Filtern
Dokumenttyp
- Zeitschriftenartikel (7) (entfernen)
Sprache
- Englisch (7)
Schlagworte
- Geopolymers (3)
- Alkali-activated materials (2)
- Alkali-activation (2)
- Blended cement (2)
- Brick clay (2)
- Calcined clay (2)
- Acid resistance (1)
- Additive manufacturing (1)
- Alkali activated materials (1)
- Bio-based materials (1)
Organisationseinheit der BAM
Paper des Monats
- ja (1)
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.
Additive manufacturing of alkali-activated materials currently attracts a lot of attention, because of the possibility to produce customized high-performance elements for a range of applications, potentially being more resource-efficient than conventionally produced parts. Here, we describe a new additive manufacturing process for alkali-activated materials that is based on selective laser-heating of lithium aluminate/microsilica slurries. The new process-material combination allows to manufacture elements with complex geometries at high building rates and high accuracy. The process is versatile and transferrable to structures of sizes differing by orders of magnitude. The mechanical strength of the obtained materials was in the range of values reported for conventional metakaolin-based geopolymers, and superior to what has been hitherto reported for alkali-activated materials produced by additive manufacturing. This mechanical performance was obtained despite the fact that the degree of reaction of the lithium aluminate and the microsilica was low, suggesting that significant reactions took place only at the surface of the microsilica particles.
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.