Geopolymers are new binders resulting from the activation of an aluminosilicate source by an alkaline solution. It is well-established that the precursors that are used affect geopolymer formation. ...This study focuses on how to control the geopolymer setting time with an alkaline silicate solution. To conduct this investigation, various geopolymer mixtures based on different alkaline silicate solutions were characterized in terms of viscosity and setting time. To understand the link between geopolymer formation and the properties of the solution, the ionic conductivity and the viscosity of the solutions were measured. The solutions were also characterized by Raman spectroscopy. The characterization of the geopolymer mixture shows that the properties of the alkaline silicate solution highly depend on the viscosity and setting time of the geopolymer. Therefore, the alkali molar concentration (3 < M < 10 mol·L−1) influenced the properties of the solution (silicate species, ionic conductivity, and viscosity). A relationship among the chemical composition, the silicate species of the solution and the geopolymer setting time is highlighted depending on the nsinAl+nM ratio. For example, a ratio of nsinAl+nM>1.1 permits a slower geopolymerization reaction, and the setting time of the geopolymer mixture is higher.
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•The alkali molar concentration modifies the silicate species.•For a given metakaolin, the silicate solution can permit to adjust the geopolymer setting time.•The geopolymer setting time is dependant of the silicate species of the solution.
This paper focuses on the effects of alkaline solution reactivity and metakaolin properties on geopolymer formation. To examine these effects, several geopolymer samples were synthesized from two ...alkaline solutions and four metakaolins. The structural evolution of the formed geopolymers was investigated using FTIR spectroscopy and pH values during the material formation. The mechanical properties were measured using compression tests. The results show that the type and amount of siliceous species and non-bridging oxygen atoms control the alkaline solution reactivity. The effect of the metakaolin reactivity is more significant when it is activated with a poorly reactive alkaline solution. However, the alkaline solution governs the reaction when it is highly reactive. Therefore, the extent of depolymerization of the alkaline solution and the reactivity of metakaolin are crucial parameters that control the rate of polycondensation and the compressive strengths of geopolymer materials.
•The amount of low order species controls the reactivity of the alkaline solution.•The polycondensation rate depends on the metakaolin and the alkaline solution.•The metakaolin reactivity is responsible for generating one or several networks.•A highly reactive alkaline solution governs the geopolymerization reaction.
The present study aims to determine the influence of aluminum and calcium availability from two different aluminosilicate sources (COx argillite and sediment), thermally treated at 750 °C, on the ...formation, the structure and the working properties of the resulting alkali-activated materials. Despite their similar chemical composition, some differences in terms of aluminum coordination, amorphous phase and calcium availability were detected between the two studied aluminosilicate sources. Indeed, sediment sample exhibits higher reactive aluminum and lower reactive calcium compared to argillite sample. Moreover, FTIR spectroscopy has revealed that available calcium interacts with free silicon and aluminum and alkali cation in excess (sodium or potassium) leading to the formation of several networks (SiOAl and SiOCa bonds). For both aluminosilicates, the calcium content (0.33 < Ca/Si < 0.36) seems to be not sufficient to produce C-S-H hydrated phases. It was also demonstrated that the low availability of aluminum in addition to the high availability of calcium favor the precipitation of secondary reaction products in the detriment of the geopolymer network. These products fill the voids inducing a decrease of the porosity and the pore size but also lead to higher heterogeneity that decreases the mechanical properties of the final material. However, the higher availability of aluminum and the lower availability of calcium in presence of high alkaline conditions favor the polycondensation reaction.
•Sediment and argillite react differently despite their similar chemical composition.•Available calcium favors the precipitation of secondary reaction products.•Available aluminum enhances the polycondensation reaction.•There is a relation between the pore size and the mechanical strength.
