1,721,025 research outputs found
Surfactant-Assisted Ball Milling--A New Techniquefor Preparing Rare-Earth Permanent Magnet Nanomaterials
No full textRare-earth permanent magnet nanomaterials have huge potential applications in different technology areas. This paper reviews the surfactant-assisted ball milling (SABM) technique which can be used to produce rare-earth permanent magnet materials and soft magnetic materials, including nanoparticles, nanoflakes and nanostructured bulk materials. The effects of milling time and temperature on the morphology and magnetic properties of the nanomaterials have been investigated systematically. It has been found that low-temperature ball milling leads to a better control of the nanoparticle morphology produced. On the other hand, surfactants play an important role in the milling process although the mechanism needs to be further studied
Review of thermal and environmental performance of prefabricated buildings: Implications to emission reductions in China
No full textPrefabricated buildings recently became more prominent providing significant benefits including contribution to emission reductions at scale and the manufacturing of building components to high accuracy. However, in China, the proportion of prefabrication is lower than that in many developed countries. The prefabricated wall is one of the most important precast elements and its thermal performance has significant impacts on buildings' energy consumption and environmental performance. This work reviews the thermal performance of prefabricated walls encompassing survey of two common structures, precast concrete sandwich walls and lightweight steel-framed walls. The applicability and limitations of methods frequently used to determine the thermal resistances of these two walls are presented. This article also shows a literature review on the implications of prefabricated buildings to China's emission reductions. Compared to conventional buildings, the contribution of prefabricated buildings on China's emission reduction has been recognized. The potentials on building energy efficient and waste minimisation were also acknowledged. It is concluded that the prefabrication has been generally considered to be a more sustainable method in building sector. The energy-saving potential of prefabrication was demonstrated based on life cycle analysis and thermal performance evaluation. It was found the thermal performance of buildings has multiplied by adopting prefabricated façade elements for building retrofitting. More studies related to thermal performance of prefabricated walls with different structures shall be covered in future research. It is also suggested to develop a more representative result of energy performance by adopting prefabrication, with all dimensions of prefabricated buildings' feature considered.</p
Influence of curing conditions on alkali-activated mortars intended for concrete repair
No full textRepair mortars are costly materials with high level of Portland cement and various additives which questions their eco-efficiency. In this respect, cement free material solutions such as alkali-activated mortars based on ground granulated blast furnace slag (BFS) are gaining interest for structural repair. The aim of this research is to study blast furnace slag as a precursor for producing ambient cured alkali-activated repair mortars. To achieve this purpose, a total of four mixtures were prepared using four different molar ratios of the silicate solution. The fresh and hardened properties of the produced mortars including flow retention, dry density, compressive and flexural strength are studied comparing ambient-cured and sealedcured specimens. Furthermore, the tensile bond strength between the repair mortars and a grit blasted concrete substrate was verified (after 7 days sealed + 21 days ambient curing). The results demonstrate, for the tested configurations, the feasibility of the alkali-activated repair mortar, including a good adherence. Given the limited scope of the tests, more work is needed to confirm the observations further
Mitigating the autogenous shrinkage of alkali-activated slag by internal curing
No full textAlkali activated slag (AAS) has shown promising potential to replace ordinary Portland cement as a binder material. Synthesized from industrial by-products, AAS can show high strength, thermal resistance and good durability. However, AAS has been reported to exhibit high autogenous shrinkage. Autogenous shrinkage is a critical issue for building materials since it can induce micro- or macro-cracking when the materials are under restrained conditions. Hence, this work aims at mitigating the autogenous shrinkage of AAS by means of internal curing. The influences of internal curing on microstructure formation and autogenous shrinkage are investigated. The results show that internal curing provided by superabsorbent polymers is a promising way to reduce the autogenous shrinkage of AAS
Drying shrinkage of alkali-activated slag concrete with natural/recycled aggregates
No full textEach year a large amount of construction and demolition waste (CDW) is generated in the European Union. For sustainability development the CDW is recycled and re-used. To promote the use of CDW, recycled aggregates from CDW were incorporated in alkali-activated concrete (AAC), which mainly consisted of secondary materials or industrial by-products. This study investigated the influence of recycled aggregates on workability, compressive strength and drying shrinkage of slag-based AAC. Properties of conventional concrete with natural/recycled aggregates were also tested for comparison. The results showed that the pre-saturated recycled aggregates only slightly affected the workability of conventional concrete or AAC. Recycled aggregates reduced compressive strength of both conventional concrete and AAC due to extra water for pre-saturation of the recycled aggregates. The mass loss of the concrete specimens upon drying was greater for low-strength concrete than for moderate strength concrete. The recycled aggregates increased the mass loss and drying shrinkage of AAC. For conventional concrete, low-strength concrete had a higher drying shrinkage compared with moderate strength concrete. On the contrary, for AAC in this study, low-strength concrete had a lower drying shrinkage compared with moderate strength concrete
