Mechanical Properties and Durability of Composite Cement Pastes Containing Phase-Change Materials and Nanosilica.

Escalating global surface temperatures are highlighting the urgent need for energy-saving solutions. Phase-change materials (PCMs) have emerged as a promising avenue for enhancing thermal comfort in the construction sector. This study assessed the impact of incorporating PCMs ranging from 1% to 10% by mass into composite Portland cement partially replaced by fly ash (FA) and nanosilica particles (NS). Mechanical and electrochemical techniques were utilized to evaluate composite cements. The results indicate that the presence of PCMs delayed cement hydration, acting as a filler without chemically interacting within the composite. The combination of FA and PCMs reduced compressive strength at early ages, while thermal conductivity decreased after 90 days due to the melting point and the latent heat of PCMs. Samples with FA and NS showed a significant reduction in the CO2 penetration, attributed to their pozzolanic and microfiller effects, as well as reduced water absorption due to the non-absorptive nature of PCMs. Nitrogen physisorption confirmed structural changes in the cement matrix. Additionally, electrical resistivity and thermal behavior assessments revealed that PCM-containing samples could reduce temperatures by an average of 4 °C. This suggested that PCMs could be a viable alternative for materials with thermal insulation capacity, thereby contributing to energy efficiency in the construction sector.

Enhancing Concrete Durability and Strength with Fly Ash, Steel Slag, and Rice Husk Ash for Marine Environments.

The effect of an alternative source of silica, based on class F fly ash mixed with blast furnace slag and activated by rice husk ash (RHA), to produce concrete exposed to marine environments was evaluated. Four mixtures activated by the combination of 85% NaOH 14M + 15% RHA were manufactured to achieve a liquid/solid ratio of 0.20. Fly ash was incorporated into the steel slag mixture at addition percentages of 20, 40, 60, and 80%, and evaluated at 28, 900, and 1800 days for pore and chloride ion absorption. In general, including rice husk ash in the mixture of fly ash and steel slag significantly affected mechanical performance because it was possible to obtain concrete with high mechanical resistance. Concerning the durability evaluation, the effect of the activator generated by rice husk ash was observed, and the increase in steel slag added to the cementitious samples improved the capacity of the material to resist the penetration and diffusion of chloride ions.

Alkaline Activation of Binders: A Comparative Study.

Binders formulated with activated alkali materials to replace Portland cement, which has high polluting potential due to CO2 emissions in its manufacture, have increasingly been developed. The objective of this study is to evaluate the main properties of activated alkali materials (AAM) produced by blast furnace slag, fly ash, and metakaolin. Initially, binders were characterized by their chemical, mineralogical and granulometric composition. Later, specimens were produced, with molarity variation between 4.00 and 5.50, using the binders involved in the research. In preparing the activating solution, sodium hydroxide and silicate were used. The evaluated properties of AAM were consistency, viscosity, water absorption, density, compressive strength (7 days of cure), calorimetry, mineralogical analysis by X-ray diffraction, and morphological analysis by scanning electron microscopy. The results of evaluation in the fresh state demonstrate that metakaolin has the lowest workability indices of the studied AAM. The results observed in the hardened state indicate that the metakaolin activation process is optimized with normal cure and molarity of 4.0 and 4.5 mol/L, obtaining compressive strength results after 7 days of curing of approximately 30 MPa. The fly ash activation process is the least intense among the evaluated binders. This can be seen from the absence of phases formed in the XRD in the compositions containing fly ash as binder. Unlike blast furnace slag and metakaolin, the formation of sodalite, faujasite or tobermorite is not observed. Finally, the blast furnace slag displays more intense reactivity during thermal curing, obtaining compressive strength results after 7 days of curing of around 25 MPa. This is because the material's reaction kinetics are low but can be increased in an alkaline environment, and by the effect of temperature. From these results, it is concluded that each precursor has its own activation mechanism, observed by the techniques used in this research. From the results obtained in this study, it is expected that the alkaline activation process of the types of binders evaluated herein will become a viable alternative for replacing Portland cement, thus contributing to cement technology and other cementitious materials.

