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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.

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K 2 O–Al 2 O 3 –SiO 2 system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.

Soluble silica content and the concentration of hydrogen peroxide were used to achieve porous geopolymers with nano, meso and milliporosity. The use of rice husk ash as a partial replacement for metakaolin allows to maintain the SiO 2 /Al 2 O 3 ratio between 1.93 and 2.71 and achieve layered bulks with the desired moisture buffering capacity. Stereo Optical Microscopy, Environmental Ecanning Electron Microscopy, Mercury Intrusion Porosimetry, and Microtomography were used with the aim of correlating the intrinsic characteristics of the porous network to the moisture buffering capacity. A detailed analysis of the results permitted to link the moisture buffering capacity to the SiO 2 /Al 2 O 3 ratio and the gel pores/large pores ratio. The saturation regime of these layered porous systems was found between 20 and 27%, while the daily moisture buffering capacity varied from 0.37 to 4.82% considering cycles of desorption and absorption. It was revealed that specimens with a high volume of gel pores (nano and meso) and optimal aeration with millipores are ideal matrices for hygroscopic applications. The porous matrices were found promising for the design of building systems with passive thermal comfort.

Porous composites with the principal class of porosity in the range of those presented in the literature as ideal for the moisture control capacity of building environment are described. In the course of the design of the matrices, micrometric pores are introduced to give to the pore systems a bi- or multimodal characters with the aim of improving the phases percolation during the course of desorption and make the moisture accumulation–desorption behavior of the porous composites essentially function of weather and environment. The porous composites present size of pores in the range 0.001 - 1 μ m for the gel pores and peak centered at 10 μ m for the micrometric pores which insure the matrices efficiency in moisture control capacity and durability. The results of cycles of moisture absorption–desorption in the course of various seasons of the year permit to identify the activities of gel pores meanly efficient in the extreme environment: absorption when the temperature is under 11 ∘ C ; relative humidity is > 60 % and desorption when the temperature is above 18 ∘ C . At ambient conditions, the pores more active are micrometric pores, while gel pores enter in activity only in the extreme environment conditions. The proposed porous geopolymer composites appeared promising candidates for the management of the moisture while improving the thermal insulation of residential building particularly in the regions with important fluctuation of weather. The use of geopolymerization process for the production of those porous composites, the choice of recycling industrial and municipal inorganic wastes appears ideal solution, environmentally friendly, eco-efficient and sustainable for the design of newly materials for the moisture control capacity in building environment. Graphical Abstract

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the [K.sub.2]O-[Al.sub.2][O.sub.3]-Si[O.sub.2] system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 µm to 1 mm) and fine powder alumina (grain size in the range 0.1-100 µm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25-900°C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000°C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000°C to 1150 or 1200°C by adding 75 wt% a-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.

Crystals of the pyroxene group (diopside, augite and enstatite, hedenbergite), series of crystals with the general formula (Mg x Fe 1– x ) 2 SiO 4 having various geometry, identified as spinel (and olivine), and plagioclase crystals from anorthite to anorthoclase that grow together in mass having thin parallel groves embedded in a complex matrix together with calcium alumina silicate grains were found to be the descriptive microstructure of fired volcanic ash. Quartz grains were rarely present as confirmed by dilatometry analysis, XRD, SEM and DTA. The presence of dendrites continuously growing to pyroxene crystals indicated the precipitation/crystallization of these crystals from matrix and regions of glass concentration enhance by ions diffusion. Rings of Ti-rich iron micro-crystals observed around spinel (and olivine) suggested the probable nucleating role of these micro-crystals for the precipitation/crystallization phenomenon. The various types of crystals formed, the difference in their geometry and size and their interlocking mechanism result in a contiguous and dense structure with relevant characteristics at relative low temperature (1125–1150 °C) confirming volcanic ash as a promising alternative raw material for vitrified ceramic products. It was concluded that controlled precipitation/crystallization of raw volcanic ash results on microstructure similar to that of glass-ceramic materials. The observation of fracture surface allowed comparison of fracture mechanics of volcanic ash ceramic to that of conventional vitrified ceramics.

