Contributions to the study of porosity in fly ash-based geopolymers. Relationship between degree of reaction, porosity and compressive strength

The main contribution of this paper relates to the development of a systematic study involving a set of parameters which could potentially have an impact on geopolymer properties: curing temperature, type of activating solution, alkali metal in solution, incorporation of slag (Ca source) and type of slag used. The microstructures, degrees of reaction, porosities and compressive strengths of geopolymers have been evaluated. Geopolymers prepared with soluble silicate presented a more compacted and closed structure, a larger amount of gel, lower porosity and greater compressive strength than those prepared with hydroxides. On the other hand, Na-geopolymers were more porous but more resistant than K-geopolymers. Although there is an inverse relation between degree of reaction and porosity, between compressive strength and porosity it is not always inversely proportional and could, in some cases, be masked by changes produced in other influencing parameters.Contribuciones al estudio de la porosidad de geopolímeros basados en cenizas volantes. Relación entre grado de reacción, porosidad y resistencia a compresión. La principal contribución de este documento es el desarrollo de un estudio sistemático implicando una serie de parámetros que podrían afectar a las propiedades de los geopolímeros: temperatura de curado, solución activadora, metal alcalino de la solución, incorporación de escorias (fuente de calcio) y tipo de escorias. Se han evaluado: microestructura, grado de reacción, porosidad y resistencia a compresión. Los geopolímeros preparados con silicatos presentaron un microestructura más densa y compacta, una mayor cantidad del gel geopolimérico, menor porosidad y mejores propiedades mecánicas que los preparados con hidróxidos. Los geopolímeros preparados con sales de sodio fueron más porosos pero más resistentes que los preparados con sales potasio. Aunque existe una relación inversa entre el grado de reacción y la porosidad, en algunos casos, la relación entre resistencia y porosidad es inexistente ya que puede estar enmascarada por cambios producidos por otros parámetros que afecten a la reacción.Ministerio de Ciencia y Tecnología CTM2010-1991

Tratamento de lixiviado de aterro sanitário com carvão ativado obtido de resíduos de café

A set of experiments were carried out in order to establish and evaluate the potential of activated carbon, produced from coffee waste in adsorption process, in the depuration of landfill leachate. Different reagents were studied in the activation of carbon: HCl, HCl + H2O2, H3PO4, H3PO4 + H2O2, all with an impregnation rate of 1:1. The activated carbon that showed the best global results was activated with H3PO4, obtaining a 51.0, 32.8, 66.0, 81.0 and 97.1% elimination of chemical oxygen demand, ammonia, total chlorine, bromine and copper, respectively. This activated carbon has a total pore area of 4.85 m2/g and a median pore diameter of 65.32 micrometers. When different loads of this carbon were placed in a stirrer system in contact with landfill leachate, with the aim of evaluating the effect of the adsorption load and contact time, the concentration of ammonia decreased from the beginning of the adsorption process to the end of it, and the removal of ammonia increased with the increase in the adsorbent load. However, the trend of the amount adsorbed per unit mass decreased with increased dosage. The model Freundlich equilibrium isotherm fits experimental data adequately, giving R2 values of 0.95, 1/n of 0.5183, and a K value of 7.08*10-5 L/g, being favourable for adsorption process.Foi realizada uma série de experimentos para estabelecer e avaliar o potencial de carvão ativado produzido de resíduos de café por meio de um processo de adsorção na depuração do lixiviado de aterro sanitário. Foram estudados diferentes reagentes para a ativação do carvão: HCl, HCl + H2O2, H3PO4, H3PO4 + H2O2, todos com taxa de impregnação de 1:1. O carvão que apresentou os melhores resultados foi ativado com H3PO4, obtendo uma eliminação de 51,0; 32,8; 66,0; 81,0; e 97,1% de demanda química de oxigênio e concentração de amônia, cloro total, bromo e cobre, respetivamente. O referido carvão ativado tem uma área total de poro de 4,85 m2/g e diâmetro médio de poro de 65,32 micrômetros. Com o objetivo de avaliar o efeito da carga de adsorção e do tempo de contato, foram colocadas diferentes cargas desse carvão em um sistema de agitação em contato com o lixiviado de aterro sanitário. Observou-se que a concentração de amônia diminuiu e que sua remoção foi maior à medida que a carga adsorvente aumentava. No entanto, a quantidade adsorvida por unidade de massa diminuiu com o aumento da dosagem. Os dados experimentais adequaram-se corretamente ao modelo de isoterma de Freundlich, obtendo-se valores de R2 de 0,95, 1/n de 0,5183, e um valor de K de 7,08*10-5 L/g, considerados favoráveis para o processo de adsorção.Ministerio de Educación (Chile) 1114021

Landfill leachate treatment using combined fish scales based activated carbon and solar advanced oxidation processes

