International Concrete Abstracts Portal

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.

Water desorption from superabsorbent polymers (SAP) into cement-based pastes was characterized by neutron radiography imaging to promote the understanding of the mechanisms behind internal curing of concrete. Two anionic SAP samples were used which differed in their inherent sorption kinetics in cement pore solution (SAP 1: self-releasing; SAP 2: retentive). Portland cement pastes with W/C of 0.25 and 0.50 and a paste additionally containing silica fume (W/C = 0.42, SF/C = 1/10) were investigated. Desorption from SAP 1 initiated immediately. SAP 2 released water into all the matrices as well, even in the cement paste with the high W/C of 0.50. In the other two pastes, which require internal curing by principle, SAP 2 retained its stored liquid for as long as the dormant period of cement hydration. Intense desorption then set in and continued throughout the acceleration period and even beyond. These findings explain the pronouncedly higher efficiency of SAP 2 as an internal curing admixture when compared to SAP 1.

The dispersing effectiveness of three polycarboxylate-based superplasticizers (PCE) was investigated in two blended cement systems containing entirely different SCMs; fly ash (FA) and calcined marl (CM) at replacement percentages of 20% and 60%. The methods of investigation employed include rheological studies, hydration profiling up to 24h, and packing density analysis. Generally, replacing clinker phases by FA decreased the dynamic yield stress and delayed hydration of the pastes due to increased PCE to clinker ratios, regardless of PCE type. Little variation except for cement with 60% FA replacement (FA60) was observed on the Bingham viscosity. On the other hand, CM competed with clinkers not only for water, but also for PCEs even in CM20, reducing the fluidity of the paste but maintaining a similar initial rate of hydration of the pastes. PCE possessing intermediate side chain lengths proved to be more effective for CM systems than PCEs possessing long side chains.