Alkali-activated mineral wools

Mineral wools –a general term for stone wool and glass wool– are the most common building insulation materials in the world. The amount of mineral wool waste generated in Europe totaled 2.3 Mt in 2010 – including wastes from mineral wool production and from construction and demolition industry. Unfortunately, mineral wools are often unrecyclable due to their fibrous nature (Figure 1) and low density. Thus, the utilization of mineral wool waste in post-consumer products remains low. Please click Additional Files below to see the full abstract

Impact of calcium hydroxide on physical characteristics and leaching stability of metakaolin-based geopolymer waste form containing radioactive borate waste

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Characterization of an aged alkali-activated slag roof tile after 30 years of exposure to Northern Scandinavian weather

Alkali-activated materials (AAMs) have been known as an alternative cementitious binder in construction for more than 120 years. Several buildings utilizing AAMs were realized in Europe in the 1950s–1980s. During the last 30 years, the interest towards AAMs has been reinvigorated due to the potentially lower CO2 footprint in comparison to Portland cement. However, one often-raised issue with AAMs is the lack of long-term studies concerning durability in realistic conditions. In the present study, we examined a roof tile, which was prepared from alkali-activated blast furnace slag mortar and exposed to harsh Northern Scandinavian weather conditions in Turku, Finland, for approximately 30 years. Characterization of this roof tile provides unique and crucial information about the changes occurring during AAM lifetime. The results obtained with a suite of analytical techniques indicate that the roof tile had maintained excellent durability properties with little sign of structural disintegration in real-life living lab conditions, and thus provide in part assurance that AAM-based binders can be safely adopted in harsh climates. The phase assemblage and nanostructural characterization results reported here further elucidate the long-term changes occurring in AAMs and provide reference points for accelerated durability tests and thermodynamic modelling

Phase evolution of C-(N)-A-S-H/N-A-S-H gel blends investigated via alkali-activation of synthetic calcium aluminosilicate precursors

Stoichiometrically-controlled alkali-activated pastes containing calcium-(sodium) aluminosilicate hydrate (C-(N)-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gels are produced by alkali-activation of high-purity synthetic calcium aluminosilicate powders. These powders are chemically comparable to the glass in granulated blast furnace slag, but without interference from minor constituents. The physiochemical characteristics of these gels depend on precursor chemical composition. Increased Ca content of the precursor promotes formation of low-Al, high-Ca C-(N)-A-S-H with lower mean chain length as determined by quantification of solid state nuclear magnetic resonance spectra, and less formation of calcium carboaluminate ‘Alumino-ferrite mono’ (AFm) phases. Increased Al content promotes Al inclusion and reduced crosslinking within C-(N)-A-S-H, increased formation of calcium carboaluminate AFm phases, and formation of an additional N-A-S-H gel. Small changes in precursor composition can induce significant changes in phase evolution, nanostructure and physical properties, providing a novel route to understand microstructural development in alkali-activated binders and address key related durability issues

Synthesis of stoichiometrically controlled reactive aluminosilicate and calcium-aluminosilicate powders

Aluminosilicate and calcium-aluminosilicate powders are synthesised via an organic steric entrapment route under conditions permitting strict stoichiometric control, utilising polyvinyl alcohol and polyethylene glycol as polymeric carriers. Polyethylene glycol is superior to polyvinyl alcohol for synthesis of calcium-aluminosilicate powders via this method, producing a more controllable product which generated less fine ash during calcination. This paper presents detailed description of synthesis and characterisation of the powders produced through this approach, including new insight into the nanostructures within the calcined powders. Aluminium environments are a mixture of 4-, 5- and 6-coordinated, while silicon is tetrahedral and shows a broad range of connectivity states. The powders are X-ray amorphous, display a high degree of homogeneity, and thus offer potential for utilisation as precursors for synthesis of hydrous aluminosilicates in the quaternary CaO-Na2O-Al2O3-SiO2 system

Reactivity tests for supplementary cementitious materials: RILEM TC 267-TRM phase 1

A primary aim of RILEM TC 267-TRM: “Tests for Reactivity of Supplementary Cementitious Materials (SCMs)” is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well

Examination of alkali-activated material nanostructure during thermal treatment

The key nanostructural changes occurring in a series of alkali-activated materials (AAM) based on blends of slag and fly ash precursors during exposure to temperatures up to 1000 °C are investigated. The main reaction product in each AAM is a crosslinked sodium- and aluminium-substituted calcium silicate hydrate (C-(N)-A-S-H)-type gel. Increased alkali content promotes the formation of an additional sodium aluminosilicate hydrate (N-A-S-(H)) gel reaction product due to the structural limitations on Al substitution within the C-(N)-A-S-H gel. Heating each AAM to 1000 °C results in the crystallisation of the disordered gels and formation of sodalite, nepheline and wollastonite. Increased formation of N-A-S-(H) reduces binder structural water content after thermal treatment and correlates closely with previous observations of improved strength retention and reduced microcracking in these AAM after heating to 1000 °C. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)-A-S-H/N-A-S-H gel blends which are fundamental for the development of new fire-resistant construction materials

