Rheological behavior of fresh inorganic polymer paste: Polymer bridging effect of the alkali silicate solution

Inorganic polymers (IP), produced by alkali activation of a glassy precursor, have been mainly investigated on their microstructure and mechanical strength properties. However, it is important to understand how the IP flow behaves under shear conditions, in particular when pumping is required. The activating solution is one of the main parameters influencing rheology. Therefore, the physical effect of the silicate structure on the rheology was investigated by varying the SiO2/Na2O molar ratio from 1.4 to 2.0 in the activator. The elastic and rheological properties of the IP were measured with a rheometer. In order to investigate the activator silicate structure and IP polymerisation development, Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance Spectroscopy (NMR) were performed. A decrease in elasticity was monitored for IP with a low SiO2/Na2O ratio as a result of the dissolved species, which can be correlated to NMR. The FTIR spectra implied that an activating solution with a higher SiO2/Na2O ratio resulted in the formation of a 3D silicate network with Q3 and Q4 crosslinks. The presence of a network modifier in the activating solution, such as Na, resulted in more Q1 and Q2 crosslinks. A higher stress, at a shear rate of 0.1 s-1; was observed in IP which consisted of a 3D silicate network as a result of the polymer bridging effect between the particles. A stronger shear thinning was observed in an IP with a higher SiO2/Na2O ratio, due to the steric hindrance from the entangled silicates. The rheological data of the IP can be fitted with the Herschel-Bulkley model

Inorganic polymers from a plasma convertor slag: Effect of activating solution on microstructure and properties

Plasma processing of materials is a technology now also employed in the management of municipal solid wastes, often mixed with industrial residues. The specifics depend per case, but typically the process delivers energy, in the form of a gas or heat, a metal-rich fraction as well as a slag. The slag, containing mainly Si-, Fe-, Ca- and Al-oxides, is almost completely amorphous after rapid cooling and thus could possibly be used as precursor in the synthesis of inorganic polymers (IP). The latter is explored in the present work. Slag resembling the composition of refuse-derived fuel ash was mixed with various Na-silicate activating solutions, and the effect of SiO₂/Na₂O as well as H₂O/Na₂O molar ratio on the synthesis and mechanical properties of the prepared IP was investigated. It was found that for SiO₂/Na₂O molar ratios of 1.2 and H₂O/Na₂O molar ratio of 30.8, the mechanical strength of casted IP reached almost 90 MPa after 90 days. Further decrease in the SiO₂/Na₂O ratio, accompanied by decrease in the H₂O/Na₂O ratio, increased the early strength and the released reaction heat, but had no effect on the late strength. In addition to that, crack formation was pronounced. The increase of the concentration of activating solution, by means of reducing the water content level, i.e. H₂O/Na₂O, resulted in an increase of the released reaction heat as well as an increase of the mechanical strength, up to 112 MPa at 90 days. The above results are relevant to a range of metallurgical slags and other vitreous by-products and contribute towards more high added-value applications.status: publishe

On the foaming kinetics for the synthesis of porous inorganic polymers

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The development and assessment of alkali activated paving blocks

In recent years great attention has been placed by the building sector on alkali-activated technology based on metakaolin, fly ash and ground granulated blast furnace slag (GGBFS), but also on emerging precursors such as by-products from non-ferrous metallurgy. The present work focuses on the development of alkali-activated binders from two slags, one from primary and one from secondary copper production, which were finely milled, blended with GGBFS, and activated with K-based alkali silicate solution with a 1.7 SiO2/K2O molar ratio. The aggregate to paste mass ratio was 2. The mixtures were cast, and cured for a designated time at room temperature and 60% RH. The so-obtained paving blocks were then tested in accordance with European standard for concrete paving blocks. The following properties were measured: splitting tensile strength, abrasion resistance, slip and skid resistance, resistance to freeze-thaw and resistance to freeze-thaw in the presence of de-icing salts. Their properties were compared to those of commercially available concrete paving blocks, and it was found that the performance of the alkali-activated pavers was generally comparable with the concrete pavers, while certain properties (e.g., abrasion resistance, freeze-thaw resistance and freeze-thaw resistance in presence of de-icing salts) were considerably better

