International Concrete Abstracts Portal

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.

The objective of this study was to establish a recycling system for cement sludge by hydration control and an evaluation of the residual cement content of sludge. The adsorption behavior of sodium gluconate on cementitious materials was clarified in this study. The saturated absorption behavior between sodium gluconate and alite follows a Langmuir adsorption isotherm. The delayed hydration time depends on the concentration of residual sodium gluconate. When the residual concentration of sodium gluconate was over 0.018–0.020 mass%, cement hydration did not proceed. The relationship between the heat liberation after 1 day and the amount of non-hydrated alite as calculated by X-ray diffractometry is linear. The amount of non-hydrated alite can be estimated by using calorimetric data. Finally, the amount of non-hydrated alite in sludge water can be estimated by using calorimetric data, magnesium hydrate hexahydrate (Mg(NO3)2·6H2O).

Self compacting concretes for precast applications were scaled-down to concrete-equivalent grouts where only the surrounding paste around bigger particles was studied in a rheometer. Low shear rate steady-state flow curves feature a non-monotonous variation of shear stress versus shear rate, with a minimum stress obtained for a critical rate. The thixotropy description initially developed by Roussel, Le Roy and Coussot1 was successfully applied to the data in order to model this behaviour. Extending the study over different superplasticizers showed that their influence depends on the molecular architecture. Specific concrete tests were developed in order to assess workability and formwork-filling retention while applying as little energy as possible to the material. The trends observed at the rheometer scale were confirmed showing that structure build-up kinetics has a major influence on concrete placing and that superplasticizers may help control it up to some extent.

It is well established among concrete producers that specific cements seem to be incompatible with most PCE products, thus causing excessive PCE dosages or even a total failure of the PCE. This effect is commonly referred to as “cement incompatibility” of PCE. The study here investigates the reasons for such incompatibility. First, it was found that only cements which upon contact with water instantaneously form large amounts of ettringite exhibit such incompatibility phenomenon. Their characteristics are elevated C3A content (> 7 wt.-%) and high initial heat of hydration. Second, it was observed that PCEs strongly influence early ettringite crystallization by acting as morphology modifying agent. Most PCEs transform common micro meter-sized ettringite into nano-sized crystals which bring about a huge surface area and thus require abnormal dosages of PCE to achieve dispersion. Such nano-sized particles can be separated from the cement paste by centrifugation where it appears as a viscous, gel-like top layer. From five chemically different PCE polymers tested, one (a modified APEG type) was identified as extremely compatible with all cement samples, whereas three other ones (two conventional MPEG and one APEG type) exhibited pronounced incompatibility with C3A rich cements. An IPEG PCE showed moderate cement compatibility. The phenomenon of cement incompatibility occurs only when the PCE is present in the mixing water, and disappears when PCE is added in delayed mode. Finally, a simple and quick test to identify cement–PCE incompatibility is proposed.

Use of pozzolanic materials such as natural zeolite as portland cement replacement helps to reduce amount of CO2 emission due to clinker production. Natural zeolite also improves mechanical and durability properties of concrete. It is common to use natural zeolite as a rheological modifying admixture in flowing concrete. However, many cases were reported that zeolite blended cements showed severe workability loss. The object of the analysis is to investigate compatibility of different chemical-based superplasticizers and effect of superplasticizers’ combination on workability retention of concrete made with zeolite blended cement. The results show that combination of lignosulfonate admixture with naphthalene and polycarboxylate based admixture not only reduces the superplasticizer’s demand to achieve certain workability retention, but also helps to reduce slump loss.