Revisão teórica sobre o potencial da adição do óxido de grafeno em materiais cimentícios

O óxido de grafeno tem sido apontado como um material de reforço para compósitos de cimento. Além de ser um material com boa dispersão em água, ele apresenta elevado módulo de elasticidade, elevada resistência à tração, elevada superfície específica e alta condutividade térmica e elétrica. Visando aprofundar o conhecimento acerca do uso do óxido de grafeno em compósitos de cimento, o presente trabalho apresenta uma ampla revisão bibliográfica sobre os efeitos da adição do óxido de grafeno em compósitos de cimento Portland. A metodologia empregada para a seleção dos artigos acadêmicos foi o ProKnow-C, (Knowledge Development Process – Constructivist), ferramenta de revisão literária que permite a evidenciação do estágio atual do conhecimento acadêmico relacionado ao tema de estudo. Por meio desta metodologia de busca e seleção de artigos, foi possível selecionar, a partir de uma amostra bruta de 1354 artigos, os 47 artigos mais relevantes que apresentam as alterações nas propriedades mecânicas, no comportamento reológico e na microestrutura de compósitos de cimento com adição de óxido de grafeno, bem como técnicas de dispersão utilizadas. Dessa forma, as principais conclusões, as lacunas atuais de pesquisa e as oportunidades de estudo foram sumarizadas, constituindo uma base de referência para orientar trabalhos futuros envolvendo o uso de óxido de grafeno para a preparação de compósitos de cimento.Palavras-chave: Óxido de grafeno. Cimento. Técnicas de Dispersão. Microestrutura. Propriedades mecânicas e reológicas.

Synthesis and characterization of portland cement manufactured with carbon nanotubes

Exportado OPUSMade available in DSpace on 2019-08-11T01:08:39Z (GMT). No. of bitstreams: 1 33.pdf: 10953929 bytes, checksum: 3f414196045e0270633e0bbf0affe7f3 (MD5) Previous issue date: 7O cimento Portland (PC) é um dos produtos mais consumidos no mundo. Seus derivados (concreto, argamassa, pasta) apresentam características satisfatórias quanto à compressão, entretanto o mesmo não ocorre com relação à tração. Os nanotubos de carbono (NTCs) possuem elevada resistência à tração, sendo deste modo candidatos para reforçar estruturalmente materiais cimentícicos. Várias tentativas foram realizadas no mundo para desenvolver processos envolvendo a produção de compósitos a partir da mistura física de cimento e de nanotubos de alta qualidade. Atualmente estes processos são ainda inviáveis para produzir material de construção em grande escala. Os problemas a isto associados estão relacionados à escala e custo de produção, além da dispersão e ligação dos nanotubos na matriz de cimento. Para tentar resolver estes problemas, neste trabalho foi desenvolvido um processo de síntese in-situ de nanotubos e nanofibras de carbono em clínquer e sílica ativa. Além disso, resíduos da siderurgia como carepa de laminação de aço e pó de aciaria foram utilizados para melhorara as características dos produtos. Os produtos da síntese foram caracterizados por microscopia eletrônica de varredura, por análise termogravimétrica e por resíduo por queima. Estes produtos apresentaram grande heterogeneidade em morfologia. Foi desenvolvido também um processo de funcionalização in-situ dos nanotubos via amônia. Os materiais nano-estruturados foram adicionados aos cimentos CP-III e CP-V em uma concentração de 0,3 % para realização de análises físico-químicas convencionais de cimento. O tempo de pega apresentou um leve aumento no cimento CP-V, mas os demais parâmetros não sofreram alterações significativas pela adição de clínquer nano-estruturado. Argamassas foram preparadas para testar as resistências à compressão e à tração dos compósitos, este último por flexão ou por compressão diametral. Aumentos nas resistências à compressão e à tração foram observados em argamassas preparados com 0,3 % de nanotubos em relação ao peso do cimento, e com aditivos plastificantes a base de policarboxilato e polinaftaleno além de lignosulfonato. Resultados promissores também foram obtidos com o uso de peróxido de hidrogênio como agente de funcionalização. A adição de sílica ativa nano-estruturada também provocou aumento de resistência mecânica dos compósitos. Análises por BET e por picnometria a hélio mostraram aumento da área superficial específica e redução dos diâmetros dos poros dos compósitos.Portland cement (PC) is one of the most consumed products of the world. Its derivates (concrete, mortar, paste) have good compressive characteristics, but on the other hand have poor tensile behavior. Carbon nanotubes have exceptionally high tensile strength and are therefore candidates for structural reinforcement of cement materials. Many tentative have been reported to develop composites with the physical mixture of high quality nanotubes and cement. These processes today are still unviable for large scale production of construction material. The problems are linked to the scale and costs of production and the dispersion and bond of the nanotubes to the cement matrix. In order to solve these problems in present work an in-situ synthesis process was developed to produce nanotubes and nanofibers on clinker and silica fume particles. Steelmaking by-products, such as steel mill scale and converter dust were also added to improve product characteristics. The synthesis products were characterized by scanning electron microscopy, thermogravimetric analysis and loss on ignition. The products showed highly heterogeneous morphology. An in-situ functionalization process was also developed based on ammonia. The nano-structured materials were added to Brazilian CP-III and CP-V type cements in 0.3 % concentration to perform common physical and chemical cement analysis. Setting time of CP-V suffered a slight delay, but other characteristics were not altered significantly after the addition of nano-structured clinker. Mortars were prepared in order to determine compressive and flexural or splitting tensile strength of the composites.Gains in the compressive and tensile strengths were observed of mortars incorporating 0.3 % nanotubes prepared with a combined polycarboxylate and polynaphtalene and a lignosulfonate based plasticizer. Positive results were also observed with the use of hydrogen peroxide as functionalizing agent. The addition of nano-structured silica fume also resulted in increase of the mechanical strength of the composites. BET and helium pycnometry analysis of the mortars showed an increase in specific surface area and reduction of mean pore diameter of the composites

