ALBERT

Description: This paper reports on the effect of the temperature used to activate kaolinite-based paper industry waste on the hydration products obtained when the activated waste was mixed with a lime solution. After activation at temperatures ranging from 500° to 700°C, clay waste exhibited high pozzolanic activity. Analysis with instrumental techniques such as X-ray diffraction, differential thermal analysis, infrared spectroscopy, and scanning electron microscopy showed that higher temperatures favored the formation of calcium aluminate hydrates, hydrotalcite-like compounds, and strätlingite, while at lower temperatures, CSH gels constituted the majority hydrated phase. According to these findings, reaction kinetics differed substantially from the results obtained at temperatures of 700°–800°C, particularly in terms of phase quantity and timing of appearance. This study shows that metakaolinite can be obtained from clay waste at temperatures of under 700°C. The hydrated phases forming under these conditions are the same as more observed at higher activation temperatures (〉700°C). The main differences found were the sequence of phase formation and the quantity of hydrates detected.

Description: Glass nanocomposites, fabricated using borosilicate glass microspheres and antimony tin oxide (ATO) nanoparticles, were previously reported to have formed segregated networks at the boundaries of the glass particles. This resulted in an electrically conducting composite at low volume fractions (~0.5–0.8 vol%) of ATO nanoparticles. The wide range of electrical response in these borosilicate glass composites containing networks of varying concentration of ATO was examined using impedance spectroscopy. The electrical resistance of these composites varied over a range of around 12 orders of magnitude and exhibited several different types of insulator and conductor behavior. The formation of the ATO network was identified and tracked by scanning electron microscopy images and energy dispersive X-ray spectroscopy (EDS) scans. Detailed impedance spectroscopy analysis using all of the dielectric functions (impedance, permittivity, electric modulus, and admittance) was found to be an excellent method for detecting the development of the network and the effect that processing variables can have on its formation and the overall electrical properties of the nanocomposites.

Description: This study examines the composition of certain hydrates (calcium silicate, aluminum silicate, and related phases) produced by the pozzolanic reaction of waste-paper sludge that had previously been activated at different temperatures. It summarizes and compares the evolution of the oxide compounds, and records their stability intervals. Changes to their mineralogical composition were analyzed using X-Ray Diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The hydration products from 1 to 360 days of curing time were CSH gels, hydrotalcite-type compounds (LDH), and stratlingite ( C 2 ASH 8 ). CSH gels were employed as substrates for growing other materials and their morphologies were modified from fibrous to hexagonal layers. The composition of the LDH-type compounds observed in the carbonate group varied with changes in curing time. Two LDH-type compound types were identified: (a) with and (b) without magnesium. Stratlingite was the only stable material after long-curing times.

Description: Glass composites containing percolated segregated networks of conducting antimony tin oxide (ATO) nanoparticles were fabricated through the use of a hot-pressing technique, which resulted in glass microspheres deforming into faceted polyhedra with the ATO located at the edges. Once the ATO percolated, it was shown that minor changes in the processing parameters could cause drastic differences in the electrical properties (as much as 4–5 orders of magnitude in some cases). This study aims to investigate how the hot-pressing processing conditions, that is, temperature and pressure, can influence the electrical properties of percolated glass/ATO composites. Glass composites containing 4.8 wt% ATO, which is a concentration higher than the percolation threshold, were hot pressed at several different temperatures (550°C–675°C) and pressures (5.8–23.4 MPa) and were examined using impedance spectroscopy. A comprehensive equivalent circuit model was developed based on the microstructure of the composites and the individual impedance behavior of the materials involved. It was found that the physical arrangement as well as the individual properties of the glass, the ATO nanoparticles, and the different interfaces between ATO (point contact vs partially sintered vs glass coated) all contributed to the measured response in a quantitative way. The equivalent circuit model was successful in fitting all of the impedance behaviors at different temperature and pressures thus revealing the influence of the processing conditions on the electrical properties of percolated ATO/glass composites.

