# Frost Resistance of Coal Gangue Aggregate Concrete Modified by Steel Fiber and Slag Powder
## 4 Conclusions  
In this paper, the recycled coal gangue was used as concrete aggregate, and the improvements of steel fibers (with different types and content) and slag powder (with different content) on the frost resistance of CGAC were studied. The following conclusions can be drawn from the presented investigation:  
* After adding proper types and amounts of steel fibers, the compressive strength loss and splitting tensile strength loss of CGAC reduced, which was beneficial to its frost resistance. However, the mass loss of CGAC increased. Compared with other mix proportions, CGAC with 1 vol.% USF suffered the smallest compressive strength loss and splitting tensile strength loss, with values of 14.2% and 14.7%, respectively. * The RDME of steel-fiber-reinforced CGAC fluctuated between 100.0% and 103.4% after the F-T test when the content of steel fiber was no less than 1 vol.%, which may be due to the speed change of ultrasound in CGAC or other factors. Thus, the RDME tested by the ultrasonic method may not be suitable to evaluate the frost resistance of steel-fiber-reinforced CGAC. * The effect of slag powder on frost resistance of CGAC at an early age was not obvious. This may be due to the slow pozzolanic reaction of slag powder and less hydration products filling the concrete pores, as the cement was partially replaced by unactivated slag powder. * The addition of HESF can obviously optimize the internal structure of CGAC. Just 1 vol.% HESF can increase the quantity and specific surface area of air-voids by 27.8% and 9.9%, respectively, and decrease the spacing factor and mean chord length of air-voids by 19.6% and 17.7%, respectively, thus improving the frost resistance of steel-fiber-reinforced CGAC. However, the slag powder cannot effectively optimize the air-voids structure of CGAC at an early age. In summary, the replacement of aggregate with coal gangue will reduce both the use of natural aggregate and the impact of solid waste on the environment. Meanwhile, the incorporation of steel fibers can improve the frost resistance of CGAC, optimize its air-void structure, and reduce the mechanical degradation of CGAC caused by frost damage. Besides, replacing partial cement with slag powder can provide a feasible and effective approach to the utilization of such industrial waste without reducing the performance of concrete too much. This study provides a reasonable prospect for the CGAC to be widely used in cold regions. Conceptualization, D.L. and Y.W. ; Methodology, D.L. and Y.W. ; Validation, D.L. and Y.W. ; Formal analysis, D.L. and S.Z. ; Investigation, S.Z. ; Resources, D.N. and Z.S. ; Data curation, D.L., Y.W., and S.Z. ; Writing--original draft preparation, D.L. ; Writing--review and editing, D.N. and Z.S. ; Supervision, D.N. and Z.S. ; Project administration, D.N. and Z.S. ; Funding acquisition, D.N. and Z.S. All authors have read and agreed to the published version of the manuscript. This work was financially supported by the National Natural Science Foundation of China (Grants No. 51808438, No. 51878549), and the Opening Fund of State Key Laboratory of Green Building in Western China (Grant No. LSKF202002). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & \begin{tabular}{c} **Quantity of** \\ **Voids** \\ \end{tabular} & \begin{tabular}{c} **Mean Chord** \\ **Length (mm)** \\ \end{tabular} & \begin{tabular}{c} **Air Content** \\ **(\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Specific Surface** \\ **Area (mm\({}^{-1}\))** \\ \end{tabular} &
\begin{tabular}{c} **Spacing Factor** \\ **(mm)** \\ \end{tabular} \\ \hline CGAC & 769 & 0.232 & 3.19 & 17.23 & 0.367 \\
10\% slag powder & 751 & 0.228 & 3.06 & 17.54 & 0.367 \\
40\% slag powder & 662 & 0.257 & 2.21 & 21.36 & 0.425 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Air-void characteristic parameters of CGAC with different cement substitution rates. **Acknowledgments:** The work presented herein was conducted in the State Key Laboratory of Green buildings in Western China at Xi'an University of Architecture & Technology.