Potential application of MSWI bottom ash as substitute material in Portland cement concrete: Filler or binder
No full textIn recent years, the rapid industrialization and urbanization led to the explosive growth of municipal solid waste incineration (MSWI) bottom ashes (BA) production. However, most of them are directly landfilled, which not only brings environmental burden but also results in loss of potential resources. Present researches have proved that MSWI BA could be utilized as a replacement in Portland cement concrete. However, several drawbacks such as volume expansion, leaching behaviour, and relatively lower strength have been reported. In this study, as-received BA was pretreated to remove the metallic aluminium which is responsible for the hydrogen-induced expansion when blended in OPC concretes. Subsequently, the treated BA samples were used as a substitution for cement at the replacement level of 10%. Micronized sand (M300) was selected as reference materials to investigate the role of treated BA in blended cement system, either as filler or binder material. In the experimental program, the hydration process of different mixtures was monitored by isothermal calorimeter and hydration products were determined by X-ray diffraction (XRD) and Thermalgravimetric analysis (TGA). Results showed that the pretreatment effectively removed the metallic aluminum in BA and no severe expansion or strength decrement were detected. The treated BA showed limited reactivity comparing with Portland cement, however, it still worked better than micronized sand as a filler substitution
Multi-level chemical characterization of dutch fine recycled concrete aggregates: a comparative study
No full textIn the Netherlands, beside cement replacement with more green cement types, there is also an urgent need for alternative materials for natural sand in new concrete in order to make it circular. Furthermore, the recyclers have raised questions regarding upscaling and the potential of fine recycled concrete aggregates (fRCA) in structural concrete elements since the availability of recycled construction rubble is increasing. The variations in their chemical and physical properties and lack of standards for their quality evaluation is the main reason for not yet using fRCA in new concrete. In this paper, an in-depth characterization of different fRCA is performed in order to define their chemical properties. The properties can be eventually related to concrete mix design and performance (next step), so that fRCA can be classified as a material that can be used in more advanced applications. This is achieved with a multi-level chemical characterization of individual and total fractions (0-0.25 mm, 0.25-4 mm and 0-4 mm) for finding type and content of the original sand and cement phases and potential contamination of selected fractions. The tests include quantification of element composition with X-ray fluorescence (XRF), qualitative and quantitative phase analysis with X-ray diffraction (XRD) and Rietveld method. In addition, cement paste content, chlorides and sulfates of each type of fRCA was measured in order to evaluate contamination of studied material. It was shown that fRCA from different origins have similar chemical and mineralogical composition and contain comparative chloride content. The chemical composition testing can provide a first line control regarding composition and potential contamination of fRCA. After that, it can be decided which additional tests are necessary to be done in order to evaluate the suitability of fRCA for replacement of primary natural fine aggregates in new concrete
Lattice Boltzmann simulation of chloride transport in alkali-activated slag
No full textIn this work, a numerical model is proposed to study chloride transport in alkali-activated materials. This model is based on multiple-relaxation-time lattice Boltzmann method, where particle distribution function is introduced to simulate the chloride binding and diffusion. This model takes into account diffusion, homogenous reaction between chloride ions and diffusive solid, and heterogeneous reaction between chloride ions and non-diffusive solid. The accuracy of the model is confirmed by a benchmark simulation of transient reactive transport problem. As a demonstration, this model is then applied to simulate chloride transport in alkali-activated slag paste with varying alkaline activators. The influence of alkali content, silica content, curing age and chloride binding on the chloride transport property is briefly discussed
Assessment of freeze-thaw resistance of cement based concrete with ground glass – pozzolan through X-ray microtomography
No full textOver the last few years, the United States has experienced a shortage of fly ash and slag that consequently created a need for an alternative material that is locally available, sustainable, and provides desirable concrete properties. Recent studies have shown that Ground Glass Pozzolan (GGP) offers favorable attributes as a supplementary cementitious material (SCM) for concrete. However, there are limited studies demonstrating freeze-thaw (FT) resistance of concrete with GGP, as well as assessing the FT resistance in relation with the air-void system of GGP mixtures. In response, this study aimed to evaluate both macro- and micro-level behavior of GGP on FT resistance, and characterize mixtures with different contents of GGP. Six concrete mixtures were evaluated: three mixtures with 20, 30, and 40% GGP as cement replacements and three other reference mixtures with 30% fly ash and 40% slag and 100% Ordinary Portland cement (OPC). Following ASTM standards, concrete beam samples were tested for accelerated FT resistance and dynamic modulus of elasticity up to 1000 cycles. All concretes showed high FT resistance with a durability factor over 90% and, consequently, minimal deterioration and scaling. Core samples extracted from the FT conditioned beams were scanned with the X-ray micro-tomography (CTscan) to identify air-void parameters. Through image analysis a quantification of air-void parameters was obtained, and their relationship to FT resistance was established. Using CT scan analysis, we demonstrated that concretes with the highest cement replacement with GGP and slag developed the most desirable spacing factor and specific surface for FT resistance
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