International Concrete Abstracts Portal

Portland cement is a multi-phase material, which can be simplified as a two-phase system with alite and C3A as the main constituents determining early properties. Alite is the most abundant phase and in a first approximation it is responsible for the development of mechanical strength during hydration, while C3A mainly affects the plastic behavior before set. In portland cement, superplasticizers are preferentially adsorbed onto C3A and its hydrates rather than alite, due to the different interaction with the mineral surfaces. Three different polycarboxylate superplasticizers (PCEs) were studied, based on copolymers of methacrylic acid and MPEG-methacrylate and characterized by different side chain length and different charge density. Their affinity to C3A and alite surfaces was determined through adsorption measurements on alite/gypsum and alite/gypsum/C3A mixtures. The results of the adsorption tests indicated that the charge density of PCEs, expressed as the ratio between carboxylic groups to ester groups, is the main parameter affecting the adsorption of the PCEs: the lower the charge density, the lower the adsorption on both the phases. The same parameter affects the induction period of alite phase, as demonstrated by in situ XRPD dissolution kinetics experiments, both in the presence and in the absence of C3A. These results can be put in relation with both the hindrance of adsorbed PCE molecules in the dissolution kinetic of alite and the concentration of PCE molecules in solution in conditions of saturation.

Slump loss of fresh concrete was a common issue in engineering construction, especially under high temperature and long distance transportation conditions. Therefore, slow-release polycarboxylate superplasticizers (PCEs) have been widely used to reduce the slump loss in various engineering projects. In this study, three kinds of PCEs with different proportions of hydroxyl ester groups (HEG) were synthesized and characterized by 1H-NMR and Gel Permeation Chromatography (GPC). The effects of the HEG content on dispersion retention, adsorption kinetics and zeta potential of fresh cement suspensions were systematically investigated to figure out the mechanism. For PCEs with the same molar ratio of carboxyl group and reactive polyether, the dispersion retention ability of PCEs is improved with the increasing of HEG ratio. HEG in PCEs can be slowly converted to carboxyl groups in the alkaline environment of cement suspension, which could enhance the adsorption of PCE molecules onto the surface of cement particles. Despite major of the initially adsorbed-PCE molecules might have been embedded in hydration products, free PCE molecules with released carboxyl groups in the solution can continuously adsorb onto the surface of cement particles and play a role in dispersion. This explains why slow-release PCEs have a dispersion retention effect on cement particles within a certain time.

The interaction of methoxy-capped poly(ethylene glycol) polymers (MPEG) and a poly(methacrylic acid) anionic polymer (PMA) from water onto sodium Montmorillonite (Na-Mmt) particles untreated or treated by calcium chloride was studied at 20°C. In the absence of Ca2+, MPEGs are able to intercalate by displacing the water molecules present in the interlayer space, as shown by XRD and TGA analyses. In contrast, the adsorbed amount of PMA remains low. The saturation of Mmt with Ca2+ prevents MPEG intercalation through replacing sodium by a stronger water coordinator in the interlayer space, but slightly increases PMA adsorption possibly through a calcium bonding mechanism. This was confirmed with PCE superplasticizers and Na- and Ca-saturated Mmt clays. Whatever the PCE, a larger amount was consumed on Na-Mmt than on Ca-Mmt. This confirms the occurrence of two consumption mechanisms: (i) a superficial adsorption via cation bonding of the carboxylate groups with anionic sites on clay surfaces, (ii) intercalation of ether units of the grafts in the interlayer space by displacement of water molecules coordinated to the exchangeable cations.

The composition of the aqueous solution in alkali-activated binders, i.e., the high alkalinity and the high ionic strength challenge chemists to design molecules exhibiting the same plasticizing effects as in cementitious materials. The highest difficulty probably lies in alkali-silicate activated systems due to the presence of multivalent silicate oligomers in solution. Reported here are new insights about the adsorption of polymers in presence of various concentrated electrolyte solutions in order to mimic the harsh conditions present in geopolymer pastes. In order to eliminate the problem of the reactivity of such systems, TiO2 nanoparticles were used as a model substrate. The adsorption of polymer molecules as well as the specific adsorption of monovalent and divalent ions is revealed. Those results are compared to the rheological characteristics of alkali-hydroxide or alkali-silicate activated geopolymers. The conclusions which can be drawn from the model system fit qualitatively very well with the classical slump tests done on real systems.

The calcination temperature in cement manufacturing can be reduced by the addition of sulphate and fluoride containing compounds and it is possible that sulphate and fluoride ions elute to the suspension after calcination. This paper describes the influence of sulphate and fluoride ions on the action of polycarboxylate based superplasticizer in cement paste. When the amount of K2SO4 or KF was increased, the viscosity of the cement paste with superplasticizer increased. The amount of adsorbed superplasticizer was decreased by K2SO4 addition but increased by KF addition. The fluidity with polycarboxylate based superplasticizer containing more functional groups was less affected by K2SO4 addition. In contrast to the case of K2SO4 addition, the increase in the degree of viscosity by KF addition was not dependent on the amount of functional groups. The specific surface area increased with K2SO4 or KF addition.