The scaling resistance of slag-blended cements can be critical when tested according to the procedure described in the present ASTM C 672 standard. As soon as slag replacement is greater than 20 percent, the scaling resistance of concretes made with a slag-blended cement deteriorates rapidly. But when the initial water curing procedure is lengthened from 13 to 27 days, the slag-blended samples have enough time to become more fully mature and they easily pass the scaling test, even when the substitution rate is as high as 80 percent. Lengthening of the present initial water-curing period from 13–27 days should be considered in the case of slag-blended cements, and most probably for other blended cements as well. This would allow for concretes made with a blended cement to reach the same degree of maturity as concretes made with pure Portland cement when they are submitted for the first time to freezing and thawing cycles in the presence of deicing salts.
For each tonne of cement used, the cement industry emits an average of 0.9 t of CO2, which contributes to the greenhouse effect. To satisfy the demands of the concrete industry for cementing materials, new environmental requirements, and the implementation of a sustainable development policy, the use of supplementary cementitious material as a replacement of part of the Portland cement has proven to be an interesting avenue that has not yet been fully explored. Granulated blast-furnace slag has been and is being used as a supplementary cementitious material in replacement of cement in many countries. In Canada, its proportion is usually limited to 20-25% of cement replacement owing to a significant decrease in early age compressive strength as well as a lower scaling resistance. In this study, we have tried to show that by reducing the water:cement ratio we can increase cement replacement by slag up to 50% without harming its short-term compressive strength and scaling resistance. The concretes that were prepared had a workability comparable to that of the reference concrete without slag, sufficient compressive strength to allow demoulding after 24 h, very low chloride ion permeability even at 28 d, as well as very good freeze-thaw and scaling resistance, as long as it is water-cured for a slightly longer period.Key words: high-performance concrete, blast-furnace slag, sustainable development, superplasticizers, workability, durability, silica fume.
The use of viscosity-enhancing admixtures increases the homogeneity of cement-based materials and leads to greater uniformity of hardened properties. These admixtures increase the yield value and viscosity rheological parameters; however, their impacts on enhancing the thixotropic behavior of the cement paste and concrete are unknown. The effect of both classes of admixtures on the rheological parameters can be quite different. Common viscosity-enhancing admixtures function by imparting structure to the liquid phase, whereas thixotropy admixtures function by imparting structure to the solid phase. The effect of welan gum and cellulose-based viscosity-enhancing admixtures and propylene carbonate (PPC) thixotropic admixture on the performance of cement grout made with a water-cement ratio (w/c) of 0.40 was investigated. The mixtures had equal initial flow-time consistencies and were tested to determine their rheological parameters, thixotropy, bleeding, setting time, heat flux, and strength development. The effect of combined additions of the thixotropic admixture with a low dosage of viscosity-enhancing admixture on slump retention, stability, setting time, and strength development was also evaluated for flowable concrete with a w/c of 0.41. Grout and concrete mixtures made with the thixotropic admixture exhibited a greater demand for high-range water-reducing admixtures and losses in fluidity than those made with a viscosity-enhancing admixture. The use of PPC led to considerable improvement in time-dependent stability characteristics, including static bleeding and surface settlement. On the other hand, the increase in thixotropy was not effective in improving stability characteristics determined shortly after mixing and prior to the development of thixotropy. This included resistance to pressure filtration and washout. The combined additions of PPC and low concentrations of a cellulosebased viscosity-enhancing admixture were very efficient, however, in enhancing stability compared with similar mixtures made with either admixture. No adverse effects on setting or strength development were observed with these combinations.
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