Hybrid Cements and Construction Elements Based on Alkaline Activation with Sodium Sulfates from Fly Ash and Construction and Demolition Waste.

This article demonstrates the possibility of producing hybrid cementitious materials (pastes, mortars, concretes, and precast elements) based on fly ash (FA) and construction and demolition wastes (CDW) using alkaline activation technology. Sodium sulfate was used as an activator and fine and coarse aggregates were obtained from CDW residues. An addition of Portland cement (OPC) (10 to 30%) allowed for improvement in the mechanical behavior of the hybrid cements and them to be cured at room temperature (25 °C). The FA and CDW cementitious materials obtained compressive strengths of 37 MPa and 32 MPa, respectively. The compressive strength of FA and CDW alkali-activated concretes at 28 days of curing was 22 MPa and 18 MPa, respectively, which identifies them as structural concretes according to NSR-10 title C in Colombia. The potential use of these concretes was validated by obtaining and classifying precast materials.

Strength and Microstructure Assessment of Partially Replaced Ordinary Portland Cement and Calcium Sulfoaluminate Cement with Pozzolans and Spent Coffee Grounds.

Supplementary cementitious materials are considered a viable and affordable way to reduce CO2 emissions from the cement industry's perspective since they can partially or nearly entirely replace ordinary Portland cement (OPC). This study compared the impact of adding spent coffee grounds (SCGs), fly ash (FA), and volcanic ash (VA) to two types of cement: OPC and calcium sulfoaluminate cement (CSA). Cement samples were characterized using compressive strength measurements (up to 210 days of curing), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), attenuated total reflection infrared spectroscopy, and hydration temperature measurements. In all the studied systems, the presence of SCGs reduced compressive strength and delayed the hydration process. CSA composite cement containing 3.5% SCGs, 30% FA, and 30% VA showed compressive strength values of 20.4 MPa and 20.3 MPa, respectively, meeting the minimum requirement for non-structural applications. Additionally, the results indicate a formation of cementitious gel, calcium silicate hydrate (C-S-H) in the OPC-based composite cements, and calcium alumino-silicate hydrate (C-A-S-H) as well as ettringite in the CSA-based composite cements.

Evaluation of the Effect of Binary Fly Ash-Lime Mixture on the Bearing Capacity of Natural Soils: A Comparison with Two Conventional Stabilizers Lime and Portland Cement.

This study evaluates a binary mixture of fly ash and lime as a stabilizer for natural soils. A comparative analysis was performed on the effect on the bearing capacity of silty, sandy and clayey soils after the addition of lime and ordinary Portland cement as conventional stabilizers, and a non-conventional product of a binary mixture of fly ash and Ca(OH)2 called FLM. Laboratory tests were carried out to evaluate the effect of additions on the bearing capacity of stabilized soils by unconfined compressive strength (UCS). In addition, a mineralogical analysis to validate the presence of cementitious phases due to chemical reactions with FLM was performed. The highest UCS values were found in the soils that required the highest water demand for compaction. Thus, the silty soil added with FLM reached 10 MPa after 28 days of curing, which was in agreement with the analysis of the FLM pastes, where soil moistures higher than 20% showed the best mechanical characteristics. Furthermore, a 120 m long track was built with stabilized soil to evaluate its structural behavior for 10 months. An increase of 200% in the resilient modulus of the FLM-stabilized soils was identified, and a decrease of up to 50% in the roughness index of the FLM, lime (L) and Ordinary Portland Cement (OPC)-stabilized soils compared to the soil without addition, resulting in more functional surfaces.

Microstructural and Mechanical Characteristics of Alkali-Activated Binders Composed of Milled Fly Ash and Granulated Blast Furnace Slag with µ-Limestone Addition.

