International Concrete Abstracts Portal

A combination of ground granulated blast furnace slag, fly ash and calcium hydroxide has been used to immobilize acid nuclear wastes in the form of a hardened system in the absence of any aggregate. An aqueous solution of NaOH (19 M) was used for two purposes: first, to neutralize the acid waste and to transform it in a very basic system (pH>13); second, to activate the hardening of the binder. Due to the liquid-solid ratio lower than 0.70, the fresh mixture is relatively stiff. Superplasticizers based on sulfonated naphthalene and melamine or acrylic polycarboxylate do not work to fluidify this system. D-Sorbitol as well as other alditols are very effective in terms of higher workability and lower workability loss of this cementitious system. Surprisingly this chemical admixture increases the compressive strength of this cementitious grout at both early and longer ages at a given waterbinder ratio. The properties of this cementitious system in the absence and in the presence of D-sorbitol as chemical admixture (0.2-0.5% by weight of binder) have been studied in the form of workability of the fresh mixture, early and later compressive strength, leachability by water and X-ray diffraction analysis of the hydrated products of the hardened system.

The relationship between the number of individual functional groups present in lignosulfonate molecular structures and the performance of lignosulfonates in fresh cement pastes was investigated. Lignosulfonate fractions from three different pulping processes (sulfite, sulfate and organosolv) were included in this study. The dispersing, set-retarding and air-entraining effects of these fractions in ordinary Portland cement pastes were studied. Elemental composition, methoxyl and sulfonate group contents were determined analytically. An algorithm was developed to generate model molecular structures representing individual lignosulfonate fractions. Each model structure was based on the results of the chemical analysis, the model structural segment typical of each particular lignin and additional literature data. The numbers of sulfonate, carboxyl, phenolic hydroxyl, aliphatic hydroxyl and methoxyl groups in each proposed molecular structure were determined. The numbers of C-C and C-O-C inter-unit bonds in each molecule were calculated as well. Correlations between selected functional group counts and the dispersing, set- retarding and air-entraining effects of lignosulfonates were determined. For the dispersing effect, methoxyl was the most positively correlated and carboxyl was the most negatively correlated group. Sulfonate had a very low correlation with the dispersing effect. For the set-retarding effect, the C-C inter-unit bond was the most positively correlated and sulfonate was the most negatively correlated. For the air-entraining effect, carboxyl was the most positively correlated and aliphatic hydroxyl was the most negatively correlated group. A low correlation was found between sulfonate and the air-entraining effect. The results are interpreted from the perspective of cement hydration processes and the implications on the understanding of lignosulfonate interactions with cement-water systems are discussed.

Dynamic and static stability of self-consolidating concrete (SCC) affect the production, transport, and overall performance of the concrete. Viscosity- modifying admixtures (VMAs) are often incorporated to enhance the stability of SCC, especially in the case of cast-in place concrete. The influence of a new type of polysaccharide, diutan gum, on key characteristics of SCC targeted for the construction and repair of concrete infrastructure is examined. The investigation compares the performance of SCC made with diutan gum and different types of high-range water reducing admixtures (HRWRAs), including polynaphtalene sulphonate (PNS)-based and two polycarboxylate polymer (PCP)-based HRWRAs. The effect of admixture combination on workability, rheology, stability, and setting time was investigated. The robustness of optimized VMAHRWRA system and key durability parameters of the concrete were also determined. Test results indicate that the use of diutan gum increases the plastic viscosity and yield stress of SCC. Regardless of the VMA and HRWRA combination, the use of diutan gum can significantly decrease segregation and bleeding, and lead to greater homogeneity of the concrete during the dormant period of cement hydration. The performance of SCC made with diutan gum depends on the type of PCP in use. Out of two PCP-based HRWRA, one system resulted in lower HRWRA demand, increase in viscosity with the increase in VMA content, and greater stability. The use of PCP and diutan gum is shown to lead to adequate robustness where the SCC can tolerate small changes in sand moisture content without significant effect on SCC properties. SCC made with medium dosage of diutan gum and PCP is shown to develop adequate air-void system and excellent frost durability and resistance to de-icing salt scaling.

Recycled-aggregate concrete is known for its higher tendency to shrink with respect to concrete prepared with ordinary aggregate, at least when both coarse and fine recycled aggregate are used. In this work, an attempt was made to manufacture recycled-aggregate concrete that is less sensitive to shrinkage. Seven different concrete mixtures were prepared with the same water-to-cement ratio of 0.45 by various kinds of coarse aggregate: ordinary natural gravel, recycled-concrete aggregate from a suitable treatment of precast-concrete scraps, or a recycled-rubble aggregate from a crushing plant in which rubble from building demolition is treated. Different kinds of water-reducing admixtures were also tested: the first one was based on polycarboxylate polymer, the other two were also based on polycarboxylate polymer incorporating a shrinkage reducing group. The latter two are characterized by a different formulation to assure either a set-accelerating or a retarding effect. The pure superplasticizing admixture was added at a dosage of 0.8%, by mass of cement, while the multifunction admixtures were added at a dosage of 1.6% and 2.0%, by mass of cement, for the accelerating and the retarding types, respectively. Compressive strength tests were carried out at different curing times, and free-drying shrinkage was measured up to 70 days of age. The results were positive, particularly in terms of very low shrinkage of recycled-aggregate concrete containing the shrinkage reducing admixture.

Natural stones have always represented major aesthetic factors in building construction. One of the more ancient and important applications, is the production of decorative floorings. The most famous example in the flooring field is the "Terrazzo alla Veneziana". In recent years, the extensive exploiting of quarries and consequent environmental problems have reduced the availability of good low cost quality stone. In the present work, artificial superplasticized aggregates have been produced and used as decorative elements in this new "Terrazzo" flooring. The substitution of natural stones with these artificial materials is a first application of new practice. These aggregates are produced by combining inorganic binders, finely divided carbonaceous and/or siliceous minerals, superplasticizers and water. The use of nanostructural polycarboxylate superplasticizers allows to obtain very low water to cement ratios and, consequently, the final products are characterised by physical and mechanical properties, similar to those of natural stones. Additional ingredients, such as pigments and other materials have been added in order to obtain artificial aggregates with the desired properties and outstanding aesthetic characteristics.