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- Alkali-activation (3)
- Geopolymers (3)
- Alkali-activated materials (2)
- Blended cement (2)
- Brick clay (2)
- Calcined clay (2)
- One-part formulation (2)
- One-part geopolymers (2)
- Acid resistance (1)
- Alkali activated materials (1)
Organisationseinheit der BAM
- 7 Bauwerkssicherheit (5)
- 7.4 Baustofftechnologie (5)
- 6 Materialchemie (2)
- 6.3 Strukturanalytik (2)
- 7.1 Baustoffe (1)
A silica residue from waste treatment of chlorosilane production was used together with solid sodium aluminate to test its applicability for the production of one-part geopolymers. The blend was activated with water and cured at 70 °C. The degree of reaction and strength were determined after 1, 3, and 7 days. The reaction products were analyzed by XRD and SEM/EDX. Until the third day of curing the degree of reaction of the residue reached 51 % and the strength was 8.9 MPa. The reaction product was identified as geopolymer containing zeolite A. Thus, the results confirmed that the residue may be used in the production of geopolymers. However, after 3 days of curing no further progress of reaction was observed and the strength slightly decreased, which was attributed to changes in the structure of the geopolymeric gel. It was further observed that even harsh vacuum drying left some water (presumably zeolitic water and surface hydroxyl groups) in the geopolymer.
Reaction products and strength development of wastepaper sludge ash and the influence of alkalis
(2014)
Wastepaper sludge ash (WSA) from a newsprint paper mill was investigated for its mineralogical composition and its reaction products and strength development after activation with water and sodium and potassium hydroxide solutions. The results showed the WSA to consist of calcite, free lime, gehlenite, tricalcium aluminate, belite, talc, quartz and probably a glassy phase. The principle reaction product was monocarboaluminate (CO3–AFm) for the water- as well as for the alkali-activated WSA. Formation of monocarboaluminate and strength gain was more rapid for the alkali-activated WSA until 1 day of curing. However, afterwards reactions proceeded much slower when alkali solutions were used, leading to an about twice as high compressive strength for water-activated WSA mortars after 28 days of curing. The observed behavior is tentatively ascribed to a less uniform microstructure of the alkali-activated WSA. Significant differences between NaOH- and the KOH-activated WSA were not observed.
One-part geopolymers offer advantages over conventional geopolymers with regard to handling and storage of feedstocks. However, they often suffer from a low degree of reaction, a high amount of crystalline byproducts, and consequently low strength. In this study, one-part geopolymers were produced from rice husk ash (RHA) and sodium aluminate, and investigated by XRD, ATR-FTIR, SEM and compressive strength testing. The compressive strength of the material was 30 MPa, i.e. significantly higher than for comparable one-part geopolymers. This is attributed to an almost complete reaction of the RHA and the absence of crystalline byproducts (zeolites) in the hardened geopolymer.
This contribution presents the results of structural and compressive strength investigations on cured andhigh-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens weresynthesized from two different silica sources, sodium aluminate and water. The phase content as well asthe compressive strength of the cured composites varied depending on the starting mix-design and thesilica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reactionproducts. One of the silica feedstocks yielded significantly higher compressive strength (19 MPa), whilethe other one appears to cause less variation in phase content. Strength testing indicated an improvementon heating up to
200–400 °C (28 MPa) followed by a moderate decrease up to 700 °C. Above 700 °C the sys-tems underwent new phase formation and shrinkage (volume decrease) deformations. After exposureat 1000 °C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexago-nal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the hightemperature phase transformations varied.
One-part alkali-activated (geopolymer) mortars based on three different silica-rich starting materials and sodium aluminate, with and without ground granulated blast furnace slag (GGBFS) addition, were tested regarding sulfuric acid resistance according to DIN 19573:2016-03 (70 days at pH = 1). Corresponding pastes were characterized by XRD, SEM, chemical analysis, 29Si MAS NMR and 1H-29Si CPMAS NMR after water storage and after acid exposure. The mortars exhibited a high resistance against sulfuric acid attack, with the best ones conforming to the requirements of DIN 19573:2016-03. The analytical results showed that this was due to precipitation of silica gel at the acid-mortar interface, which formed a mechanically stable layer that protected the subjacent mortar and thus inhibited further degradation. The addition of GGBFS decreased the acid resistance via formation of expansive calcium sulfate phases.
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.
Alkali-activated materials are ideal for the repair of concrete structures in harsh environmental conditions due to their high durability in chemically aggressive environments. However, slag-based mortars, in particular, are prone to shrinkage and associated cracks. In this respect, the application of steel fibres is one solution to reduce the formation of shrinkage induced cracks and to improve post cracking behaviour of these mortars. This study investigated the influence of two different types of steel fibres on the tensile properties of two alkali-activated mortars. Direct tensile tests and single fibre pull-outs were performed to analyse the determining failure modes both on macro and micro scale. Mechanical testing was accompanied by non-destructive testing methods such as digital image correlation and acoustic emission for a detailed analysis of the fracture process.
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.
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.
Mixing and curing of geopolymers and related alkali-activated materials without storage and handling of highly alkaline solutions possesses advantages regarding safety and economic viability. One possible approach is to produce these materials from solid silica feedstocks and solid sodium aluminate, and subsequent mixing with water. We present a comparison between geopolymers and aluminosilicate gel-zeolite composites synthesized by this route from different silica feedstocks (by-product silica from chlorosilane production, microsilica, rice husk ash) and with different SiO2/Al2O3 ratios, using results from XRD, NMR, SEM, thermal analysis, mechanical and acid resistance testing. The use of rice husk ash favors formation of a fully amorphous geopolymer with high strength. Utilization of the other silica feedstocks leads to formation of aluminosilicate gel-zeolite composites, the amount and kind of zeolites depending on the feedstock and the SiO2/Al2O3 ratio. These composites show beneficial dehydration behavior, viz. no distinct dehydration step of thermal strain, with the phase assemblage after heating to 1000°C controlled by the starting composition. Mortars produced from both, the geopolymers as well as the composites, exhibit high resistance to sulfuric acid attack, making them promising materials for the construction and the repair of industrial and sewer structures.