Concrete is the most used construction material, needing large quantities of Portland cement. Unfortunately, Ordinary Portland Cement production is one of the main generators of CO2, which pollutes the atmosphere. Today, geopolymers are an emerging building material generated by the chemical activity of inorganic molecules without the Portland Cement addition. The most common alternative cementitious agents used in the cement industry are blast-furnace slag and fly ash. In the present work, the effect of 5 wt.% µ-limestone in mixtures of granulated blast-furnace slag and fly ash activated with sodium hydroxide (NaOH) at different concentrations was studied to evaluate the physical properties in the fresh and hardened states. The effect of µ-limestone was explored through XRD, SEM-EDS, atomic absorption, etc. The addition of µ-limestone increased the compressive strength reported values from 20 to 45 MPa at 28 days. It was found by atomic absorption that the CaCO3 of the µ-limestone dissolved in NaOH, precipitating Ca(OH)2 as the reaction product. SEM-EDS analysis showed a chemical interaction between C-A-S-H- and N-A-S-H-type gels with Ca(OH)2, forming (N, C)A-S-H- and C-(N)-A-S-H-type gels, improving mechanical performance and microstructural properties. The addition of µ-limestone appeared like a promising and cheap alternative for enhancing the properties of low-molarity alkaline cement since it helped exceed the 20 MPa strength recommended by current regulations for conventional cement.

Assessment of the Corrosion of Steel Embedded in an Alkali-Activated Hybrid Concrete Exposed to Chlorides.

Hybrid alkali-activated cements (HAACs), also known as cements with high percentages of alkali-activated supplementary materials, are alternative cements that combine the advantages of ordinary Portland cement (OPC) and alkali-activated systems. These cements are composed of a minimum of 70% precursor material and a maximum of 30% OPC mixed with an alkaline activator. This article evaluates the corrosion performance of reinforced HAAC concrete based on fly ash (FA) under exposure to chlorides (FA/OPC, 80/20). Its performance is compared with that of a binary alkali-activated cement (AAC) based on FA and granulated blast furnace slag (GBFS) (FA/GBFS, 80/20). The tests performed on the concrete matrix correspond to the compressive strength and permeability to chloride ions. Using accelerated corrosion techniques (impressed voltage) and electrochemical tests after immersion in 3.5% NaCl, the progress of the corrosive process in the reinforcing steel is evaluated. The FA/OPC exhibit a better corrosion performance than the FA/GBFS concrete. At the end of the exposure to chlorides, the FA/OPC hybrid concrete presents the best performance, with a 49% lower corrosion rate than that of the FA/GBFS. Note that according to the polarization curves, the values of the proportionality constant B in the alkaline-activated concretes differ from the values recommended for concrete based on OPC.

Alkali-Activated Hybrid Cements Based on Fly Ash and Construction and Demolition Wastes Using Sodium Sulfate and Sodium Carbonate.

This article demonstrates the possibility of producing alkali-activated hybrid cements based on fly ash (FA), and construction and demolition wastes (concrete waste, COW; ceramic waste, CEW; and masonry waste, MAW) using sodium sulfate (Na2SO4) (2-6%) and sodium carbonate (Na2CO3) (5-10%) as activators. From a mixture of COW, CEW, and MAW in equal proportions (33.33%), a new precursor called CDW was generated. The precursors were mixed with ordinary Portland cement (OPC) (10-30%). Curing of the materials was performed at room temperature (25 °C). The hybrid cements activated with Na2SO4 reached compressive strengths of up to 31 MPa at 28 days of curing, and the hybrid cements activated with Na2CO3 yielded compressive strengths of up to 22 MPa. Based on their mechanical performance, the optimal mixtures were selected: FA/30OPC-4%Na2SO4, CDW/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, and CDW/30OPC-10%Na2CO3. At prolonged ages (180 days), these mixtures reached compressive strength values similar to those reported for pastes based on 100% OPC. A notable advantage is the reduction of the heat of the reaction, which can be reduced by up to 10 times relative to that reported for the hydration of Portland cement. These results show the feasibility of manufacturing alkaline-activated hybrid cements using alternative activators with a lower environmental impact.

