Caracterização de cinzas de biomassa e potencial de aplicação como material pozolânico ou ativador alcalino
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Data
2022-04-05
Autores
Assis, Letícia Freitas
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Universidade Estadual Paulista (Unesp)
Resumo
O concreto é o material artificial mais utilizado no mundo (10 Gt/ano). Nos primeiros onze meses de 2021, o consumo mundial de cimento aumentou 3,6% em relação a 2020. Estima-se que a fabricação do cimento é responsável por 8% das emissões antropogênicas globais de CO2, consome 85 kWh de energia elétrica e 3,1 GJ de energia térmica por tonelada de cimento. Quase 30% da emissão de CO2 já foi reduzida nos últimos 25 anos por meio da adoção de fornos mais eficientes, e, portanto, existe pouca margem para melhorias adicionais. Visando o desenvolvimento sustentável a indústria cimenteira pode utilizar material pozolânico para fabricação de cimentos com menores proporções de clínquer e, consequentemente, reduzir o consumo de energia e emissões de CO2. Paralelo a isso, existe a possibilidade da utilização de um aglomerante alternativo que não utiliza o cimento Portland, denominado aglomerante ativado alcalinamente (AAA). Estudos feitos com resíduos agroindustriais (cinzas de biomassa) têm apresentado resultados satisfatórios e são amplamente disponíveis já que grandes quantidades de resíduos são geradas. Nesse contexto, esta pesquisa propõe estudar algumas cinzas de biomassa, e seus potenciais pozolânicos e alcalinos. Para tal, resíduos da agroindústria (palha de milho, casca de soja, casca de café, casca de laranja, casca de banana, folha de bananeira e folha de bambu) foram calcinados a 600 °C durante 1 hora em mufla, foram moídos e peneirados na peneira 0,75 mm e denominadas cinzas teste. Então, as cinzas teste e as cinzas parâmetro (cinza da casca de arroz e cinza do bagaço de cana) foram caracterizadas por meio da Espectrometria de Fluorescência de Raios X (FRX) e Difração de Raios X (DRX). Após a caracterização das cinzas, objetivando analisar a reatividade pozolânica e o potencial alcalino, foram preparadas suspensões com seis diferentes proporções de cal/cinza (2,0:8,0; 2,5:7,5; 3,0:7,0; 3,5:6,5: 4,0:6,0 e 4,5:5,5), e estas foram monitoradas através de medidas de condutividade elétrica e pH a 60 ºC, durante 7 dias consecutivos. Com isto, observou-se uma redução nos valores de pH e condutividade elétrica, no caso das cinzas com propriedades pozolânicas, e um aumento, no caso das cinzas com potencial alcalino. A cinza da folha de bambu apresentou comportamento pozolânico. A cinza da casca de café e de banana foram capazes de atuar como ativador alcalino em argamassas, proporcionando resistência média à compressão comparável às obtidas com as argamassas ativadas com reagentes químicos comerciais. A cinza da casca de café proporcionou 34,27 MPa de resistência à compressão, isso corresponde ao aumento de 181,96% quando comparada à resistência atingida pela argamassa padrão sem ativador e 8,53% comparada à argamassa ativada com hidróxido de sódio. Já a cinza da casca de banana, gerou 47,61 MPa de resistência à compressão, que significa um aumento de 16,31% em relação a argamassa padrão ativada com hidróxido de potássio, 50,80% em relação a argamassa ativada com hidróxido de sódio e 291,76% comparada a argamassa controle sem ativador alcalino. A análise de pH e condutividade elétrica das cinzas da casca de café e da casca de banana demonstrou que a liberação de álcalis foi quase instantânea, indicando que os álcalis ficam disponíveis imediatamente para que a cinza atue como ativador alcalino. Sendo assim, foi concluído que a cinza da folha de bambu é uma cinza pozolânica e as cinzas da casca de café e da casca de banana são cinzas que podem ser utilizadas como ativador alcalino alternativo na elaboração de argamassas e concreto.