Description: Ceramic composite rods of alumina filled with varying volume fractions of silicon carbide whiskers ( SiC w ) were made by extrusion and pressureless-sintering. The electrical response was measured over a wide range of frequency (1 mHz–1.8 GHz) and was compared to that of disks made by dry-pressing and pressureless-sintering the same starting powder blends. Extrusion resulted in increased whisker alignment and electrical conductivity along the processing direction compared to dry-pressing. Pressureless sintering resulted in anisotropic shrinkage and the rods having lower density ( D ), dc conductivity (σ dc ), and SiC w –SiC w connection quality compared to fully dense hot-pressed disks. Porosity ( P  = 1 −  D ) increased with SiC w loading and was linearly related to the rod elastic modulus, which was determined via sonic resonance. SiC w percolation in the extruded rods required greater SiC w loading, took place over a wider compositional range, and resulted in significantly decreased σ dc for the rods compared to hot-pressed disks. The σ dc of percolated rods having composition near the percolation threshold was sensitive to small changes in porosity and was correlated to the elastic modulus. The weak percolation transition was correlated to a gradual evolution of the complex permittivity spectra. As SiC w content was increased, such spectra exhibited dc-conductivity tails in the loss which grew in magnitude, and dielectric relaxations which grew in strength, shifted to lower frequency, spread out over wider frequency ranges, and seemed to evolve toward power-law behavior for the dielectric constant.

Description: The transformation process of clay minerals into pozzolanic products for use as active additives in cement matrixes has been closely studied by both the scientific community and the cement industry. Sourcing these additions from recycled waste products is widely prioritized in environmental policies, because of their associated environmental benefits. This article reports an exhaustive analysis of slate waste in Spain, for its eventual use as an alternative source of pozzolans, based on activated phyllosilicates. The analysis examines the effect of activation conditions on mineralogy and the formation and evolution of the hydrated phases that form during the pozzolanic reaction at 28 days. The results show good pozzolanic activity in the starting slate wastes activated between 800°C and 1100°C of temperature for 2 h of retention, on which basis 1000°C and 2 h were recommended as the more suitable activation conditions for these activated slate wastes (ASW), as total destruction of all phyllosilicates is ensured under those conditions. The hydrated phases formed during the pozzolanic reaction in the ASW/ Ca (\ OH ) 2 system were calcium silicate hydrate gels (CSH), randomly interstratified chlorite (Cl) /smectite with variables containing chlorite/smectite ratios 30% chlorite, 40% chlorite, 50% chlorite, 60% chlorite, respectively, and monosulfoaluminate ( C 3 A · SO 4 Ca ·12 H 2 O ) directly related to the activation process used in this article. All of these findings support the viability of slate waste for use as a pozzolanic addition.

Description: The percolation threshold in a composite depends on the processing conditions used to fabricate it along with the size and shape of the filler and the matrix. In this study, borosilicate glass microspheres were used as the matrix material, and nanosized antimony tin oxide (ATO) particles were used as the filler. The glass microsphere/ATO composites were fabricated by hot pressing at temperatures in the range between the glass transition temperature and the softening temperature of the glass to control the viscosity. The pressure and temperature applied allowed the ATO to be confined to the spaces between certain glass particles, forming percolating networks at low volume fractions of the ATO. The viscous flow of the glass allowed for the composite to have near full densities, while allowing for the nanoparticle segregation to occur. Even though apparently similar microstructures were made using different heating schedules, the percolation behavior and electrical conductivity showed noticeable differences. The percolation threshold ranged from 0.1 to 2.5 phr (parts per hundred glass) and the change in electrical conductivity was around seven to nine orders of magnitude. The differences were attributed to the interaction of the segregated ATO particles with one another. The electrical properties were examined using ac impedance spectroscopy along with current atomic force microscopy (C-AFM), which allowed for valuable insights in the structure–property–processing relationships in these materials.

Description: This study reports on exhaustive scientific research into the influence of the activation temperature of inert waste from drinking water treatment plants for use as supplementary cementing material in cements. The effect of activation temperature on the mineralogy of the reactive products resulting from pozzolanic activity and on the evolution of the hydrated phases formed during the pozzolanic reaction at 28 d of curing was analyzed with the assistance of different instrumental techniques such as X-ray fluorescence, X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, and scanning electron microscopy. The results show that all the activated products (based on metakaolinite) presented high pozzolanic activity at all ages of the reaction (up to 90 d), although 600°C at 2 h are the recommended ideal activation conditions from an energetic and economic viewpoint. The activation temperature (600°C–900°C for 2 h of retention) plays an important role in the reaction kinetics in activated drinking water waste/Ca(OH) 2 systems. The hydrated phases identified under these activation conditions were very similar, but with important differences in the crystalline aluminates phases content. Thus, the formation of stratlingite ( C 2 ASH 8 ) is favored at low temperatures (〈800°C); whereas at higher temperatures (at 900°C), tetra calcium aluminate hydrate ( C 4 AH 13 ) appears as the only crystalline phase. Finally, this type of treatment of drinking water waste (based on kaolinite) is ideal to obtain future pozzolans based on recycled metakaoline, a product that is currently listed in international standards for the manufacture of commercial cements.