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- Cement paste (2)
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Time-resolved in situ investigation of Portland cement hydration influenced by chemical admixtures
(2016)
Numerous admixtures are used in the building practice to customize the properties of the cement paste during application. The influences of admixtures on the course of cement hydration and formation of hydrate phases have to be considered. Polycarboxylate ether (PCE) based polymeric superplasticizers (SPs) are known to retard the setting of the cement paste. The extent of the retardation differs depending on the molecular structure of the SP. Additionally, the presence of a stabilizing agent (SA) in the cement paste has a retarding side effect on the setting. The initial cement hydration processes and the detailed mechanisms of the retardation influenced by PCEs, as well as their interactions with particular SAs, are insufficiently understood. Up to now, only the results of phenomenological studies were taken into account to describe this retardation process. A detailed structure analysis monitoring the change of the phase composition during the hydration was never applied. Both SP and SA affect the adsorption of the sulphate ions on the clinker particles, causing changes in the formation of ettringite during the initial hydration, and are therefore a crucial part of the setting process itself. Here, the initial hydration of cement influenced by the interaction of SP and SA was monitored in situ by synchrotron X-ray diffraction. The high time resolution of the measurements allowed a continuous detection of the hydrates formed. The hydration was followed from the starting point of water addition and for couple of hours afterwards. The hydration of the levitated cement pellets containing starch as SA was initialized by adding aqueous solutions of different commercial SPs. Changes in the ettringite formation were detected in comparison to the reference hydration of pure cement.
Pathway of a damaging mechanism - Analyzing chloride attack by synchrotron based X-ray diffraction
(2015)
Typically, the changes of the phase compositions due to the chemical attack are studied in-situ only by chemical analysis or microscopy. In this study, the chloride transport and binding in the cement matrix in different cementitious materials was analyzed by synchrotron based X-ray diffraction (SyXRD) and energy dispersive X-ray spectroscopy (EDX). Sample materials consisting of cement paste were embedded in high concentrated sodium chloride solution over different time spans. Afterwards, the phase and chemical compositions were determined. The high spatial resolution and the information about the chloride distribution offer a detailed view of chloride binding in the cement matrix and allow the conclusions about the degradation mechanisms. The results are discussed related to the influence of different supplementary cementitious materials on the damaging mechanism.
The complex matter of external sulphate attack on cement-based construction materials is still not completely understood. The concentration of sulphate is a crucial factor for the formation of secondary phases and phase transitions of cement hydrates due to sulphate ingress into the microstructure. The sulphate attack on building materials for high and low sulphate concentrations was monitored by laboratory experiments. Hardened cement paste consisting of ordinary Portland cement (CEM I) were exposed to aqueous solutions of sodium sulphate for 18 months. Three sample compositions were used for this research, including different supplementary cementitious materials (SCM). The phase composition was determined for different time spans by high resolution synchrotron X-ray diffraction. Cross sections of exposed cement prisms were investigated as a representation of the microstructural profile. Based on the data, a temporal and spatial determination of the stages of the sulphate attack and the deterioration course was possible. Cement matrices blended with slag showed the highest resistance against sulphate attack.
The exact mechanisms of the phase transitions caused by a combined sulphate-chloride attack are discussed controversially. The main points concern the mutual influences of sulphate and chloride ions during the secondary binding processes of these anions within cement hydrate phases. We simulated combined sulphate-chloride attack under laboratory conditions using solutions containing NaCl and Na2SO4 in different concentrations. Three sample compositions were used for the preparation of the specimens. In two of them, 30% of Portland cement was replaced by supplementary cementitious materials (fly ash, slag). The phase distribution in the samples was determined using synchrotron X-ray diffraction. The analysis with high spatial resolution allows the localisation of the secondary phase formation in the microstructural profile of the sample. A mechanism of the phase developments under combined sulphate-chloride attack is derived.
High resolution synchrotron X-ray diffraction (SyXRD) was applied for a microstructural profile analysis of concrete deterioration after sulfate attack. The cement matrices consist of ordinary Portland cement and different amounts of supplementary cementitious materials, such as fly ash, natural pozzolana and granulated blast furnace slag. The changes of the phase composition were determined along the direction of sulfate ingress. This approach allows the identification of reaction fronts and zones of different phase compositions and conclusions about the mechanisms of sulfate attack. Two reaction fronts were localized in the initial 4 mm from the sample surface. The mechanism of deterioration caused by the exposition in the sulfate-bearing soil is discussed. SyXRD is shown to be a reliable method for investigation of cementitious materials with aggregates embedded in natural environments.