Working mechanism of a poly-beta-naphthalene sulfonate (NS) and polycarboxylate (PC) superplasticizer (SP) types was studied from the point of solution chemistry and initial hydration of cement. Test results showed that the dispersing characteristics of SP can be represented by two parameters, which are the critical dosage (CD) and dispersing ability (DA). When used at lower dosage than CD, SP did not affect the fluidity of paste. However, beyond CD value, the fluidity of paste showed a linear increase with SP dosage. DA is defined as the fluidity increment per unit of SP dosage. CD is closely related to the initial hydration heat, which corresponds to the surface area of initial hydrates. For NS type, CD is affected by the absorption ratio of NS into initial hydrates. The absorption amount is decreased by increasing the sulfate ion concentration supplied from the alkaline sulfate in cement or by delaying the addition of NS. In the case of PC, the absorption amount was negligible. DA of PC value is related to the sulfate ion concentration. SP adsorption on hydrates is in equilibrium state with SP concentration in solution phase. Furthermore, sulfate ion and SP showed a competitive adsorption on hydrates. More sulfate ion in solution phase changes the adsorption equilibrium of SP to a low adsorbed state. The decrease of adsorption amount of SP decreases the paste fluidity.

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The results obtained show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.

The growing use of polycarboxylate-based superplasticizing polymers can be attributed to the numerous advantages they provide to the production and quality of concrete mixtures. The ability of polycarboxylates to fluidify and maintain the workability of concrete, while having minimal impact on setting characteristics, has contributed to increased strength and durability, and has allowed for more economical completion of numerous concrete handling operations. With the increased application of polycarboxylates in concrete, more frequent opportunities exist for these polymers to be used with other chemical admixtures such as conventional water reducing, set retarding, and set accelerating agents as well as other functional admixtures. The resulting admixture combinations have resulted in a wide range of interactions. This paper discusses the highly synergistic strength increase observed between a condensed polyacrylic acid-aminated polyether-based superplasticizer and several calcium salts typically used in admixture formulations.

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.

The time dependence of the interaction between hydrating cement particles and a poly-naphthalene sulfonate (PNS) superplasticizer has been investigated using rapid response calorimetry and other physico-chemical approaches. The study focuses on the processes which occur during the first instants following the immersion of the cement particles into the solution, in the presence, or absence, of the PNS superplasticizer. Specifically, the investigation aims to elucidate the dominant phenomena in the coupled processes taking place in the water/cement/sulfate/PNS system, and the consequences of these phenomena on 1- the reaction rates and products and 2- the cement-superplasticizer compatibility The systems discussed here are cement pastes at W/C=3 containing cements having highly different alkali sulfate contents and a normal PNS super-plasticizer. The kinetics of the initial reactions (O-30 min.) are monitored by fast response adiabatic calorimetry; the superplasticizer adsorption, and variations in the ionic composition of the interstitial solution, are also determined at short time intervals during the same period. The evolution of the hydrate phases as a function of time is monitored through XRD analyis.