This two-part paper surveys the frequency and voltage control ancillary services in power systems from various parts of the world. In this first part, the nomenclature used to describe active power ...reserves across 11 systems is first reviewed in order to facilitate the comparison of frequency control ancillary services. The essential technical features of frequency and voltage control ancillary services are then described. Finally, the technical requirements adopted in eight jurisdictions (North America, continental Europe, Germany, France, Spain, the Netherlands, Belgium, and Great Britain) are compared. The companion paper surveys the economic features of these ancillary services
Materials such as ceramic matrix composites are developed for mechanical applications at high temperature, but their cost remains a limitation. Consequently, the use of acid-based geopolymer matrices ...may be an alternative to reduce costs.
In this study, the sample was prepared from metakaolin and phosphoric acid. Fourier Transform InfraRed and Nuclear Magnetic Resonance spectroscopies, X-Ray Diffraction and thermal measurements were used to understand the structural evolution of acid-based geopolymers (binders) during consolidation and after thermal treatments.
According to the results, the consolidation of the binder has been divided into four steps: the dissolution of the metakaolin, the polycondensation reactions forming AlPO4 entities and hydrated phases, the breakdown of SiOAl bonds with formation of various hydrated silica networks and finally the completion of the networks. After a thermal treatment at 1000 °C, the binder consists of AlPO4 phases, quartz and vitreous silica.
•The polycondensation mechanism of acid-based geopolymers is highlighted.•The different networks formed are evidenced from spectroscopy and XRD data.•Several hydrated phases and AlPO4 polymorphs crystallize during heat treatment.
PM2.5 filter samples have been collected in three megacities at the middle and lower reaches of the Yangtze River: Wuhan (WH), Nanjing (NJ), and Shanghai (SH). The samples were analyzed using ...ultra-high-performance liquid chromatography (UHPLC) coupled with Orbitrap mass spectrometry (MS), which allowed for detection of about 200 formulas of particulate organosulfates (OSs), including dozens of formulas of nitrooxy-organosulfates, with various numbers of isomers for each tentatively determined formula at each location. The number of aliphatic OS formulas represented more than 78 % of the detected OSs at the three locations, while aromatic OSs were much less numerous. OSs with two to four isomers accounted for about 50 % of the total OSs on average in these megacity samples, and the percentage of OSs with six and more isomers in the WH sample was more significant than those in the SH and NJ samples. Additionally, the molecular formula, average molecular weight, and degrees of oxidation and unsaturation of tentatively assigned OSs were compared. The results indicate that the OSs between NJ and SH shared higher similarity, and the characteristics of OSs in SH varied diurnally and seasonally. OSs derived from isoprene, monoterpenes, and sesquiterpenes were abundant in samples from the three megacities and could be produced through both daytime photochemistry and NO3 night-time chemistry. The reaction pathways leading to isoprene-derived OSs probably varied in those locations because of the different NOx levels. In addition, a number of OSs that might be formed from polycyclic aromatic hydrocarbons were also detected, which underlies the importance of anthropogenic sources for this class of compounds.
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•Metakaolins reactivity impacts the behavior in the presence of alkaline solution.•Highly reactive metakaolin accelerates the consolidation of the material.•The metakaolin reactivity ...can generate the formation of one or several networks.•Nanostructure variations after consolidation influence the mechanical properties.•A descriptive model of the mechanism of geopolymer formation was proposed.
Geopolymer materials are obtained by the alkaline activation of aluminosilicate sources, the best of which is metakaolin. However, every raw material is different, and very few comparative studies have been done on different metakaolin sources. The aim of this work is to develop methods for the prediction of the working properties of geopolymer materials based on the reactivity of the metakaolin employed. Infrared spectroscopy showed direct relationships between the wettability, the Si/Al ratio and the kinetics of conversion of Si–O–Si bonds to Si–O–Al bonds. Moreover, it was demonstrated that the presence of impurities and the reactivity of the metakaolin can generate the formation of one or several networks. Finally, a descriptive model of the mechanism of geopolymer formation was proposed that takes into account the quality of metakaolin used.
