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Cubes and slab specimens prepared from three concrete mixtures containing three types of fly ash (FA), Classes F, C, and F/C, and a control portland cement concrete (PC) were cast and moist cured for either 7 or 28 days. Following 14 days of air drying, the cubes were immersed for 21 days in, and the slabs ponded for 12 months with, 15 percent NaCL solution. Chloride-ion distribution profiles were determined at four depth intervals. For all four concretes, the chloride ion permeability decreased when the length of moist curing was increased from 7 to 28 days. However, after 12 months of testing, the two mixes containing FA Class F and F/C exhibited such low chloride-ion contents at depths below 25 mm that the length of moist curing had little influence on chloride-ion reduction in contrast to that found for PC concrete, and to a lesser degree, concrete containing FA Class F. Use of all thre FA materials resulted in remarkable improvement in concrete impermeability when compared with PC concrete.

It is well known that reduction of free lime in reinforced concrete by air carbon dioxide neutralization and calcium carbonate formation makes steel reinforcement more susceptible to corrosion, since the reduction of pH eliminates the passivity conditions and can promote iron corrosion. On the other hand, the addition of pozzolan also causes a reduction of free lime by producing calcium silicate and calcium aluminate hydrates. It may therefore seem that fly ash addition, which improves many aspects of concrete durability, can specifically reduce the iron protection from corrosion promoted by reduction of pH. Concretes containing different amounts of ordinary portland cement and fly ash were produced. Fly ash (an additional 20 percent by weight of cement), was used either to replace cement or as an additional ingredient without any cement reduction. Plain and reinforced specimens were manufactured. All the specimens were kept in a carbon dioxide enriched (30 percent by volume) room to accelerate the carbonation process. The carbonation depth on plain concrete specimens and electrochemical measurements on reinforced specimens were carried out. The results indicated that fly ash addition reduces the carbonation rate when used without cement reduction, whereas it accelerates the process when used to replace cement. The electrochemical measurements of electric potential and polarization resistance are not significantly affected by the presence of fly ash, especially when added without cement reduction. The visual observation of iron reinforcement indicated that fly ash addition does not substantially modify the corrosive aspects, even when it replaces cement.

Adequate curing is essential for all concrete, whether it contains fly ash or not, if the potential properties of concrete are to be fully realized. However, since the long-term benefits associated with the pozzolanic reaction have become more evident in well-cured concrete, it has been generally considered that concrete containing fly ash has a greater susceptibility to poor curing than plain concrete. Tests were carried out at the Department of Civil Engineering of the K. U. Leuven on a series of mortar mixes with a range of fly ash-cement ratios to study the effect of curing on the strength development of mortar. Mortar specimens were subjected to a range of moist-curing periods prior to air-storage. Compressive strength was determined at various ages. The results confirm the importance of curing, with reductions in curing period resulting in lower strength. The strength of the mortar containing fly ash appears to be more sensitive to poor curing than the plain mortar.

Class F fly ash was used as the basic granulating material. Catalysts and binders were added to evaluate the behavior of granulated material. The accelerated curing method was also considered. Results indicate that granulation rate depends closely on the slant angles of the disc, the revolving rate, the methods of adding admixtures, and granulation time. Though aluminum powder reduced unit weight and raised the strength of fresh particles, it had a detrimental effect on other properties. Addition of hydrophilic seed reduced the granulation time and increased productivity. The results showed that at a constant relative humidity, the higher the temperature, the more rapid and higher the strength development. It is important to maintain constant temperatures or low strengths may result. Normal steam curing, autoclave curing, and microwave steam curing have beneficial effects on the strength of fly ash lightweight aggregates. The differences in curing results are due to differences in mix proportioning of aggregates and duration of curing.

The admixtures of condensed silica fumes (CSF) and phosphogypsum (neutralized and dehydrated at 400 C) were used together with fly ashes as blended cement components to improve early strengths and other properties. The cements with the initial 15 to 50 percent low-calcium PFA content (SiO2 + AL2O3 + Fe2O3 - 83.3 percent) or 15 to 70 percent high calcium PFA content (22.1 percent CaO) were mixed with the additional components just mentioned. Standard tests at normal curing were made, as well as measurements after the low-pressure steam treatment at 70 C. All cements mixed with CSF showed standard compressive strengths about 13 to 20 MPa higher than the reference mortars. More detailed studies of the hardening process were also carried out using calorimetry, DTA, TG, XRD, and porosimetry, which showed acceleration of the hydration process due to pozzolanic properties of CSF. Reduction of total porosity and pore size was also found. The same positive effect of CSF was observed in the case of mortars treated at 70 C. This additive improves significantly the pozzolanic properties of low-calcium PFA. At standard curing, activated phosphogysum addition brings about a decrease in the hydrated calcium silicates. A substantial amount of ettringite forms and partially inverts into monosulfate after 28 and 90 days of hardening. At accelerated curing, the mortars containing phosphogypsum show a significantly higher degree of hydration than the reference mortar. The results relating to pastes and mortars have been confirmed for concretes. Therefore, one can conclude that the admixtures studied, particularly CSF, have positive influence on the properties of PFA concretes and help to augment the effect of PFA content in these concretes.