International Concrete Abstracts Portal

When the historical buildings of the Republic of Venice were erected all the construction materials and the corresponding techniques were always carefully selected. Even at that time architects were aware of the importance of the durability of buildings in a very hostile environment such as that existing in Venice characterized by permanent humid air and capillary rise of salty water from the foundations. In particular cementitious materials were adopted in agreement with the empirical rules of the Romans’ experience and adapted for the particular Venetian environment. The present paper examines two specific cementitious materials which became very popular throughout the world: the stucco plaster and the terrazzo concrete. At the time of the Republic of Venice, chemical admixtures were not available. Therefore, masonry artisans and architects developed a special know-how to manufacture durable materials. This was based on the use of mineral admixtures and natural substances, the invention of innovative binders and the development of special application techniques. After the advent of portland cement and especially of chemical admixtures, modem stucco plaster and terrazzo concrete can be produced with different (not necessarily better) properties and at higher rates of productivity. The present paper examines the microstructural aspect, the composition, the performance, and the manufacturing process of the original and modem materials.

Anti-washout admixtures are used to enhance stability of cement-based systems, such as in the case of concrete intended for underwater repair of marine structures, self-leveling and segregation-free concrete for above-water placement, and for shotcrete. Mixtures modified with an anti-washout admixture can also incorporate silica fume or fly ash to enhance fresh and hardened properties. Anti-washout admixtures are relatively new, and there is a lack of systematic data comparing the efficiency, advantages, and limitations of using them in concrete, especially in silica fume or fly ash concrete. A laboratory investigation was undertaken to evaluate the effect of cementitious material combinations on fresh and hardened properties of fluid concrete made with three commonly used anti-washout admixtures. Each anti-washout admixture was used at two concentrations corresponding to relatively low and medium dosages that are typically used in concrete. The anti-washout admixtures included a powder-based product, a liquid-based cellulosic material, and a liquid-based microbial polysaccharide. The mixtures were prepared with 100% of a Type 10 cement, a blended silica fume ‘cement containing 8% silica fume, as well as a Class F fly ash used at 20% replacement. All mixtures were made with a 0.41 water-to-cementitious materials ratio and had initial slump values of 230 + 5 mm (9.1 + 0.2 in.). The influence of fly ash and silica fume incorporation on slump retention, washout resistance, bleeding, segregation, surface settlement, setting time, compressive and splitting-tensile strength developments was evaluated for the fluid mixtures. The paper summarizes the results of the experimental study that involved the comparison of data obtained from 21 concrete mixtures.

The diversity of concreting methods in recent years has led to a demand for long retardation of concrete setting. Conventional research has achieved retardation for up to about 3 days by the addition of a set retarder, but few studies have dealt with concrete that is placable after a much longer period. This paper reports the effects of the set-retarder dosage and temperature on the slump, period of set retardation, time limits for superplasticizing and compressive strength of concrete. The use of concrete with long-retarded setting is discussed as well.

It is broadly recognized that the adsorption of super-plasticizers on cement particles is a key factor in determining the rheology of concrete. In order to avoid the problems linked to the hydration of cement, the adsorption of super-plasticizers is often studied on unreactive model powders. However, in order for the model system to remain as close as possible to cement, the surface should have a similar charge and a similar chemical nature. Furthermore, the pH of the solution should be close to that of the hydrating cement (about 12.5). Under these conditions, cement has been shown to have a positively charged surface. The model powders used in this study were Mg(OH)2 and dead burnt MgO, which have nominal isoelectric points of 12.0 and 12.4 respectively, and which are chemically similar to Ca(OH)2 and CaO. The surface charge of such model suspensions was studied as a function of added superplasticizer. These were either commercially available or currently under development, ranging from strongly to very weakly ionic. Adsorption isotherms for two polymeric super-plasticizers, with similar structures but with different ionic group spacing, have been measured for both MgO and Mg(OH)2 at pH 12 and 11.3 respectively and between 10 and 40°C. Results showed a strong temperature dependence for the adsorption of the less ionic polymer on MgO.

Results describing investigations of the kinetics of hydration and struc-ture formation of cement systems (C3 S, C3 A, C3 A + gypsum, and portland ce-ment) in the presence of naphthaleneformaldehyde type superplasticizers (SNF ) of different compositions are discussed Using the individual components of SNF sodium polymethylenenaphthalenesulfonates ( PNS ) with polycondensation degrees ranging from 2 to 17, the basic dependences of the kinetics of the processes on the average molecular mass of the admixture ( Mn were shown In the case of C3 S the retarding effect of SNF on hydration and structure formation processes depends lineally on Mn , as a general rule, whilst for C3 A the retarding effect is more pronounced for mid-condensated components of superplasticizer. This difference is due to the different adsorption mechanism of the superplasticizer on the above-mentioned minerals. The unusual form and the analogy of ther-mokinetic curves for C3 A and C3 A - gypsum hydration in the presence of PNS are the first experimental proof that organic-mineral compounds are formed in these systems.