Influence of Structural Characterization of C3S-C3A Paste under Sulfate Attack

The durability of C₃S-C₃A paste with varied C₃A content (0%, 5%, 10%, and 20%) against sulfate attack at various attack ages (3 d, 7 d, 28 d, and 180 d) was investigated in this study through the examinations of corrosion product composition, Ca/Si and Al/Si of calcium-(aluminum)-silicate-hydrate (C-(A)-S-H) gel, formation and evolution of microstructure, migration and transformation of Al containing phase products, and pore structure. The results indicated that sulfate attack can promote the hydration reaction in C₃S-C₃A paste, thus accelerating the production of C-(A)-S-H gel in the paste. With the increase of C₃A content, the acceleration effect becomes more significant. In addition, sulfate attack led to the dealumination and decalcification of C-(A)-S-H gel, resulting in the reduction of the gelling power of C-(A)-S-H gel. The degree of dealumination and decalcification of C-(A)-S-H gel increases with the increase of C3A content. At the same time, free Al and Ca promote the formation of expansive products such as ettringite and gypsum. Finally, under the action of sulfate, the pore characterization of C₃S-C₃A paste deteriorated, showing a decrease in specific surface area, cumulative pore volume and average pore diameter.

Evolution of Hierarchically Porous Nickel Alumina Catalysts Studied by X-Ray Ptychography

The synthesis of hierarchically porous materials usually requires complex experimental procedures, often based around extensive trial and error approaches. One common synthesis strategy is the sol-gel method, although the relation between synthesis parameters, material structure and function has not been widely explored. Here, in situ 2D hard X-ray ptychography (XRP) and 3D ptychographic X-ray computed tomography (PXCT) are applied to monitor the development of hierarchical porosity in Ni/Al₂ O₃ and Al₂ O₃ catalysts with connected meso- and macropore networks. In situ XRP allows to follow textural changes of a dried gel Ni/Al₂ O₃ sample as a function of temperature during calcination, activation and CO₂ methanation reaction. Complementary PXCT studies on dried gel particles of Ni/Al₂ O₃ and Al₂ O₃ provide quantitative information on pore structure, size distribution, and shape with 3D spatial resolution approaching 50 nm, while identical particles are imaged ex situ before and after calcination. The X-ray imaging results are correlated with N₂ -sorption, Hg porosimetry and He pycnometry pore characterization. Hard X-ray nanotomography is highlighted to derive fine structural details including tortuosity, branching nodes, and closed pores, which are relevant in understanding transport phenomena during chemical reactions. XRP and PXCT are enabling technologies to understand complex synthesis pathways of porous materials.

Macro Photography as an Alternative to the Stereoscopic Microscope in the Standard Test Method for Microscopical Characterisation of the Air-Void System in Hardened Concrete: Equipment and Methodology

The determination of the parameters that characterize the air-void system in hardened concrete elements becomes crucial for structures under freezing and thawing cycles. The ASTM C457 standard describes some procedures to accomplish this task, but they are not easy to apply, require specialised equipment, such as a stereoscopic microscope, and result in highly tedious tasks to be performed. This paper describes an alternative procedure to the modified point-count method that is described in the Standard that makes use of macro photography. This alternative procedure uses macro-photographic images that can be obtained with a quite standard photo camera and it is successfully applied to a large set of samples and presents some advantages over the traditional method, since the required equipment is less expensive and provides a more comfortable and less tedious procedure for the operator.