Lightweight masonry block without Portland cement / Bloco de alvenaria leve sem cimento Portland

ABSTRACT Huge amounts of fly ash - a substance that does not conform to the ASTM C618 classification due to its chemical properties - have been abandoned in landfills around the world, despite their self-cementing property. It has not been used in concrete making applications due to its large amounts of free lime and sulfate contents. The fly ash in these plants is dumped in landfills, causing serious environmental hazards. Fly ash is disposed to the landfills by belt conveyors after being humidified with water. Therefore, the fly ashes humidified in the landfill areas are hydrated in nature. This hydration is further intensified in landfills by rain and snow. Thus, the free lime content of fly ash decreases due to its long hydration process. In this work, the lightweight masonry blocks were produced by mixing normal and hydrated fly ashes or normal, hydrated fly ash and lime without Portland cement. The compressive strength, water absorption, sorptivity, density, porosity, and thermal conductivity values of the samples produced were determined. The results obtained from these tests showed that lightweight masonry blocks could be produced by using these waste materials in building applications.

RESUMO Enormes quantidades de cinzas volantes - uma substância que não está de acordo com a classificação ASTM C618 devido às suas propriedades químicas - foram abandonadas em aterros sanitários ao redor do mundo, apesar de sua propriedade de autocimentação. Essas substâncias não têm sido usadas em aplicações de fabricação de concreto devido às suas grandes quantidades de cal livre e teores de sulfato. A cinza volante dessas usinas é despejada em aterros sanitários, causando sérios riscos ambientais. Essas cinzas são descartadas em aterros por correias transportadoras após serem umedecidas com água. Portanto, as cinzas volantes umedecidas nas áreas do aterro são hidratadas na natureza. Essa hidratação é ainda mais intensificada em aterros, pela chuva e pela neve. Assim, o teor de cal livre nas cinzas volantes diminui devido ao longo processo de hidratação. Neste trabalho, blocos de alvenaria leves foram produzidos pela mistura de cinzas volantes normais e hidratadas, ou cinza volante normal hidratada e cal sem cimento Portland. Foram determinados os valores de resistência à compressão, absorção de água, sensibilidade, densidade, porosidade e condutividade térmica das amostras produzidas. Os resultados obtidos nesses testes mostraram que blocos de alvenaria leves podem ser produzidos usando esses materiais residuais em aplicações de construção.

Solidification of municipal solid waste incineration fly ash and immobilization of heavy metals using waste glass in alkaline activation system.

Hazardous heavy metals in Municipal Solid Waste Incineration (MSWI) fly ash are a threat to the environment and ecosystems. The objective of the work is to investigate the solidification of MSWI fly ash and the immobilization of the heavy metals through alkaline activation reaction with waste glass as an additive. Compressive strength measurement, X-ray diffraction (XRD), 29Si nuclear magnetic resonance spectroscopy (29Si NMR) and scanning electron microscope (SEM) were performed to evaluate the solidification effect and characterize the microstructure of alkali-activated MSWI fly ash-based mortars. The leaching test, back-scattered electron microscopy (BSE) and X-ray photoelectron spectroscopy (XPS) were conducted to determine the heavy metals' immobilization effect and their immobilization forms. It was found that waste glass addition effectively reinforced the solidification of MSWI fly ash and immobilized the heavy metals. With 40% addition of waste glass, the compressive strength reached a maximum of 3.55 MPa. The immobilization efficiency of Cr increased with the addition of waste glass, while that of Cu, Pb, Zn and Cd is dependent on the eluant final pH, which decreased with the decrease of eluant final pH. The main immobilization forms include physical encapsulation, the formation of alkaline environment and the generation of silicate compounds.

