Pozzolanic durability

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). 

FGD scrubber material Stabilized base - Stabilized or fixated FGD scrubber material has been used successfully for road base construction, at a number of different sites in Florida, Pennsylvania, Ohio, and Texas - Stabilization or fixation of FGD scrubber material can be accomphshed by the addition of quicklime and pozzolanic fly ash, Portland cement, or selfcementing fly ash - The FGD scrubber sludge is dewatered before the addition of stabilization or fixation reagents - Additional fixation reagents may need to be added for stabilized base construction in order to meet compressive strength or durability requirements 66-80... 

In addition to strength, the addition of pozzolanic materials, and in particular metakaolin, leads to a refinement of the pore structure (Khatib and Wild, 1996) and significantly reduces the permeability of the resulting concrete (Caldarone and Gmber, 1995). This, in due course, will improve the overall durability and resistance against chemical attack, including sulfates (Wild et al, 1997). 

As in the pozzolanic reaction free calcium hydroxide is consumed, and is replaced by phases of extremely low water solubility lime mortars combined with natural or artificial pozzolanas attain a high degree of durability and water resistance if allowed to be precured for a sufficiently long time. Thus, not surprisingly, many stractures built in ancient times—especially by the Romans—using these binders have been preserved until the present day, even when constracted to be used as aqueducts. 

They can be used alone with lime for low-cost bulk filling operations such as backfilling in old mine workings or to extend and improve the properties of Portland cement. Their use reduces permeability and heat of hydration, leading to improved durability. The downside is that strength development is slower. Analysis is important to discover the likely reactivity of the pozzolan and to determine whether it contains anything likely to be deleterious to its intended use. 

Pozzolanic reaction courses slower than reactions between clinker components and water however its speed can be increased by varying the composition and surface area of the pozzolanic additive. Pozzolans more commonly used to increase durability of cement composites with cellulosic fibers are silica fume, metakaolin fly ash and slag. 

Gutierrez et al. [50] investigated the effect of replacing part of Portland cement by pozzolans as silica fume, fly ash and metakaolin on the mechanical properties of cement mortar reinforced with synthetic and natural fibers. The mechanical behavior and durability of composite materials was improved by replacing 15 wt% of cement with metakaolin or silica fume however, because of its low pozzolanic power, fly ash had no effect on the durability of these materials. Khorami and Ganjian [62] increased flexural strength in 20% of cementitious materials reinforced with straw fibers and eucalyptus, by replacing 5 wt% of Portland cement with silica fume. 

Rajczyk, K., Giergiczny, E., Glinicki, M. A. (1997) The influence of pozzolanic materials on the durability of glass fibre reinforced cement composites , in Brittle Matrix Composites 5, A. M. Brandt, V. C. Li and I. H. Marshall eds, Cambridge and Warsaw Woodhead Publishing-Bigraf, pp. 103-12. 

The role of individual supplementary cementing materials and additives has been described in the previous sections. The effect of these materials, viz., silica fume, slags, rice husk ash, fly ash, metakaolinite, or natural pozzolans on the properties of concrete such as workability, setting, rate of hydration, strength development, heat, durability, etc., is known to... 

An important development in concrete mix design has been the addition of so-called supplementary cementitious materials. Two classifications apply to these compoimds Pozzolans, such as fly ash and silica fume, react with the cement hydration products, notably calcium hydroxide hydraulic materials, such as granulated blast furnace slag, undergo direct hydration reactions. As these materials are of lower cost than conventional cement and essentially represent environmental waste products, there are obvious incentives for blending them in concrete mixes. As pointed out by Hansson, these materials can improve the strength and durability of concrete, with the important proviso that the concrete is cured adequately. 

Retrofitting of masonry joints with mortar is a common intervention work of restoration projects. Before designing the mortar s composition, a systematic analysis of existing mortars is made to meet requirements for compatibility and a decision is made about strength demand and durability. The selected traditional type binding system, such as lime-pozzolan, may also include a small percentage of brick bust or cement and admixtures to meet the compatibility and... 

The use of conventional pozzolanic materials has been studied extensively [29,84,110-112], in particular the use of fly ash. These additives did not prevent a loss in strength and toughness, but they did slow down the rate of loss. Singh and Majumdar [112] reported that the best results, in terms of strength and toughness retention, were obtained with a 40% fly ash content. They pointed out, however, that these high level additions were accompanied by some reduction in the initial properties. The extent of the improvement in durability varied with different fly ashes and natural pozzolans [84,110]. Leonard and Bentur [84] reported that such differences could not be correlated with the pozzolanic activity, but rather with the effect of the fly ash on the interfacial microstructure developed. 

The improvements in durability with the addition of conventional pozzolans such as fly ash were relatively modest. Thus, special attention has more recently... 

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