The depuration of landfill leachate (LL) was evaluated by solar advanced oxidation processes (AOPs) (photo-Fenton + O3 and photo-Fenton + O2) and adsorption processes, using activated carbon made from fish scales, as either post-treatment or pre-treatment of advanced oxidation process. Also, the effect in the reduction of toxicity of LL depurated under these methods was evaluated. The activated carbon Brunauer-Emmett-Teller (BET) surface area is 1.8329 m2/g and the adsorption average pore width is 12.79833 nm, considered as a mesoporous material. When AOPs were used as treatment and adsorption was used as post-treatment of the LL, a removal of 67%, 98%, 98.9%, 83.3% and 99.6% of chemical oxygen demand (COD), colour, copper, iron and ABS254, respectively, was obtained. When the adsorption process was applied as AOP pre-treatment, the removal of organic matter was better in comparison to the results of the adsorption process as AOPs post-treatment. This resulted in a removal of COD, colour, copper, iron and ABS254 of 75.4%, 99.4%, 94.4%, 68.3% and 98.1%, respectively. When the AOPs were followed the adsorption process, a germination index (GI) of 126%, 122% and 116% was obtained. This was higher than the results obtained by LL treated with PF + O2 in a solution concentration of 1%, 3% and 10%, respectively. When PF + O3 was used at a concentration solution of 1%, 3% and 10%, similar GI values were obtained (122%, 113% and 114%, respectively). The EC50 is very low for raw LL, but this value increases as the LL is treated. A very high EC50 is obtained for the adsorption process and for AOPs following the adsorption process, going from 1.8 to 21.0 V/V. Adsorption as pre-treatment helps removing colour and metals from LL, thus enhancing the performance of solar AOPs

The Incorporation of Ladle Furnace Slag in Fire Insulating Gypsum-Based Materials

Ladle slag, a byproduct of steel manufacturing, exhibits inherent reactivity and undergoes hydration when exposed to water. Nevertheless, these reaction byproducts often remain metastable, leading to microstructural alterations when incorporated into cementitious materials, thereby limiting the recycling potential of ladle slag. This study explores the fire insulating capacity and the physical, mechanical, and leaching characteristics of gypsum-based materials with substantial quantities of ladle slag in instead of gypsum. The mechanical strength of the specimens declines as the ladle slag content increases. Nevertheless, the percentage decrease in compressive strength at various temperatures (300 °C, 500 °C, and 700 °C) is less pronounced when higher amounts of ladle slag are used. Fire-resistant properties, assessed using the EN 1363-1 standards, diminish with increasing slag proportions; although the inclusion of ladle slag introduces certain endothermic processes that positively affect the fire insulating capacity, resulting in a 20% reduction when 60%wt of slag is employed. Notably, no gas emissions were observed during the fire test, indicating the absence of environmental hazards. In conclusion, ladle slag does not pose a leaching threat to the environment, making it a viable and sustainable alternative to gypsum in gypsum-based materials

Fly ash based geopolymeric foams using silica fume as pore generation agent. Physical, mechanical and acoustic properties

Authors would like acknowledge the help received by CITIUS (General Research Services) from University of Seville, especially the XRD laboratory.The aim of this work is the development of a porous geopolymeric foam with sound absorbing properties using silica fume as the pore generation agent. The samples were manufactured using a coal combustion fly ash as source material for the geopolymerization reaction, an alkaline solution as activating solution and silica fume as pore forming agent. Three parameters were studied: silica fume proportion (0, 20 and 40 wt%), activating solution (potassium silicate and potassium hydroxide) and setting temperature (40 and 70 °C). Once the samples were prepared, the experimental study of the most important physical, mechanical and acoustic features were carried out. The increase in the proportion of silica fume in the mixture and setting temperature produced a reduction in setting time and a raise of open void porosity, reducing the compressive strength at 28 days up to a half but increasing the sound absorption. The influence of activating solution in open porosity and sound absorbing properties was not very important

Propiedades de paneles geopoliméricos basados en ceniza volante y metacaolín bajo ensayos de resistencia al fuego

This paper presents the results of a study about the effect of fire on geopolymer paste composed of fly ashes, metakaolin and sodium silicate. 2 cm thick, 28 cm high and 18 cm wide panels were filled with the paste obtained. After 28 days of curing at 20 °C and 45% of relative humidity, different tests were carried out in the geopolymers: physico-chemical (density, water absorption, porosity), mechanical (flexural and compressive strength), fire resistance and environmental (leaching and radioactivity). The panels manufactured have been compared with other commercial panels in order to determine the recycling possibilities of fly ashes in manufacturing new fire-insulating geopolymers. The panels obtained can be utilized for the production of interior wall materials, with a good physical, mechanical, fire resistant properties without any environmental problem.Este documento presenta los resultados de un estudio sobre el efecto del fuego sobre pastas de geopolímeros compuestas de cenizas volantes, metacaolín y silicato sódico. Con la pasta obtenida se han rellenado paneles de dimensiones 2 cm de espesor, 28 cm de altura y 18 cm de ancho. Tras 28 días de curado a 20 °C y un 45% de humedad relativa, diferentes ensayos fueron realizados en los geopolímeros obtenidos: fisicoquímicos (densidad, absorción de agua, porosidad), mecánicos (resistencia a compresión y a flexión), de resistencia al fuego y medioambientales (lixiviación y radioactividad). Los paneles fabricados han sido comparados con paneles comerciales para determinar las posibilidades de reciclaje de las cenizas volantes para la fabricación de nuevos productos geopoliméricos con propiedades aislantes al fuego. Los paneles obtenidos pueden ser utilizados para la producción de paredes interiores, con buenas propiedades físicas, mecánicas y de resistencia al fuego sin ningún problema medioambiental