Effect of Cathodic Protection on Reinforced Concrete with Fly Ash Using Electrochemical Noise.

Corrosion of steel reinforcement is the major factor that limits the durability and serviceability performance of reinforced concrete structures. Impressed current cathodic protection (ICCP) is a widely used method to protect steel reinforcements against corrosion. This research aimed to study the effect of cathodic protection on reinforced concrete with fly ash using electrochemical noise (EN). Two types of reinforced concrete mixtures were manufactured; 100% Ordinary Portland Cement (OCP) and replacing 15% of cement using fly ash (OCPFA). The specimens were under-designed protected conditions (-1000 ≤ E ≤ -850 mV vs. Ag/AgCl) and cathodic overprotection (E < -1000 mV vs. Ag/AgCl) by impressed current, and specimens concrete were immersed in a 3.5 wt.% sodium chloride (NaCl) Solution. The analysis of electrochemical noise-time series showed that the mixtures microstructure influenced the corrosion process. Transients of uniform corrosion were observed in the specimens elaborated with (OPC), unlike those elaborated with (OPCFA). This phenomenon marked the difference in the concrete matrix's hydration products, preventing Cl- ions flow and showing passive current and potential transients in most specimens.

Resistance to Chemical Attack of Hybrid Fly Ash-Based Alkali-Activated Concretes.

The environmental impacts related to Portland cement production in terms of energy consumption, the massive use of natural resources and CO2 emissions have led to the search for alternative cementitious materials. Among these materials, alkali-activated cements based on fly ash (FA) have been considered for concrete production with greater sustainability. In the present article, the chemical durability properties (resistance to sulphates, chloride permeability, and resistance to carbonation) of a hybrid alkali-activated concrete based on fly ash-ordinary Portland cement (FA/OPC) with proportions of 80%/20% were evaluated. It is noted that the FA was a low-quality pozzolan with a high unburned carbon content (20.67%). The results indicated that FA/OPC concrete had good durability with respect to the OPC concrete, with 95% less expansion in the presence of sodium sulphate and a 2% strength loss at 1100 days, compared with the 56% strength loss of the OPC concrete. In addition, FA/OPC showed lower chloride permeability. On the contrary, the FA/OPC was more susceptible to carbonation. However, the residual compressive strength was 23 MPa at 360 days of CO2 exposure. Based on the results, FA/OPC, using this type of FA, can be used as a replacement for OPC in the presence of these aggressive agents in the service environment.

Effects of aluminum on the expansion and microstructure of alkali-activated MSWI fly ash-based pastes.

Alkaline activation is of great potential in the solidification of municipal solid waste incineration (MSWI) fly ash, but the metallic aluminum in the ash inhibits its application. This work studies the effects of residual metallic aluminum on the expansion and microstructure of alkali-activated MSWI fly ash-based pastes. Based on the results obtained, an optimized preparation process is suggested. Characterizations of the pastes include expansion ratio, morphology (SEM), mechanical strength and microstructure (XRD and FTIR). It is confirmed that MSWI fly ash could be solidified through alkaline activation when using a small amount of coal fly ash to adjust the reactive silica and aluminum ratios. In the optimized preparation, sodium hydroxide was added separately, so that expansion in the pastes was significantly mitigated, the formation of geopolymer gel was improved and the compressive strength of the pastes increased.

Effect of Silica Fume and Fly Ash Admixtures on the Corrosion Behavior of AISI 304 Embedded in Concrete Exposed in 3.5% NaCl Solution.

