Analytische Chemie
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Over the course of the last decades, concrete construction has significantly changed from mass application towards sophisticated more innovative, future oriented, and performance based materials technology. Today concrete no longer consists of cement, water, and aggregates, but can be enhanced significantly by using supplementary additions and chemical admixtures. Advancements in admixtures eventually facilitated the development of new concrete types such as self-compacting concrete (SCC) and high performance concrete (HPC) by uncoupling the workability from the water-cement ratio (w/c).
While for traditional concrete types the major mixture parameter for the workability and strength were the w/c and the grading of the aggregates, for SCC and HPC the performance parameters are more complicated and the performance depends highly on the powder components. This makes the system more sophisticated and in parallel more prone to scatter during the production. The complexity and difficulty to predict the effects of changes in the process and mixture makes it very difficult to identify the right potentials for savings.
Nowadays, superplasticizers (SPs) are widely used to increase fluidity and reduce water content in concrete; thus, allowing better workability for final applications. The present study will focus on the hydration effect using comb shape polycarboxylates (PCEs), which are known to allow a very low water/cement ratio (w/c of 0.20) or less.Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules.This encouraged us to assess the potential of these methods, and particularly fluorescence, for the investigation of the interactions that occur at the interface between hydrate surfaces of cement particles and PCE at a very early stage of concrete formation and to differentiate between the impact of PCE’s molecular structures on such interactions.
Besides their plasticizing effect, superplasticizers (SPs) are known to retard the hydration of inorganic systems such as cement. Despite their frequent use, the understanding of these highly complex systems is still limited and the relevant parameters, which control the interaction between SPs, and cement components and reaction products are in the focus of ongoing research activities.[1] Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules. The potential of these methods to study processes at the interface between (hydrated) particles and the fluid phase at a very early stage of concrete formation could reveal possible mechanisms of interaction.
This investigation focuses on the study of organic/inorganic mixtures consisting of cement (CEM) and cement phases (C3S and C3A) in the presence of polycarboxylate ether and organic dyes in aqueous solution (particularly alkali resistant dyes) at a water to powder ratio of 1. Diffuse reflectance as well as steady state and time resolved fluorescence spectroscopy of the above mentioned mixtures were evaluated. Based upon changes of the intensity of the reflectance and fluorescence signal and spectral changes of the dye, acting as optical reporter, a model for the interactions of dye, PCE and cement (including different cement phases) was derived which describes the very first stage of cement hydration.
Chemical admixtures like superplasticizers or stabilizing agents are of ever increasing importance for modern concrete technology. Although such admixtures have meanwhile become common practice in many applications of concrete technology, the understanding of these highly complex systems is still limited and the relevant parameters, which predominantly control the interaction between the superplasticizer and the cement components, have not been identified yet.
Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules. This encouraged us to assess the potential of these methods, and particularly reflectance and fluorescence measurements, for the study of the interactions that occur at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation. Special emphasis is dedicated to search for and identify differences between commonly used superplasticizers.
Here, we focus on hydration effects using commercial comb shape polycarboxylate ethers (PCEs) with different charge densities, which are known to allow a very low water/cement ratio (w/c of 0.20 or less) while maintaining good workability. Based upon changes of the intensity of the reflectance and fluorescence signal and spectral effects of a dye, acting as optical reporter, a model for the interactions of dye, PCE molecules and cement nanoparticles in the very first phase of cement hydration is derived
Rice husk as (RHA) is an eco-friendly material, which can be used as a supplementary cementitious material (SCM) in cement and concrete. Due to the high water demand for the material, superplasticizers (SPs) are essential to improve the performance. However, the interaction between the SPs and RHA systems is limited. This paper investigates the interaction of the binders with three SPs, i.e. two polycarboxylate ethers (PCEs) and one lignosulphonate (LS). The investigations are performed on blended systems of mortar containing various percentages of RHA and limestone powder (LSP). LSP is used in this research to improve the workability of the mortar. The results from the zeta potential (ZP) shows that the SPs are extremely dependent on the pH of the suspension. At higher pH values such as in a cementitious system, the ZP becomes less negative indicating that the ions in the suspension interact with the carboxyl groups in the backbone of the polymers thus reducing the surface charges of the SP. The mini-slump flow shows that the workability of the blended mortar systems is significantly improved with the addition of SPs. LS systems with increasing RHA is observed to have similar workability as the control mix over time. The compressive strength test results show increased strength for all mortar specimens with added RHA and LSP at later ages.
Effect of rheological parameters on rice husk ash blended pastes with various superplasticizers
(2017)
Rice husk ash (RHA) is a low-energy efficient material with high pozzolanic characteristics compared to other supplementary cementitious materials (SCMs). The replacement of cement with RHA has extensive improvement on the mechanical properties of concrete. However due to the porous nature of the RHA particles, it requires a higher water demand when replaced with Portland cement. Therefore, superplasticizers (SPs) are essential to reduce the water demand and improve the workability of the mixes. There is very little information about the effect of RHA with added SPs in its fresh state. Hence, this paper investigates the interaction of RHA pastes with three types of SPs, i.e. one polycarboxylate-based SP and two different types of lignosulphonates. This paper presents results of the investigation related to the flow behaviour of blended mixes with increasing percentage of RHA. The yield stress for the pastes with and without the addition of SPs is investigated over time as well as 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.