Analytische Chemie
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- 2017 (2)
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- Englisch (2)
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- CO2 reduction (2) (entfernen)
The development of low carbon cements has gained great importance over the last decades. In the effort to limit the carbon content related to its production, cement is often replaced with supplementary cementitious materials (SCMs). Commonly used SCMs such as fly ash and slag have undergone extensive research, but are currently limited in supply and availability. Among the researched SCMs, calcined clays have proven to be a suitable alternative pozzolan due to their worldwide availability and extremely low calcination temperatures when compared to clinker. This poster presents the results from an ongoing project, aimed at obtaining pozzolans, available in sufficient quantities for Portland-pozzolana cement production in Germany. The poster shows the analyses of seven brick clays from various deposits in central Germany. The chemical and mineral composition of the brick clays was evaluated through inductively coupled plasma – optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), and thermogravimetric analysis (TG/DTG). The particle size distribution (PSD) was also evaluated by laser granulometry. The results show that the brick clays contain a substantial amount of kaolinite, illite and smectite minerals, the relative proportions differing significantly between deposits. The suitable calcination temperature range of the clays is also evaluated through TG/DTG analysis to obtain the optimum degree of dehydroxylation from which the bricks clays can be suitable for use as a pozzolan.
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.