Resumo em Inglês:

Abstract Palygorskite (Pal) is a natural clay mineral with fibrous morphology and high surface area. Depending on the geological origin, it presents impurities, such as quartz and carbonates, which can harm some of its properties. Therefore, this work seeks to define a viable methodology for the purification of a Brazilian Pal. Two types of mixing processes (sonication and milling) and two types of dispersing agents (sodium silicate and sodium polyacrylate) were investigated. In addition, a subsequent acid activation with hydrogen peroxide and sulfuric acid was performed for complete purification. The viability of the purification of Pal was confirmed by X-ray diffraction, X-ray fluorescence, and thermogravimetric analyses. The sonication mixture process and the use of sodium polyacrylate as a dispersing agent were more effective. In addition, BET analysis showed an increase in the surface area of Pal, and scanning electron microscopy confirmed the permanence of its fibrous morphology after the purification steps.

Resumo em Inglês:

Abstract The spinel ferrite system Ni1-xZnxFe2O4 (x=0-0.7, Δx=0.1) was prepared by wet chemical co-precipitation method using sulfates of respective metal ions. The electrical resistivity and dielectric properties of the prepared samples have been studied as a function of temperature, frequency, and Zn content (x). The DC resistivity (ϱ) was measured in the temperature range of 300 to 800 K. The curves so plotted show two phases viz. ferrimagnetic and paramagnetic, wherein the transformation from former to later phase took place at the Curie temperature, TC. The values of TC were found to decrease with the increase in x. The values of activation energy (ΔE<0.2 eV) depicted the possibility of the hopping mechanism governing the conduction within the ferrites. The ferrite samples exhibited semiconducting behavior, where the electrical resistivity (ρ) decreased with the increasing temperature. The dielectric constant (ε’) and dielectric loss (ε”) were studied as a function of composition, temperature, and frequency.

Resumo em Inglês:

Abstract Given the potential of acid-activated clays in adsorptive and catalytic processes, several studies focusing on acid matrices have grown over the years, making it possible to study vermiculite, a mineral composed of a type 2:1 unit cell with tetrahedral and octahedral sheets. Therefore, this work aimed to study vermiculite treated with acids for the production of matrices. In this work, the treatment of natural vermiculite-hydrobiotite, with nitric acid in concentrations of 0.25, 0.50, 0.75, and 1 mol.L-1, followed by the dealumination process with sulfuric acid 25% v/v, aiming the production of acid matrices for applications as support for biocatalysts, chemical adsorbents, among others. The produced materials were characterized using different techniques, including X-ray fluorescence spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and isotherm adsorption using the BET method. The results showed that after acid leaching the vermiculite showed a high SiO2 content, high Brønsted and Lewis surface acidity, good adsorption/desorption performance, and significant growth of the specific surface area, qualifying the vermiculite as a promising material for future applications in catalytic and biocatalytic processes. The acid concentration had a considerable influence on the compositional and textural modification of vermiculite-hydrobiotite.

Resumo em Inglês:

Abstract Refractory ceramic materials were manufactured, mainly composed of magnesia grains (coarse and medium fraction) and CaZrO3/MgO as a binding phase produced from the reaction sintering between CaMg(CO3)2 (dolomite) and m-ZrO2 (monoclinic zirconia), which constituted the finer fraction. Different relationships between the proportion and the size of the different aggregates were studied. The microstructure of the materials was characterized in terms of density, crystalline phases, grain phase distribution, and morphology. A combination of X-ray diffraction analysis and scanning electron microscopy with microanalysis was used. The mechanical behavior was determined in terms of the room temperature dynamic Young’s modulus and the 3-point bending modulus of rupture (MOR) at room temperature. All the materials remained with porosities above 30%. The microstructure developed was similar between the sintered materials, characterized with MgO coarse grains bonded by CaZrO3/c-ZrO2 phases, and dispersed irregularly shaped pores. The MOR was directly related to the finer fraction content in the composition, where a higher quantity of finer fraction resulted in a higher MOR.

Resumo em Inglês:

Abstract Atrazine’s adsorptive capabilities make the use of clays practical and effective. Three types of organophilic clays modified by cationic surfactants with the ability of one of these clays to adsorb atrazine in an aqueous medium are discussed in this work. The modification in clays was done with the surfactants cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, and dodecyltrimethylammonium chloride. The X-ray diffraction data indicated different lamellar expansions for the three organophilic clays obtained when compared to in natura clay, proving the presence of quaternary ammonium salts in the interlamellar space. There was also a decrease in the specific surface area showing less accessibility to the lamellar spaces due to the presence of surfactants. The thermogravimetric analysis showed a negative variation profile in all the clays. The adsorption test showed a better capacity for atrazine adsorption in the organophilic clay of 64% when compared with 36% of in natura clay, and it followed the Freundlich isotherm model.

