Xiao Zhao

Mechanical properties and microstructure analysis of fly ash geopolymeric recycled concrete.

Six mixtures with different recycled aggregate (RA) replacement ratios of 0%, 50% and 100% were designed to manufacture recycled aggregate concrete (RAC) and alkali-activated fly ash geopolymeric recycled concrete (GRC). The physical and mechanical properties were investigated indicating different performances from each other. Optical microscopy under transmitted light and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) were carried out in this study in order to identify the mechanism underlying the effects of the geopolymer and RA on concrete properties. The features of aggregates, paste and interfacial transition zone (ITZ) were compared and discussed. Experimental results indicate that using alkali-activated fly ash geopolymer as replacement of ordinary Portland cement (OPC) effectively improved the compressive strength. With increasing of RA contents in both RAC and GRC, the compressive strength decreased gradually. The microstructure analysis shows that, on one hand, the presence of RA weakens the strength of the aggregates and the structure of ITZs; on the other hand, due to the alkali-activated fly ash in geopolymer concrete, the contents of Portlandite (Ca(OH)(2)) and voids were reduced, as well as improved the matrix homogeneity. The microstructure of GRC was changed by different reaction products, such as aluminosilicate gel.

BUCKLING BEHAVIOR OF SHORT MULTI-WALLED CARBON NANOTUBES UNDER AXIAL COMPRESSION LOADS

We investigate the buckling behaviors of short multi-walled carbon nanotubes (MWCNTs) under axial compression by using molecular mechanics (MM) simulations. The effects of the number of walls, length and chiral angle of MWCNTs on the buckling behaviors are examined. The results show that the buckling behaviors of short MWCNTs are rather different from single walled carbon nanotubes (SWCNTs) and slender MWCNTs. Moreover, it is observed that the buckling strains of short MWCNTs vary inversely proportional to the number of nanotube walls. For slender MWCNTs, the buckling strains fluctuate as the number of walls increase. It increases for beam-like buckling mode, decreases for shell-like buckling mode and is approximately constant for the shell-beam-like buckling mode. The increase in the length of MWCNT has also led to a significant decrease of the buckling strain for short MWCNTs. However, chirality does not have a significant effect on the buckling strain of MWCNTs nor alter the buckling mode of short MWCNTs.