# Properties of a Lightweight Fly Ash-Slag Alkali-Activated Concrete with Three Strength Grades
## 3 Experimental Results
### Fracture Properties
The fracture energy (\(G_{\text{F}}\)) of the different mixtures was calculated by Equation (4) and listed in Table 6. The fracture energy of the NAAC ranged from 121 to 154 N/m, and that of LAAC varied between 93 and 100 N/m at 28 d. The value of GF of LAAC and LOPC was much lower than that of NAAC and NOPC, indicating that the contribution of the aggregate interlock action to GF in LAAC and LOPC was much less than that in NAAC and NOPC. It was noted that the fracture energies of the NAAC and LAAC were basically the same as those of the corresponding OPC concretes with the same compressive strength. However, [PERSON] et al. [18] found that the fracture energy of alkali-activated fly ash paste was lower than that of OPC paste; thus, they pointed out that the fracture energy of an FA-based AAC was lower than that of OPC concrete. Figure 9a show the relationship between fracture energy and compressive strength. It can be found that the fracture energy of the AAC increased with the increase in compressive strength, which was consistent with the fracture test results of the OPC concrete [49]. However, the increase rate in \(G_{\text{F}}\) was lower in LAAC than in NAAC, which was attributed to the fact that the NWA itself and ITZs between the matrix and NWA was stronger than the LWA and ITZs between the matrix and round LWA particles. [PERSON] et al. [7] pointed out that the fracture energy of a thermally-cured FA-based AAC was similar to that of OPC concrete and increased with compressive strength at a higher rate than that of the OPC concrete. [PERSON] et al. [20] found that, with the increase in compressive strength, the improvement degree of the fracture energy of an ambient-cured slag-based AAC was lower than that of its cement counterpart. [PERSON] et al. [19] reported that adding a small proportion of BFSP improved the fracture energy of a low-calcium fly ash-based AAC cured at room temperature, and the increasing rate of fracture energy with compressive strength was not significantly different from that of OPC concrete. The relationship between the \(G_{\text{F}}\) values and compressive strength of the above results for the heat-cured and room temperature-cured NAACs is also plotted in Figure 9a. It can be seen that the test results from various literature were quite discrete, which was attributed to the fact that the types and sizes of coarse aggregates and paste materials in the different studies were quite different. The CEB-FIP committee [50] recommended a formula for calculating the fracture energy of OPC concrete as follows:  
\[G_{\text{F}}=(0.0469D_{\text{max}}^{2}-0.5D_{\text{max}}+26)f_{\text{c}}^{0.7} \tag{11}\]  
where \(D_{\text{max}}\) = maximum aggregate size (mm); and \(f_{\text{c}}\) = compressive strength (MPa). As presented in Figure 9a, the CEB-FIP equation cannot accurately predict the experimental values of fracture energy of the NAAC and LAAC. A similar form of the CEB-FIP formula was used as a regression equation for predicting the fracture energy of AAC as a function of the compressive strength:  
\[G_{\text{F}}=(0.0469D_{\text{max}}^{2}-0.5D_{\text{max}}+k_{1})f_{\text{c}}^{ k_{2}} \tag{12}\]  
where \(k_{1}\) and \(k_{2}\) are regression parameters. The comparison results in Figure 9a showed that the predictions by Equation (12) were close to the test results of the NAAC and LAAC samples. The mean \(K_{\text{IC}}\) value and characteristic length \(l_{\text{ch}}\) for each concrete mixture are also given in Table 6. The results showed that the \(K_{\text{IC}}\) value of the AAC was higher than that of its cement counterpart with a similar compressive strength, while the \(l_{\text{ch}}\) value of the AAC specimens was lower than that of corresponding OPC concrete. This demonstrated that the crack resistance of the AAC was higher than that of its cement counterpart, and the AAC was more brittle than the OPC concrete with the same compressive strength. [PERSON] et al.