Influence of main components of accelerators on mechanical property and hydration of Portland cement in a dry-hot geothermal environment

Abstract

The behavior of accelerators in the shotcrete of tunnel structure is significantly affected by the high geothermal tunnel environment. The objective of this study is to investigate the influence of main components of alkali-free accelerators (aluminum sulfate, AS) and alkaline accelerators (sodium aluminate, NA) on mechanical property and hydration of Portland cement in a dry-hot geothermal environment of 80 ℃ (DH). Setting time test, compressive strength test, XRD, TGA, MIP and SEM were performed on cement systems with AS and NA. Results show that relative to standard curing environment, the cement hydration was promoted and the pore structure at early stage was optimized in DH, thereby significantly accelerating the setting time of cement pastes and improving the early compressive strength development of mortars. In contrast, the later cement hydration was retarded and pore structure was detrimental in DH, resulting in abnormally slow development of later strength and even reduction. Additionally, the stability of hydration products such as ettringite (AFt) and monosulfoaluminate hydrate (AFm) would decline in DH. DH could accelerate the dehydration and decomposition of AFt, and promote the transformation process from AFt to AFm at early stage. After 28 days, AFm would convert into hemicarboaluminate due to carbonation. The addition of AS elevated the hydration degree and optimized the pore structure, especially at later stage, thus exhibiting more stability in long-term strength. Nevertheless, the incorporation of NA would bring about serious deterioration phenomenon in DH, and the compressive strength almost stagnated after 1 day. This work provides a guidance for the real application of accelerators in the shotcrete of tunnel structure in a dry-hot geothermal environment.