International Concrete Abstracts Portal

Due to the high carbon dioxide emissions linked to concrete production and a rapidly increasing demand for cementitious materials, particularly in the global South, it is inevitable to use cement in concrete more efficiently. This requires chemical admixtures to enhance the overall performance of the binder and to cope with the negative rheological influences of supplementary cementitious materials that are used to replace ordinary Portland cement. However, particularly in the growing economies of the Southern hemisphere, where a massive part of the future construction activities will take place, the supply chains for performance-enhancing chemical admixtures are often poor, and local production facilities are lacking today. This paper presents case studies of polysaccharide-based alternative admixtures such as acacia gum, cassava starch, and the gum of the bark of Triumfetta pendrata A. Rich, which can be used effectively as superplasticizer, robustness enhancer, and thixotropy incorporating agent, respectively. Their modes of operation are discussed based on their spread flow, zeta potentials, and hydrodynamic diameters in the presence and absence of calcium ions.

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 influence of various types of surfactants on the rheological properties and on the rates of bleeding and sedimentation of limestone and cement pastes as been examined as function of surfactant concentration. The surfactants selected include both low foaming and foaming compounds to enable a distinction of the effects inherent to the surfactants, from those due to air entrained in the pastes. Over the range of concentration examined, the rheological characterization performed shortly after mixing showed little change in the paste rheology due to the presence of the surfactants, in the absence of entrained air. As expected, with entrained air, the rheological parameters are all moderately altered. On the other hand, the stability of the CaCO3 pastes, evaluated through their bleeding and sedimentation kinetics, is markedly affected by the surfactants. At short times (<1 hr), the stability is only marginally changed; at longer times, the paste stability is substantially decreased through cooperative bleeding and sedimentation behavior. Similar effects were observed in lime-saturated CaCO3 pastes, though less pronounced due to the higher stability of these pastes. In relatively stable cement pastes, the bleeding and sedimentation were initially similar to the limestone pastes, but no cooperative destabilization was observed, with or without surfactants. The influence of the surfactants on bleeding and sedimentation kinetics was attributed to the formation of channels in the pastes, a process facilitated by the surfactants. The variations observed with surfactant molecular and solution properties and concentration are discussed; a plausible mechanism is suggested to explain surfactant-induced effects.

The growing use of fluid concrete increases the need for understanding the conditions under which these materials can undergo bleeding and segregation. However, the interfacial and colloidal phenomena, which control water and solids migration in cementitious systems, are inherently complicated by the hydration of the cement components. Hence, to unravel the specific role of chemical admixtures on the stability of cement-based systems, the mode of action of these admixtures should also be investigated in dense colloidal slurries of ‘un-reactive’ minerals. Several highly insoluble minerals, having specific surface areas comparable to that of a Portland cement, were thus evaluated for this purpose. The state of flocculation of these materials in dilute and concentrated slurries was examined through sedimentation and rheological measurements under various conditions, and the results compared to observations on similar slurries containing cements. The comparison showed that calcium carbonate (CaCO3) exhibits surface and colloidal properties very similar to “un-hydrating” cement particles. In fact, CaCO3 pastes can be made to accurately reproduce most of the kinetic properties of a cement paste, including bleeding, sedimentation and all dynamic viscosity parameters. It is therefore proposed that CaCO3 pastes can be used to adequately model ‘physical-type’ effects occurring in cementitious systems at very early stage of hydration, i.e., in the first hour.

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.