Influence of hoop restraint on microstructure and phase composite of cement paste filled steel tube under external sulfate attack

The deterioration of cement-based material caused by external sulfate attack (ESA) is a major durability problem affecting concrete structures. This study aims to investigate the effect of hoop restraint of steel tube on micro-structure and phase composition of cement pastes prepared with ordinary Portland cement (OPC) and high sulfate-resistance Portland cement (SRPC) under ESA for 540 days. The effects of hoop restraint on the change in micromorphology, phase composition and microstructure of cement hydration products under ESA were investigated by SEM-EDS, XRD, 29Si and 27Al MAS NMR spectroscopy and low-field NMR. Moreover, the effects of hoop restraint on local mechanical properties of the cement pastes under ESA were evaluated by micro -hardness measurement. Results indicate that hoop restraint significantly inhibits the formation of micro -cracking and limits the permeation rate of sulfate ions, leading to a reduction in the amount of ettringite and gypsum crystals. 27Al NMR analysis shows that the hoop restraint has significant "delay effect" on the transfer of Al phase transition from monosulfoaluminate (AFm) and third aluminate hydrate (THA) to ettringite. 29Si NMR test reveals that hoop restraint can significantly slow down decalcification and dealuminization of C-(A)-S-H gel, leading to a slow reduction in Al[4]/Si ratio of C-(A)-S-H, together with the lower mean chain length (MCL) of the silicate chain. The MCL reduces by 34.7 % and 9.0 %, and Al[4]/Si ratio increases by 41.9 % and 8.3 % for OPC and SRPC pastes with steel tube compared to the equivalent free ones, respectively. Additionally, hoop restraint results in better quality pore structure of cement pastes with steel tube than the equivalent free ones.