Reaction pozzolans

Methods of preventing alkali-aggregate expansion reactions in concrete consist of avoidance of reactive aggregates, use of cement with alkalis less than 0.6% Na20 equivalent, the use of pozzolanic materials, sealing of... 

By consuming the Ca(OH)2 the pozzolan makes it unavailable for sulphation and carbonation reactions and increases the density of the concrete, which lowers its permeability and reduces the risk of chloride attack. 

We have demonstrated that supercritical C02 can be used to accelerate the natural carbonation reactions in unmodified Portland cements, and that this treatment enhances the physical properties of the cement. Further, it has been proven that the use of supercritical C02 allows the replacement of Portland cement powder with inexpensive, lower-grade pozzolans, such as fly ashes. 

Lime/fly ash pozzolanic processes combine the properties of lime and fly ash to produce low-strength cementation. Kiln dust processes involve the addition of kiln dust to eliminate free liquids and usually form a low-strength solid. Lime-based processes for solidification use reactions of lime with water and pozzolanic (siliceous) materials, such as fly ash or dust from cement kilns, to form concrete, called a pozzolanic concrete. Wastes of desulfurization of gases and other inorganic wastes can be immobilized by this method. 

Uchikawa (UI7) reviewed the hydration chemistry of pfa and other composite cements. Pfa cements differ from pure Portland cements notably in (i) the hydration rates of the clinker phases, (ii) CH contents, which are lowered both by the dilution of the clinker by pfa and by the pozzolanic reaction, (iii) the compositions of the clinker hydration products and (iv) formation of hydration products from the pfa. The two last aspects cannot be wholly separated. 

Of the pfa characteristics that influence reactivity, the glass content appears to be much the most important, but specific surface area, glass composition and the effect of stress in the glass caused by the crystalline inclusions may also be relevant (U17). Of external factors, the RH, temperature (C43) and alkali content of the cement are probably the most important. Sulphate ion may also enhance reactivity by promoting the removal of AF from the glass (U17). The rates of the pozzolanic reaction and of strength development are more sensitive to temperature than are those of hydration and strength development for pure Portland cements (e.g. Ref. H52). 

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

Traetieberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with CH probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the CH content passed through maxima usually within the first day before beginning to decrease in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for CH content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements. 

Menzel s results were thus explained. Addition of a small amount of quartz brings the bulk Ca/Si ratio to 2.0, which is that of u-C,S hydrate. This phase is relatively dense and crystallizes as rectangular tablets the product is porous and weak. With larger amounts of quartz, pozzolanic reaction occurs the CFl is consumed, formation of u-C,S hydrate is avoided, and a C-S-Fl of low Ca/Si ratio is formed. The porosity is reduced, and a strong material results. The optimum addition of quartz is the maximum that can be taken up further additions act only as a diluent, and the strength decreases. Table 11.1 gives crystal data for u-C S hydrate and other phases. 

H53.H62). The properties were attributed to a combination of effects. The particles of microsilica, being much finer than those of the cement, partially fill the spaces between the cement grains, and this, together with the superplasticizer, allows the latter to pack more uniformly. They also provide nucleation sites for hydration products, undergo pozzolanic reaction and probably improve the paste aggregate bond. 

In pozzolanic reaction, the alkali silicate gel is formed in an environment rich in Ca and, except in a narrow zone close to the reacting surface, is quickly converted into C-S-H. In ASR, it is formed in an environment poor in Ca ", and massive outflows of gel may result. The cement paste cannot supply Ca " fast enough to prevent much of this gel from persisting for long... 

These hypotheses are not mutually exclusive, though the general slowness of pozzolanic reactions renders (3) unlikely. The evidence is insuHicient to determine their relative importance, which could probably be assessed from a combination of expansion data, pore solution analyses and SEM of polished sections with accompanying X-ray microanalyses made in parallel on a series of mortars of suitable ages and compositions. 

It should be noted that lime reacts with clay particles. This leads to strength increase by pozzolanic and carbonation cementation processes. Cation exchange and pozzolanic reactions result in strength increase. The level of reactivity and hence strength gained in soil-lime mixtures depends on the level of pozzolanic product created. The chemical reaction between soil and lime can be presented as below ... 

Before concluding this subject, mention is made here of two more novel approaches for using SBE. Pollard et al. (143) have reported that SBE can be used to prepare a pseudo-graphitic char suitable as a low-cost replacement for activated carbon in the stabilization/solidification of industrial wastes. In their process, they char 2 1 blends of SBE and ZnCl2 at 450°C/1 hour and then activate the material at 600°C/1 hour. The resultant hybrid material is as effective as activated carbon for fixing toxic organics and, because of its aluminosilicate framework, exhibits additional pozzolanic activity in the cement-based stabilization/solidification reactions in which these materials are used. Very recently (144), Bohling reported on... 

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