Abstract
Thermal energy storage is a key component in harnessing renewable resources, compensating for the energy variations across time scales. A popular strategy for thermal energy storage is storing thermal energy in hot water tanks, which are generally made of copper, stainless steel, and vitreous enamel-lined carbon steel. However, these materials usually suffer a high production cost and short life cycle. UHPC with superior strength and durability holds the potential as a construction material for hot water tanks, which are commercially available and affordable for large-scale applications. During the charging process of hot water tanks, the UHPC structures are thus loaded by a long-term temperature-pressure load (autoclaving condition). However, the influence of long-term autoclaving on UHPC is still unclear. Therefore, the influence of long-term autoclaving at 200°C on the mechanical properties and microstructure of UHPC is studied here. The effect of the long-term autoclaving depends on the UHPC compositions. The compressive strength can stay robust owing to the accelerated formation of hydrates, while the flexural strength is vulnerable to the long-term autoclaving. Katoite, hydroxylellestadite, and scawtite are identified as the new hydrates in the autoclaved UHPC with typical components. The transformation of amorphous C-S-H into more ordered phases results in the low flexural strength and the undensified interface between the matrix and steel fibres. The partial replacement of cement by fly ash mitigates the detrimental effect of the long-term autoclaving. The incorporation of fly ash provides additional silica and increases the ratio of silica to cement, leading to more poorly crystallized C-S-H with a low Ca/Si ratio, which benefits microstructure densification and mechanical strength. The decrease of Ca/Si ratio and the increase of Al by fly ash accelerate the decomposition of katoite and hydroxylellestadite and formation of tobermorite. This study clarifies the influence of the long-term autoclaving on UHPC and provides guidance for developing an applicable and sustainable UHPC as the construction material for hot water tanks.
Highlights
Ultra-high performance concrete (UHPC) is an innovative and promising cementitious composite (Fehling et al, 2014)
The partial replacement of cement by fly ash leads to a slight decrease in compressive strength, which is different from the influence of fly ash in normal concrete
In UHPC with a relatively low w/b ratio, cement hydration and pozzolanic reaction compete with water, and the active cement clinker phases take precedence over fly ash
Summary
Ultra-high performance concrete (UHPC) is an innovative and promising cementitious composite (Fehling et al, 2014). The ultra-high performance typically comprises two aspects: ultrahigh durability and ultra-high mechanical properties (Richard and Cheyrezy 1995; Matte and Moranville 1999; Fehling et al, 2014). Steel fibre is a popular type for UHPC reinforcement, and its influence on mechanical performance, which depends on many factors, such as geometry, distribution, and content, has been extensively studied (Park et al, 2012; Yoo et al, 2017; Zhang et al, 2018). Short-term hydrothermal curing at 60 and 90°C can lead to the more densified microstructure and fibre-matrix interface in UHPC, exhibiting higher mechanical strength (Garas et al, 2012). For the UHPC exposed to elevated temperatures (fire resistance experiment), the hydration products may decompose and porosity increases, causing cracks and the degradation of bond strength between the matrix and fibres (Ahmad et al, 2019; Huang et al, 2019)
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