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- Accelerated carbonation (2)
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The thermal gravimetric analysis (TG) is a common method for the examination of the carbonation progress of cement-based materials. Unfortunately, the thermal properties of some components complicate the evaluation of TG results. Various hydrate phases, like ettringite (AFt), C-S-H and AFm decompose almost simultaneously in the temperature range up to 200 °C. Additionally, physical bound water is released in the same temperature range. In the temperature range between 450 °C and 600 °C the decomposition of calcium hydroxide and amorphous or weakly bound carbonates takes place simultaneously. Carbonates, like calcite, from limestone powder or other additives may be already contained in the noncarbonated sample material. For this research an attempt was made to minimise the influence of these effects. Therefore, differential curves from DTG-results of non-carbonated areas and areas with various states of carbonation of the same sample material were calculated and evaluated. Concretes based on three different types of cement were produced and stored under accelerated carbonation conditions (1 % CO2 in air). The required sample material was obtained by cutting slices from various depth of previously CO2-treated specimen and subsequent grinding. During the sample preparation, a special attention was paid that no additional carbonation processes took place. As reference method for the determination of the carbonation depth the sprayed application of phenolphthalein solution was carried out. Microscopic analysis where examined to confirm the assumptions made previously. Furthermore, the observed effect of encapsulation of calcium hydroxide by carbonates caused by the accelerated carbonation conditions was examined more closely.
While calcined clays in general have been credited with a great potential to mitigate CO2 emissions related to cement production and consumption, calcined brick clays are currently understudied in this regard. In the present work, two brick clays, a low-grade kaolinitic clay, and a mixed clay composed of 50% brick clay and 50% low-grade kaolinitic clay were studied regarding transformations on calcination, and strength and durability performance as well as pore structure of mortars made with the blended cements. All calcined clays exhibited pozzolanic reactivity, with the performance of the brick clays inferior to the low-grade kaolinitic clay. However, the mixed clay performed very similar to the low-grade kaolinitic clay, which points to a viable option for optimal use of brick clays in cementitious systems. The carbonation resistance of the blended cement mortars was generally worse than that of the plain Portland cement mortar, as expected, but the former exhibited a significantly improved chloride penetration resistance. The latter improvement was due to pore structure refinement in the blended cement mortars, compared to the Portland cement mortar.
Rising temperatures worldwide pose an increasing challenge for safe and healthy living conditions. Particularly inner cities have been affected by these environmental changes because of the materials used to build houses, streets and infrastructure. The most common building material is concrete. It shows a specific heat capacity, while the heat conductivity for Standard concrete is low. Thus, the use of concrete generates a high capacity of heat storage. In addition, extensive soil sealing also contributes to the temperature rise of inner city areas compared to
their surroundings. To mitigate this so-called urban heat island effect, a self-cooling concrete paver was developed. This paver is able to store water. The evaporation of the water at elevated temperatures provides a cooling effect. This paper focuses on determination of this new paver’s capability to cool the surface and the surrounding. The new paver’s cooling Qualities were analysed in a series of laboratory tests. To prove the results outside of laboratory conditions, two fields (12 m × 8 m) with self-cooling and reference pavers were installed in Spain.
This paper presents and discusses the results of the tests. Correlation between reduced surface temperature of the self-cooling concrete pavers and the air temperature is examined.
The most common method for determination of the carbonation depth is the sprayed application of indicator solutions, Phenolphthalein in particular. Since the aerosols of Phenolphthalein are suspected to be carcinogenic, efforts have been intensified to use alternative indicators for the determination of carbonation depth. The subject and aim of this research paper are to examine indicator solutions from Alizarin Yellow R, Thymol Blue and a Mixed Indicator. These indicator solutions are commercially available and are not suspected to be carcinogenic.
For the purpose of this research, samples previously stored under accelerated carbonation conditions (1% CO2) were examined by thermogravimetric analysis (TGA) and examination of the pH value. Additionally, the electrolytic back-titration for determination of total CO2 amount was carried out comparatively to TGA.
The use of calcined clays as supplementary cementitious materials (SCMs) has been identified as a viable option to decrease the CO2 emissions related to cement production. However, while extensive data is available about kaolinitic clays in this context, other clays such as illitic clays appear to be under-studied. Therefore, in the present study, two illitic clays were compared to two low-grade kaolinitic clays in terms of transformations in the calcination temperature range 650–900 °C, and performance of the calcined clays in blended cement pastes as measured by strength evolution, heat release, hydrated phase formation and portlandite consumption. The illitic clays required a higher calcination temperature for complete dehydroxylation of their illite than what is necessary for dehydroxylation of kaolinite. These higher calcination temperatures also led to particle sintering, significantly decreasing the specific surface area of the illitic clays, particularly for the clay with the higher Fe2O3 content. Nevertheless, while the kaolinitic clays generally exhibited the best performance as SCM, the illitic clay with lower Fe2O3 content performed similar to the kaolinitic clays when calcined at optimum temperature and applied at a moderate substitution rate. These findings demonstrate that several different clays have the potential to be used as SCM and indicate possible routes to identify suitable deposits for this purpose.
Properties of alkali-activated mortars with salt aggregate for sealing structures in evaporite rock
(2021)
Concrete structures for sealing of tunnels in the host rock are an essential part of systems for nuclear waste storage. However, concretes based on blended cements or magnesium oxychloride cements, which are commonly considered for this application, can deteriorate severely due to a significant heat of hydration and associated deformation and cracking. Alkali-activated materials (AAMs) offer a potential solution to this problem because of their low heat release during hardening. To explore their suitability for the construction of sealing structures in evaporite rock, various AAMs with salt aggregate were studied regarding fresh properties, heat release, mechanical properties and microstructure. The heat of reaction of the AAMs was up to 55% lower than that of a blended cement designed for sealing structures, indicating significant benefits for the intended application. Other relevant properties such as mechanical strength and permeability depended strongly on the mix-design of the AAMs and curing conditions.
Die Bestimmung der Rauheit von Bauteiloberflächen stellt eine wichtige Messaufgabe im Bauwesen dar. Vor allem bei der Betoninstandsetzung, im Straßenbau und bei der Herstellung von Stahlbeton-Halbfertigteilen ist die Rauheit – gemessen als Rautiefe – eine wichtige Kenngröße. Zurzeit werden Messungen der Rautiefe mittels volumetrischer Verfahren (Sandflächenverfahren) durchgeführt. Diese bereits seit mehreren Jahrzehnten angewandten Verfahren sind jedoch auf horizontale bzw. wenig geneigte und trockene Oberflächen beschränkt. Darüber hinaus hängen die erzielten Ergebnisse sehr stark von der angewandten Prüfvorschrift, den darin vorgeschriebenen Prüfmitteln und Geräten und dem jeweiligen Anwender ab.
Als Alternative zu den volumetrischen Verfahren wurde an der Bundesanstalt für Materialforschung und -prüfung (BAM) ein automatisiertes, laserbasiertes Messverfahren entwickelt, mit dem Rautiefen von Bauteiloberflächen in Anlehnung an bereits standardisierte Verfahren bestimmt werden können. Der vorliegende Beitrag befasst sich zunächst mit den konventionellen Prüfverfahren, beschreibt die wichtigsten Zusammenhänge und zeigt die Grenzen dieser Methoden auf. Im zweiten Teil wird das an der BAM entwickeltes Messsystem, als zielführendes Alternativverfahren für die konventionellen Methoden, vorgestellt und dessen Leistungsfähigkeit näher beschrieben.