Concrete is the most used artificial material globally (10 Gt/year). In the first eleven months of 2021, world cement consumption increased by 3.6% compared to 2020. It is estimated that cement manufacturing is responsible for 8% of global anthropogenic CO2 emissions, consumes 85 kWh of electricity, and 3 .1 GJ of thermal energy per ton of cement. Almost 30% of CO2 emissions have already been reduced in the last 25 years through the adoption of more efficient ovens, and therefore there is little scope for further improvement. Aiming at sustainable development, the cement industry can use pozzolanic material to manufacture cement with lower proportions of clinker and, consequently, reduce energy consumption and CO2 emissions. At the same time, there is the possibility of using an alternative binder that does not use Portland cement, called alkaline activated binder (AAA). Studies carried out with agro-industrial residues (biomass ash) have shown satisfactory results and are widely available since large amounts of residues are generated. In this context, this research proposes to study some biomass ashes and their pozzolanic and alkaline potentials. For this purpose, agro-industry residues (corn husk, soybean hulls, coffee husks, orange peels, banana peels, banana leaves, and bamboo leaves) were calcined at 600 °C for 1 hour in a muffle furnace, ground, and sieved in a 0.75 mm sieve and called test ash. Then, the test ash and the parameter ash (rice husk ash and sugarcane bagasse ash) were characterized using X-Ray Fluorescence Spectrometry (FRX) and X-Ray Diffraction (XRD). After the ashes characterization, aiming to analyze the pozzolanic reactivity and the alkaline potential, suspensions were prepared with six different proportions of lime/ash (2.0:8.0; 2.5:7.5; 3.0:7, 0; 3.5:6.5: 4.0:6.0 and 4.5:5.5), and these were monitored through measurements of electrical conductivity and pH at 60 ºC, for 7 consecutive days. It was observed a reduction in the values of pH and electrical conductivity, in the case of ashes with pozzolanic properties, and an increase, in the case of ashes with alkaline potential. Bamboo leaf ash showed pozzolanic behavior. Coffee and banana peel ashes were able to act as an alkaline activator in mortars, providing average compressive strength comparable to those obtained with mortars activated with commercial chemical reagents. The coffee husk ash provided 34.27 MPa of compressive strength, which corresponds to an increase of 181.96% when compared to the strength achieved by the standard mortar without an activator and 8.53% compared to the mortar activated with sodium hydroxide. The banana peel ash generated 47.61 MPa of compressive strength, which means an increase of 16.31% in relation to standard mortar activated with potassium hydroxide, 50.80% in relation to mortar activated with sodium hydroxide, and 291.76% compared to control mortar without an alkaline activator. The analysis of pH and electrical conductivity of ashes from coffee husk and banana peel showed that the release of alkali was almost instantaneous, indicating that the alkali is immediately available for the ash to act as an alkaline activator. Therefore, it was concluded that the bamboo leaf ash is a pozzolanic ash and the coffee husk ash and the banana peel ash can be used as alternative alkaline activators in the preparation of mortars and concrete.
Concrete is the most used artificial material globally (10 Gt/year). In the first eleven months of 2021, world cement consumption increased by 3.6% compared to 2020. It is estimated that cement manufacturing is responsible for 8% of global anthropogenic CO2 emissions, consumes 85 kWh of electricity, and 3 .1 GJ of thermal energy per ton of cement. Almost 30% of CO2 emissions have already been reduced in the last 25 years through the adoption of more efficient ovens, and therefore there is little scope for further improvement. Aiming at sustainable development, the cement industry can use pozzolanic material to manufacture cement with lower proportions of clinker and, consequently, reduce energy consumption and CO2 emissions. At the same time, there is the possibility of using an alternative binder that does not use Portland cement, called alkaline activated binder (AAA). Studies carried out with agro-industrial residues (biomass ash) have shown satisfactory results and are widely available since large amounts of residues are generated. In this context, this research proposes to study some biomass ashes and their pozzolanic and alkaline potentials. For this purpose, agro-industry residues (corn husk, soybean hulls, coffee husks, orange peels, banana peels, banana leaves, and bamboo leaves) were calcined at 600 °C for 1 hour in a muffle furnace, ground, and sieved in a 0.75 mm sieve and called test ash. Then, the test ash and the parameter ash (rice husk ash and sugarcane bagasse ash) were characterized using X-Ray Fluorescence Spectrometry (FRX) and X-Ray Diffraction (XRD). After the ashes characterization, aiming to analyze the pozzolanic reactivity and the alkaline potential, suspensions were prepared with six different proportions of lime/ash (2.0:8.0; 2.5:7.5; 3.0:7, 0; 3.5:6.5: 4.0:6.0 and 4.5:5.5), and these were monitored through measurements of electrical conductivity and pH at 60 ºC, for 7 consecutive days. It was observed a reduction in the values of pH and electrical conductivity, in the case of ashes with pozzolanic properties, and an increase, in the case of ashes with alkaline potential. Bamboo leaf ash showed pozzolanic behavior. Coffee and banana peel ashes were able to act as an alkaline activator in mortars, providing average compressive strength comparable to those obtained with mortars activated with commercial chemical reagents. The coffee husk ash provided 34.27 MPa of compressive strength, which corresponds to an increase of 181.96% when compared to the strength achieved by the standard mortar without an activator and 8.53% compared to the mortar activated with sodium hydroxide. The banana peel ash generated 47.61 MPa of compressive strength, which means an increase of 16.31% in relation to standard mortar activated with potassium hydroxide, 50.80% in relation to mortar activated with sodium hydroxide, and 291.76% compared to control mortar without an alkaline activator. The analysis of pH and electrical conductivity of ashes from coffee husk and banana peel showed that the release of alkali was almost instantaneous, indicating that the alkali is immediately available for the ash to act as an alkaline activator. Therefore, it was concluded that the bamboo leaf ash is a pozzolanic ash and the coffee husk ash and the banana peel ash can be used as alternative alkaline activators in the preparation of mortars and concrete.
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Palavras-chave
Caracterização, Resíduos agroindustriais, Materiais pozolânicos, Ativação alcalina, Condutividade elétrica e pH, Characterization, Agro-industrial waste, Pozzolanic materials, Alkaline activation, Electrical conductivity and pH