# Properties of a Lightweight Fly Ash-Slag Alkali-Activated Concrete with Three Strength Grades
## 3 Experimental Results
### Uniaxial Compression
As shown in Figure 4b,c, the strength grades had an obvious effect on the stress-strain behavior of fly ash-slag-based AAC. With the increase in uniaxial compressive strength, the modulus of elasticity (stiffness) and axial strain corresponding to peak stress of NAAC and LAAC increased, and the descending section of the \(\sigma\)-\(\epsilon\) curve became steeper, which was common in OPC concrete [42]. The modulus of elasticity and Poisson's ratio of the OPC concrete mainly depend on the type and volume fraction of the aggregates [43]. In Table 5, the modulus of elasticity of the LOPC and LAAC with a compressive strength grade of 35 MPa was about 35.5% and 20.5% lower than that of the NOPC and NAAC with a similar compressive strength grade, respectively. This can be attributed to the fact that the stiffness of the LWA particles is generally lower than that of the NWA. In addition, compared with the spherical LWA particles, a stronger bond can be established between the NWA with an angular shape and cementitious materials (OPC or alkali-activated mortar). Figure 4: Compressive stress–strain curves of the tested prisms: (**a**) stress–strain curves of different mixtures; (**b**) stress–strain curves of NAAC; (**c**) stress–strain curves of LAAC. The measured elastic modulus of this study and those from other research [12; 16; 24; 39; 44; 45] are plotted in Figure 5a, indicating significant differences in the experimental data. This is due to different raw materials, different type and concentration of the alkali activator and the different type and volume fraction of the lightweight aggregate used by the different researchers. However, similar to OPC concrete, the elastic modulus of the AAC increases with the increase in concrete strength grade. The modulus of elasticity of the OPC concrete can be estimated from the ACI 318 code [39] as  
\[E=w^{1.5}\times 0.043\times 10^{-3}f_{\rm c}^{1/2} \tag{7}\]  
where \(E\) = modulus of elasticity of the concrete at 28 days (GPa); \(w\) = unit weight (kg/m\({}^{3}\)); and \(f_{\rm c}\) = average compressive strength at 28 days (MPa). The experimental values of the elastic modulus for the NAAC varied in the range of 19.03-29.32 GPa, which was below the predicted value of NOPC in ACI building code 318. The experimental values of the elastic modulus for the LAAC vary in the range of 12.0-20.43 GPa, which became close to the predictions for the LOPC of the ACI building code 318 as the compressive strength increases. Based on all the data obtained in this study (containing different aggregates), the following formula was determined to predict the elastic modulus of the fly ash-slag-based AAC:  
\[E=mf_{\rm c}^{n} \tag{8}\]  
where both \(m\) and \(n\) are parameters. The values of \((m,n)\) for the NAAC and LAAC in this study by the least-squares linear regressions were (6.51, 0.368) and (1.31, 0.721), respectively. As shown in Table 5, the Poisson's ratio of all the AACs in this study slightly decreased with the increase in strength grades, and the values of Poisson's ratio for the NAAC and LAAC were very close to the typical value of their cement counterparts with a similar compressive strength. The test results of [44] showed that the range of Poisson ratio of an FA-BFSP-based AAC was between 0.23 and 0.26, which was slightly higher than that of the OPC concrete with normal strength (0.11-0.21). [PERSON] et al. [45] revealed that the Poisson's ratio of an FA-BFSP-based AAC increased with the increase in BFSP content. [PERSON] et al. [16] reported that the Poisson's ratio of a palm oil fuel ash-slag-based LAAC was slightly lower than that of NAAC. Due to the limited number of samples in this study, this paper did not attempt to establish a model of the relationship between the Poisson's ratio and strength class of the NAAC and LAAC. The peak strains of the AAC with different strength grades are shown in Figure 5b.