Towards Sustainable Inorganic Polymers: Production and Use of Alternative Activator

Inorganic polymers are considered to have a much smaller CO2 footprint than traditional Portland cements. The possible environmental impact reduction was calculated to be up to 77% of a CEMI ordinary Portland cement (OPC)-based mortar, while the CO2 emissions were reduced by up to 83%. The highest contribution to the total CO2 footprint of inorganic polymers is related to the manufacturing of alkali activators. Within this context, the current work discusses the synthesis of inorganic polymers made from Fe-rich precursor and alternative silicate solutions. The obtained results indicate that there is no significant difference between an inorganic polymer prepared from alternative and commercially available solutions, which confirms that the waste glass can be use as alternative raw material in the production of sodium silicate

Susceptibility of mineral phases of steel slags towards carbonation: mineralogical, morphological and chemical assessment

Process limitations have thus far prevented mineral carbonation of alkaline wastes from been widely applied. These barriers are caused by inefficient processing, but also by mineralogical aspects inherent to the materials. Better understanding and predictability of the effects of mineral carbonation on alkaline materials could be obtained by studying the carbonation susceptibility of constituent minerals separately, allowing for detailed and accurate analysis of their reaction kinetics and maximal conversions and of the carbonate products formed. For this purpose, this paper presents the synthesis and carbonation of the seven most abundant alkaline minerals found in AOD, CC and BOF slags, namely: åkermanite (Ca2MgSi2O7), bredigite (Ca7Mg(SiO4)4), cuspidine (Ca4Si2O7F2), β– and γ–C2S (Ca2SiO4), merwinite (Ca3Mg(SiO4)2), and srebrodolskite (Ca2Fe2O5). Two experimental approaches to mineral carbonation of increasing levels of process severity are utilized: (mild) incubator carbonation, and (accelerated) pressurized slurry carbonation. In addition, the slags and two free oxides (CaO and MgO) are equally carbonated and evaluated. Data regarding CO2 uptake, mineral conversion and formed carbonate and non-carbonate products in the samples were obtained through TGA, QXRD (Rietveld refinement) and SEM techniques. Reduction in material basicity and evolution of particle morphology were also assessed. The synthesized mineral purities (> 70 wt% target mineral phase) were found sufficient for more accurate assessment of carbonation behaviour of the individual minerals. Bredigite was found to be the most reactive mineral under all processing conditions; C2S and wollastonite were more reactive under slurry carbonation, while srebrodolskite and calcium monoferrite were found to be more reactive under moist carbonation. Merwinite and diopside had the slowest carbonation conversions. Calcite and aragonite were the dominant carbonate products formed, whereby aragonite formation was promoted in Mg-containing materials. The morphology of aragonite crystals and the packing density of its product layer were found to vary depending on the parent mineral. Characteristic slag carbonation products, not observed as extensively from synthetic mineral samples, were magnesian calcite from slurry carbonation, and monohydrocalcite and vaterite from moist carbonation. Wollastonite was the main crystalline non-carbonate product, occurring predominantly from slag carbonation, while silica-rich amorphous matter formed in all samples proportionally to CO2 uptake. Free lime, when present, controlled material basicity above pH 12, while silicates were found to typically possess pH in the range of 11.3–11.9, and Ca-carbonates eventually controlled the pH of well carbonated samples to values under 10.status: publishe

Transformation of stainless steel slag toward a reactive cementitious binder

Argon oxygen decarburization (AOD) slag represents more than 50 wt% of the slag from stainless steel production. Although some applications are available,e.g., as aggregates for road constructions or fertilizers, they are characterized by low economic value and limited applicability. In order to increase the economic value of AOD slag, alternative applications have been proposed, e.g., as partial or full replacement for Ordinary Portland Cement (OPC). The work presented here investigates whether the adaptation of the AOD slag chemistry within a high temperature process leads to an improvement of its hydraulic properties and thereby can demonstrate its potential to be converted into a hydraulic binder suit able for OPC replacement. For this purpose, three synthetic AOD slags with basicities(CaO/SiO2) of 2.0, 2.2, and 2.4 were synthesized, and the effect of the CaO/SiO2 ratio on the material stability, the amount of tri-calcium silicate formed, and their hydraulic properties investigated. X-ray diffraction, scanning electron microscope(SEM), and isothermal calorimetry analysis were used to characterize the microstructure and the hydraulic activity. The results show that the proposed method is indeed a promising way to stabilize a stainless steel AOD slag and con-vert it into a hydraulic binder.status: publishe

