International Concrete Abstracts Portal

This research was aimed at the use of agroresources as admixtures for mortar and concrete. Thirteen modified starches from potato, wheat, corn and waxy corn were investigated. They were compared to methylhydroxyethylcellulose (MHEC) and Welan gum for the following properties: thicknening effect, water retention, setting time and 28-day compressive strength. The study was carried out on standard mortars and the starch dosage was either 0.75 % or 0.25 % of the total dry matter (sand + cement). The results obtained show that the recommended dosage is 0.25 %, which is still economic compared to MHEC and Welan gum. The best performances were obtained with waxy corn starches and, to a less extent, with potato starches. The latter performed well except for water retention.

Certain di-phosphonate terminated monofunctional polyoxyethylene are widely used as concrete superplasticizers. In order to understand its action mode, its effect on hydration of pure tricalcium silicate suspensions has been investigated by conductimetry, isothermal calorimetry and ionic and total organic carbon (TOC) analyse of the liquid phase. The polyoxyethylene di-phosphonate modifies the nucleation and growth process of C-S-H by reducing the number of initial nuclei, decreasing the growth rate in the accelerated period and increasing the rate during the difision limited period comparatively to control samples. The evolution of ionic concentrations in the solution during hydration reveals an apparent increase of the critical supersaturation required to nucleate both C-S-H and portlandite. This is due to the formation of a calcium-diphosphonate polyox complex. The calcium complexation constant and pKa of the phosphonate polyox have been determined from pH and conductivity studies of calcium-hydroxide and sodium-hydroxide polymer solutions and well account for portlandite solubility in presence of phosphonate. The polyox phosphonate does not seem to adsorb on C3S surface but is rather adsorbed on C-S-H. This is probably the origin of the decrease of the growth rate leading to the modification of the texture of C-S-H and subsequently the modification of the rate during the diffusion limited period.

In connection with environmental problems, the utilization of industrial materials waste, municipal refuse, sludge or its incineration ashes as raw materials for cement has recently begun. If their utilization amount increases in the future, it will result in the increase in aluminate phase. It is necessary to clarify the influence of superplasticizers on the hydration of aluminate phase. In addition, the fluidity of the cement paste which the sulfate ion content is high, is decreased by addition of comb-type superplasticizers. This is also closely related to initial hydration of cement. In order to clarify the action mechanisms of comb-type super-plasticizers, this paper discusses the influence of super-plasticizers on the early hydration of C,A-calcium sulfates by quantitative XRD compared with the case of naphthalene type superplasiticizer. Early hydration of C,A is accelerated by adding of comb-type super-plasticizers in C,A-calcium sulfates systems. In the case of comb-type superplasticizers, SO,“- contents in gel formed around C,A is decreased. It is necessary to control the early hydration reaction of aluminate phase in order to control the fluidity of the cement with comb-type superplasticizers.

In the absence of admixtures, the workability of calcium aluminate cement (CAC) concretes is similar to that of Portland cement concrete. However, the classic types of superplasticisers for Portland cement concrete, lignosulphonates and polynaphthalene sulphonates, have only a modest effect on the workability of concrete made from calcium aluminate cements. Consequently, the placement of CAC concretes at water to cement ratios below about 0.4 can be difficult and necessitates the use of a high cement content (1, 2) . In contrast, a ‘new generation’ superplasticisers (poly carboxylate polyox) has been found to be highly effective in improving the rheology of calcium aluminate cement concrete. If used alone they can also severely retard the setting time, but this effect can be overcome if they are used in combination with other admixtures. This paper discusses the effect of superplasticisers on the flow and setting time of CAC mortars.

Depending on the nature of the materials used, the characteristics of both fresh and hardened concrete can be systematically altered with concrete admixtures. Special investigations were carried out on different polymers used as superplasticizers to determine the mode of action. The emphasis of the investigations was on the adsorption behaviour of the polymers on different cements phases and their influence on the rheological properties of mortar. The adsorption behaviour was investigated by High Performance Liquid Chromatography and the rheological properties were determined in standard mortar mixtures by the flow table spread measurement and by using the Viskomat NT to determine the flow characteristics. Additionally pore solutions were analized Based on these results some interesting correlations co used to explain the mode of action of superplasticizers.