Fly Ash Hydration

This hypothesis is based upon the studies of Greenberg [25, 26], which has shown in his works that the silanol groups are the active sites on the surface of silica on which the calcium iorrs from the liquid phase are chemisorbed. Orrly the second elementary event is the release of silicon ions to the liquid phase and 

In the NaOH solution the solubihty of silica is the function of its specific surface areaiS  

Finally, the mechanism of the process can be reduced to the dissolution of fly ash glass or ciystalline zeohte, in the case of natural pozzolana. There is an opinion that the pozzolanas composed of zeolites are more reactive than the vitreous ones [27], This can be related to the high porosity of zeolites and their ion exchange ability [41], Therefore the bonding of calcium ions occurs veiy quickly and pozzolana is transformed into the alumina-siUca gel. 

The C-S-H formed has lower C/S ratio than in the Portland cement paste. Take-moto and Uchikawa [23] studied the chemical composition of C-S-H phase binding C3S crystals with fly ash particles. They found a significant decrease of C/S ratio from approximately 2 in the vicinity of C3S to the very low value near the fly ash grain surface (Fig. 8.5). 

Uchikawa [18] determined the composition of C-S-H phase in the Portland cement paste and in the pastes with 40% fly ash and slag addition after 4 years of hydration. The results are given in Table 8.1. 

---

E. the thixotropy, such as fly ash, hydrated hme, kaolin, diatomaceous earth and other raw or calcined pozzolanic materials and various rock dusts. [40, 42]. 

Class E—inorganic materials of very high specific surface area they increase the content of fine particles in the paste and therefore its thixotropic properties. The examples are siliceous fly ash, hydrated Ume and kaolin. 

Berry, E.E. et al. (1989) Beneficiated fly ash hydration rrricrostmcture and strength development in Portland cement systems. ACl SP-114, pp. 241-273. 

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 calculated failure functions for fly ash, hydrated lime and olivine sand which were used for outlet design, are illustrated in Fig. 2. A linear failure function was assumed, and least sum of squares regression was used to fit the measured data points. For olivine sand, individual failure functions were calculated from the strength measurements of the respective shear cells. For fly ash, and hydrated lime, as indicated by the correlation coefficients (in... 

Fig. 2. Failure functions for fly ash, hydrated lime, and olivine sand.

A comparison of the experimentally determined critical arch spans /outlet widths and the critical outlet widths calculated from the Jenike method [2] are given in table 2, for fly ash, hydrated lime and olivine sand. In table 2 at filling represents the measured maximum arch span which occurred prior to sustained flow of the stored bulk solid. The intervals on the predicted outlet widths are at the 95% confidence limit determined from statistical analysis of the failure function data. The recommended mass flow design lines [2] correspond with hopper half angles of 37, 28, and 30 degrees respectively for fly ash, hydrated lime and olivine sand. 

Figure 2.18 shows an example of the use of calorimetry to measure the activity of a biomass fly ash in water without portland cement at 20°C. This approach is sometimes useful to isolate the comparably low fly ash heat of hydration. Figure 2.19 shows a similar example where three different samples of high-calcium fly ash are hydrating in a simulated portland cement environment including 50% calcium hydroxide with 0.5 M NaOH at w/c of 1.0, thus using heat of fly ash hydration as a test method for fly ash activity in a well-defined chemical environment, excluding interference from Portland cement clinker. 

Figure 2.18 Biomass fly ash hydration power in water, comparing two reactive samples with one nonreactive sample (Ash 3) (a) 0-8 h, capturing the initial hydration (b) 0-170 h capturing later reactivity. Note the different scales of the y axes to highlight the difference in hydration between the two reactive biomass ashes starting at approximateiy iOO h.

Figure 2.19 Three different samples of high-calcium fly ash hydrating in a simulated port-land cement pore solution environment, with excess water, calcium hydroxide and 0.1 M NaOH. (a) Initial hydration 0-2 h after mixing, (b) Residual hydration 0-24 h after mixing. Note that the scales of the y axes are different in a and b.

The significant increase in the use of supplementary cementing materials (such as fly ash and slag) in the last decade has dictated the need for an admixture that can offset the slowed hydration that results when such materials are incorporated in concrete. Strong basic salts such as sodium aluminate, alkali hydroxides, silicates, sulfates and thiosulfates have shown some promise. A number of proprietary admixtures which claim to catalyze the pozzalanic and thereby increase the rate of hydration are now marketed. 

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... 

A thorough understanding of the chemical and mineralogical composition of CCPs is necessary for proper management of these materials. This chapter will cover (1) the composition of coal (2) the formation of CCPs (3) the physical, chemical, and mineralogical characteristics of CCPs (4) characterization of North American fly ashes (5) hydrated minerals in fly ash/ water pastes (6) sulphur scrubbing products and (7) environmental impact of CCPs. 

The principal hydration product identified by XRD in moderate-Ca Fort Union fly ash/water... 

Fig. 8. Scanning electron micrograph of hydrated Class-C lignite fly ash (from Fort Union coal). Eltringite crystals are the small bumps seen on the surface of the fly ash particles.

Fig. 9. Scanning electron micrograph of hydrated Powder River Basin high-Ca fly ash. The phases identified by XRD included ettringite (bumps on surface), monosulphate, and stratlingite (the latter are both platy).

One such Ca-aluminate hydrate mineral was observed in fly ash-water pastes (Tishmack 1999). The fly ash analysed in the study was washed with deionized water and Na-EDTA to selectively remove the soluble S-bearing minerals prior to being blended with additional CaO and water. The XRD analysis of the hydrated pastes showed peaks that closely matched the pattern of hydrogamet with one notable exception, a strong peak at 32.1° 2-theta. The phase may be a tetracalcium aluminate hydrate prototype AFm mineral noted to form in systems that have a low S C3A composition (Highway Research Board 1972). It may also be possible that the phase had a composition close to that of hydrogamet with some substitution of Si for Al. 

The hydration behaviour of high-Ca fly ashes, such as PRB fly ash, makes these materials a valuable replacement for Portland cement in concrete. It is commonly added at levels as high as... 

---