A porous geopolymer based on aluminum-waste with acoustic properties

Paval, a solid waste stream from the aluminum industry, is used as a pore generation agent in geopolymers. Paval was mixed with coal combustion fly ash, as a geopolymeric precursor, and activated with alkaline solution with the aim of obtaining porous geopolymers to be used as noise barriers. Both geopolymeric and pore generation reactions happen simultaneously. Aluminum from Paval can react with water and OH from the geopolymerization activating solution, producing hydrogen. The hydrogen gas released generates a highly porous material. The influence of the fly ash-paval proportion and the setting temperature on open porosity, compressive strength and noise-absorbing properties were evaluated. To better understand these influences, the setting time, volume expansion and mineral composition were also studied. The obtained results showed that a higher Paval content (fly ash-Paval ratio 50:50) and setting temperature (70 C) produced a lower setting time and higher volume expansion, increasing the open porosity and improving acoustic properties, but reducing the compressive strength. The material manufactured under these conditions showed similar amorphous phase content to the non-porous geopolymers made without Paval. On the other hand, the obtained materials did not raise environmental concerns in a normalised leaching test

Valorization of Bayer Red Mud in a Circular Economy Process: Valuable Metals Recovery and Further Brick Manufacture

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).In this work, the recovery of valuable metals from Bayer red mud using hydrometallurgical techniques and the subsequent use of the solid remaining after leaching as the principal component of the fired bricks were analyzed. Water, sulfuric acid, and sodium hydroxide were used as leaching agents. Different L/S ratios and contact times were also tested. According to technical, economic, and environmental considerations, the optimal conditions to recover valuable elements from red mud were 2 M H2SO4, in contact for 24 h, with an L/S ratio = 5. Under these conditions, high leaching yields of valuable elements such as La (47.6%) or V (11%) were achieved. After the leaching process, the remaining solid was mixed with clay and water to produce bricks. Two doses of red mud (50 and 80% w) and two different sintering temperatures (900 and 1100 °C) were tested. When the proportion of treated RM in the mix was increased, the compressive strength of the bricks was reduced, but it was increased as the sintering temperature was increased. The environmental safety of the bricks manufactured (leaching of heavy metals and radionuclides) was also studied, and it was found that it was more favorable when red mud was treated instead of fresh red mud being used

Environmental risks and mechanical evaluation of recycling red mud in bricks

More and more by-products are being used in certain materials, especially in the construction industry. Natural construction materials contain amounts of heavy metals and radionuclides, but when by-products are used in these kinds of materials, this could lead to a growth in their concentrations and have a negative impact on public health.In this paper, red mud was used as a raw material (as a clay substitute) to manufacture fired bricks. Physical, mechanical, radiological and heavy metal leaching properties of fired bricks with a replacement ratio of up to 80 wt% of clay to red mud are discussed. In addition, the effect of different sintering temperatures (1173K and 1373K) was analyzed, and results showed that the higher the temperature produced, the higher the mechanical strength.To environmentally characterize materials, they were subjected to two different leaching tests: a batch test for raw materials and a monolithic test for the bricks, respectively. The results obtained were compared with the limits stated for several heavy metals by the European Landfill Directive. Results showed that red mud gives leachate concentration values for Cr higher than the limits stated for non-hazardous by-products. Bricks do not exhibit the same problem in the samples containing a high RM proportion and manufactured at a low sintering temperature (1173K), although in the case of V, a high concentration is observed.The contents of radionuclides such as Ra-220, Th-232 and K-40 of the final construction materials were analyzed and compared with different indexes. This paper indicates the maximum amounts of RM that can be used to replace clay for the manufacture of fired bricks without environmental risk

Development of fly ash-based geopolymers using powder sodium silicate activator

Geopolymer manufacturing by means a ready-mix procedure using powder sodium silicate as activator is described in this paper. Its characteristics are compared with conventional geopolymers using aqueous sodium silicate. Degree of reaction, mercury intrusion porosity and compressive strength were measured at different curing times. Three durability tests were carried out. The geopolymer obtained using powder activator showed slightly lower mechanical properties and better durability results than those found for geopolymers based on aqueous activators. In addition, the manufacture of geopolymers based on powder silicate was easier and simpler than the more common procedure using sodium silicate solutions, so the ready-mix manufacturing could be an alternative to produce geopolymers.Junta de Andalucía US.20-1