Carbonation resistance of alkali-activated GGBFS/calcined clay concrete under natural and accelerated conditions

The carbonation resistance of alkali-activated materials (AAMs) is a crucial parameter for their applicability in concrete construction, yet the parameters influencing it are insufficiently understood to date. In the present study, the carbonation resistance of alkali-activated concretes with varying fractions of ground granulated blast furnace slag (GGBFS) and calcined clay (i.e., high, intermediate, and low Ca contents) were assessed under natural and accelerated conditions. Corresponding hardened AAM pastes were studied using X-ray diffraction, thermogravimetry, Raman microscopy, and mercury intrusion porosimetry. The carbonation resistance of the concretes at natural CO2 concentration depended principally on their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio. The remaining variability for similar ratios was caused by differences between the pore structures of the AAMs. For concrete with favorable water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio and pore structure, the carbonation resistance was comparable to that of Portland cement concrete. The relationship between carbonation coefficients obtained under accelerated and natural conditions differed for concretes with high and low fractions of calcined clay, indicating that accelerated carbonation testing is less suitable to study the carbonation of low-Ca AAMs

Geopolymer-bound intumescent coatings for fire protection

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 [1]. 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 [2]. Two base formulations, a meta­kaolin/silica-based mix, adapted from Krivenko et al. [3], 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. Please click Additional Files below to see the full abstract

Structure, acid-resistance and high-temperature behavior of silica-based one-part geopolymers and geopolymer-zeolite composites

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 [1, 2]. 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 [3]. 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 \u3e 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. [1] Sturm, P., Greiser, S., Gluth, G. J. G., Jäger, C. & Brouwers, H. J. H. Degree of reaction and phase content of silica-based one-part geopolymers investigated using chemical and NMR spectroscopic methods. J. Mater. Sci. 50, 6768–6778 (2015). [2] Sturm, P., Gluth, G. J. G., Brouwers, H. J. H. & Kühne, H.-C. Synthesizing one-part geopolymers from rice husk ash. Constr. Build. Mater. 124, 961–966 (2016). [3] Sturm, P., Gluth, G. J. G., Simon, S., Brouwers, H. J. H. & Kühne, H.-C. The effect of heat treatment on the mechanical and structural properties of one-part geopolymer-zeolite composites. Thermochim. Acta 635, 41–58 (2016)

APPLICATION OF STEEL FIBRES IN ALKALI-ACTIVATED MORTARS

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

Steel reinforcement corrosion in alkali-activated fly ash mortars

Corrosion of steel rebars in concrete presents one of the main deterioration mechanisms limiting service life of the reinforced structures. The corrosion is accompanied by an expansion of the corrosion products causing high pressures, concrete cracking and finally spalling of a cover layer. Critical chloride concentration, loss of alkalinity and modeling of the steel corrosion are in researchers\u27 spotlight for decades, however reinforcement corrosion in alkali activated materials is insufficiently described and understood yet. In this work, the steel reinforcement corrosion in alkali-activated fly ash mortars is investigated in terms of electrochemical behaviour of the reinforced mortars exposed to aggressive environments such as leaching, carbonation and chloride ingress. A selected geopolymer mixture based on hard coal fly ash activated with sodium hydroxide and sodium silicate solutions is used for the steel reinforcement-corrosion experiments. The formation of passive layer on the steel rebars is observed after approx. two weeks of hardening at laboratory temperature. However, alternative heat-treatment at 80°C for several hours leads to immediate formation of the passive layer as well as to a faster strength gain (80 MPa after 24h at 80°C). Chloride-induced corrosion, leaching and carbonation resistance of the alkali activated fly ash-based concrete is studied, where leaching in deionized water or carbonation under natural conditions (~0.04 % CO2) for 300 days did not lead to corrosion of the embedded steel. On the other hand, accelerated carbonation under 100 % CO2 atmosphere lead to depassivation within two weeks. Please click Additional Files below to see the full abstract