Valorisation of stainless steel slags as a hydraulic binder

This work is focused on exploring various cold and hot stage treatment paths of stainless steel slag as a tool to improve its hydraulic properties. At a cold stage, mechanical and chemical activation was applied on industrial stainless steel slag; and it was found that both activation methods effectively improve the reactivity of the studied slag. In addition, the detailed investigation of hydration on two major phases, γ – C₂S and merwinite, revealed that their hydration resulted in the formation of C – S – H gel, typically formed during the hydration of OPC. Regarding the hot stage treatment, the combination of the chemistry modification with the addition of fly ash at 30 wt. % and fast cooling by means of water quenching resulted in a complete amorphisation of the material. Ultimately, the produced material possessing similar properties to granulated blast furnace slag could be used as a latent hydraulic material in blended cements.status: publishe

Mineralogical effects on the intensified mineral carbonation of steel slags: kinetics, conversion, basicity and products

Mineral carbonation, the reaction of carbon dioxide with alkaline minerals, is an attractive carbon sequestration approach owing to the geochemical stability of carbonates. When applied to low-value or hazardous industrial alkaline residues, it can result in the reduction of basicity and leaching, enabling valorisation or safe disposal. However, process limitations including high energy intensity, low reaction conversion, and slow reaction kinetics, have thus far prevented mineral carbonation of wastes from been widely applied. These barriers are caused by inefficient processing, but also by mineralogical aspects inherent to the materials. Alkaline waste materials are typically composed of several mineral phases that may or may not be susceptible to mineral carbonation, and which, if reactive to CO2, may exhibit varying degrees of carbonation kinetics and influence on the material’s basicity. The formation and character (e.g. thickness, porosity) of passivating layers can also be affected by the relative solubility of the minerals. Most carbonation studies to date, however, have focussed on the chemical composition of these materials rather than on their mineralogical composition. The commonly used Steinour equation, for example, relies solely on the amounts of alkali oxides to predict CO2 uptake capacity. Though stoichiometrically accurate, this prediction can be overly optimistic, causing doubts whether carbonation processes are ineffective in reaching complete conversion (due to insufficient process severity or formation of passivating layers), or if the unreacted material is inert to carbonation. In the present work, three types of steel slags are utilized for mineral carbonation: argon oxygen decarburization slag (AOD), continuous casting slag (CC), and basic oxygen furnace slag (BOF). This study aims to provide insight on the carbonation behaviour of various minerals commonly found in steel slags, including: lime, portlandite, periclase, brucite, γ- and β-dicalcium silicates, srebrodolskite, bredigite, cuspidine, merwinite, åkermanite, and gehlenite. The carbonation products, which include various forms of calcium and magnesium carbonates (e.g. calcite, aragonite...), silica and intermediate products (e.g. wollastonite) are also characterized, both with respect to composition and to the process conditions that promote their formation (temperature, ionic concentrations, sonication). Three experimental approaches of increasing levels of process severity have been tested and optimized, and are discussed in this work: (i) mild wet carbonation; (ii) accelerated slurry carbonation, and (iii) rapid high temperature direct carbonation. A comparison is made of the characteristics of these processes and the results achieved, with respect to carbonation kinetics, mineral conversion and basicity reduction. In addition to carbonation of the innate slags, high purity single minerals (synthetic and geological) were also prepared and tested to reveal in more detail their carbonation behaviour. Mineralogical assessment was made by Rietveld refinement technique of XRD diffractograms. The understanding of the mineralogical behaviour of alkaline materials towards mineral carbonation can enable the optimal development of intensified processes that better meet energy and economical demands of carbon sequestration technologies, as well as materials valorisation requirements. The outcome includes better choice of carbonation technology, optional hot-stage tuning of residue composition, and a priori prediction of carbonation effects, such as CO2 uptake, basicity and leaching behaviour.status: publishe