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Wan K.,Jiangsu Key Laboratory of Construction Materials | Wan K.,Nanjing Southeast University | Xue X.,Nanjing Southeast University
Materials Characterization | Year: 2013

This paper aims at illustrating the potential of lab source X-ray CT for studying the damage behavior of cement based materials through in situ load experiments. This approach permits quantifying the microstructure prior and during loading. The load damage is separated from the specimen deformation using an image interpolation method. A quantitative relationship between external load and internal specimen damage is analyzed using the statistical information of gray scale values of the CT data. Local damage degrees are defined on 3D subset, and the 3D spatial distribution of damage information is clarified in this research. © 2013 Elsevier Inc. Source


Wan K.,Jiangsu Key Laboratory of Construction Materials | Wan K.,Nanjing Southeast University | Xu Q.,Nanjing Southeast University
Science China Technological Sciences | Year: 2014

Porosity is one of the most important parameters for cement-based materials, which influences the mechanical property, transport property, and durability. The spatial and frequency distributions of local porosity of cement pastes are characterized using X-ray micro-tomography data and treating methods. The 3D spatial distributions for three cement paste specimens with different water cement (w/c) ratios show reasonable heterogeneity. The probability analysis also reveals this heterogeneity: the representative volume element (RVE) size based on porosity maps decreases with w/c ratio firstly, then increases with w/c ratio; and the heterogeneity on the characterized probe size or on the RVE size increases with w/c ratio. Average porosities obtained using the CT method are further compared with those by traditional methods. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg. Source


Rong Z.,Jiangsu Key Laboratory of Construction Materials | Sun W.,Nanjing Southeast University | Xiao H.,Nanjing Southeast University | Jiang G.,Nanjing Southeast University
Cement and Concrete Composites | Year: 2015

In this research the effects of nano-SiO2 particles on the mechanical performance, hydration process and microstructure evolution of ultra-high performance cementitious composites were investigated by different methods. The results showed that the compressive and flexural strength increased with the increase of the nano-SiO2 content up to 3% and due to agglomeration of nano-SiO2 particles, the mechanical properties decreased slightly when the nano-SiO2 content was more than 3%. The hydration process was accelerated by the addition of nano-SiO2. The porosity and the average pore diameter decreased with the increase of the nano-SiO2 content and aging. The microstructure was more homogenous and dense for nano-SiO2 specimens as compared to the control specimen. All of these improvements could be mainly attributed to the pozzolanic and filler effects of nano-SiO2. © 2014 Elsevier Ltd. All rights reserved. Source


Rong Z.D.,Jiangsu Key Laboratory of Construction Materials | Rong Z.D.,Nanjing Southeast University | Sun W.,Jiangsu Key Laboratory of Construction Materials | Xiao H.J.,Jiangsu Key Laboratory of Construction Materials | Wang W.,Jiangsu Key Laboratory of Construction Materials
Construction and Building Materials | Year: 2014

The hydration process of ultra-high performance cementitious composite (UHPCC) is distinctly different from common concrete owing to its low water-binder ratio (w/b). The effects of silica fume and fly ash on the hydration process and microstructure were investigated via different characterization methods in this work. The results indicated that the hydration process could be accelerated at the beginning by addition of silica fume, and fly ash, on the other hand, retarded hydration mainly in the dormant and acceleration periods. When they were incorporated together in cement, the result differed significantly from cement hydration at higher w/b (>0.35). As a result of Ca(OH)2 consumption by pozzolanic reaction, the porosity reduced (<4%) and the interface strengthened as the curing time increased. Nanoindentation results revealed that most hydration products of UHPCC were UHD C-S-H and unhydrated cement. The unhydrated cement particles were surrounded by UHD C-S-H, implying that the UHPCC had excellent mechanical performance and durability. © 2013 Elsevier Ltd. All rights reserved. Source


Wan K.,Jiangsu Key Laboratory of Construction Materials | Wan K.,Nanjing Southeast University | Xu Q.,Nanjing Southeast University | Li L.,Nanjing Southeast University | And 2 more authors.
Construction and Building Materials | Year: 2013

Calcium leaching of cement-based materials is one of durability concerns in concrete structures. Calcium leaching leads to high porosity and porosity gradient near the wet surfaces of cement-based materials. In this research, the 3D porosity distributions of partly leached cement pastes with different water-to-cement ratios are characterized for the first time, and the porosity evolution caused by calcium leaching are discussed. The results are verified using the average porosities of the partly leached specimen. From the quantitative porosity results, large porosities over 60% are observed on the leached parts, and very sharp porosity gradients are observed on the leaching fronts. The results of current research can be used for the verification of calcium leaching models and for understanding the leaching mechanism. © 2013 Elsevier Ltd. All rights reserved. Source

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