Purpose - The paper aims to focus on microwave (2.45 GHz) assisted SHS (MA-SHS) preparation of NiAl intermetallic coatings on titanium substrates conducted in single mode applicator in order to promote the formation of a complex Ni-Al-Ti interface. This enhances the NiAl coating adhesion to the Ti substrate and presents high hardness, high toughness and the capability of stopping the fracture propagation.Design methodology approach - Numerical modelling, coupling electromagnetic and heat transfer, allowed to demonstrate that the interface cooling rate can be controlled immediately after SHS using microwave heating, benefiting from the possibility of conveying energy to the newly formed intermetallic compounds, despite an adverse temperature gradient which would negatively affect conventional heating techniques, based exclusively on heat transfer. Experimental validation of the modelling results confirmed that by altering the synthesis conditions (load geometry, microwave power, auxiliary microwave absorbers) the thickness of the Ni-Al-Ti layer can be controlled.Findings - The growth of the interface layer can be ascribed to the formation of a liquid phase (ternary eutectic) which progressively consumes NiAl and Ti from the substrate. In case of MA-SHS, the liquid phase presence can be prolonged during cooling, thus explaining the formation of the thick interface layer.Practical implications - Microwave selective heating can be used to initiate the SHS without affecting the metallic substrate, which is only heated locally by the reaction products, thus preserving its properties.Originality value - Coupling numerical simulation and experimental activity demonstrated that the different microstructures obtained by MA-SHS are a result of the peculiar temperature profile, favoured by microwave volumetric and selective heating of the reacting powders.

Ultrasonic pulse velocity testing and image analysis were used to predict the thermal stability of cordierite–mullite refractories. Two compositions used as substrates in fast firing of porcelain whiteware, characterized by different microstructure and crack propagation behavior, were investigated. Fracture strength and fracture toughness values were obtained from three point bending test and chevron notched specimen technique, respectively. The measurement of the ultrasonic velocity was used to assess the material degradation with increasing number of thermal-shock cycles and specimen damage was monitored using image analysis to obtain further evidence of material degradation. The correlation between thermo-mechanical properties, ultrasonic velocity, microstructure, crack-propagation behavior and thermal-shock resistance was discussed. A remarkable similarity was found between the variation of ultrasonic velocity (when measured through the length of the refractory plates) and fracture strength with number of thermal shock cycles. On the other hand, the development of surface microcracking, as monitored by image analysis, is in good agreement with the variation of K IC with the number of thermal-shock cycles. The variation of the ratio with number of thermal-shock cycles shows the highest gradient of the investigated trends and it is proposed as a promising parameter to differentiate refractory materials regarding their different thermal shock behavior. Service life prediction models for refractory plates, from measured values of ultrasonic velocity and surface damage analysis, were proposed and validated.

The effect of alkali-silicate glassy matrix as replacement for feldspar in soft and hard porcelain compositions was studied. SEM and X-ray diffraction analysis were used to evidence phase evolution. For each composition, the influence of soaking time was evaluated. The difference in chemical composition (amount of alkali and alumina) between the two types of porcelain studied influenced the final microstructure: density, pore size and shape, and mullite content. Quartz dissolution was more important in soft porcelain where the mullitization was limited by the low amount of alumina compared to hard porcelain. Replacing the feldspar by alkali-silicate glassy matrices with similar chemical composition, the amount of secondary mullite and mechanical properties increased in both soft and hard compositions.

The influence of the introduction of ZrO2 in concentrations of up to 5 mol% into a 30K2O-70SiO2 base glass composition has been investigated with the aid of differential thermal analysis (DTA), FT-IR spectroscopy, as well as chemical durability tests (pH, conductivity and AES-Inductively Coupled Plasma measurements) and refraction index determination. Several differences have been found by comparison with similar compositions belonging to the Li2O—ZrO2—SiO2 system. The higher ionic radius and the lower field strength of K+ with respect to Li+ is responsible for higher stability towards crystallisation and lower chemical durability. Moreover, even though Zr4+ increases glass polymerisation (higher glass transition temperature, chemical durability and refraction index), the presence of high water content as highlighted by spectroscopy measurements, seems to weaken the glass structure.