Phase assemblage and mechanical strength of ultra-high performance concrete under long-term autoclaving: Fid-Bau Portal

Phase assemblage and mechanical strength of ultra-high performance concrete under long-term autoclaving

Tian, Hongwei

This research studied the behavior of Portland cement-based materials under autoclaving at 200 C. The cement hydration achieves a higher degree under autoclaving due to the transformation of amorphous C-S-H to more ordered phases, releasing more water for hydration. However, some calcium aluminoferrite phase keeps partially unhydrated after longterm autoclaving, probably due to the retardation by the hydrate layer. A pure cement paste generates the crystalline C-S-H with a high Ca/Si ratio. Quartz powder can effectively decrease the matrix Ca/Si ratio, leading to the crystalline C-S-H with a relatively low Ca/Si ratio. Further increase of quartz powder results in the formation of poorly crystallized C-S-H. The addition of fly ash can increase the effective Al in the matrix, leading to the formation of hydrogarnet. The composition of hydrogarnet depends on the contents of Si and Al in the matrix. High contents of Si and Al favor Si-rich hydrogarnet. A high water/solid ratio can accelerate the dissolution of raw materials, leading to a different hydrate assemblage. Moreover, in the case of a low Ca/Si ratio, a high water/solid ratio favors the transformation of the C-S-H precursor to tobermorite. For the samples with the same contents of fly ash and quartz powder, a low water/solid ratio generates the hydrogarnet with a higher Si content (more anhydrous). For autoclaved UHPC, the compressive strength can stay robust because a large amount of silica (silica fume and quartz powder) prevents the formation of crystalline hydrates with high densities and leads to tobermorite and poorly crystallized C-S-H as the main hydrates. However, the flexural strength is vulnerable to autoclaving, probably due to the transformation of amorphous C-S-H to more ordered phases. The UHPC performance is improved by partially replacing cement with limestone powder, benefiting from the decreased Ca/Si ratio and the more poorly crystallized C-S-H with a low Ca/Si ratio. However, excessive poorly crystallized C-S-H may magnify the thermal mismatch between matrix and aggregate, leading to low mechanical strength. The addition of fly ash can also mitigate the detrimental effect of longterm autoclaving by increasing the Si and Al contents and inducing tobermorite and poorly crystallized C-S-H as the main hydrates. Meanwhile, the content of fly ash should also be controlled as excessive Al results in the massive formation of hydrogarnet. The mechanical strength is not strictly related to the porosity of UHPC under autoclaving, and the hydrate assemblage overweighs porosity on the mechanical strength. Appropriate incorporation of SCMs in UHPC can decrease the production cost, improve sustainability, and intensify the performance of UHPC under autoclaving.