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Organisationseinheit der BAM
In Produktionspausen können in Betonwerken nennenswerte Lagerungsintervalle entstehen. Auf Baustellen und in den Instandsetzungen werden Zement- und Trockenmörtelsäcke teilweise über längere Zeiträume gelagert. Auch im Zusammenhang mit Zementimporten und zunehmend filigran abgestimmten Betonrezepturen die bereits bei kleinen Änderungen der Ausgangsstoffe sensibel reagieren, nimmt die Relevanz der Zementlagerung zu. Aus diesem Grund wurde der Einfluss der Zementlagerung auf die Eigenschaften von Betonen in einer Versuchsreihe an der Bundesanstalt für Materialforschung und -prüfung (BAM) ermittelt.
SCM potential in Africa
(2012)
Clearing the way
(2012)
Sub-Saharan Africa is populated by more than half a billion people,
and rapid urbanization is creating challenges in the areas of housing,
roads, railways, power supply, dams and water pipelines —
aspects of infrastructure that are critical to the well being of the
population. This situation indicates that cement and concrete will
play a major role in the future construction technology in Africa, despite
the fact that it does not have a long-lasting tradition over most
of the continent. Although the continent is rich in natural resources
for the production of mineral binder systems, many sub-Saharan
African countries exhibit challenging boundary conditions, that have
to be well understood in order to use cement and concrete technologies
in a sustainable and reasonable way.
Polysaccharides provide high potential to be used as rheology modifying admixtures in mineral binder systems for the construction industry such as concrete or mortar. Since superplasticizers have become state of technology, today, concrete is more and more adjusted to flowable consistencies. This often goes along with the risk of segregation, which can be effectively avoided by adding stabilising agents supplementary to superplasticizers. Stabilising agents are typically based on polysaccharides such as cellulose, sphingan gum, or starch. Starch clearly distinguishes in its effect on rheology from other polysaccharides, mainly due to the strong influence of amylopectin on the dispersion and stabilisation of particles. Based on rheometric investigations on cementitious and limestone based dispersions with different volumetric water to solid ratios, the mode of operation of modified potato starch is explained in comparison to a sphingan gum. It is shown that the stabilising effect of starch in a coarsely dispersed system is mainly depending upon the water to solid ratio and that above a certain particle volume threshold starch mainly affects the dynamic yield stress of dispersions, while plastic viscosity is affected only to a minor degree. Sphingans operate more independent of the particle volume in a coarsely dispersed system and show significantly higher effect on the plastic viscosity than on the yield stress. In systems incorporating superplasticizers, influences of both stabilising agents on yield stress retreat into the background, while both observed polysaccharides maintain their effect on the plastic viscosity.
Superplasticizers (SPs) have been employed in concrete technology for decades to improve the workability of concrete in its fresh state. The addition of SPs in cement-based systems affects the early properties. Although the interaction of the cement particles with various SPs has been extensively researched, there still exists limited research on the interaction of SPs with supplementary cementitious materials such as rice husk ash (RHA). This paper investigates the rheological properties and early hydration kinetics of RHA-blended systems with three types of SPs, a polycarboxylate ether (PCE) and two lignosulphonates (LS-acc and LS-ret). In rheological properties, the addition of SP causes an initial improvement of workability as the yield stress is significantly reduced. The pastes with PCE and LS-acc show a slight increase of yield stress over time whereas pastes with LS-ret tend to lower the yield stress slightly over time, further improving the workability. Without SP, pastes with RHA show a lower yield stress but an increase in plastic viscosity as cement is further replaced with RHA. The addition of the LS SPs is observed to lower the plastic viscosity but remains constant with further replacement of cement with RHA. This indicates that LS SPs further adsorbs on RHA particles and hydration products produced causing dispersion of the particles
within the system. In early hydration kinetics, pastes with PCE retard hydration and the degree of retardation is further increased with LS SPs. In the presence of RHA, the retardation of LS SP systems is significantly reduced. The pastes with PCE show more ettringite in the SEM micrographs, but is observed to be shorter needles. This indicates an initial good workability for PCE. However, C-S-H and CH were observed to be low in quantity, whereby the pastes with LS show more nucleation sites for C-S-H and CH. The ettringite needles in the LS systems were similar in quantity and more elongated in some cases but not abundant as in the PCE systems.
Structural design and application have always been linked to the compressive strength of concrete as the main relevant criterion. This was justifiable in the past, where concrete consisted of water, ordinary Portland cement and aggregates, but this concept is no longer relevant for modern and more sustainable cement and concrete.
Despite these new developments, existing standards, guidelines and academic curricula have not been much updated and are still used worldwide. There is a need to change this situation by proper education of the users.
This overview describes the challenges that arise at a user Level from the higher complexity of modern concrete, and defines needs and requirements for enhanced applicability of sustainable concrete concepts.
Furthermore, recommendations are given on how better concrete practice can be communicated to all the involved parties, from civil and design engineers to constructors and site-appliers on the construction site.