Time filter

Source Type

Somerton, Australia

Ismail I.,University of Melbourne | Ismail I.,University Malaysia Sarawak | Bernal S.A.,University of Melbourne | Provis J.L.,University of Sheffield | And 2 more authors.
Materials and Structures/Materiaux et Constructions | Year: 2013

Sulfate attack is recognized as a significant threat to many concrete structures, and often takes place in soil or marine environments. However, the understanding of the behavior of alkali-activated and geopolymer materials in sulfate-rich environments is limited. Therefore, the aim of this study is to investigate the performance of alkali silicate-activated fly ash/slag geopolymer binders subjected to different forms of sulfate exposure, specifically, immersion in 5 wt% magnesium sulfate or 5 wt% sodium sulfate solutions, for 3 months. Extensive physical deterioration of the pastes is observed during immersion inMgSO4 solution, but not in Na2SO4 solution. Calcium sulfate dihydrate (gypsum) forms in pastes immersed in MgSO4, and its expansive effects are identified as being particularly damaging to the material, but it is not observed in Na2SO 4 environments.A lowerwater/binder (w/b) ratio leads to a greatly enhanced resistance to degradation by sulfate attack. Infrared spectroscopy shows some significant changes in the silicate gel bonding environment of geopolymers immersed in MgSO4, attributed mostly to decalcification processes, but less changes upon exposure to sodium sulfate. It appears that the process of 'sulfate attack' on geopolymer binders is strongly dependent on the cation accompanying the sulfate, and it is suggested that a distinction should be drawn between 'magnesium sulfate attack' (where both Mg2+and SO4 2-are capable of inducing damage in the structure), and general processes related to the presence of sulfate accompanied by other, non-damaging cations. The alkali-activated fly ash/slag binders tested here are susceptible to the first of these modes of attack, but not the second. © RILEM 2013. Source

Ismail I.,University of Melbourne | Ismail I.,University Malaysia Sarawak | Bernal S.A.,University of Melbourne | Bernal S.A.,University of Sheffield | And 6 more authors.
Cement and Concrete Composites | Year: 2014

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C-A-S-H type gel, while silicate-activated fly ash are dominated by N-A-S-H 'geopolymer' gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C-A-S-H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N-(C)-A-S-H is also identified, as part of the Ca released by slag dissolution is incorporated into the N-A-S-H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure. © 2013 Elsevier Ltd. All rights reserved. Source

Bernal S.A.,University of Sheffield | Bernal S.A.,University of Melbourne | Bernal S.A.,University of Valle | San Nicolas R.,University of Melbourne | And 7 more authors.
Cement and Concrete Research | Year: 2014

The structural development and carbonation resistance of three silicate-activated slags (AAS) with varying MgO contents (< 7.5 wt.%) are reported. AAS with lower MgO content reacts faster at early age, forming gismondine and C-A-S-H type gels, while in slags with higher MgO content (> 5%), hydrotalcite is identified as the main secondary product in addition to C-A-S-H. Higher extent of reaction and reduced Al incorporation in the C-S-H product are observed with higher MgO content in the slag. These gel chemistry effects, and particularly the formation of hydrotalcite, seem to reduce the susceptibility to carbonation of AAS produced with higher MgO contents, as hydrotalcite appears to act as an internal CO2 sorbent. This is evidenced by an inverse relationship between natural carbonation depth and slag MgO content, for paste samples formulated at constant water/binder ratio. Thus, the carbonation performance of AAS can be enhanced by controlling the chemistry of the precursors. © 2013 Elsevier Ltd. Source

Feng D.,University of Melbourne | Provis J.L.,University of Melbourne | Van Deventer J.S.J.,University of Melbourne | Van Deventer J.S.J.,Zeobond Pty. Ltd.
Journal of the American Ceramic Society | Year: 2012

Precursors for the preparation of one-part geopolymers are synthesized by thermal activation of albite with sodium hydroxide and sodium carbonate, then cooling and crushing the resulting product. Albite is stable under thermal treatment up to 1000°C, but is able to be converted to depolymerized, disordered, and X-ray amorphous geopolymer precursors in the presence of sodium hydroxide or sodium carbonate at elevated temperatures. The geopolymer precursors react with the addition of water (i.e., form a "one part geopolymer mix"), forming geopolymers with acceptable compressive strength. One-part geopolymers synthesized via thermal activation of albite with NaOH show a higher compressive strength than those produced with Na 2CO 3at the same dosage. Some crystalline sodium-aluminosilicate hydrates (zeolites) are also formed in addition to geopolymer gel in the geopolymers synthesized from albite activated by NaOH, compared to predominantly amorphous phases in the samples activated by Na 2CO 3. The activation of natural aluminosilicates including albite by thermal treatment with alkalis has great potential in the development of novel one-part mix geopolymers. © 2011 The American Ceramic Society. Source

Hajimohammadi A.,University of Melbourne | Provis J.L.,University of Melbourne | Van Deventer J.S.J.,University of Melbourne | Van Deventer J.S.J.,Zeobond Pty. Ltd.
Chemistry of Materials | Year: 2010

The effect of the rate of alumina release during the reaction of a one-part (just-add-water) geopolymer mix on growing geopolymer gels is investigated by coupling time-resolved and spatially resolved infrared spectroscopic analysis. The rate of alumina release from different precursors has previously been identified as a critical controlling factor in the formation of mechanically strong and durable geopolymers; however, its influence on the nanostructure of the geopolymer gel has never before been directly analyzed. Gel microstructure and nanostructure are able to be observed by synchrotron radiation-based infrared microscopy (SR-FTIR) with hierarchical clustering analysis, conducted in conjunction with the in situ attenuated total reflectance (ATR) FTIR technique to provide temporal resolution. The SR-FTIR technique provides the opportunity to analyze the chemistry of the heterogeneous geopolymer binder at a level of detail that previously has not been available. Although spatially averaged (ATR-FTIR) infrared results show similar spectra for well-cured samples with different alumina release rates, SR-FTIR shows that the gels are markedly different, with less unreacted silica particles remaining in geopolymer gels with slow alumina release, but greater homogeneity when alumina is released more rapidly. Rapid release of alumina is shown to impede the dissolution of silica particles in the early stages of the reaction; thus, the participation of alumina in forming geopolymer gels appears to be more beneficial to geopolymer nanostructural development when it becomes available in the later stages of the reaction process. © 2010 American Chemical Society. Source

Discover hidden collaborations