Effects of steam on the compressive strength and microstructure of cement paste cured under alternating ultrahigh temperature

Abstract

Alternating ultrahigh temperature (AUHT) method for curing cement paste was established to investigate the effects of dry and steam conditions on the compressive strength of the cement paste. X-ray diffraction, thermogravimetry, scanning electron microscopy, and nitrogen adsorption were used to test the phases, chemical structure, microstructure, and pore structure of the cement paste in order to elucidate the mechanisms by which the curing conditions influenced the compressive strength of the cement paste cured at AUHT. The results showed that, when the curing time reached nine days under AUHT-dry conditions, the compressive strength of the cement paste decreased rapidly and continuously. However, when the cement paste was cured under AUHT-steam conditions, its compressive strength was highly stable. When the curing time increased to 21 days, the compressive strength of the cement paste under AUHT-dry conditions decreased to 25.8 MPa, which is only 89% of the compressive strength of the cement paste cured under AUHT-steam conditions. According to the chemical structure, microstructure, and pore structure results, under AUHT-dry conditions, xonotlite formed in the cement paste and its microstructure changed from “network” and “bulk” to “highly-porous spongy-type assembly”, which increased the porosity of the cement paste and reduced its compressive strength. Under AUHT-steam conditions, some new products formed, increasing the total content of hydration products and maintaining their “bulk” microstructure. Therefore, the porosity of the cement paste cured under AUHT-steam conditions was lower than that of the cement paste cured under AUHT-dry conditions, but the compressive strength was higher.