The Effects of High Temperatures on Concrete Performance based on Nanostructural Changes in Calcium Silicate Hydrate (C-S-H)

Concrete behavior at high temperatures has significant implications for structural safety under specific loads and for measuring the load-bearing capacity of a structure for its continued utilization; however, its behavior varies with changes in temperature. Thus, this paper aims to investigate the effect of different temperatures on the strength parameters of concrete made with Portland cement from a nanostrucrual viewpoint based on the nanostrucrual changes in C-S-H. Accordingly, 300 samples were cured for 1, 3, 7, 14, and 28 days in a moisture room. After that, all samples were exposed to temperatures of 25, 50, 100, 200, 300, 500, 700, and 900 degrees Celsius for two hours. The changes in length and weight, compressive strength, and cracking behavior in the concrete samples were studied. Moreover, scanning electron microscopy (SEM) was used to analyze the microstructural behavior of samples at different temperatures. Based on the results, the behavior of C-S-H nanostructure causes the changes in length, weight, and compressive strength of the samples to be dependent on the C-S-H nanostructure behavior. With the onset of C-S-H decomposition due to heat, the compressive strength and weight decrease and the cracks spread. The results indicated that the compressive strengths of the 14- and 28-day samples reduced from 262 kg/cm2 and 270 kg/cm2 to 36 kg/cm2 and 44 kg/cm2, respectively. SEM analysis indicated that this reduction was due to the complete decomposition of the C-S-H nanostructure and Portlandite in the cement structure.