International Concrete Abstracts Portal

The difference in the retarding effects induced by sugar alcohols on the hydration of pure tricalcium silicate and white cement pastes is investigated. The polyols studied which are stereoisomers (D-glucitol, D-galactitol and D-mannitol) generate a lower retarding effect on the hydration of white cement than on the hydration of pure tricalcium silicate. The presence of aluminate and sulfate phases in white cement pastes is shown to reduce the retarding effect induced by the molecules. Moreover, these alditols strongly complex aluminate in solution and adsorb on hydrating cement. The interactions of polyols with the anhydrous and/or hydrated aluminate phases and their effects on the hydration kinetics of white cement are discussed.

One of the main problems associated with supplementary cementitious materials (SCMs), which are used as clinker or cement replacement, is the slow strength development compared to pure OPC. This is especially evident in the early stages of cement hydration and may cause significant problems for the customers. Therefore, the demand for new and powerful accelerators only having a marginal influence on the workability of the concrete, is rising. These types of accelerators may find their application in normal ready-mixed concrete but, much more evident, in precast applications. In this paper, a new accelerator is presented, which can significantly improve the early strength (up to 2 days) of concrete. In addition to this, the components of this new approach do not bear any potential risk of corrosion for steel, be it normal reinforcement or prestressed steel.

Interactions between cements with calcined clay and superplasticizers based on polycarboxylates were investigated. All cements contained the identical clinker and sulfate carriers as well as a systematic variation of the type and proportion of calcined clay. Investigations were carried out on fresh cement paste as well as on fresh concrete by means of pore solution analyses, zeta potential experiments and rheological as well as consistency measurements. Depending on the mineralogical composition of the clay and on the calcining conditions, the specific surface area of the calcined clay varied significantly. With an increasing proportion of calcined clay in the cement, the fresh cement paste’s pore solution composition changed and, as a result, so did the zeta potential. Depending on the type and proportion of calcined clay in the cement, the specific sorption behavior of the superplasticizers diminished and consequently, their particular plasticizing effects.

Viscosity-modifying admixtures (VMA) are often introduced in the formulation of modern factory-made mortars in order to prevent segregation and to improve the homogeneity and workability of cement-based system. Among VMAs, organic admixtures, and more especially polysaccharides such as cellulose ethers (CE), are widely used, since they improve both rheological property and water retention capacity of the mortars. The present study examines the influence of chemical composition and structure of guar gum derivatives on water retention capacity (WR) and rheological behavior of fresh state Portland-based mortars. The investigation was also completed by adsorption isotherms. For this, original guar gum, HydroxyProplyl Guars (HPG) and hydrophobically modified HPGs were selected. The effect of the molar substitution (MSHP) and the degree of substitution (DSAC) was investigated. The results highlight that chemical composition of HPGs has a remarkable effect on fresh state properties of mortars. The original guar gum does not impact both WR and rheological behavior. Increasing MSHP leads to an improvement of the WR and the stability of mortars while the hydrophobic units further enhance WR and lead to a decrease in the yield stress and an increase in the resistance to the flow of admixed mortars.

In many countries there are regions with very cold winter climate. To provide for year-round building and construction works antifreezing admixtures must be used. When suitable technology and high performance antifreezing admixtures have been used, ready mix concrete may be delivered and placed at temperature as low as -25 °C. The transfer from “summer” concrete mix to “winter” one implies the addition of optimal amount of antifreezing admixture. In principal, both separate superplasticizer and antifreezing admixture dosing and usage of blended (combined) admixture is possible. In Russia the usage of blended plasticizing-antifreezing admixtures is predominant historically. The prime blended admixtures on the base of sodium formate didn’t freeze down to -15 oC and allowed secure concrete hardening with dosages about 10% of commercial product. At present day such admixtures are unsuitable neither by the dosage level (danger of ASR) nor by the freezing-point. The method for elaboration of blended antifreezing admixture (BAFA) with extreme freezing-point was developed. These admixtures allow building and construction works at a temperature as low as -25 oC with dosages about 1-1.3% for PCE-based admixtures and 1.8-2.2% for PNS-based admixtures. It is important that these dosages ensure superplasticizing effect. Non-chloride non-plasticizing admixture with freezing-point below -40 oC was developed also. The kinetic of concrete strengthening at various temperature regimes has been thoroughly studied.