## 6 Conclusions  
Based on the experimental results, the following conclusions can be drawn regarding material characterization and methods used:  
1. The ratio of compressive strength under the heat curing condition and the standard curing \((f^{\prime}_{c})_{H}/(f^{\prime}_{c})_{S}\) slightly increased as the heat curing age increased up to 2 days, beyond which it was insignificantly affected by the heat curing age, where \((f^{\prime}_{c})_{H}\) is 28-day compressive strength of UHPC at different heat curing ages and \((f^{\prime}_{c})_{S}\) is that of the counterpart UHPC specimen cured under standard condition. This confirms that 2-day heat curing is sufficient for UHPC to gain high strength.
2. At the same heat curing age, higher values of \((f^{\prime}_{c})_{H}/(f^{\prime}_{c})_{S}\) were observed for UHPC mixtures with more contents of fly ash and lower water-to-binder ratio (_W/B_). The higher content of fly ash used with the higher values \((f^{\prime}_{c})_{H}/(f^{\prime}_{c})_{S}\) was gained with time of heat curing age, particularly UHPC using 70% fly ash content.
3. The preference of the addition of fly ash for enhancing the compressive strength of UHPC requires the early heat curing procedure which can be recommended as at least 2 days under 90 \({}^{\circ}\)C hot water. The strength gain rate of UHPC cured under heat curing condition was insignificantly affected by fly ash contents, indicating that the rates at 3 days ranged between 0.92 and 0.99 for all the specimens. Overall, the compressive strength of UHPC with fly ash under heat curing condition mostly reached its 28-day strength at just 3 days. The FA content can be increased up to 50% for UHPC with early heat curing while considering a comparable value to the compressive strength of UHPC without FA and cured under standard curing condition.  
Figure 10: Embodied CO2 emissions of UHPC with different FA contents under different curing conditions.  
* For predicting the compressive strength development of UHPC mixtures cured under heat curing condition, the _fib_ 2010 model gives underestimation at early ages and overestimation at long-term ages, irrespectively of fly ash contents. The predictions obtained from the proposed models are in good agreement with test results measured at different ages. Thus, the proposed models have a good potential to assess reliably the compressive strength development of UHPC (with different fly ash contents (up to 70% fly ash content)) and cured under different conditions.
* The model is applicable to HVFA UHPC with a time from 3 to 180 days incorporating 0%-70% class F fly ash, cement type I, _W/B_ from 0.12 to 0.18 by mass, compressive strength from 80 to 165 MPa, curing under standard condition and heat treatment.
* The CO2 emission of UHPC mixtures decreased in proportion to FA content, indicating that a 50% reduction can be obtained with the cement replacement of 50% FA.  
Methodology: P.S.D., N.V.T., L.T.T., N.C.T., C.V.H., and J.-H.M.; modeling: J.-H.M.; validation: P.S.D. and N.V.T.; writing--original draft preparation: P.S.D., N.V.T., N.C.T., V.H.C. and J.-H.M.; writing--review and editing: J.-H.M., L.T.T. and N.V.T.; project administration: N.V.T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National University of Civil Engineering, Vietnam, under grant number 220-2018/DHXD-TD.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.