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- Alkali-activated materials (5)
- Geopolymers (4)
- Acid resistance (3)
- Fire proofing (2)
- Fire protection (2)
- Acoustic emission (1)
- Alkali activated materials (1)
- Alkali-activated slag (1)
- Alkali-activated slag mortar (1)
- Aluminium hydroxide (1)
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Slags from the nonferrous metals industry have great potential to be used as feedstocks for the production of alkali-activated materials. Until now, however, only very limited information has been available about the structural characteristics of these materials. In the work presented herein, synthetic slags in the CaO–FeOx–SiO2 system, representing typical compositions of Fe-rich slags, and inorganic polymers (IPs) produced from the synthetic slags by activation with alkali Silicate solutions have been studied by means of X-ray absorption near-edge structure (XANES) spectroscopy at the Fe K-edge. The iron in the slags was largely Fe2+, with an average coordination number of approximately 5 for the iron in the amorphous fraction. The increase in average oxidation number after alkali-activation was conceptualized as the consequence of slag dissolution and IP precipitation, and employed to calculate the degrees of reaction of the slags. The degree of reaction of the slags increased with increasing amorphous fraction. The iron in the IPs had an average coordination number of approximately 5; thus, IPs produced from the Fe-rich slags studied here are not Fe-analogs of aluminosilicate geopolymers, but differ significantly in terms of structure from the latter.
Alkali-activated slag materials are known for their high Chloride penetration resistance. This makes them potentially applicable as repair systems for damaged steel-reinforced concrete structures, which are exposed to chloride attack. For this purpose, the influence of the activator composition, i.e. the SiO2 and Na2O concentration of the alkaline solution on a) the compressive strength, b) shrinkage and mass change and c) the resistance against chloride penetration of four alkali-activated slag mortars (AASM) were studied. An ordinary Portland cement-based mortar was used as the reference sample. Increasing SiO2 and Na2O concentrations increased the strength, shrinkage and mass loss of the AASMs. The resistance of the mortars against chloride penetration was evaluated using the non-steady-state migration coefficient Dnssm obtained from NT BUILD 492. The results indicate that the Dnssm is related to differences in the pore solution of the AASMs rather than to differences in their microstructure. An upcoming study of the authors is going to evaluate this hypothesis by the accelerated chloride penetration (diffusion) test.
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 geopolymer-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 geopolymer-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 compo¬sites, exhibit high resistance to sulfuric acid attack, making them promising materials for the construction and the repair of industrial and sewer structures.
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.
One-part geopolymers (OPGs) are a sort of alkali-activated materials (AAMs) which production avoids the use of highly-alkaline activator-solutions and contributes to a better acceptance of alternative mineral binders in terms of safety-related and economic aspects. In the present contribution OPGs were synthesized by blending silica sources (two industrial silicas and two biogenic silicas) with sodium aluminate and only water must be added to initiate the hardening, i.e. mixing is performed in the same way as for conventional Portland cements. The OPGs were characterized by XRD, and SEM and the degrees of reaction of the silicas were determined by a chemical dissolution method.
The industrial silicas led to the formation of geopolymer-zeolite composites, that contained, besides geopolymeric gel, crystalline tectosilicates (e.g. zeolite A and hydrosodalite) and depending on the starting composition also unreacted silica. The biogenic silicas provided a higher reactivity and avoided the formation of crystallite by-products. The differences in the microstructures caused differences in the mechanical strength of the specimens.
The treatment of the OPG composites at moderate elevated temperatures revealed promising behavior on thermal dehydration in terms of shrinkage and residual strength up to 700 °C. Above 700 °C sintering and partial melting occurred, and new phase formation commenced. After exposure to 1000 °C the specimens appeared virtually amorphous or formed stuffed silica structures of nepheline- or carnegieite-type type.
The investigations of the OPG based mortars on their resistance against sulfuric acid in accordance with DIN 19573 (Appendix A) revealed very high residual strengths up to 78 % after treatment with H2SO4 (pH 1) for 70d. A mechanism of dissolution of the primary aluminosilicate reaction products of the pastes and the precipitation of a silica gel that protects the remaining aluminosilicates and decelerates further corrosion was found to be the main reason for the good performance under acidic conditions. The addition of CaO-containing feedstocks enhanced hardening, but at a certain content the resistance against sulfuric acid decreased, due to the formation of gypsum on exposure to sulfate.
In addition, the mortars exhibited excellent shrinkage behavior as well as good bond to concrete substrates with pull-off strength up to > 3 MPa. The workability of the fresh mortars provided very good manual applicability; automatic applications such as sprayed and spun mortars will require further optimization regarding rheological properties. In summary, the studied OPG are promising materials for the construction and the repair of concrete structures, such as sewers, that are affected by biogenic sulfuric acid corrosion.
Intumescent coatings for fire protection offer advantages over (non-intumescent) cementitious coatings and boards regarding speed of construction, architectural aesthetics, sometimes costs, and other features. However, conventional organic intumescent coatings as well as soluble silicate (waterglass) coatings form foams with low mechanical stability, and the latter coatings generally suffer from low resistance against humidity. Therefore, the search for novel intumescent coatings for more demanding conditions (e.g., abrasive environments) is a necessity in the context of steadily increasing requirements of society and industry.
