Cements with Fly Ash Addition

The mortars produced from cements with fly ash show, however, lower strength development at early age. Simultaneously, one should take into account the inevitable rise of concrete temperature, which favors the pozzolanic reaction and improves the strength. At 20 °C the pozzolanic reaction commences not before 10-14 days [132, 133]. 

another drawback of fly ash cements becomes evident— the veiy low strength increase at low temperatures. The improvement of strength development can be achieved by very fine fly ash grinding. 

Taking into account the properties of fly ash cements, as well as the acceleration of pozzolanic reaction at higher temperature, maity authors recommend to apply this cement in the production heat treated precast elements,. For example Dalziel [138] found that the thermal treatment of cement with 20 % of fly ash at temperatme of 65 and 85 °C result in higher strength of mortar already after three days and this trend is continued during all smdied period. The flexural strength increase was particularly evident (Fig. 7.22). This behaviour was proved by the other authors [113]. 

Concretes from cements with fly ash addition exhibit a higher sensitivity to the moisture deficiency at early age. They should be carefully cured protected against the moisture loss, because of the shrinkage cracks risk. 

The increased resistance to the aggressive environment is an advantageous feature of these materials. It is linked with Ca(OH)2 content lowering in the paste, but primarily to the reduction of larger pores share, that means the permeability decrease. The ion exchange ability decreases and this counteracts the corrosion reactions progress. Obviously, these properties are developing with the fly ash content increase, and in the case of sulphate attack— with CjA content decrease in cement clinker. The apparent diffusion coefficient of Na and Q decrease with siUceous fly ash addition is shown in Fig. 7.23 [139]. 

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There are traditional prejudices against the use of slag cement or Portland cement with fly ash addition in reiirforced concretes, however, they have not been proved experimentally [311, 352], On the contrary, a quite different behaviour of these materials was reported presumably this can be the consequence of reduced permeability of concrete and the resulting lower diffusion (see Table 6.9). However, apphcation of higher content and higher class cement has a positive influence on the dnrabihty of reinforcement. 

The three component cements with fly ash addition, apart from granulated blastfurnace slag CEM II/B-(S-V), have the advantageous properties first of all fairly good strength development. 

The authors [142] are reporting that the same advantage can be achieved in concrete production using cement with fly ash addition, with simultaneous warranty of its quality and optimum properties, which are under constant control. These authors are reminding also that the new ASTM C 1157 Standard edited in 1998, does not restrict the type and percentage of mineral addition, under the condition that cement complies with other standard requirements [142],... 

Fly ash, as it is a large-volume industrial waste, is both cheap and abundant, so that there is an economic incentive to use fly-ash-modified cements. In addition, C02 is also produced as a waste by-product of industrial processes (power generation, cement manufacture, etc.), and its permanent sequestration into cement is an added environmental benefit. A fully carbonated Portland cement permanently sequesters about 130 L of C02 per kilogram of cement. Figure 15.8 shows the structural and chemical modifications produced in cemented fly ash microspheres as a result of the supercritical C02 treatment. As is the case with fly ash, kiln dusts are primarily siliceous, so that the same benefits can be derived from their use as modifiers in immobilization and S/S matrices. 

Both cements (Table 7) compile with the requirements of the Yugoslav standard JUS B.C1.011, concerning chemical composition. Portland fly ash cement PPC has the expected higher insoluble residue and the loss on ignition regarding Portland cement OPC because of the fly ash addition. Other constituents were not significantly changed with the fly ash addition. 

Fly ash addition was obviously (Table 8) increasing the demand for the water for the standard consistence and prolonging the setting time, but has no influence on the other cement characteristics. All characteristics were in the compliance with the Yugoslav standard JUS B.C1.011. 

C-S-H in cement pastes reveals the C/S ratio in the range from 1.75 to 1.85 and these value do not alter neither with time nor with w/c, as it has been establish by Rayment and Lachowski [51] in numerous experiments. In the pastes of cement with slag or fly ash addition this C/S ratio is much lower. 

