Issue 71

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

Specific density [g/cm 3 ]

CEM I 42.5R

Fly ash

Silica Fume

River sand

VC111-P

Biochar

Recycled fines

3.07

2.28

2.22

2.64

1.07

1.44

2.80

Table 2: Specific density of used materials. Thermogravimetric analysis of the biochar and recycled fines was performed using a TG5500 TA instrument with a heating rate of 10 °C min − 1 . The materials were analyzed in air and inert atmosphere (nitrogen), with gas flow set at 25 ml/min. The TGA plots are presented in Fig. 1. Plots of the biochar and recycled fines in both atmospheres are presented in Fig. 1a and Fig. 1b, respectively.

Figure 1: Thermogravimetric analysis of the biochar (a) and recycled fines (b), in air and nitrogen flow.

The TGA plot of biochar (in nitrogen) can be divided into three segments: 25-150 °C, 322-384 °C, and 385-900 °C. The first segment corresponds to water evaporation, around 4 %wt. The second segment clearly shows a decrease in weight – 5 %wt. This segment corresponds to the carbonization of remaining organic residues. The last stage corresponds to the thermal decomposition and carbonization of organic residues. The decrease was measured at 23 %wt. The TGA analysis performed in the air can be divided into two main segments: water evaporation and burning carbon/organic materials. The first stage shows a similar weight decrease to the analysis performed in an inert atmosphere. On the other hand, the second stage starts earlier at 281 °C and ends at 533 °C, during which material loses 89 %wt. After TGA in the air, the combustion residue remains, making up to 3 %wt. As presented in Fig. 1b, using air or inert gas does not influence the weight of the recycled fines during heating. In the plot, three segments can be distinguished corresponding to the water evaporation (25-150 °C), decomposition of the CSH phase (413-466 °C), and decomposition of calcium carbonate (553-750 °C). During this segment, material lost around 8 %wt, 2 %wt, and 7 %wt, respectively. The analysis of material weight decrease during temperature changes shows that recycled fines are more stable regarding atmosphere changes. The second conclusion is that biochar can burn entirely after exposure to higher temperatures, leaving additional air bubbles. Based on the recommendations from the literature [19] and previous studies [5], a reference 3D printed concrete (3DPC), designated as REF, was designed. Subsequently, four mixes with biochar and four with recycled fines were developed, replacing 1.25%, 2.5%, 5%, and 10% of the cement volume, respectively. The mixes were designated BCx and RFx for the biochar and recycling fines mixes, respectively, where x indicated the replacement level. The exact compositions of all mixes are presented in Tab. 3.

93