## 1 Introduction  
Ultra-high-performance concrete (UHPC) has commonly been considered as a new class of concrete that has gained a strong interest in research and application since the late 1990s [1; 2; 3] with outstanding characteristics including high fluidity, compressive strength (over 120 MPa [4]), high modulus of elasticity, low permeability, and excellent durability compared to conventional concrete and high strength concrete [5; 6; 7]. Because of these outstanding properties, application of UHPC can create many possibilities such as lighter structures, larger-span structures, hybrid structures, new design, and new products with a potential for a better economy and resource consumption in comparison with conventional concrete, steel, and other building materials [2; 5; 8]. In Vietnam, research on UHPC has been also implemented since 2011 and several results have been published elsewhere [9; 10; 11]. Manufacturing UHPC usually needs high material costs, high cement content (around 1000 kg/m\({}^{3}\)), and high-temperature curing. These are all limiting factors for the broader application and sustainability of UHPC. Replacement of a certain amount of cement content in UHPC composition is a scientific and practical topic that UHPC research groups have been undertaking [12; 13; 14] to reduce costs as well as to minimize the negative impacts on the environment caused from the cement production. In particular, the potential for the partial replacement of cement by coal-fired fly ash (FA) discharged from coal-fired power plants has being increased recently in Vietnam. More than 20 power plants with a capacity of 13,100 MW in Vietnam generate an amount of approximately 15.7 million tons of coal ash annually and mainly has been dumped in landfills. The number of power plants is increasing every year and it is forecasted that thermal energy will account for more than 50% of the total electricity generation in 2030. These power plants will consume approximately 171 million tons of coal and discharge a huge amount of coal ash, significantly exceeding the current consumption of coal ash of about 3-4 million tons per year [15]. Therefore, development of UHPC using FA has the \"double benefit\" of enhancing the applicability of UHPC as it reduces the amount of cement used and also reduces the amount of FA dumped in landfills near the coal-burnt thermal power plants in Vietnam. High-volume fly ash (HVFA) concrete, which was firstly introduced by [PERSON] at Canada Center for Mineral and Energy Technology (CANMET), Ottawa in the 1980s, and defined as FA replacement above 50% cement content, addresses the aforementioned issue. FA levels of around 15%-20% by mass of total cementitious material in structural concrete have become generally accepted worldwide in normal practice, even in some recent attempts to apply in producing UHPC [16; 17]. In the past, concrete containing high volumes of low-calcium FA was mostly used in mass concrete works such as roller-compacted concrete dams and highway base courses, where high strength and high workability are not required. As stated by [PERSON] [18], half and more substitution of FA is possible to produce sustainable and high-performance concrete with lower water demand, good workability, minimized cracking due to thermal and drying shrinkage, and improved durability. However, cost-effective and less-energy intensive curing methods of UHPC using HVFA have not been sufficiently studied. The compressive strength of UHPC is enhanced significantly by using a very high cement content and silica fume (SF) combined with very high superplasticizer dosage to allow excessively lower water-to-binder ratios. This is also supported by heat curing for the concrete after setting. The development of compressive strength of UHPC is mostly dependent on the hydration of cement and pozzolanic reactions of mineral admixtures at lower temperatures and higher temperatures. Due to a slower rate of hydration, it has become inevitable to induce thermal regime to enhance the reactions.