PREPARAÇÃO E CARACTERIZAÇÃO DE NANOCOMPÓSITOS COM NANOTUBOS DE CARBONO DISPERSOS EM SURFACTANTES

A dispersão efetiva dos NTCs para preparação de compósitos de cimento é crucial para o alcance de ganhos de resistência. O comportamento hidrofóbico desse nanomaterial faz com que ele se aglomere em presença de água o que leva a formação de grânulos. Pesquisas envolvendo incorporação de NTCs em pastas de cimento têm sendo desenvolvidas, e o presente trabalho realizou experimentos de análise de dispersão em meio aquoso e preparação e caracterização de pastas de cimento com NTCs. Como resultado, foi observado que os NTCs se mostram dispersos em meio aquoso na presença de aditivos plastificantes a base de lignosulfonato e policarboxilato. No entanto, as pastas de cimento preparadas por esse método de dispersão não apresentaram ganhos de resistência significativos, sugerindo pouca eficiência de dispersão para esse fim

Influence of Carbon Nanotubes on the Mechanical Behavior and Porosity of Cement Pastes Prepared by A Dispersion on Cement Particles in Isopropanol Suspension

Cement composites prepared with nanoparticles have been widely studied in order to achieve superior performance structures. The incorporation of carbon nanotubes (CNTs) is an excellent alternative due to their mechanical, electrical, and thermal properties. However, effective dispersion is essential to ensure strength gains. In the present work, cement pastes were prepared incorporating CNTs in proportions up to 0.10% by weight of cement, dispersed on the surface of anhydrous cement particles in isopropanol suspension and using ultrasonic agitation. Digital image correlation was employed to obtain basic mechanical parameters of three-point bending tests. The results indicated a 34% gain in compressive strength and 12% in flexural tensile strength gains, respectively, as well as a 70% gain in fracture energy and 14% in fracture toughness in the presence of 0.05% CNTs were recorded. These results suggest that CNTs act as crack propagation controllers. Moreover, CNT presence contributes to pore volume reduction, increases the density of cement pastes, and suggests that CNTs additionally act as nucleation sites of the cement hydration products. Scanning electron microscopy images indicate effective dispersion as a result of the methodology adopted, plus strong bonding between CNTs and the cement hydration product. Therefore, CNTs can be used to obtain more resistant and durable cement-based composites

