# Design Procedures for Sustainable Structural Concretes Using Wastes and Industrial By-Products
## 4 Discussion
### Mechanical Properties  
#### 4.2.1 Compressive Behavior of EAFS Mixtures  
The average compressive strength, \(f_{cm}\), and the average elasticity modulus, \(E_{cm}\), obtained when considering REF1 as the reference mixture are presented in Figure 11. The same results but considering REF2 as the reference mixture are presented in Figure 12. The failure modes of representative EAFS mixtures are shown in Figure 13.  
Figure 11a shows that the EAFS3B mixture exhibited a strength at 28 days of 26.29 MPa, which is relatively low when compared to the strength of the EAFS8B mixture, for the same distribution modulus, \(q\). However, an improvement in the behavior, both in the fresh and in the hardened state, was noted when compared to the EAFS3A mixture. After preparing the EAFS5C mixture, it was found that it would not be advantageous to repeat the EAFS4 mixture for a lower \(q\), since the EAFS4 mixture as the EAFS3A mixture, was very dense and showed poor mechanical properties (Figure 13a,b). The EAFS4 mixture, like the EAFS5C mixture, also contains in its composition a larger number of EAFS aggregate fractions, and the results obtained for the EAFS5C mixture at 28 days show no significant advantages over the EAFS8B and 8C mixtures, which were composed of fewer fractions of EAFS aggregate (less laborious and costly), exhibiting compressive strengths and elasticity modulus as high as the EAFS5C mixture (Figure 11). By comparing the characteristic compressive strength of cylinders at 28 days, it was possible to confirm that the mixtures showing a strength class equal to or higher than the REF1 were the EAFS8B, 8C, 5C and 8D mixtures. Taking into account the data gathered from the algorithm based on the modified Andreasen and Andersen particle packing model, it was found that the adoption of more EAFS aggregate fractions leads to lower \(RSS\) values, and the resulting target curves show better agreement. Therefore, it was expected that the EAFS5C mixture at 28 days, as the EAFS mixture with the largest number of EAFS aggregates fractions, would have reached the best compressive strength characteristics. As shown in Figure 11a, the EAFS5C mixture is among the ones showing higher compressive strength, classifiable as C30/37 strength class. However, the EAFS8B, 8C and 8D mixtures at 28 days, with slightly greater values of \(RSS\), also showed high values of compressive strength, reaching the strength class obtained with the EAFS5C mixture. It may be concluded that the EAFS8B, 8C and 8D mixtures were more advantageous over the EAFS5C mixture, since good performances in the hardened state were reached with a lower number of EAFS aggregate fractions required, which makes them more practical and economically viable. To some extent, the particularly porous and irregular nature of the EAFS aggregates may justify these results. The mechanical properties obtained at 90 days were slightly higher than the ones obtained at 28 days after casting (Figure 11), except for EAFS6, which did not contain FA additions. As commonly accepted, one of the characteristics of the FA is to confer  
Figure 10: Images of representative specimens during slump test of REF and EAFS mixtures: (**a**) REF1 mixture with a slump class S3; (**b**) REF2 mixture with a slump class S4; (**c**) EAFS mixture with a slump class S1; (**d**) EAFS mixture with a slump class S2/S3; (**e**) EAFS mixture with a slump class S4. additional strength in the long term, and at 90 days the effect of this type of addition is already observed. Figure 11a,b show that the EAFS mixtures with a distribution modulus equal to 0.35 did not reach desirable characteristics in terms of strength and stiffness, since according to Eurocode 2 [58] the average values of concrete cylinder compressive strength and elasticity modulus for a strength class C30/37 are 38 MPa and 33 GPa, respectively. On the other hand, EAFS mixtures with a distribution modulus equal to 0.20, 0.25 and 0.15 were much closer or even reached the mechanical requirements, except for EAFS3B.