Resumo em Inglês:

Abstract The physical and mechanical properties of foamed geopolymers were determined. The geopolymers were made with two different metakaolins (from Metacaulim do Brasil - MB, and Pantano Grande/RS - MPG) as precursors, with NaOH and Na2SiO3 as activators, and with the addition of foam, which was produced with the foaming agent diethanolamide of coconut fatty acid with a minimum of 80% concentration. Also, two times (2.5 and 4 h) were used for the calcination of MPG at 750 °C. The results showed that all factors (precursors type, alkali content, silica modulus, and foam content) had a significant influence on the response variables, which presented the following variations: compressive strength of 0.36 to 3.23 MPa; thermal conductivity from 0.22 to 0.42 W/(m.K); saturated bulk density from 1176 to 1364 kg/m3; dry bulk density of 619 to 864 kg/m3; air void from 46.4% to 62.5%; and water absorption from 55.7% to 95.4%. It was also concluded that the precursor type was the factor that had the most influence on the properties of foamed geopolymers, with MPG calcined for 4 h being the best, as greater compressive strength and low thermal conductivity were obtained.

Resumo em Inglês:

Abstract The properties of self-leveling mortars (SLMs) in the fresh and hardened state are strongly affected by their formulation. In this study, the influence of three different quartz sands (different physical characteristics) was investigated in SLMs, as well as the replacement of these sands by crushed basalt filler. The tests performed were: spreading, density in the hardened state, absorption by immersion, flexural strength, compressive strength, dynamic modulus of elasticity, water capillary absorption, and drying shrinkage. The results obtained show that the particle size distribution (PSD) of sand influenced most of the properties. The shape and texture of the sand grains did not seem to have much influence on the properties of SLMs when the PSD was different. The powdery (fine) material content of sand seemed to affect most notably properties in the fresh state and shrinkage. The increase in the content of sand replacement by filler decreased the workability of SLMs, however, it may improve the initial flexural and compressive strengths and reduce the drying shrinkage.

Resumo em Inglês:

Abstract In order to reduce the impacts of the industrial effluent on the environment, silicon carbide hollow fiber membranes were prepared by the precipitation-immersion technique and sintered at 1450 and 1500 °C. The membranes were characterized by X-ray diffraction, their surface structure was characterized by scanning electron microscopy and atomic force microscopy, pore size distribution and porosity, mechanical properties, and flow measurements with distilled water and effluent generated by the indigo blue industry. The sintered membranes presented crystalline phases of silicon carbide and aluminum oxide. The tubes presented defect-free microstructure and uniform porous surface, with porosity above 50%. The silicon carbide membrane presented significant reductions of the solutes and colloidal particle contents in the effluent. The membranes sintered at 1500 °C proved to be more efficient for reductions of the turbidity and color of the effluent. Silicon carbide hollow fiber membrane is an interesting alternative for the treatment of effluents from the textile industry.

Resumo em Inglês:

Abstract Calcium aluminate cement (CAC) can induce the development of high green mechanical strength to refractory castables in a short period of time (24 h). However, the production of this hydraulic binder is energy-intensive and releases a large content of carbon dioxide (CO2), which adds to global warming. Thus, aiming to develop an alternative binding system, this study investigated the combined addition of aluminum lactate (0.25-1.0 wt%) and calcium carbonate (3.2 wt%) to alumina castables. The prepared samples were analyzed using different experimental techniques, such as cold and hot modulus of rupture, hot elastic modulus, X-ray diffraction, and apparent porosity. According to the results, the lactate addition to the castables improved their green mechanical strength after drying at 110 °C/24 h (4.89 MPa) when compared to the one containing plain calcium carbonate (1.72 MPa). Furthermore, the carbonate-containing refractories presented cold and hot modulus of rupture similar or even superior to the castable containing cement in a wide temperature range (600-1500 °C).

Resumo em Inglês:

Abstract The use of supplementary cementitious materials (SMCs) is one of the three essential factors for the sustainability of the cement industry. To reduce CO2 emissions, several studies have introduced new methods for the utilization of industrial wastes such as the addition into cementitious materials. The objectives of this study were to chemically and physically characterize refractory ceramic industry waste and determine its viability as an SMC. The pozzolanic characteristics or SMC characteristics of the refractory waste were investigated using standard tests. The results revealed that the refractory waste was mainly composed of Al2O3 and SiO2. The mineralogical analysis revealed that the mullite was the major phase of the waste, followed by small amounts of cristobalite and quartz phases. The results of the modified Chapelle test and pozzolanic activity index with lime after 7 days indicated that the waste did not meet the minimum pozzolanicity required by the standard. However, the waste improved the mechanical resistance of the cement specimens containing the waste after 28 days compared with the reference sample, indicating the promising potential of the material for this application. This result indicated that the refractory waste shows a filler type effect, and thus, can be employed as an SCM.

