Research on the degradation mechanism of various composite cementitious materials exposed to cryogenic temperatures

Green composite cementitious materials based on low carbon requirements have broad application potential in the field of cryogenic engineering and construction. In this study, the stability of various composite cementitious materials at cryogenic temperatures (-165 degrees C) was evaluated. The macromechanical behavior of the relative compressive strength of four hardened slurries with different deterioration degrees caused by cryogenic treatment was investigated. X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and Si-29 magic-angle spinning nuclear magnetic resonance (Si-29 MAS NMR) spectroscopy were used to analyze the structural stability of the hydration products after the cryogenic treatment. Results indicate the compressive strength of the hardened paste at -165 degrees C decreased to varying degrees compared to the peak strength, even under drying conditions, and the magnitude of the strength reduction decreased as the amount of supplemental cementitious materials (SCMs) increased. The hydration products of the hardened cement paste with a large dose of SCMs exhibited excellent stability at -165 degrees C, and the increase in Ca2+ and bound water in the hydration products was positively correlated with the Ca/Si ratio after cryogenic treatment, as determined by quantitative XRD and TGA analyses and also substantiated by FTIR. Via the experimental and deconvolution results of Si-29 MAS NMR, the mean chain lengths (MCLs) gradually decreased with increasing Ca/Si ratios at ambient temperature. Furthermore, after exposure to cryogenic temperatures, the low-Ca/Si silicate chains slightly aggregate owing to cryogenic treatment, and cryogenic temperatures can increase the susceptibility to disaggregation of low-Al or Al-free silicate chain structures. When Ca/Si >1.5, Q(2) (1Al) was more easily fractured, and the fracture of the silicate chain was likely the essential reason for the Ca2+ spillover at cryogenic temperatures. Al incorporation effectively inhibited the decalcification of the hydration products, and its incorporation was related to the SCMs dosage. Therefore, the use of a large admixture of SCMs as concrete materials for AC-LNG tanks is likely preferable.