Chemie und Prozesstechnik
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The analysis of the chemical composition of cement-based materials like concrete is a crucial task in civil engineering. Concrete is a multi-phase material made of cement, water, aggregates, additives and admixtures. During the damage assessment of concrete structures, the external ingress of harmful species like chlorine, alkalis or sulfates needs to be determined. In order to provide a fast and precise analysis of concrete the application of LIBS offers unique possibilities as the technique can detect all elements at air atmosphere.
To determine the condition of concrete, the information about the chemical composition, the ingress of harmful species as well as the evaluation of characteristic values like the used cement type or the type of aggregates are needed. This work presents the two-dimensional evaluation of concrete samples using LIBS and multivariate data analysis of atomic and molecular emission. For the evaluation of the main components different classification methods will be applied to identify the heterogeneity of the samples. With respect to their specific phase, different species will be quantified using multivariate regression (PLS) and important figures of merit will be presented. All results will be verified using Standard procedure of wet chemistry (XRF, potentiometric titration and ICP-OES).
In civil engineering, the investigation of existing infrastructure is of major importance for maintaining and ensuring stability of the structures. To ensure durability, uniform regulations and standards apply e.g. the European standard EN 206-1. In some countries, the EN-standard is supplemented by additional standards, as in Germany with DIN 1045-2. Here, specific application rules are described, e.g. for the cement type, to ensure the resistant to different exposures. Therefore, the knowledge of the materials originally used is important in assessing the condition of existing concrete structure. Unfortunately, these are often unknown and must be determined retrospectively. Therefore, we present the application of the laser-induced breakdown spectroscopy to distinguish between different types of cement. Spectral information’s are used to build a classification model. First, the accuracy of the classification is analyzed on ten pure laboratory cement samples. To investigate possible sources of error, the model was then applied to cement samples with different moisture content. The study shows that LIBS is a promising tool for distinguishing between cement types. For further industrial application, however, factors influencing the LIBS signal must be included to ensure a robust model.
The presence of polycarboxylate ether (PCE) based superplasticizers (SPs) has enormous influence on the early hydration of cement. C3A as the most reactive phase of Portland cement plays a significant role in early hydration reactions and affects the rheological performance. Therefore, this paper presents experimental results about the influence of delayed addition of PCEs on the Hydration of cement and C3A-gypsum pastes investigated by isothermal heat flow calorimetry. Complementary in-situ XRD was carried out on C3A pastes to analyze hydration and phase changes related to the addition of PCE.
Cement pastes with a delayed addition of PCE showed less Retardation compared to simultaneous addition. The alteration caused by PCE is much more pronounced for C3A-gypsum mixes. With a delayed addition of SP, the hydration of C3A is less retarded or even accelerated. It is obvious that there is less retardation the later the addition of SP. Furthermore, the PCE alter the hydration of C3A when added delayed and exhibit changes in kinetics and hydration rates. XRD results showed that more C3A is dissolved in the presence of PCE. Also, the gypsum depletion occurs earlier in the presence of PCE and even faster with delayed addition. Without PCE AFm starts to form just after the gypsum depletion. However, in the presence of PCE AFm already starts to form at the beginning of the hydration. Due to the faster gypsum depletion in the presence of PCE, also the transformation from ettringite into AFm begins earlier, but takes longer as without SP.
The presence of polycarboxylate ether (PCE) based superplasticizers (SPs) has enormous influence on the early hydration of cement. C3A as the most reactive phase of Portland cement plays a significant role in early hydration reactions and affects the rheological performance. Therefore, this talk presents experimental results about the influence of delayed addition of PCEs on the Hydration of alite and C3A-gypsum pastes investigated by isothermal heat flow calorimetry. Complementary in-situ XRD was carried out on C3S and C3A-gypsum pastes to analyze hydration and phase changes related to the addition of PCE. Cement pastes with a delayed addition of PCE showed less retardation compared to simultaneous addition. The alteration caused by PCE is much more pronounced for C3A-gypsum mixes. With a delayed addition of SP, the hydration of C3A is less retarded or even accelerated. It is obvious that there is less retardation the later the addition of SP. Furthermore, the PCE alter the hydration of C3A when added delayed and exhibit changes in kinetics and Hydration rates. XRD results showed that more C3A is dissolved in the presence of PCE. Also, the gypsum depletion occurs earlier in the presence of PCE and even faster with delayed addition. Without PCE AFm starts to form just after the gypsum depletion. However, in the presence of PCE AFm already starts to form at the beginning of the hydration. Due to the faster gypsum depletion in the presence of PCE, also the transformation from ettringite into AFm begins earlier, but takes longer as without SP.
