Study on Mechanical and Microscopic Properties of Fly Ash Cement‐Based Materials in High Geothermal Environment

Abstract

Through simulating a hot and humid environment of high-ground-temperature tunnel and taking advantage of mechanics performance testing, differential thermal analysis (DTA) technology, and scanning electron microscope (SEM) observation technology, the macroscopic and microscopic properties of cement-based materials and their performance improvement measures can be investigated. Also, the mechanism of improvement can be revealed from the perspective of the hydration degree and microstructure. The experimental results indicate that under the hot and humid environment of high ground temperature, adding fly ash could efficiently weaken the early high-temperature curing effect of cement-based materials, improving their physical properties. But the mechanical strengths of cement-based materials go through a transformation from increase to decline with the rise of fly ash, appearing a maximum value in the process. Meanwhile, the differential thermal analysis and the microstructure analysis show that a hot and humid environment enhances the pozzolanic activity of fly ash and an appropriate ratio of fly ash could improve the microstructure of cement slurry, but when fly ash covers more than 30% of the mix of cement-based materials, the quantity of the alkaline exciting agent, calcium hydroxide, is not enough to meet the needs of the active effect of fly ash and the hydration degree of fly ash becomes extremely low. Last, under a hot and humid environment of high ground temperature, fly ash saturation in cementing material should range from 20% to 30% in mass of total binder, which makes for long-term performance of cement-based materials.

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“…The presence of fly ash, Ca­(OH) 2 , Na 2 SO 4 , and other alkaline substances simultaneously enhances the activity, resulting in the production of numerous hydration products. This process consumes a substantial amount of free water and combined water, thereby reducing the fluidity of the cement slurry. , The PVA fibers are devoured by the cement slurry in a similar manner as the water content. When the PVA content exceeds 0.6%, excessive PVA causes agglomeration, resulting in uneven mixing of the cement paste and reduced fluidity.…”

“…This process consumes a substantial amount of free water and combined water, thereby reducing the fluidity of the cement slurry. 41,42 The PVA fibers are devoured by the cement slurry in a similar manner as the water content. When the PVA content exceeds 0.6%, excessive PVA causes agglomeration, resulting in uneven mixing of the cement paste and reduced fluidity.…”