# Properties of a Lightweight Fly Ash-Slag Alkali-Activated Concrete with Three Strength Grades
## 5 Conclusions  
In this study, an FA-BFSP-based AAC was prepared with different cementitious materials and W/S ratios at room temperature to evaluate the effects of the mixed ratios and aggregate types on its mechanical and fracture properties. In addition, the morphology and microstructure of the ITZs that combined with the different coarse aggregates and cementitious materials were observed by SEM. Based on the experimental data analysis, the following conclusions can be drawn:  
1. LAAC was prepared and manufactured in this study and exhibited 28-day compressive strengths from 23.99 to 43.84 MPa, corresponding to dry apparent densities of 1825 to 1867 kg/m\({}^{3}\), respectively, satisfying the requirements of a structural lightweight concrete. The strength grades or the UPVs of the LAAC mainly depend on the designed W/S ratios and the volume content of the coarse and fine aggregates.
2. The 28-day fracture energy, fracture toughness and characteristic length of the LAACs made of sintered fly ash ceramics are 93-100 N/m, 0.48-0.74 MPa-m\({}^{1/2}\) and 150-192 mm, respectively, corresponding to compressive strengths from 26.10 to 50.29 MPa, while the fracture parameters of the NAACs are 121-154 N/m, 0.62-1.02 MPa-m\({}^{1/2}\) and 187-275 mm, respectively, corresponding to compressive strengths from 23.99 to 43.84 MPa. In the case of similar strength grades to the corresponding cement concrete, the fracture properties of the NAAC and LAAC are approximately the same as that of the corresponding NOPC and LOPC, respectively.
3. The regression analysis models of the splitting tensile strength, elastic modulus and fracture energy of the NAAC and LAAC are established, which fit well with the experimental results, and can be used for analysis and prediction of AAC mechanic and fracture performance
4. In the microstructure analysis, an obvious interface area is observed in the concrete with cement as cementitious material. The LWA has a slight wall effect at the interface and, compared with the conventional aggregate, the water absorption rate is higher.  
Figure 12: SEM image of the interfacial transition zone (ITZ) between the binder and aggregate: (a) NOPC; (b) LOPC; (c) NAAC; (d) LAAC.  
However, the pore size of the interface area decreases and the interface density improves when the alkali-activated FA and BFSP are used as the cementitious material.  
Conceptualization, H.W., Y.W. and B.C.; methodology, H.W., B.C., H.C. and L.W.; formal analysis, H.W., Y.W., H.C. and B.C.; writing--original draft preparation, H.W., Y.W., H.C. and L.W.; writing--review and editing, H.W., Y.W., and B.C.; visualization, B.C.; supervision, H.W.; project administration, H.W.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Guangxi University under contract no. XGZ160701, National Natural Science Foundation of China (Grant no. 51768003; 51868005) and the Science and Technology Planning Project of GuangXi Province (Grant no.2017 GXNSFAA198360).  
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.  
All authors declare that they have no conflict of interest.