A vidéki megélhetés jellemzői és tipikus modelljei a leghátrányosabb helyzetű kistérségekben = Characteristics of Rural Subsistance and Typical Models in the Most Disadvantaged Small Regions in Hungary

A tanulmány a hátrányos kistérségekben élők társadalmi, szociális és munkaerő-piaci jellemzői alapján vizsgálja vidéki megélhetés színvonalát, differenciáltságát, valamint a megélhetés, a „válságkezelés” megoldásait, sajátos típusait. Az elemzés, egyrészt jelzi az országos és regionális tendenciákat, másrészt a leghátrányosabb helyzetű 33 kistérség társadalmi-gazdasági jellemzőit részletezi, harmadsorban pedig, a családok szintjén kérdőívek segítségével méri fel, a jövedelemszínvonalat befolyásoló tényezőket, feszültségeket. A vizsgálat kiemelten foglalkozik a vidéki megélhetés és a mezőgazdaság kapcsolatával, az ágazat helyi erőforrásokat mobilizáló szerepével és formáival (önellátás, „helyi léptékű gazdasági tevékenységek szervezése, szociális földprogramok működtetése, falusi közösségek formálása). A tanulmány rávilágít a szociálpolitikai, a munkaerőpiaci és az önkormányzati rendszer újszerű feladataira, annak érdekében, hogy a hátrányos vidéki térségekben élők további leszakadása fékezhető, megakadályozható legyen. = This study assesses the standard, the differentiation and the solutions and peculiar types of rural subsistence and “crisis management” on the basis of the social and labour market characteristics of people living in the disadvantaged small regions. The analysis, on the one part, points out the national and regional trends, while revealing in details the socio-economic properties of the 33 most disadvantaged small regions, assessing, on the other part, the factors and tensions influencing the amount of incomes at the level of families, through questionnaires. The study lays great emphasis on examining the relationship between rural living and the agriculture, the sector’s role and forms able to mobilise the local resources (self-supply, organisation of “local-scale” economic activities, operation of social land programs, development of rural communities). The study accentuates the new tasks of the social policy, labour market and municipality system in arresting and avoiding further back lagging of the population living in the disadvantaged rural areas

Using Converter Dust to Produce Low Cost Cementitious Composites by in situ Carbon Nanotube and Nanofiber Synthesis

Carbon nanotubes (CNTs) and nanofibers (CNFs) were synthesized on clinker and silica fume particles in order to create a low cost cementitious nanostructured material. The synthesis was carried out by an in situ chemical vapor deposition (CVD) process using converter dust, an industrial byproduct, as iron precursor. The use of these materials reduces the cost, with the objective of application in large-scale nanostructured cement production. The resulting products were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) and were found to be polydisperse in size and to have defective microstructure. Some enhancement in the mechanical behavior of cement mortars was observed due to the addition of these nano-size materials. The contribution of these CNTs/CNFs to the mechanical strength of mortar specimens is similar to that of high quality CNTs incorporated in mortars by physical mixture

Sodium Silicate from Rice Husk Ash and Their Effects as Geopolymer Cement

Sodium silicate is a commonly used activator in geopolymer that is produced commercially. In this study, rice husk ash (RHA) from agricultural waste was used to synthesize sodium silicate as an activator for geopolymer cement. This white ash was applied for producing sodium silicate with different molarities (8, 10, and 12) and then used to synthesize fly ash-based geopolymer cement. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) were applied to investigate the micro-characteristics of the geopolymerization products. Bulk density, water absorption, compressive strength, flexural strength, and fracture toughness were carried out to measure and evaluate the geopolymers with sodium silicate. The combination of 10 M NaOH with sodium silicate increased the compressive strength by 16.21% and the flexural strength and fracture toughness by 81.6%. However, sodium silicate combined with 12 M NaOH decreased compressive strengths by 13.23% and flexural strength and fracture toughness by 61.94%. The lowest water absorption value of 12.3% was obtained in a geopolymer paste using sodium silicate combined with 10 M NaOH, and the largest was 13.3% for sodium silicate combined with 8 M NaOH. The microstructure analysis showed the hydrated calcium alumina silicate gel (C–A–S–H) and the SEM image also revealed a compact geopolymer matrix. Thus, it can be concluded that sodium silicate from rice husk ash can be utilized as an activator or reactive material to produce geopolymer cement with a good geopolymer network

