish standards by Sanchez de Rojas and Frias (1995) gave interesting results, which are shown in Figure 4.2. The total amount of heat of OPC mortar is compared with that liberated by mortars with 30% cement replacement with fly ash and SF. The replacement with fly ash decreased the hydration heat due to its slower pozzolanic activity while that... 

The influence of fly ash, ground blast-furnace slag and other micro-fillers on the properties of high performance concrete is positive. As for ordinary concretes, these mix components densify the structure, and because of their pozzolanic properties they take part in hydration processes. Partial replacement of Portland cement by fly ash and ground slag enables a decrease in the cost of materials, improves the workability and reduces the heat of hydration. In practice, the majority of concrete structures are made with binary or ternary blended cements, which means that more than one additional binder is used with Portland cement. 

For the fly ash the Jenike design criterion estimates that their is no minimum critical outlet width for flow, which compares well, with experimental observations, which found the instantaneous outlet width to be below the minimum obtainable with the test silo. For the hydrated lime and olivine sand, the actual critical outlet widths at emptying were of a similar size to that of fly ash, but the Jenike method over predicted this by almost an order of magnitude. 

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