PSI - Issue 81

Oleh Bordiuzhenko et al. / Procedia Structural Integrity 81 (2026) 78–83

81

3. Results and discussion The test results of pressed composite specimens obtained from the studied concrete waste powders are presented in Table 3. The specimens were formed under a pressing pressure of 20 MPa, which is quite acceptable in the technology of building materials, particularly autoclaved silicate materials. Analysis of the obtained results shows that the highest compressive strength at the age of 28 days (13.4 MPa) was demonstrated by the specimens made from the dispersed fraction of concrete waste B1. With a bulk density of 1.62 g/cm³ and a water resistance coefficient of 0.92, it can be concluded that pressed artificial stone may be produced, exhibiting characteristics comparable to those of conditionally effective ceramic products. The results indicate that the strength of the pressed concrete waste powders decreases in the following order: B1 > B3 > B2 > B4. This reduction clearly correlates with the age of the original concrete and its actual strength (essentially with the initial cement content). Therefore, it can be stated that concrete waste originating from younger concretes, with higher strength and higher cement content, potentially exhibits a better hardening capacity under pressing conditions due to the presence of unhydrated cement paste particles.

Table 3. Properties of pressed composites.

Compressive strength, MPa at age of

Water resistance coefficient (28 days)

Dry bulk density g/cm³

Concrete samples

1 hour

1 day

7 days *

28 days *

B1 B2 B3 B4

3.1 2.9 3.2

4.6 3.8 4.1 3.5

11.5

13.4

1.62 1.59 1.61 1.56

0.92 0.87 0.91 0.88

6.7 8.6 4.2

8.7

10.2

3

5.6

*Specimens were stored above water.

The change in strength values of the specimens over time shows a stable growth trend, which clearly indicates the occurrence of structure-forming processes. The strength development kinetics are similar for specimens of all four types of waste (Fig. 3), with the peak strength gain observed at 7 – 10 days. Further growth continues at a lower rate.

16

C1 C2 C3 C4

14

12

10

8

6

4

Compressive strength, MPa

2

0

1 h

1 day

7 days

28 days

Age

Fig. 3. Strength development kinetics of pressed specimens over time.

Bulk density is determined mainly by the pressing pressure and the particle size distribution of the dispersed fraction of concrete waste. As seen from the data in Table 3, at constant pressing pressure, the bulk density of the specimens varied slightly and was about 1.6 g/cm³. The values of the water resistance coefficients for all specimens of concrete waste (about 0.9) indicate sufficiently high water resistance of the obtained materials. The curing conditions of the specimens were also studied. After pressing, the specimens were stored in air-dry conditions, above water (in a desiccator), and fully immersed in water (after preliminary curing in air for 1 day). As the results show (Fig. 4), air-dry curing is not optimal. Obviously, due to the relatively high porosity of the specimens, which can reach 25 – 30%, the composites dry out relatively quickly under low relative humidity conditions, causing a lack of water for structure-forming processes. The best strength results