In this contribution, we present results on intumescent aluminosilicate coatings for fire protection that form foams with significantly increased mechanical strength. Two base formulations, a metakaolin/silica-based mix, adapted from Krivenko et al., and a silica/corundum-based mix, developed at Curtin University, as well as formulations modified with additives (Al(OH)3, Mg(OH)2, B2O3, Na2B4O7), were applied to steel plates (75 mm × 75 mm) and exposed to simulated fire conditions (fire curve according to ISO 834-1:1999). Temperature-time curves were recorded to assess the degree at which the coatings insulated the substrate. In addition, XRD, TG, oscillatory rheometry, and SEM were employed to characterise the coatings.
The coatings were observed to partly expand during hardening due to H2 formation. When the hardened coatings were exposed to elevated temperatures they intumesced as expected, with the degree and nature of expansion dependent on the formulation. Oscillatory rheometry provided insights into the intumescent processes in an apparently brittle material. It revealed that the hardened aluminosilicate coatings became viscous (loss factor > 1) at 75–225 °C, in the temperature range of major water release, as opposed to a “standard” metakaolin-based geopolymer, which continued to behave as a solid. This explains the intumes¬cent behavior of the coatings, i.e. further expansion and foam formation. Microstructural analysis confirmed pore expansion and coalescence; XRD showed that the phases formed after heating (max. temperature ~840 °C) were of ceramic-type.
The fire protection (defined here as the time for the steel substrate to reach the critical temperature of 500 °C) depended mainly on the thick¬ness of the fully expanded coating, i.e. after intumescence. An alumino¬silicate coating free of additives with an original thickness of 12 mm was able to protect the steel for >30 min. The addition of 10 % anhydrous borax (Na2B4O7) caused a significant improvement, such that an original coating thickness of only 6 mm was sufficient to protect the steel for ~30 min. This was caused by the formation of sodium metaborate dihydrate (NaB(OH)4) in the coating that led to a significantly extended dehydration plateau in the temperature-time curve at ~100 °C during the fire exposure.
Alkali-activated fly ash mortars were studied with regard to durability-relevant transport coefficients and the electrochemical behaviour of embedded carbon steel bars on exposure of the mortars to leaching, carbonation and chloride penetration environments. The transport coefficients differed considerably between different formulations, being lowest for a mortar with BFS addition, but still acceptable for one of the purely fly ash-based mortars. Leaching over a period of ~300 days in de-ionized water did not lead to observable corrosion of the embedded steel, as shown by the electrochemical data and visual inspection of the steel. Exposure to 100 % CO2 atmosphere caused steel depassivation within approx. two weeks; in addition, indications of a deterioration of the mortar were observed. The results are discussed in the context of the different reaction products expected in high- and low-Ca alkali-activated binders, and the alterations caused by leaching and carbonation.
The influence of starting materials and synthesis route on the properties and the structure of cementitious sodium aluminosilicate gels is not fully understood, partly due their amorphous nature and the fact that they often contain residual reactants, which can make the results of single-pulse NMR spectroscopy applied to these materials difficult to interpret or ambiguous. To overcome some of these limitations, 29Si{27Al} TRAPDOR NMR as well as 27Al{29Si} and 27Al{1H} REDOR NMR spectroscopy were applied to materials synthesized by the one-part alkali-activation route from three different amorphous silica starting materials, including rice husk ash. The latter led to formation of a fully amorphous sodium aluminosilicate gel (geopolymer), while the materials produced from the other silicas contained amorphous phase and crystalline zeolites. Application of the double-resonance NMR methods allowed to identify hydrous alumina gel domains in the rice husk ash-based material as well as significantly differing amounts of residual silica in the three cured materials. Four-coordinated Al existed not only in the aluminosilicate gel framework but also in a water-rich chemical environment with only a small amount of Si in proximity, likely in the alumina gel or possibly present as extra-framework Al in the aluminosilicate gel. The results demonstrate how the employment of different silica starting materials determines the phase assemblage of one-part alkali-activated materials, which in turn influences their engineering properties such as the resistance against chemically/biologically aggressive media.
The present study aims to investigate the use of geopolymer mortars as passive fire protection system for steel structures. Coal fly ashes were used as aluminosilicate source and perlite was employed as aggregate to obtain a lightweight system. In addition, a geopolymer mortar containing quartz aggregate was produced for comparison. The geopolymer mortars were applied on stainless steel plates and exposed to both, cellulosic and hydrocarbon standard fire curves, according to ISO 834-1 and EN 1363-2, respectively. Acoustic emission measurements were conducted to analyze cracking phenomena during the high temperature exposure. The resulting temperature-time curves showed that the investigated system is effective in retarding the temperature rise of the steel plates. When the cellulosic fire curve was applied, a 20 mm [0.79 in.] thick layer of lightweight geopolymer mortar protected the steel substrate from reaching the critical temperature of 500 °C [932 °F] for at least 30 minutes, avoiding the rapid decrease of its mechanical properties and thus representing an important safety measure against accidental fires. No spalling phenomena on heating were detected; however, significant cracking was observed on cooling.