International Concrete Abstracts Portal

The implementation of cement materials is improved by the addition of a polymer called a superplasticizer. In this presentation, we look for the impact of the polymer architecture on the physico-chemistry properties of cement slurries. Initially, we focus on the adsorption of sodium polymethacrylate grafted by poly(ethylene oxide) chains on different types of cement using macroscopic and microscopic techniques. Finally, we measure the fluidity of different cement slurries with the use of new tools such as helicoidal ribbon geometry to explain how polymer architecture controls the performance concrete formulations.

The combination of different type of blending materials with portland cement such as natural pozzolans, fly ashes, and slags provides high flexibility in designing concrete mixtures with several technical advantages and low cost. However, their addition influences the properties of fresh concrete and causes problems of compatibility in blended cement systems containing superplasticizers that are usually used either for reduction of water demand or for increase of workability. In this study, two of the widely used type of superplasticizers, one based on sulphonated naphthalene formaldehyde condensate (SNF) and another based on polycarboxylate polymers (PC), were tested with ten blended cement mixtures containing 20, 30, 50, or 80% portland cement replacement with one or more of the above-mentioned supplementary cementitious materials. The action of superplasticizer on cement pastes was monitored by making zeta potential, pH, and temperature measurements as well as by using DTA-TG method for determining the hydration and hardening process. Porosity and strength development were also measured at different ages. The aim of the research work was to find which type of superplasticizer was more suitable for each type of binder system. The highest reduction of water demand was achieved with polycarboxylate polymer superplasticizer. Results show that compared to the plain portland cement system, the decrease of water/binder ratio with superplasticizer addition was lower in blended cements.

Viscosity-enhancing agents (VEA) are often used to improve the cohesiveness and stability of self-consolidating and underwater concrete. However, because of various types of interactions occurring between the VEA polymers and other components of fresh cementitious systems, the beneficial effects of the VEA is found to depend on the nature of both the VEA and the other the other components, particularly the superplasticizer (SP). Hence, different VEA-SP combinations are found to yield different dose-response effects in application. To investigate the origin and consequences of VEA-SP interactions, the influence of two common VEAs on the properties of cementitious and reference (limestone) systems was investigated through rheological and stability (bleeding and segregation) measurements in the presence of two typical SPs, a naphthalene-based (PNS) and a carboxylate-based (PC) polymer. The rheology of cement and powdered limestone pastes was evaluated through the Kantro mini-slump test and from measurements of yield stress and plastic viscosity in simple shear, and dynamic moduli obtained through oscillatory measurements. The bleeding and sedimentation behaviors were monitored using a multipoint conductivity method. Corresponding rheology and stability data were also obtained on mortars incorporating the same VEA-SP admixture combinations. In these systems, the different VEA-SP couples demonstrate varying compatibility which impact on their performance.

The use of polycarboxylate (PC)- and polyether (PE)-based superplasticizers often generates segregation, inadequate flowability or similar problems due to the incompatibility between the cements and admixtures used. In light of the widely varying composition of these admixtures, not all cement- and superplasticizer-related factors which could affect compatibility have been defined to date. In this study, therefore, rheological trials were conducted with a rotational viscometer and adsorption tests were conducted in a total organic carbon (TOC) analyzer to explore the compatibility between different PC - PE admixtures and cements employed in a variety of compositions and additions. Three admixtures (PC1, PC2, and PC3) with different carboxylate (CA) and polyether (PE) group contents were used, along with seven standard cements whose chemical and mineralogical compositions and active additions varied. The structural characteristic of the admixtures affecting compatibility most intensely was found to be the carboxylate (CA) to polyether (PE) group ratio. In cements with no active additions, characteristics such as fineness and the C3A to calcium sulphate and C3S to C3A ratios were also observed to have marked effect on compatibility. On the other hand, in cements with limestone or fly ash additions, no fundamental differences were identified with respect to a standard CEM I 42.5R cement in terms of admixture compatiblity. In calcium aluminate cement (CAC) the fluidizing effect of polycarboxylate superplasticizers led to very significant declines in yield stress.

The increased performance of polycarboxylate superplasticizers is generally explained by the steric hindrance they are intended to develop between cement particles. In fact, direct evidence of this is relatively scarce. The only direct measurements to date have been made by atomic force microscopy on model surfaces of magnesium oxide. In this paper, we report very recent measurements using the same technique but on surfaces of calcium silicate hydrate that constitute a more realistic model system. Furthermore, it is shown that the measured interfacial behavior of superplasticizers can be quantified by a scaling law approach borrowed and extended from polymer physics.