Geopolymer foams (highly porous materials) have emerged as one of the most exciting materials over the past few years due to their remarkable properties, low cost and green synthesis protocol, ...enabling their use in various high added-value applications. Review papers on porous geopolymers are uncommon, and the emphasis has been given to materials processing and properties, while the applications were only briefly addressed. This review aims to fill this gap by presenting a comprehensive literature survey and critical analysis of the most recent and exciting research carried out on geopolymer foams. Up to now, these bulk-type (not powders) materials have been mainly considered as thermal and acoustic insulators. However, besides addressing their use as building material, this review also shows that their use in less investigated, but environmentally and economically relevant applications (e.g. bulk-type adsorbents, pH buffering agents and catalysts), is feasible and might ensure performance and technical advantages over their powdered counterparts. The limitations, challenges and future prospects associated with the different applications are presented. This review shows the remarkable potential of geopolymer foams in high added-value applications, far beyond their historical use as Portland cement replacement, which may encourage the widespread technological use of these materials.
In this work, and for the first time, fly ash (FA)-based geopolymer (d = 2.6 mm) spheres were used to extract methylene blue from synthetic wastewaters. The influence of sorption time, dye initial ...concentration and adsorbent amount on the dye removal efficiency and uptake by the porous spheres was evaluated. The adsorbents' recyclability and their dye fixation efficiency were also considered. The initial dye concentration strongly affected the uptake and removal efficiency by the porous bodies, the former rising from 1.1 to 30.1 mg/g when the dye initial concentration jumped from 10 to 250 ppm, and the latter increasing from 82.3% to 94.3% when the dye initial concentration varied from 10 to 125 ppm. Results showed a much faster (24 h) and higher (30.1 mg/g) methylene blue uptake in comparison with the other bulk-type geopolymers reported to date (30 h; 15.4 mg/g). The cumulative methylene blue uptake shown by these innovative spheres (79.7 mg/g) surpasses all other powdered geopolymer adsorbents, being among the highest values ever reported for geopolymers. The adsorbent was successfully regenerated and reused eight times. Regeneration was found to negatively affect the MB uptake, but nevertheless, even after eight regeneration cycles a very high MB removal efficiency (83%) was maintained. The use of these bulk-type waste-based geopolymer adsorbents is a low-cost, more eco-friendly, safer and easier alternative to the use of powdered adsorbents in wastewater treatment systems, since these ∼3 mm spheres may be used directly in packed beds, and were produced using significant amounts of waste material.
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•For the first time FA-based geopolymer spheres were evaluated as MB adsorbents.•Highest ever reported MB uptake (30.1 mg/g) by bulk-type geopolymer adsorbents.•Bulk-type adsorbents are a safer and easier alternative to powdered adsorbents.•These novel adsorbents can be regenerated and reused in multiple adsorption cycles.•Cumulative MB uptake (79.7 mg/g) is among the highest ever reported for geopolymers.
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•Red mud-based geopolymer spheres (2.5 mm) were synthesised for the first time.•The aluminosilicate sources of the geopolymer spheres were 100% waste-based.•The spheres pH buffering ...ability in aqueous medium was evaluated.•High OH− leaching (0.0237 mol/dm3g) and narrow pH drop (2.4 pH units) was attained.•This innovative recycling strategy may significantly reduce red mud disposal.
In this study, and for the first time, red mud (RM)-based geopolymer spheres were synthesised, with varying porosity and RM content, and then their use as pH regulators was evaluated. The aluminosilicate sources of these inorganic polymers were 100% waste-based, consisting of a mixture of RM and fly ash wastes. Geopolymer spheres containing up to 60 wt.% RM were successfully produced, while higher RM contents distorted the specimens’ spherical shape. Results showed that alkalis leaching from the spheres over time can be controlled by their porosity, while the RM content induces only minor changes to leaching. The RM-based spheres leached up to 0.0237 mol/dm3g of OH− ions from their structure, this being among the highest values ever reported for geopolymers. This allowed a continuous and prolonged pH buffer capacity with narrow pH decay over the 28 days (2.4 pH units), suggesting the use of the RM-based spheres as pH buffering materials in wastewater treatment and anaerobic digestion systems.
