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form cordierite, thus aVecting the sintering behaviour of the cordierite. Hence it seemed pertinent to investigate the Cordierite-zircon ceramic composites were fabricated eVect of ZrSiO 4 second phase particles on the toughening by die pressing a commercial cordierite powder with of cordierite, as zircon sand is a naturally abundant and the addition of up to 10 wt-% zircon (ZrSiO 4 ). inexpensive raw material with a high degree of purity. Sintering of cordierite was enhanced by the ZrSiO 4 In the present study, cordierite was fabricated with addition through glass phase formation. With ZrSiO 4 dispersions of up to 10 wt-% ZrSiO 4 in order to determine additions above 2·5 wt-% no further eVect on the the eVects of such additions on the densi cation, thermal mechanical properties of the composites was observed.expansion, and mechanical properties of the sintered comThe maximum exural strength at 2·5 wt-%ZrSiO 4 posites and to relate these to microstructural observations. addition was 84±7 MPa, about 30% higher than the The content of ZrSiO 4 was kept as low as possible in order 67±5 MPa found for pure cordierite. The strength of to avoid degrading both the dielectric constant and the cordierite at 2·5 wt-%ZrSiO 4 increased with sintering thermal expansion coeYcient of the resulting composites. temperature up to 1300°C, owing to the enhanced densi cation. Above 1300°C, however, the strength was reduced as a result of the formation of large pockets EXPERIMENTAL PROCEDURES of glassy phase. The average fracture toughness of The starting materials were a commercial grade cordierite cordierite was increased from 1·0 to 1·5 MPa m1 / 2 with powder (BL7, Rauschert Portuguesa Lda), and ZrSiO 4 the addition of ZrSiO 4 . This toughening can be attri-(Extra ne grade, RZM Europe) with the composition buted to crack de ection around ZrSiO 4 particlesThese rather than to residual compressive stresses imposed powders were characterised in terms of particle size distrion the cordierite owing to thermal expansion mismatch bution (using a Malvern Mastersizer Micro instrument) between cordierite and the ZrSiO 4 second phase. and surface area (using a QuantaChrome MS-10 instrument, BCT/497 based on the BET method).Cordierite powders with additions of 0-10 wt-% ZrSiO 4

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RESUMOEste trabalho tem por objetivo avaliar a durabilidade de uma argamassa geopolimérica exposta a ambientes altamente agressivos compostos por sulfato de sódio e ácido sulfúrico em soluções com 10% e 5% de concentração, respectivamente. A argamassa foi constituída de metacaulim, cimento Portland, hidróxido de potássio e silicato de sódio alcalino. As propriedades avaliadas foram: resistência à compressão axial e características microestruturais. A microestrutura da pasta geopolimérica foi avaliada por microscopia eletrônica de varredura e por análise termodiferencial. Os resultados indicaram que o metacaulim utilizado não possuiu propriedades satisfatórias para fins de geopolimerização, comprometendo de sobremaneira a resistência mecânica da argamassa submetida aos dois ambientes agressivos.Palavras-chave: ácido, ataque químico, durabilidade, geopolímero, sulfato. Geopolymeric mortar: study of degradation by sodium sulfate and sulfuric acid ABSTRACTThis work aims to evaluate the durability of a geopolymer mortar in contact with highly aggressive environments by sodium sulfate and sulfuric acid, in solutions with 10wt% and 5wt% of concentration, respectively. The mortar was made with metakaolin, Portland cement, potassium hydroxide and alkaline sodium silicate. The evaluated properties were: axial compressive strength and microstructural characteristics. The microstructure of the geopolymer paste was observed by scanning electron microscopy and thermodiferential analysis. The results indicated that the metakaolin did not have adequate properties for the geopolymerization, causing deleterious effects on the mechanical strength of the mortars which are submitted to both aggressive environments.Keywords: acid, chemical attack, durability, geopolymer, sulfate. INTRODUÇÃOA maioria das pesquisas na área de tecnologia de novos materiais cimentícios visa conferir maior durabilidade através da melhoria das propriedades químicas, físicas ou mecânicas dos aglomerantes. Isto pode ser feito por meio de emprego de aglomerantes que apresentam comportamento diferenciado em relação ao cimento Portland, material aglomerante mais empregado no mundo. A busca por alternativas ao cimento, sobretudo em condições de exposição a ambientes agressivos [1], visa, além da maior durabilidade, mitigar efeitos negativos do emprego deste tradicional material, sobretudo associados ao impacto ambiental atrelado à indústria cimenteira.Em termos de durabilidade, os ataques químicos por sulfatos e ácidos são freqüentes e afetam a durabilidade das estruturas de materiais cimentícios em longo prazo. O ataque por sulfatos ao material cimentício conduz à expansão, fissuração e deterioração de muitas estruturas da engenharia civil expostas ao ambiente sulfatado, tais como cais, pontes, fundações, tubulações de concreto, etc. [2]. Já o ataque por ácidos, por sua vez, acarreta lixiviação de compostos da matriz cimentícia e degradação gradativa do material.Uma alternativa ao cimento Portland, amplamente estudada nas últimas décadas, consiste no cimento ge...

Technical ceramics are being widely employed in the electric power, medical and engineering industries because of their thermal and mechanical properties, as well as their high resistance qualities. The manufacture of technical ceramic components involves complex processes, including milling and stirring of raw materials in aqueous solutions, spray drying and dry pressing. In general, the spray-dried powders exhibit an important degree of variability in their performance when subjected to dry-pressing, which affects the efficiency of the manufacturing process. Commercial additives, such as deflocculants, biocides, antifoam agents, binders, lubricants and plasticizers are thus applied to ceramic slips. Several bacterial and fungal species naturally occurring in ceramic raw materials, such as Sphingomonas, Aspergillus and Aureobasidium, are known to produce exopolysaccharides. These extracellular polymeric substances (EPS) may confer unique and potentially interesting properties on ceramic slips, including viscosity control, gelation, and flocculation. In this study, the microbial communities present in clay raw materials were identified by both culture methods and DNA-based analyses to select potential EPS producers based on the scientific literature for further assays based on the use of EPS for enhancing the performance of technical ceramics. Potential exopolysaccharide producers were identified in all samples, such as Sphingomonas sp., Pseudomonas xanthomarina, P. stutzeri, P. koreensis, Acinetobacter lwoffi, Bacillus altitudinis and Micrococcus luteus, among bacteria. Five fungi (Penicillium citrinum, Aspergillus niger, Fusarium oxysporum, Acremonium persicinum and Rhodotorula mucilaginosa) were also identified as potential EPS producers.

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