Sol–gel synthesis and characterization of lithium aluminate (L–A–H) and lithium aluminosilicate (L–A–S–H) gels

Hydrous lithium aluminosilicate (L–A–S–H) and lithium aluminate (L–A–H) gels are candidate precursors for glass-ceramics and ceramics with potential advantages over conventional processing routes. However, their structure before calcination remained largely unknown, despite the importance of precursor structure on the properties of the resulting materials. In the present study, it is demonstrated that L–A–S–H and L–A–H gels with Li/Al ≤ 1 can be produced via an organic steric entrapment route, while higher Li/Al ratios lead to crystallization of gibbsite or nordstrandite. The composition and the structure of the gels was studied by thermogravimetric analysis, X-ray diffraction, 27Al and 29Si magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. Aluminium was found to be almost exclusively in six-fold coordination in both the L–A–H and the L–A–S–H gels. Silicon in the L–A–S–H gels was mainly in Q4 sites and to a lesser extent in Q3 sites (four-fold coordination with no Si–O–Al bonds). The results thus indicate that silica-rich and aluminium-rich domains formed in these gels

Leaching, carbonation and chloride ingress in reinforced alkali-activated fly ash mortars

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 highand low-Ca alkali-activated binders, and the alterations caused by leaching and carbonation

DataSheet1_The influence of curing temperature on the strength and phase assemblage of hybrid cements based on GGBFS/FA blends.PDF

Hybrid cements are composites made of Portland cement or Portland clinker and one or more supplementary cementitious materials like slag, fly ash or metakaolin, activated with an alkali salt. To date, their hydration mechanism and the phase formation at various temperatures is insufficiently understood, partly due to the large variability of the raw materials used. In the present study, three hybrid cements based on ground granulated blast furnace slag, fly ash, Portland clinker and sodium sulfate, and an alkali-activated slag/fly ash blend were cured at 10 and 21.5°C, and subsequently analyzed by XRD, 27Al MAS NMR, and TGA. The compressive strength of the hybrid cements was higher by up to 27% after 91-day curing at 10°C, compared to curing at 21.5°C. The experimental results as well as thermodynamic modeling indicate that the differences in compressive strength were related to a different phase assemblage, mainly differing amounts of strätlingite and C-N-A-S-H, and the associated differences of the volume of hydration products. While the strätlingite was amorphous to X-rays, it could be identified by 27Al MAS NMR spectroscopy, TGA and thermodynamic modeling. The microstructural properties of the hybrid cements and the alkali-activated slag/fly ash blend as well as the compatibility between thermodynamic modeling results and experimental data as a function of curing temperature and time are discussed.</p

Material aspects of sintering of EAC-1A lunar regolith simulant

Abstract Future lunar exploration will be based on in-situ resource utilization (ISRU) techniques. The most abundant raw material on the Moon is lunar regolith, which, however, is very scarce on Earth, making the study of simulants a necessity. The objective of this study is to characterize and investigate the sintering behavior of EAC-1A lunar regolith simulant. The characterization of the simulant included the determination of the phase assemblage, characteristic temperatures determination and water content analysis. The results are discussed in the context of sintering experiments of EAC-1A simulant, which showed that the material can be sintered to a relative density close to 90%, but only within a very narrow range of temperatures (20–30 °C). Sintering experiments were performed for sieved and unsieved, as well as for dried and non-dried specimens of EAC-1A. In addition, an analysis of the densification and mechanical properties of the sintered specimens was done. The sintering experiments at different temperatures showed that the finest fraction of sieved simulant can reach a higher maximum sintering temperature, and consequently a higher densification and biaxial strength. The non-dried powder exhibited higher densification and biaxial strength after sintering compared to the dried specimen. This difference was explained with a higher green density of the non-dried powder during pressing, rather than due to an actual influence on the sintering mechanism. Nevertheless, drying the powder prior to sintering is important to avoid the overestimation of the strength of specimens to be fabricated on the Moon