For the first time biomass fly ash geopolymer monoliths were used as adsorbents for the removal of methylene blue from synthetic wastewaters. Highly porous and lightweight fly ash-based geopolymers ...were produced and then evaluated as methylene blue adsorbents. The geopolymers’ porosity strongly affects the dye extraction, a threefold increase (from 5.4 to 15.4 mg/g) being observed when the porosity rises from 40.7 to 80.6%. The maximum uptake reported here (15.4 mg/g) surpasses several other powdered adsorbents, which demonstrates the interesting potential of this innovative adsorbent. Moreover, these monolithic adsorbents can be used directly in packed beds as membranes, this being a major advantage over powdered adsorbents. Furthermore, these adsorbents were successfully regenerated and reused (up to five cycles) without compromising the performances. In fact, enhanced methylene blue uptake (up to 20.5 mg/g) was observed after regeneration. Additionally, an unexplored waste stream was used as raw material which mitigates the waste environmental footprint contributing towards a circular economy.
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•Novel porous monolithic adsorbents for methylene blue extraction were developed.•The geopolymers were produced using 66.6 wt% biomass fly ash (unexplored waste).•Methylene blue uptake by the monolithic bodies up to 20.5 mg/g was observed.•The removal efficiency can be controlled by pores' volume and connectivity.•Adsorbents can be used in packed beds, regenerated and reused (at least 5 times).
The cement industry is facing important challenges regarding the reduction of raw materials and energy demands, and CO2 emissions. Although many advances have been reached in the last decades through ...technology improvements, the use of alternative materials and fuels still can be further explored. The Brazilian cement industry is considered energetically efficient. Nevertheless, there are only few scientific studies regarding environmental concerns in the cement production in Brazil. This paper aims to analyze the environmental impacts of the Brazilian cement industry, such as resource depletion and toxicity. The life cycle assessment was used as a methodology to estimate impacts on climate change, ozone depletion, photochemical oxidant formation, particulate matter, terrestrial acidification, freshwater and marine eutrophication, metal and fossil depletion and human, terrestrial, freshwater and marine toxicity. Results clearly show that the transportation step has the largest contribution to those impacts, followed by fossil fuels production and clinkering. Together, these steps are responsible for more than 70% of CO2 equivalent and 90% of CFC-11 equivalent. Thus, the replacement of fossil fuels by alternative fuels can be an option to decrease impacts regarding fossil fuels consumption and atmospheric emissions from the kiln. However, the distances from the source of these alternative fuels to the plant must be taken into account.
This paper reports on the high sensitivity of sub-spherical In2O3-Pt nanoparticles (NPs) for detecting ppb levels of acetone, a biomarker for diabetes. The In2O3-Pt NPs, in the form of monodisperse ...metal-oxide In2O3 NPs with diameters of 6–8nm, decorated with 2wt% Pt metal NPs (2–3nm) on the surface, were synthesized by a novel non-aqueous sol–gel route. NPs samples were investigated by X-ray powder diffraction (XRPD), using the advanced whole powder pattern modeling (WPPM) method, and high-resolution transmission electron microscopy (HR-TEM). The advantage of this preparative process is that it preserves metallic platinum NPs formed during the synthesis. The highly sensitive acetone sensor based on these NPs, showed a lower detection limit as low as 10ppb or less, which is the lowest detection limit ever reported for any chemoresistive acetone sensors. This exceptional performance is likely due to the key role played by very small Pt metal NPs uniformly distributed in the In2O3-Pt nanostructure. The developed sensor would be suitable for use as a highly sensitive, practical breath acetone checker for daily diet and diabetes management and diagnosis.
This book provides an updated state-of-the-art review on new developments in alkali-activation. The main binder of concrete, Portland cement, represents almost 80% of the total CO2 emissions of ...concrete which are about 6 to 7% of the Planet's total CO2 emissions. This is particularly serious in the current context of climate change and it could get even worse because the demand for Portland cement is expected to increase by almost 200% by 2050 from 2010 levels, reaching 6000 million tons/year. Alkali-activated binders represent an alternative to Portland cement having higher durability and a lower CO2 footprint. * Reviews the chemistry, mix design, manufacture and properties of alkali-activated cement-based concrete binders * Considers performance in adverse environmental conditions. * Offers equal emphasis on the science behind the technology and its use in civil engineering.