Resumo em Português:

Resumo Os híbridos derivados de grafeno ou óxido de grafeno e o argilomineral montmorillonita são materiais cuja matriz do filossilicato oferece estabilização das camadas nanométricas das estruturas carbonáceas, além de melhoramento das propriedade físico-químicas e da biocompatibilidade. Por sua vez, o grafeno e seus derivados agregam novas funcionalidades à montmorillonita e ao aumento da reatividade. Os materiais funcionais resultantes do sinergismo entre os constituintes têm sido explorados em diferentes aplicações, como em processos de remediação ambiental por meio de adsorção e catálise, ciência de materiais, áreas biológicas e biomédicas. No âmbito dessa revisão, as principais rotas para obtenção dos híbridos de grafeno e derivados com montmorillonita são apresentadas, relacionando os fatores estruturais e interações entre os constituintes para formação dos compósitos. Ainda os principais métodos de caracterização são discutidos e as aplicações mais recentes são relacionadas às características estruturais, morfológicas, térmicas e texturais.

Resumo em Inglês:

Abstract In hybrids derived from graphene or graphene oxide and montmorillonite (MMT) clay, the phyllosilicate matrix of MMT provides stability to the carbonaceous layer structure of graphene. The formation of hybrids improves the physicochemical properties and biocompatibility of the individual components. In addition to enhancing the chemical reactivity of MMT, the synthesis of hybrids with graphene or its derivatives may offer enhanced properties. The synergistic properties of the organic and inorganic components of the hybrids result in functional materials, as demonstrated in various fields, such as environmental remediation, adsorption, catalysis, materials science, biology, and biomedical sciences. In this review, we present the main routes for synthesizing the hybrids of graphene and its derivatives with MMT. We discuss the effects of the structure and interactions between the constituents on the formation of composites. Furthermore, major characterization techniques and material properties, such as structural, morphological, textural, and thermal properties, are discussed.

Resumo em Inglês:

Abstract The effect of microwave heating on the expansion of vermiculite was studied at a level of 700 W for 4 min exposure time and compared with raw vermiculite. In this study, the characterization of vermiculite from a mine in the northeast region of Brazil was performed using thermogravimetry, N2-adsorption, cation exchange capacity, X-ray powder diffraction (XRD), infrared (IR) spectroscopy, X-ray fluorescence spectroscopy (XRF), and scanning electron microscopy (SEM). Following the microwave heating, the vermiculite was characterized using XRD, IR spectroscopy, and SEM. Microwave irradiation of the vermiculite sample caused structural changes such as loss of crystallinity and disorder, as revealed by the XRD patterns, but did not cause expansion. Results from SEM and IR spectroscopy showed that the microwave heating did not cause profound alterations to the morphology, clay structure, and chemical composition of the vermiculite.

Resumo em Inglês:

Abstract Multiphasic calcium phosphate powders were synthesized by the acidic route, using lactic acid as a chelating agent that allows the production of a stable and rich solution of calcium and phosphate ions at room temperature. After pH adjusting, without varying the concentrations of precursor solution, XRD and FTIR analyses showed different CaP phases before and after heat treatment at 1000 °C. At pH 5, brushite plate-like particles were produced, while at higher pH levels (7 to 12) nano-hydroxyapatite particles were formed. After the calcination process, partial and total decomposition of hydroxyapatite in the β-TCP phase was explained by XRF analysis due to the calcium-deficient hydroxyapatite of synthesized samples at pH 7 and 10. This work presents an important method to synthesize any desired CaP phase compositions by varying the pH level and subsequently performing heat treatment, which has a direct effect on morphology, crystallinity, and formation of different CaP powders.

Resumo em Inglês:

Abstract In this work, the sintering mechanisms of 3 mol% yttria-stabilized zirconia (TZ3Y) and TZ3Y containing 0.3 wt% of ZnO or Co3O4 were analyzed by determining the apparent activation energy of sintering from dilatometry data and observing the evolution of microstructure. Dilatometry test was performed at a heating rate ranging from 5 to 20 °C/min from room temperature up to 1500 °C. The main mechanism of each sintering additive for the studied samples was revealed from the variation of the sintering activation energy as a function of the relative density and scanning electron microscopy images. The mechanism for the ZnO additive was the formation of a liquid phase at the beginning of the sintering process, while the Co3O4 additive acted via solid-state sintering by increasing cation diffusivity.

Resumo em Inglês:

Abstract The abundantly available mica flakes were added in raw materials to manufacture pottery in Central Peninsular India during the early Iron Age (700-300 BCE) for storage and glittering. The micaceous potsherds from the excavated megalithic site of Nagpur were studied for their composition, manufacturing techniques, microstructural features, microbial colonization, and estimation of firing temperature. The analytical techniques used were micro-XRF, FTIR, XRD, FEG-SEM, mercury porosimetry (MIP), and thermal analysis. The XRF and SEM-EDX data revealed that the potteries were coated with aqueous iron oxide on either surface for red hue and subsequently fired at an estimated temperature of 700-750 °C in a reducing atmosphere as determined by vibrational and mineralogical analysis. The X-ray microscopy and FEG-SEM images showed a grey color sandwich layer representing the original clay composition between iron oxide-coated red surface layers. MIP showed that the fine ware had a high porosity of 21-26% with a void ratio (e) of 0.28-0.36. The loosely adhered mica flakes influenced the strength and stability of the pottery. The 16S rRNA gene sequencing identified the presence of Bacillus velezensis within the round and elongated pits within the potsherds drawing nutrients from minerals.