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1.
Backscattered electron (BSE) images of heat-cured concretes show alite grains surrounded by inner C-S-H gel of two distinct grey levels (referred to as two-tone inner C-S-H gel). The lighter rim forms at elevated temperature whereas the darker rim develops during subsequent exposure to moisture at 20 °C. This microstructural feature can potentially be used as an indicator to assess the curing history of a concrete. However, microstructural examinations of room-temperature concretes containing silica fume or which have been exposed to severe conditions (external sulfate, carbonation) also show distinct rims of two-tone inner C-S-H gel.The chemical compositions of the rims were determined by EDX microanalysis in the scanning electron microscope (SEM). Our results show that for heat-cured samples, the different grey levels of the two-tone inner C-S-H are caused by relative differences in microporosity and water content and not by ones in chemical composition. However, in silica-fume blended concrete, sulfate attacked or carbonated specimens the different grey levels of the two-tone inner C-S-H gel were associated with significant differences in chemical composition. This difference allows two-tone inner C-S-H gel arising from heat curing to be distinguished from that arising from these other causes. 相似文献
2.
Representative and quantitative microstructural information of cement-based materials can be obtained in the backscattered electron and X-ray modes of the scanning electron microscope (SEM). One prerequisite, of several, is to use flat specimens. Microstructures that are minimally affected by the grinding and polishing necessary to produce the flat surface can be obtained. It is essential to fill the pores of the specimen with epoxy resin prior to grinding and polishing. After hardening, the epoxy stabilizes the microstructure and enables it to withstand the stresses of grinding and polishing without alteration. In the present paper, we describe a preparation technique that we consider to have produced excellent polished specimens. The importance of epoxy impregnation is demonstrated. 相似文献
3.
Effect of atmospheric pollutants on the corrosion of high power electrical conductors: Part 1. Aluminium and AA6201 alloy 总被引:2,自引:0,他引:2
Rosa Vera 《Corrosion Science》2006,48(10):2882-2900
The aim of this study was to evaluate the joint effect of marine and industrial pollutants on the atmospheric corrosion of aluminium and its AA6201 alloy. Weight loss was determined after 4, 11, 16 and 24 months exposure being morphology and attack intensity analysed through SEM-EDX. Both materials showed the most intense attack for the highest SO2 contents. Good correlation among weight loss, attack depth and tensile strength to rupture with time and with pollutant contents was determined for both materials in most sites. The cause for low aggressiveness of the heaviest Cl− polluted atmosphere on aluminium was electrochemically demonstrated. 相似文献
4.
Jean-Marc Tulliani Laura MontanaroAlfredo Negro Mario Collepardi 《Cement and Concrete Research》2002,32(6):843-849
A case history of a severe degradation of concrete foundation plinths and piers of an about-35-year-old building located in Northern Italy is described. Significant amounts of gypsum, near ettringite and/or thaumasite were detected by X-ray diffraction analyses performed on ground concrete samples. Large gypsum crystals were mainly located at the interface between the cement paste and aggregates, as observed by scanning electron microscopy coupled to energy-dispersive spectroscopy microanalysis. The degradation effects increased with decreasing the distance of concrete structures from an absorbing well located in the courtyard of the building. The well was recognized as the sulfate source due to the microorganism metabolism of sulfur compounds present in the sewage. Consequences of this attack were a very poor bond strength between cement paste and aggregates and a severe cracking of the concrete cover of the steel reinforcement. 相似文献
5.
Lightweight aggregates (LWAs) are frequently used in concrete as well as in thermally insulating mortars and grouts, so that information on their alkali-silica reactivity (ASR) is very important. Four LWAs—expanded vermiculite, expanded clay, expanded glass and perlite—were studied regarding their ASR, using the following test methods: the accelerated mortar bar test (ASTM C 1260), the rapid chemical test (ASTM C 289) and the combined scanning electron microscopy-energy dispersive X-ray technique (SEM-EDX). According to these methods, neither the expanded vermiculite nor the expanded clay exhibited any potential ASR. On the other hand, in the case of the aggregates containing a glassy phase (expanded glass and perlite), the results of SEM-EDX analysis showed serious decomposition of aggregate texture due to ASR, although no deleterious expansion was observed in the accelerated mortar bar test. Therefore, suitable test criteria for ASR need to be defined for LWAs of this type when the AMBT method is used, as has already been suggested for slowly reactive aggregates in Australia. 相似文献
6.