Understanding geopolymers through synthetic gel systems

© 2016 Dr. Brant WalkleyAlkali-activated materials and geopolymers have received significant academic and commercial interest in recent years due to desirable physical properties, wide-ranging applications and a low CO2 embodiment. Variations in precursor chemical composition can induce significant changes in the structure and physical properties of these materials. In particular, the coexistence of calcium aluminosilicate hydrate (C-A-S-H), calcium (alkali) aluminosilicate hydrate (C-(N)-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gel frameworks, which are the main reaction products in alkali-activated materials, results in complex physiochemical interactions which dictate material properties and performance. Despite extensive academic focus on the chemistry of alkali-activated materials and geopolymers, the way in which precursor chemistry controls phase evolution and nanostructural development of these materials is not yet fully understood. The influence of precursor chemistry on gel nanostructure and phase evolution in alkali-activated materials and geopolymers is investigated by application of advanced characterisation techniques, in particular solid state MAS and MQMAS NMR, to a novel class of stoichiometrically controlled, high-purity synthetic precursors and alkali-activated binders. This chemically simplified model system provides a unique tool for fast and reliable synthesis of AAM and geopolymers of a chosen composition without interference of contaminants. Gel formation, phase evolution and nanostructural development of N-A-S-H, C-A-S-H and C-(N)-A-S-H gel frameworks within both single phase and multiphase systems is examined. The interaction of these gel frameworks with each other and with additional reaction products and how these processes are influenced by the chemical composition of the precursor materials used is also investigated. This provides for the first time a process by which the physicochemical interactions occurring during alkali-activation of aluminosilicate-based precursors and subsequent formation of cementitious materials may be accurately simulated by pure systems. Structural analysis of N-A-S-H, C-A-S-H and C-(N)-A-S-H gel frameworks within stoichiometrically controlled synthetic alkali-activated materials reveals a strong dependence of the physiochemical characteristics of the gels on precursor composition. Application of advanced multinuclear solid state multiple quantum MAS NMR to a novel class of isotopically enriched C-A-S-H, C-(N)-A-S-H and N-A-S-H gels identifies six key structural motifs present within N-A-S-H gel frameworks, which are combined to develop a new structural description of the N-A-S-H gel in these systems. The key structural motifs which comprise C-(N)-A-S-H gel frameworks are also identified, and the overall nanostructure of these gels is shown to be significantly altered with changes in bulk composition. The findings presented in this thesis show that even small changes in reaction mixture composition can induce significant changes in phase formation and evolution, dictating the nanostructure of alkali-activated materials and geopolymers. This establishes a new platform of knowledge which is crucial for developing further detailed understanding of the molecular interactions governing phase evolution and nanostructural development of alkali aluminosilicate gels, as well as controlling the material properties and durability of modern cements which gain strength from these gels

Characterisation of calcined waste clays from kaolinite extraction in alkali-activated GGBFS blends

Metakaolin is a well-studied supplementary cementitious material (SCM) and precursor in alkali-activation. The utilisation of limited kaolinite content clays generated from large-scale industrial activity, denoted as waste clays, in alkali-activation is still largely unexplored. This paper investigates the use of five samples of calcined waste clays from different locations in a kaolinite extraction site as precursors in alkali-activated cements (AACs). Calcined waste clay precursors are blended with ground granulated blast furnace slag (GGBFS) and activated with sodium silicate. The physical and chemical properties of the calcined waste clays are characterised. Both Chapelle and R3 tests for pozzolanic reactivity are examined based on calcined waste clay properties. The R3 test data obtained for sieved particle fractions &lt; 63 µm show marked improvement in the case of C4, elucidating potential calcined waste clay behaviour with prior processing in alkali activated systems. Clay particle size distribution and morphology are identified as key factors to be considered for initial calcination and in mix designs, strongly affecting both the fresh properties and workability of blended pastes. Good workability is highlighted as being crucial for achieving well-behaving calcined waste clay blended binders with resulting dense microstructures and high strength values, comparable with other reported GGBFS systems. Quartz, muscovite, and alkali-feldspar present in the calcined waste clays remain stable throughout alkali-activation. Reaction of the amorphous phase fraction present in the calcined waste clays results in the formation of additional cross-linked gel. A replacement level of up to 20 wt% calcined waste clay is found to exhibit similar compressive strengths to pure GGBFS binders, whilst achieving lower porosity within the bulk. This study provides a methodology for characterising the behaviour of calcined waste clays in alkali-activated systems and a proof of concept in the formulation of novel binders that incorporate waste clays.</p