This paper presents results about the characterisation of the biomass fly ashes sourced from a thermal power plant and from a co-generation power plant located in Portugal, and the study of new ...cement formulations incorporated with the biomass fly ashes. The study includes a comparative analysis of the phase formation, setting and mechanical behaviour of the new cement–fly ash formulations based on these biomass fly ashes. Techniques such as X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), thermal gravimetric and differential thermal analysis (TG/DTA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and environmental scanning electron spectroscopy (ESEM) were used to determine the structure and composition of the formulations. Fly ash F1 from the thermal power plant contained levels of SiO
2, Al
2O
3 and Fe
2O
3 indicating the possibility of exhibiting pozzolanic properties. Fly ash F2 from the co-generation plant contained a higher quantity of CaO (∼25%). The fly ashes are similar to class C fly ashes according to EN 450 on the basis of chemical composition. The hydration rate and phase formation are greatly dependant on the samples’ alkali content and water to binder (
w/
b) ratio. In cement based mortar with 10% fly ash the basic strength was maintained, however, when 20% fly ash was added the mechanical strength was around 75% of the reference cement mortar. The fly ashes contained significant levels of chloride and sulphate and it is suggested that the performance of fly ash–cement binders could be improved by the removal or control of these chemical species.
This work investigates the use of biomass fly ash (BFA) and an alkaline effluent (AEF), both generated from the Kraft pulp industry, in the preparation of geopolymeric binders and mortars for ...construction applications. BFA replaced the metakaolin (MK) while the AEF substituted the distilled water used to dissolve NaOH pellets. The mix design aims to maximize the amount of both the wastes and to optimize the materials properties, such as workability and mechanical performance. At the same time, also the environmental impact decreases enhancing the materials' sustainability and facilitating the circular economy. For the previously optimized BFA/MK ratio (70/30 wt.%) several NaOH/Na2SiO3 ratios and water to AEF substitutions were tested. The best performance was achieved for the alkaline activator having one part of NaOH and three parts of Na2SiO3. According to the main results, the biologic AEF can totally replace the distilled water in the NaOH preparation. Thus will increase the sustainability of the novel materials. Subsequently, the optimized binder was used – with and without AEF - to manufacture mortars with various binder/aggregate ratios. It is concluded that the formulations with a binder/aggregate mass proportion of 1:3 showed the best mechanical performance (compressive strength surpassing 20 MPa, class M20) and the replacement of distilled water with AEF did not affect the specimens final properties, but enhance the materials sustainability. Furthermore, in the pursuit of sustainability, manufacture and curing were conducted at ambient conditions (20 °C, 65% RH), avoiding any external source of energy, involving simple, reproducible, and low-cost processes.
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•Biomass alkaline effluent is used for the first time as geopolymer activator.•Biomass fly ash are used as main alumina-silicate source (70 wt.%).•Used industrial wastes are of biologic nature.•Manufacture is highly sustainable and reproducible.•Novel geopolymeric binders and mortars are suitable for structural applications.
Porous inorganic polymer (IP) monoliths were synthesised using a modified bauxite residue (BR) as the reactive fraction with the objective of using them as adsorbents for methylene blue (MB) from ...synthetic wastewater. First, unreactive, as-produced BR was transformed into a suitable glassy precursor material for IPs by blending BR with minor quantities of C and CaSiO3, thermally treating it at 1200 °C for 2 h, and quenching in water. The alkaline activation of this slag in combination with a pore foaming agent led to the formation of a highly porous microstructure with up to 85% in total porosity. The synthesised porous monoliths demonstrated high MB uptake (up to 17 mg of MB/g of IP with an initial MB concentration of 75 mg/L). A higher porosity of the monoliths, a higher pH, an increasing initial MB concentration as well as stirring of the testing solution have a positive effect on the adsorption capacity, while an optimum solution volume for adsorption was identified. Furthermore, the reuse of these novel monolithic adsorbents was demonstrated by repeating adsorption tests up to five cycles (adsorption-desorption), with a decrease in adsorption capacity of approximately 30%, but a promising cumulative uptake of about 40 mg of MB per g IP.
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•Transformation of bauxite residue into a reactive inorganic polymer precursor.•Synthesis of highly porous inorganic polymer monoliths.•Methylene blue uptake of up to 17 mg per g of inorganic polymer.•High regeneration potential of adsorbents.
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