For the pore size distribntion determirration the sample must be dried, arrd as it has been merrtioned, the properties of cemerrt paste rrricrostmcture strongly depend on the conditiorrs of drying. Marsh [59] studied the pore size distribution for the samples dried at terrrperatirre of 105 °C and the samples in which water was replaced by propane. The samples cirred one day and those with the fly ash addition after different time of cirring reveal the sirbstantial differences (Fig. 5.28). However, the results for Portland cement pastes matured for longer time are relatively similar [59]. 

The similar reduction of water permeability was found in the case of a paste from cement with silica fume, rice hush ash and slags [138, 141], Also concretes produced of cement with 35 % fly ash addition show 2-5 times lower permeability, as compared to concrete from cement with no mineral additions [139]. 

Berry and Malhotra [118] conclude that in any high quality concrete with fly ash the carbonation process is comparable to this in concrete without mineral additions. Concrete with low cement content, not snbjected to the proper curing at early age (stored at low humidity) will be undoubtedly susceptible for the action of various corrosions physical and chemical, including carbonation [118]. 

Many research were focused on the effect of fly ash, siUca fume and granulated blast furnace slag addition on the concrete freeze-thaw resistance. All these additions improve the freeze-thaw resistance, as compared with the concrete from Portland cement, however, on condition that all these eoncretes are air entrainment [80], Freeze-thaw resistance of concrete with fly ash ean be lowered in the case when it has high coal content. The limit value, increased recently to 9% (according to the European standard EN 197-1 2002/A3 2007), is too high and the 5% level should be maintained, as it was reeommended by the elder directives (previous standard EN 197-1 2002). The effeet of non-eombusted coal on the appUcabilily of fly ash is diseussed in details in Seet 7.4. 

Fig. 7.21 Relative strength of cement pastes with 30 % fly ash addition (according to [137])...

Fly ash addition is diminishing also the risk of reaction of alkalis from cement with aggregate, which was discussed in Sect. 6.4. The fly ash addition, which allows the reduction of OH ions concentration in the liquid phase of the paste to 0.3 mol/1 is protecting concrete against destraction [140]. This corresponds to about 40% of fly ash addition to cement with average alkalis content (Na2O =0.92%) or about 30% in the case of cement with lower alkalis content (Na2O =0.68%). 

At low fly ash additions— that is, below 15%—the extent of carbonation in mature fly ash concrete tends to be equal to or lower than that in similar concrete mixes with no ash, in spite of the lower calcium hydroxide content of the formed hydrated cement paste (Buttler et al., 1983 Hobbs, 1988 Goni et al, 1997). This is due mainly to the reduced permeability of the paste to CO2. However, at higher fly ash contents the resistance to carbonation is significantly reduced (Goni et al, 1997). 

Fly ash may be added directly to concrete mix as an additional admixture or as a cement replacement. It may be also added to the cement clinker before grinding. In both cases, usually the mass fraction of fly ash is limited to 30%, but additions over 50% are also reported with good results. The initial strength (at an early age) of concretes blended with fly ash is lower, up to approximately 28 days, and later recovery of strength depends on the intensity of pozzolanic reaction. Strength increase, due to active fly ashes, may be expected at the age of 90 days. 

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. 

The addition of fly ash to cement results in the formation of decreased amounts of calcium hydroxide in the hydration product. This is attributable to the dilution effect and to the consumption of calciiun hydroxide by the pozzolanic reaction with the fly ash. In Fig. 1, the amount of calcium hydroxide formed at different times of hydration in cement containing fly ash is given. The amount of Ca(OH)2 estimated by TG was found to be lower in samples containing fly ash. With the increase in the amount of fly ash, less calcium hydroxide was formed because of the pozzolanic reaction and dilution effect. Even at 60% fly ash, some lime was present in the mortar, and the pH was found to be 13.5. At this pH value, the passivity of steel is assured. It can also be observed that there is more lime at 60% fly ash than at 75% slag addition. 

There are six stabilization techniques currently available however, only two of them have found widespread application. These are cementation and stabilization through the addition of lime and fly ash.25 26 There is currently developmental work being undertaken to make use of bitumen, paraffin, and polymeric materials to reduce the degree to which metals can be taken into solution. Encapsulation with inert materials is also under development. 

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