Issue 71

K. Federowicz et alii, Fracture and Structural Integrity, 71 (2025) 91-107; DOI: 10.3221/IGF-ESIS.71.08

after 24 hours of curing. The increase in the stiffness of the cement matrix is supported by calorimetric studies, which also confirmed higher activity in mixtures with up to 2.5 vol.% recycled fines.

Figure 11: Dynamic modulus of elasticity development of mixtures with recycled fines.

For the RF500 and RF1000 mixtures, a delay in stiffness development was observed in the initial hours, which was ultimately mitigated after 20 hours of curing. These mixtures eventually achieved a DME of 26.8 GPa and 27.1 GPa, respectively.

Figure 12: Dynamic modulus of elasticity development of mixtures with biochar.

Fig. 12 presents analogous results for mixtures with biochar additives. Interestingly, all modified mixtures with biochar exhibited a delayed increase in the dynamic modulus of elasticity (DME) over time. The delay was more pronounced with higher levels of cement replacement by biochar. The mixtures with biochar achieved DME values of 26.0 GPa, 24.5 GPa, 23.9 GPa, and 21.8 GPa, respectively. The mixture with 10 vol.% biochar represented a reduction in DME of nearly 20%, which contrasts sharply with the increase in green strength for this mixture. The explanation for this can be found in the drying effect of the mixture, where biochar absorbs water. This absorption increased the strength in the initial minutes of setting but ultimately reduced the degree and rate of hydration due to the limited amount of free water.

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