This work aims to study the effect of the addition of reactive or unreactive aluminosilicate sources on the geopolymerisation reaction, and therefore on the geopolymer's working properties. To this ...end, six formulations with different metakaolin content, as well as the potential addition of mica fillers, were synthetized and investigated over time. The structural evolution of the reactive mixtures was monitored by FTIR spectroscopy and thermal analysis, whereas the resulting geopolymers were characterized using X-ray diffraction and 27Al NMR. Finally, SEM observations of the microstructures of the consolidated materials were carried out. The results show that a rise in the metakaolin content modifies the dissolution step of the geopolymer formation by decreasing the amount of consumed water from 50 to 34%. Moreover, it increases the initial viscosity by 66% and decreases the setting time by 168 min. Besides, with the addition of mica fillers, the analysis of the FTIR spectra outlines a lower reactivity of the mixture and the formation of several geopolymer networks, ultimately leading to a heterogeneous microstructure. Finally, the mechanical properties are directly related to the level of amorphous phase in the geopolymer sample, the highest values being reached for 85% of amorphous phase.
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•The metakaolin content has a strong influence on the geopolymer formation.•The addition of mica affects the reactive mixture properties.•The use of mica fillers modifies the polycondensation reactions and the microstructure of the geopolymer synthetized.
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•Setting time of geopolymer samples has been controlled with the use of additives.•The addition of boron-based compound can decrease the pH value of geopolymer sample.•Boron ...coordination seems to be the key parameter to control the geopolymer pH value.•The phosphate species perturb the polycondensation reactions.
The aim of this investigation was the development of a geopolymer grout. To achieve this goal, it is necessary to control the characteristics of the reactive mixture (i.e., between the dissolution of the mineral source and the consolidation of the binder) and its setting time. The targeted application (geological radioactive waste disposal) requires a grout with a setting time greater than 24 h, a viscosity in the range from 1 to 5 Pa·s and a moderate pH value between 10 and 11. To reach these values, a metakaolin-based geopolymer was used with the addition of mineral additives. The viscosity measurements reveal that a focused viscosity is reached and that the setting time can be easily controlled and extended above 24 h with the addition of 2.8%weight of acids (boric and phosphoric) or 10%weight of borax. The acids reduce the reactivity of the mixtures by decreasing the alkalinity and inducing slow silica/alumina dissolution. The borax extends the setting time by increasing the water content and by modifying the polycondensation reactions to form huge structural entities. The decrease in the pH value to the target (10–11) can only be reached using 10%weight of borax or 7.5%weight of lithium tetraborate due to the insertion of BIV in the geopolymer network.
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•The number of non-bridging oxygen atoms (NBOs) is directly linked to the Si/M molar ratio.•The higher reactivity, i.e. the higher number of non-bridging oxygen, is obtained for ...0.5<Si/M<1.•Raman spectroscopy provides additional information to FTIR and NMR data.
Alkaline silicate solutions are of great interest for several industrial applications. Previous studies demonstrated the influence of different parameters such as the Si/M (Na, K) molar ratio, the dilution level, the cation size and the manufacturing process on the silicate species. The aim of this study was to complete the data from previous works. This study focuses on the effects of these various parameters on the polymerization and the reactivity of silicate solutions. To evaluate the effects, several solutions obtained from various processes with different cations and Si/M ratios were studied. All the solutions were characterized using spectroscopic investigations (FTIR, Raman and NMR). FTIR spectroscopy was used to define the polymerization state of the various solutions. The experiments revealed that increases in the Si/M molar ratio, the dilution level and the cation size lead to the polymerization of the species. However, the effect of the cation is less pronounced. Moreover, Raman spectroscopy revealed an increase in oligomers for Si/M molar ratios less than 1. It was also shown that the manufacturing process causes a slight variation in the amount of the different silicate species responsible of the reactivity of the solution. Knowledge of these key parameters is important for different applications, including geopolymer fabrication. Moreover, it was shown a correlation between the results obtained by NMR and Raman spectroscopy.
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