Scanning electron microscopy (SEM) microanalyses of the calcium-silicate-hydrate (C-S-H) gel in Portland cement pastes rarely represent single phases. Essential experimental requirements are summarised and new procedures for interpreting the data are described. These include, notably, plots of Si/Ca against other atom ratios, 3D plots to allow three such ratios to be correlated and solution of linear simultaneous equations to test and quantify hypotheses regarding the phases contributing to individual microanalyses. Application of these methods to the C-S-H gel of a 1-day-old mortar identified a phase with Al/Ca=0.67 and S/Ca=0.33, which we consider to be a highly substituted ettringite of probable composition C6A2S?2H34 or {Ca6[Al(OH)6]2·24H2O}(SO4)2[Al(OH)4]2. If this is true for Portland cements in general, it might explain observed discrepancies between observed and calculated aluminate concentrations in the pore solution. The C-S-H gel of a similar mortar aged 600 days contained unsubstituted ettringite and an AFm phase with S/Ca=0.125. 相似文献
7.
F. Liu J. E. Tang T. Jonsson S. Canovic K. Segerdahl J. -E. Svensson M. Halvarsson 《Oxidation of Metals》2006,66(5-6):295-319
FIB, SEM and STEM/EDX were used to investigate X20 stainless-steel samples exposed to dry O2, or O2 containing 40% H2O, with a flow velocity of 0.5 cm/s or 5 cm/s, for 168 hr or 336 hr at 600°C. Thin protective Cr-rich (Cr,Fe)2O3 was maintained on the samples exposed to dry O2, even after 336 hr, and on the sample exposed to O2/H2O mixture with the low-flow velocity (0.5 cm/s) for 168 hr. The oxide scale formed in the latter environment contained less Cr, due to Cr loss through CrO2(OH)2 evaporation. Breakaway oxidation occurred on the samples exposed in high-gas-flow velocity for shorter time (168 hr) or in low-gas-flow velocity (0.5 cm/s) for longer time (336 hr). The breakaway scales featured a two-layered structure: an outward-growing oxide “island” consisting of almost pure hematite (α-Fe2O3), and an inward-growing oxide “crater” consisting of (Cr,Fe)3O4. The transition from a thin protective (Cr,Fe)2O3 scale to a non-protective thick scale on this martensitic/ferritic steel originated locally and was followed by rapid oxide growth, resulting in a thick scale that covered the whole sample surface. 相似文献
8.
XU Chuan WANG Jinyan JIN Haiyan ZHOU Jin WEN Cheng P 《稀有金属(英文版)》2007,26(5):463-469
Ti/Al/Ti/Au and Ti/Al/Ni/Au ohmic contacts were fabricated on AlGaN/GaN heterostructure under different temperatures of rapid thermal processing (RTP). Since abnormal resistance values were observed during the contact resistance testing,the surface morphology and contact borders of the samples were analyzed to determine the physical mechanism. Such abnormal phenomenon is found to originate from cracking of the AlGaN layer during RTP,flowing of Ti/Al metallic liquid along the crevices,and continuous reaction of the metallic liquid with AlGaN/GaN. Such processes result in abnormal conduction channels. The possible mechanism of the crevice formation was discussed,and the possible solutions to avoid the crevices were proposed. 相似文献
9.
《材料科学技术学报》2019,35(7):1444-1454
The galvanic corrosion of the Q-phase/Al couple in 0.1 M NaCl solutions has been studied using the scanning vibrating electrode technique (SVET), the scanning ion-selective electrode technique (SIET) and energy dispersive X-ray spectroscopy (EDX). The galvanic corrosion of the Q-phase/Al couple was found to be dependent on pH and immersion time. Current density maps obtained by SVET shows that the anodic oxidation processes emerge from Al in a localized manner in pH 2 and 6 solutions but is initiated in a uniform manner in pH 13 solution, whereas, the cathodic processes are more homogeneously distributed over the Q-phase at pH 2, 6 and 13. It is seen that the Q-phase remains cathodic in the Q-phase/Al couple in acidic, neutral and alkaline solutions indicating that the galvanic polarity of the Q-phase is independent of pH. The effect of the galvanic corrosion was largest at pH 2 and 13 compared to pH 6. The pH map obtained by SIET indicates that the galvanic activity of the Q-phase/Al couple proceeds via heavy alkalization of the Q-phase surface with the generation of appreciable amounts of OH− ions. The enrichment of Cu indicated by EDX is responsible for the observed cathodic activity of the Q-phase in the Q-phase/Al couple. 相似文献
10.