## 4 Discussion  
### Impact of Variations in Concrete Mix Composition on Mechanical Properties  
The mechanical properties at the age of 28 days show that replacing natural aggregate with recycled aggregate had different effects on the Young's modulus and compressive strength (Figure 7). The SCC0 mix had the highest Young's modulus. Replacing the coarse aggregate resulted in a reduction in Young's modulus by 25% for mixes SCC10 and SCC20, regardless of whether only one or both fractions of the coarse aggregate were replaced. This reduction is primarily due to the lower stiffness of the recycled aggregate compared to the natural aggregate. The compressive strength in the SCC10 mix was reduced by 11% and in the SCC20 mix by 9%. Replacing 50% of the cement with pulverised coal fly ash in mix SCC30 resulted in a further 36% reduction in stiffness and 56% reduction in compressive strength compared to mix SCC0. This could be due to the slower pozzolanic reactions that dominate at an age of more than 28 days [59], but also to the lower stiffness of fly ash particles compared to cement particles [60; 61]. The reduction of the cement paste volume by lowering the water content by 40 litres and the reduction of the capillary porosity due to the lower \(w/c\) ratio both increased the stiffness of the cement paste matrix in the SCC40 and SCC50 mixes. However, the introduction of recycled aggregate resulted in a reduction in Young's modulus by 13% and 14% for the SCC40 and SCC50 mixes, respectively, compared to SCC0 mix. At the same time, the compressive strength was reduced by 2% in the SCC40 mix and increased by 14% in the SCC50 mix. It is noteworthy that the replacement of 4/8 mm aggregate in the SCC20 and SCC40 mixes had almost no effect on the Young's modulus compared to the replacement of only 8/16 mm aggregate, but at the same time it increased the compressive strength (Figure 7a). For the normal concrete mix NC2, the replacement of both coarse aggregates also resulted in an increase in compressive strength compared to the NC1 mix. This could be related to the lower stiffness of the recycled aggregate, which reduces the stress concentrations, i.e., reduces the difference in the stiffness of the cement matrix and the aggregate, resulting in a more favourable stress distribution under loading [62]. [PERSON] et al. numerically determined that the magnitude of tensile and shear stresses in the interfacial transition zone increases rapidly with the increase in the modulus of elasticity of the aggregate [63]. This also means that the introduction of fly ash into the cement matrix in the SCC30 mix increased the difference between the stiffness of the aggregate and the matrix, resulting in higher stress concentrations compared to the SCC20 mix and consequently contributing to lower compressive strength. In the SCC60 mix, all the above effects are present, resulting in a 42% reduction in Young's modulus and 48% reduction in compressive strength compared to the SCC0 mix. [PERSON] et al. compared the compressive strength of SCC with natural fine aggregate and with 100% replacement of the fine aggregate with recycled aggregate and found a 47% reduction in strength [52]. At an age of 2 days, the SCC10 and NC1 mixtures exhibited a higher compressive strength than the SCC20 and NC2 mixtures. The \"crossover\" in compressive strength for these mixtures therefore took place in the period between 2 and 28 days. This could also be related to the gradual increase in stiffness of the cement matrix caused by cement hydration and the associated redistribution of stress between the matrix and aggregates. The increase in compressive strength in the mixtures SCC20, SCC50 and NC2 compared to the mixtures SCC10, SCC40 and NC1 shows that the replacement of all coarse aggregates by RCA is more favourable than the replacement of only one coarse fraction. The results allow the conclusion that incorporation of RCA reduces the compressive strength.