Somerton, Australia
Somerton, Australia

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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.

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.

Provis J.L.,University of Melbourne | Duxson P.,Zeobond Pty. Ltd. | van Deventer J.S.J.,University of Melbourne | van Deventer J.S.J.,Zeobond Pty. Ltd.
Advanced Powder Technology | Year: 2010

This paper presents a brief review of the role of particle technology in the development of low-CO2 aluminosilicate 'geopolymer' binders and concretes as an alternative to traditional Portland cement-based materials. The role of particle shape in particular is highlighted, both in the context of its effect on paste rheology and on water demand. The spherical particles of fly ash and the platy particles of metakaolin show opposite effects in each of these areas, and this must be understood and controlled if an effective geopolymer concrete is to be designed. The angular particles of blast furnace slag are also important in determining paste rheology and porosity. The selection of the correct combination of aggregate gradings is critical in maximising concrete durability, as the ability of aggregates to pack sufficiently densely in a hardened concrete product then hinders the ability of aggressive external agents to migrate into the concrete and cause structural damage to either the binder or the embedded steel reinforcing. © 2009 The Society of Powder Technology Japan.

Provis J.L.,University of Melbourne | Myers R.J.,University of Melbourne | White C.E.,University of Melbourne | Rose V.,Argonne National Laboratory | And 2 more authors.
Cement and Concrete Research | Year: 2012

Durability of alkali-activated binders is of vital importance in their commercial application, and depends strongly on microstructure and pore network characteristics. X-ray microtomography (μCT) offers, for the first time, direct insight into microstructural and pore structure characteristics in three dimensions. Here, μCT is performed on a set of sodium metasilicate-activated fly ash/slag blends, using a synchrotron beamline instrument. Segmentation of the samples into pore and solid regions is then conducted, and pore tortuosity is calculated by a random walker method. Segmented porosity and diffusion tortuosity are correlated, and vary as a function of slag content (slag addition reduces porosity and increases tortuosity), and sample age (extended curing gives lower porosity and higher tortuosity). This is particularly notable for samples with ≥ 50% slag content, where a space-filling calcium (alumino)silicate hydrate gel provides porosity reductions which are not observed for the sodium aluminosilicate ('geopolymer') gels which do not chemically bind water of hydration. © 2012 Elsevier Ltd. All rights reserved.

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.
Cement and Concrete Research | Year: 2011

The effect of silica availability on geopolymer binder formation is investigated in the geothermal silica-sodium aluminate-water system, using sodium silicate solution as an additional, highly available silica source. Time-resolved and spatially-resolved FTIR data are combined to provide a mechanistic understanding of the role of silica availability in controlling geopolymer nucleation and gel growth behaviour. A higher degree of alumina contribution to geopolymer gels and newly formed crystal phases is observed in systems with higher silica availability. Gel nucleation is observed to take place in the region immediately surrounding the solid silica source particles when no dissolved silica is initially supplied. Conversely, mixes which initially contain dissolved silica show nucleation in bulk regions, and involving more of the Al which is rapidly released from the sodium aluminate precursor. These differences in nucleation lead to a more chemically heterogeneous binder in the case where silica is released more gradually. © 2011 Elsevier Ltd. All rights reserved.

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.

Bernal S.A.,University of Melbourne | Provis J.L.,University of Melbourne | Brice D.G.,Zeobond Pty Ltd. | Kilcullen A.,University of Melbourne | And 3 more authors.
Cement and Concrete Research | Year: 2012

The carbonation resistance of alkali-activated binders is often tested via accelerated test protocols designed for Portland cements, without questioning whether the tests replicate the mechanisms observed in service. Thus, validation of accelerated methods is required to enable realistic prediction of material performance. Changes in pore solution equilibria cause the formation of sodium bicarbonates during accelerated carbonation, compared with hydrous sodium carbonates in natural carbonation. This shifts the carbonation mechanism to favour more rapid reaction progress, to give a higher apparent degree of acceleration (compared to natural conditions) than in Portland cements. The pore solution pH under accelerated carbonation is significantly lower than at natural CO 2 concentrations, leading to a falsely short predicted service life (time to expected corrosion of embedded steel), as natural CO 2 concentrations appear not to reduce the pH below 10. Thus, accelerated carbonation testing is unduly aggressive towards alkali-activated binders, and test results must be cautiously interpreted. © 2012 Elsevier Ltd.

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.

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.

Bernal S.A.,University of Melbourne | Bernal S.A.,University of Sheffield | Provis J.L.,University of Melbourne | Provis J.L.,University of Sheffield | And 10 more authors.
Cement and Concrete Research | Year: 2013

Binders formed through alkali-activation of slags and fly ashes, including 'fly ash geopolymers', provide appealing properties as binders for low-emissions concrete production. However, the changes in pH and pore solution chemistry induced during accelerated carbonation testing provide unrealistically low predictions of in-service carbonation resistance. The aluminosilicate gel remaining in an alkali-activated slag system after accelerated carbonation is highly polymerised, consistent with a decalcification mechanism, while fly ash-based binders mainly carbonate through precipitation of alkali salts (bicarbonates at elevated CO2 concentrations, or carbonates under natural exposure) from the pore solution, with little change in the binder gel identifiable by nuclear magnetic resonance spectroscopy. In activated fly ash/slag blends, two distinct gels (C-A-S-H and N-A-S-H) are formed; under accelerated carbonation, the N-A-S-H gel behaves comparably to fly ash-based systems, while the C-A-S-H gel is decalcified similarly to alkali-activated slag. This provides new scope for durability optimisation, and for developing appropriate testing methodologies. © 2013 Elsevier Ltd.

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