The use of supplementary cementitious materials such as fly ash, slag, and silica fume improve reinforced concrete corrosion performance, while decreasing cost and reducing environmental impact compared to ordinary Portland cement. In this study, the corrosion behavior of AISI 1018 carbon steel (CS) and AISI 304 stainless steel (SS) reinforcements was studied for 365 days. Three different concrete mixtures were tested: 100% CPC (composite Portland cement), 80% CPC and 20% silica fume (SF), and 80% CPC and 20% fly ash (FA). The concrete mixtures were designed according to the ACI 211.1 standard. The reinforced concrete specimens were immersed in a 3.5 wt.% NaCl test solution to simulate a marine environment. Corrosion monitoring was evaluated using the corrosion potential (Ecorr) according to ASTM C876 and the linear polarization resistance (LPR) according to ASTM G59. The results show that AISI 304 SS reinforcements yielded the best corrosion behavior, with Ecorr values mainly pertaining to the region of 10% probability of corrosion, and corrosion current density (icorr) values indicating passivity after 105 days of experimentation and low probability of corrosion for the remainder of the test period.

Long-term leaching of As, Cd, Mo, Pb, and Zn from coal fly ash in column test.

Globally, millions of tons of coal fly ash (CFA) are generated per year, and the majority of this material is usually stored in stock piles or landfills, and in a long-term, it can be an environmental hazard if rainwater infiltrates the ashes. Long-term leaching studies of Brazilian ashes are scarce. The purpose of this study was to evaluate arsenic, cadmium, molybdenum, lead, and zinc leaching behavior from a Brazilian CFA by a column experiment designed to simulate field conditions: slightly acid rain considering seasonality of precipitation and temperature for a long-term leaching period (336 days). All elements were leached from CFA, except lead. Elements leaching behavior was influenced by leaching time, leaching volume, and temperature. Higher leachability of As and Cd from CFA during warm and wet season was observed. Results indicate a potential risk to soil and groundwater, since ashes are usually stored in uncovered fields on power plants vicinity.

Development and Characterization of Glass-Ceramics from Combinations of Slag, Fly Ash, and Glass Cullet without Adding Nucleating Agents.

Developments in the field of materials science are contributing to providing solutions for the recycling of industrial residues to develop new materials. Such approaches generate new products and provide optimal alternatives to the final disposal of different types of industrial wastes. This research focused on identifying and characterizing slag, fly ash, and glass cullet from the Boyacá region in Colombia as raw materials for producing glass-ceramics, with the innovative aspect of the use of these three residues without the addition of nucleating agents to produce the glass-ceramics. To characterize the starting materials, X-ray diffraction (XRD), X-ray fluorescence (XRF), and Scanning Electron Microscopy (SEM) techniques were used. The results were used to evaluate the best conditions to produce mixtures of the three waste components and to determine the specific compositions of glass-ceramics to achieve products with attractive technical properties for potential industrial applications. The proposed mixtures were based on three compositions: Mixture 1, 2, and 3. The materials were obtained through thermal treatment at 1200 °C in a tubular furnace in accordance with the results of a comprehensive characterization using thermal analysis. The microstructure, thermal stability, and structural characteristics of the samples were examined through SEM, differential thermal analysis (DTA), and XRD analyses, which showed that the main crystalline phases were diopside and anorthite, with a small amount of enstatite and gehlenite. The obtained glass-ceramics showed properties of technical significance for structural applications.

Use of anaerobic co-digestion as an alternative to add value to sugarcane biorefinery wastes.

In this study the anaerobic co-digestion (AcD) of sugarcane biorefinery by-products, i.e. hemicelluloses hydrolysate (HH) (obtained by hydrothermal pretreatment of sugarcane bagasse), vinasse, yeast extract (YE) and sugarcane bagasse fly ashes (SBFA), was optimized by means of biochemical methane potential experiments. The best experimental conditions of AcD (25-75% HH-to-vinasse mixture ratio; 1.0 g L-1 YE; 15 g L-1 SBFA and 100-0% HH-to-Vinasse; 1.5 g L-1 YE; 45 g L-1 SBFA) led to the production of 0.279 and 0.267 Nm3 of CH4 per kg of chemical oxygen demand (COD) with an energy surplus of 0.43 and 0.34 MJ kg SB-1, respectively. Adsorption experiments using SBFA were carried out and showed this residue could adsorb up to 61.71 and 17.32 mg g-1 of 5-hydroxymethyl-2-furfuraldehyde and 2-furfuraldehyde, thereby reducing toxicity and improving biogas production.