While today, engineers can choose from a wide range of rheology modifying admixtures, in some parts of the world, these are difficult to access, due to their complex processing. However, alternatives can be bio-based polymers such as polysaccharides from various sources. These are easily accessible all over the world, do not demand for complicated processing, and typically they are more sustainable than many established materials, which are crude oil-based.
The paper presents the effects of acacia gum, cassava starch and the gum of triumfetta pendrata A. Rich on the rheological performance of cementitious systems. It is shown that acacia gum can be as efficient as polycarboxylate based superplasticisers, cassava starch can reduce the yield stress slightly with little effect on the plastic viscosity, and the gum of triumfetta pendrata A. Rich increases the thixotropy of cement pastes with plasticizing polymers significantly.
Fresh cement paste is a suspension consisting of a hydraulic binder (cement), water, and numerous minor components – admixtures. Addition of admixtures aims at specific modification of properties of the fresh cement paste or hardened cementitious building material. Specific admixtures, so-called superplasticizers (SP), are used to improve the flowability of the fresh cement paste with reduced water content. The latter is the starting material for the high-strength concrete. Thus, SPs are essential for the ambitious construction projects.
However, uncontrollable retardation of the setting time in presence of SPs is occasionally observed. Obviously, SPs influence early products of the cement hydration leading to changes in the microstructure development. The hardening is thus delayed, and the quality of the resulting building material suffers. The mechanisms of the admixture action during the hydration process are still intensively investigated [1-7]. A detailed understanding of the admixture effects during the early hydration stage is the key to control and individual adjustment of the cement-based construction materials.
We use the unique combination of the wall-free sample holder and the time-resolved X-ray scattering analysis to achieve the full information about the hydrate phases formed under the influence of admixtures. We use ultrasonic levitator to start the cement hydration in levitated cement pellets [8, 9]. The sample levitation allows collection of the unimpaired information about cement hydrate phases. The most beneficial is the avoiding of the contributions of the sample holder material to the data signal.
We induce the cement hydration by adding water to unhydrated Portland cement during the data acquisition. The full phase composition of the hydrating cementitious system can be gathered in situ using wide angle X-ray scattering (WAXS). During the hydration of cement both crystalline and amorphous hydrate phases are formed. WAXS data contain the information about crystalline phases behind the Bragg reflections, whereas the amorphous hydrates influence the appearance of the background. Application of the data analysis specific for crystalline or amorphous phases is needed. The data quantification by the Rietveld method allows to conclude about the changes of the phase amounts due to the presence of admixture. The calculation of the pair distribution functions allows analysis of the amorphous hydrates. Based on this information, the SP effects and the extent of their involvement into the ongoing reactions can be concluded. A detailed understanding of the complex cement hydration process is envisioned.
The presented work discusses the accuracy of Laser Induced Breakdown Spectroscopy (LIBS) in determining the total chloride content in cement pastes. LIBS as an emission spectroscopy method is used to detect simultaneously several elements present in cement-based materials. By scanning surfaces the variability in the spatial distribution of elements can be visualised. However, for a quantification of the results, studies are necessary to characterise possible influences due to the wide variation of the chemical compositions in which cement can occur. It is shown how the calibration can be done, how the calibration samples were produced, and which statistical parameters are necessary to describe the precision of the regression. The performance of LIBS is estimated by detecting chloride in validation samples. Therefore, 55 samples and 7 ets with changing mix ompositions were produced. The presented study deals with possible influences of different mix compositions, ncluding different cations of chloride, varying w/c-ratios and the artial replacement of Portland cement with last furnace slag (50% BFS) and limestone (30% LS). Comparing the LIBS results with otentiometric titration, n accuracy of±0.05 wt%/total has been determined.