# Properties of a Lightweight Fly Ash-Slag Alkali-Activated Concrete with Three Strength Grades
## 3 Experimental Results
### Uniaxial Compression  
The failure characteristics of the OPC and AAC specimens were similar, which were inclined shear failure. The typical failure patterns are illustrated in Figure 3. The strength grade and coarse aggregate type had no obvious effect on the failure modes for AAC. The stress-strain curves of all mixtures under axial compression are shown in Figure 4, in which the positive strain is the axial compressive strain and the negative one is the tensile strain in the transverse direction. Figure 4a demonstrated that the fly ash-BFSP-based NAAC and LAAC manufactured at ambient temperature exhibit a faster stress drop during the post-peak branch. This demonstrates that the compressive toughness is lower than that of the OPC concrete. However, the whole \(\sigma\)-\(\epsilon\) curve of FA-BFSP-based NAAC and LAAC was not much different from that of corresponding OPC concretes. Previous findings [41] indicated that the \(\sigma\)-\(\epsilon\) behavior of a heat-cured, low-calcium fly ash-based AAC was almost the same as that of a typical heat-cured, high-strength OPC concrete. On the other hand, [PERSON] et al. [9] confirmed that the stress-strain behavior of a BFSP-based AAC cured at room temperature was obviously different from that of OPC concrete. The BFSP-based AAC exhibited brittle failure in the post-peak branch, while the OPC concrete of the same strength grade displayed a gradual strain softening process following the peak. The high brittleness of the BFSP-based AAC was attributed to a large number of micro-cracks in the internal structure of the alkali-activated BFSP [17]. Figure 4a also shows that the ascending branch of \(\sigma\)-\(\epsilon\) curve of the LOPC and LAAC has a more linear segment than that of the NOPC and NAAC, which is consistent with most research findings on lightweight OPC  
Figure 3: Typical failure patterns of the prism specimens: (**a**) normal-weight ordinary Portland cement concrete (NOPC); (**b**) lightweight ordinary Portland cement concrete (LOPC); (**c**) normal-weight alkali-activated concrete (NAAC); (**d**) lightweight alkali-activated concrete (LAAC) \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Slump** & **Apparent** & **Dry** & **\(f_{\mathbf{c}}\)** & **UPV** & \(f_{\mathbf{ls}}\) & **Elastic** & **Poisson’s** \\  & **(mm)** & **Wet Density** & **Density** & **(MPa)** & **(m/s)** & **(MPa)** & **(MPa)** & **Ratio** \\ \hline NOPC & 84 & 2364 & 2261 & 37.21 & 3830 & 3.57 & 29.11 & 0.19 \\ LOPC & 60 & 1987 & 1888 & 39.48 & 3717 & 3.25 & 18.77 & 0.22 \\ NAAC-1 & 223 & 2340 & 2242 & 26.10 & 3664 & 3.08 & 21.58 & 0.21 \\ NAAC-2 & 214 & 2361 & 2243 & 39.13 & 3701 & 4.01 & 25.17 & 0.20 \\ NAAC-3 & 205 & 2387 & 2296 & 50.29 & 3924 & 4.76 & 27.60 & 0.18 \\ LAAC-1 & 268 & 1992 & 1825 & 23.99 & 3234 & 2.51 & 12.95 & 0.22 \\ LAAC-2 & 245 & 2001 & 1859 & 31.71 & 3386 & 2.93 & 16.56 & 0.21 \\ LAAC-3 & 237 & 2003 & 1867 & 43.84 & 3596 & 3.69 & 20.01 & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Basic properties of the mixtures. concrete [21; 22]. From the above analysis of the test results, it can be concluded that the NAAC and LAAC with sufficient compressive strength and ductility can be produced by mixing fly ash and slag as a binder, and no thermal curing is required. As mentioned above, the W/S ratio had a significant effect on the peak strength of fly ash-slag AAC. As can be seen from Table 5, the strength of the NAAC increased by 92.7% when the W/S ratio was reduced from 0.29 to 0.21, and the strength of the LAAC increased by 82.7% when the W/S ratio was reduced from 0.26 to 0.18. The peak stress of the LAAC using sintered fly ash aggregates was lower than that of the NAAC using crushed limestone; even the liquid/solid ratio was smaller for the LAAC with a similar grade. This was the same trend as for the LOPC in this study, compared to the NOPC with a similar grade. These trends can be explained by the fact that the strength and stiffness of the LWAs were smaller than those of the NWAs [21; 22].