Synthesis of coal fly ash zeolite for the catalytic wet peroxide oxidation of Orange II.

Fly ash, a coal combustion residue produced by Termotasajero in Colombia, has been hydrothermally treated after an alkaline fusion to produce zeolite without addition of silicon or aluminum. The starting material was thoroughly mixed with NaOH, in a 1:1.2 mass ratio, to obtain a homogeneous mixture that was heated to 100 °C during different times (6, 8, and 10 h) and three zeolite samples were produced. The samples were characterized by XRD, SEM, XRF, Mössbauer spectroscopy, and N2 physisorption. According to characterization results (high surface area and appropriate morphological properties including crystallinity) and synthesis time, zeolitic catalyst synthesized with 8 h of hydrothermal treatment was selected to perform further analysis. This sample consisted of a mixture of zeolite X and zeolite A of high surface area (301 m2 g-1) and a Fe content of 6% wt/wt. The zeolite was used as a catalyst for the Fenton oxidation of Orange II. Experiments were performed in a laboratory batch reactor at 70 °C and constant pH = 3, using different concentrations of H2O2. When the stoichiometric amount of H2O2 was used, good mineralization (XTOC = 45%), complete discoloration, and oxidant consumption were obtained after 240 min of reaction. The sample retained activity after 16 h of usage. The presence of Fe in the reaction media was always detected and a homogeneous Fenton mechanism induced by surface-leached iron is suggested.

Síntese hidrotérmica de zeólitas a partir de cinza volante de carvão mineral: aplicação na adsorção de íon amônio / Hydrothermal synthesis of zeolites from fly ash of mineral coal: application of ion ammonium adsorption

RESUMO A cinza volante é o principal resíduo industrial do uso de carvão mineral na geração de vapor e energia. No Brasil, são produzidas 1,4 milhão toneladas ao ano. Essas cinzas podem ser convertidas em produtos zeolíticos por tratamento hidrotérmico alcalino. Este trabalho teve como objetivo principal realizar essa conversão hidrotérmica, a fim de obter unicamente fases cristalinas zeolíticas para a adsorção de íon amônio. Realizaram-se diversas sínteses alterando o método utilizado (clássico ou de duas fases), o tempo de reação (24 ou 30 h) e a massa de NaOH. A caracterização dos produtos e da cinza (in natura e calcinada) foi realizada por difratometria de raios X, microscopia eletrônica de varredura e, em alguns casos, análises térmica diferencial e gravimétrica (ATD-TG). Os resultados demonstraram que é possível sintetizar as zeólitas hidroxissodalita e cancrinita a partir da cinza estudada. O produto obtido pelo método de duas etapas foi utilizado na adsorção de íon amônio em solução, sendo, neste processo, o modelo isotérmico de Sips o mais adequado; alcançando um valor de capacidade máxima de adsorção de 2,71 mg.g-1.

ABSTRACT Fly ash is the main industrial waste generated by coal in steam and power plans. In Brazil, 1,4 million tons are produced every year. These ashes can be converted into zeolite products by alkaline hydrothermal treatment. The main objective of this paper was to induce this reaction which produces only zeolitic crystalline phases for ammonium ion absorption. So, some syntheses were done by different hydrothermal method (classical or two stages), work time (24 or 30 h) and many NaOH bulks. The characterization of ash (in natura and calcined) and products was performed by X-ray diffraction method, scanning electron microscopy and, in some cases, differential thermal and thermogravimetric analysis The product by two stages method was used in ammonium ion absorption in solution at Sips Mathematic Model: the highest capacity of 2,71 mg.g-1.