A közvetlen támogatások feltételezett csökkentésének társadalmi-, gazdasági- és környezeti hatásai (első megközelítés) = The Social, Economic and Environmental Impacts of the Hypothetical Reduction of Direct Payments (first approach)

Az EU költségvetés, és ezen belül a Közös Agrárpolitika (KAP) küszöbön álló, vélhetően az agrártámogatások csökkenésével járó felülvizsgálata aktuálissá tette annak elemzését, hogy a közvetlen támogatások részleges megvonása, milyen hatást gyakorolna a magyar mezőgazdaságra, a vidéken élő népesség megélhetésére, társadalmi, szociális helyzetére, a területi differenciáltság alakulására. A tanulmány regionális, kistérségi vetületben áttekintette a közvetlen támogatások területi, ágazatok közötti, illetve kedvezményezettek szerinti megoszlását. Részletesen elemezte a különböző gazdasági fejlettségű kistérségek foglalkoztatási, szociális helyzetéről. Az elemzés egyértelműen bizonyította, hogy a gazdasági fejlettség alacsonyabb szintje, és a gazdaságban a mezőgazdaság erőteljesebb szerepe szorosan összekapcsolódik. A magyar mezőgazdaság alapvető érdeke tehát a közvetlen agrártámogatások fenntartása. A támogatások csökkentése sem csupán a termelők jövedelmi pozícióját rontaná, hanem kedvezőtlen járulékos következményei is lennének a környezetre, és a mezőgazdasági termelés élőmunka igénye is jelentősen csökkenne. Minden szempontból hátrányosan érintené általában a vidéket, különösen a lemaradó és stagnáló kistérségeket és a kedvezőtlen adottságú területeket, általánosan az országnak azon részeit, ahol most is jelentősek a társadalmi-gazdasági feszültségek. = The approaching review of the EU budget and more particularly of the Common Agricultural Policy (CAP), which will probably involve the cutback of agricultural subsidies have necessitated the analysis of the potential effects of the partial withdrawal of direct payments on the Hungarian agriculture, the subsistence and the social situation of the rural population and on regional differences. The paper surveys the distribution of direct payments by regions, sectors and the types of beneficiaries in a region/micro-region dimension. It includes a detailed analysis of the employment and social situation of micro-regions at different stages of economic development. The analysis has clearly demonstrated that there are strong links between the lower level of economic development and the greater economic weight of agriculture. Therefore, the preservation of direct payments is a fundamental interest of Hungarian agriculture. Apart from worsening producers’ income position, the reduction of subsidies would also have unfavourable collateral consequences on the environment and would greatly decrease the employment in agricultural production. It would detrimentally affect rural areas in general from every respect, in particular the micro-regions whose economy is stagnating and falling behind and less-favoured areas, in general, the parts of the country with considerable social and economic tensions

Bond behavior of GFRP bars in concrete with carbon nanotubes

International audienceThis work investigates the adherence of 12.7 mm glass fiber-reinforced polymer (GFRP) bars in concrete with carbon nanotubes (CNT) through pull-out tests. Eighteen specimens were used to investigate the impact of CNT content (0% and 0.05% by weight of cement) and anchorage length (5d and 10d). Reference samples were produced using 12.5 mm steel bars. An additional fifty-four specimens were cast to evaluate the influence of CNTs on the mechanical and durability properties of the concrete. This research provides satisfactory results regarding utilizing GFRP bars instead of steel ones and incorporating CNT in concrete