*5.2. Properties of Fresh Mixtures*

Selected test results of the consistency of cement composite mixtures measured according to [28] are presented in Table 6.


**Table 6.** The flow diameter of cement composites mixes**.**

The fresh mortar with RCM obtained as a result of the thermal and mechanical treatment (650 ◦C, 60 min) of concrete rubble was characterized by slightly limited flow diameter in comparison to control mortar. The RCM, due to the developed specific surface (about 3000 cm2/g), was also characterized by high water demand. Despite this, it was not necessary to increase the amount of water in the recipe in order to thoroughly mix the ingredients and achieve the required workability.

### *5.3. Properties of Hardened Cement Composites*

#### 5.3.1. Compressive Strength

The average compressive strength results obtained for the test series in comparison to the control samples (series 11) and (series 12) are given in Figure 15.

**Figure 15.** The compressive strength of composites after 28 days.

The changes in cement composite compressive strength depending on the calcination temperature of rubble (*x*1) and treatment time (*x*2) are presented in Figure 16. The function describing the dependence of the compressive strength on the tested variables for composites with RCM is expressed in the following equation:

$$f\_{\text{fCM},28} = 36.16 + 5.48x\_1 + 0.82x\_2 + 2.24x\_1^2 + 1.18x\_2^2 \, R^2 = 0.83 \tag{2}$$

**Figure 16.** The changes in compressive strength of cement composites (MPa), depending on *x*<sup>1</sup> and *x*2.

The obtained compressive strength test results indicate that the increase in calcination temperature from 288 to 712 ◦C caused the increase in compressive strength of about 32%. The highest strength values, exceeding those obtained for the control series, were recorded for composites with RCM calcined at temperature 650 ◦C (series 3 and 4, Figure 16). The compressive strength for these series exceeded the results obtained for the control series 11 by 4 and 7%, respectively. However, in case of using the RCM calcined at lower temperatures (up to 350 ◦C), as well as in the presence of non-calcined addition, the compressive strength results of the composite did not exceed 35 and 30 MPa, respectively. Thus, only a sufficiently high temperature of rubble processing allows to obtain RCM that causes a favorable effect on the compressive strength of cement composites.

#### 5.3.2. Flexural Strength

The average flexural strength results obtained for the test series in comparison to the control samples (series 11) and (series 12) are given in Figure 17.

**Figure 17.** The flexural strength of composites with RCM.

The changes in flexural strength depending on the calcination temperature of rubble (*x*1) and treatment time (*x*2) are presented in Figure 18. The function describing the dependence of the flexural strength on the tested variables for composites with RCM is expressed in the following equation:

$$f\_{fm,28} = 6.76 + 0.75x\_1 + 0.18x\_2 + 0.24x\_1^2 + 0.24x\_2^2 \ R^2 = 0.78 \tag{3}$$

**Figure 18.** The changes in flexural strength of cement composites (MPa), depending on *x*<sup>1</sup> and *x*2.

The analysis of test results showed that the most favorable results of flexural strength of cement composites containing RCM were obtained in cases of using the recycled material after calcination at temperature higher than 650 ◦C (series 3, 4, 6), and the results were similar to those obtained in the control series. The use of RCM from rubble after thermal treatment at lower temperatures (<650 ◦C) had no significant effect on flexural strength compared to the composite with non-calcined RCM.

#### 5.3.3. Water Absorbability

The average results of water absorbability test were given in Figure 19.

**Figure 19.** The water absorbability of composites with RCM.

*Materials* **2020**, *13*, 64

The changes in cement composite water absorbability depending on the calcination temperature of rubble (*x*1) and thermal treatment time (*x*2) are presented in Figure 20. The function describing the dependence of water absorbability on the tested variables for composites with RCM is expressed in the following equation:

$$\%MA = 9.49 - 0.18x\_1 + 0.13x\_2 - 0.2x\_1^2 - 0.16x\_2^2 \, R^2 = 0.68 \tag{4}$$

**Figure 20.** The water absorbability (%) changes of cement composites depending on *x*<sup>1</sup> and *x*2.

Based on the test results, it can be concluded that a decrease in the absorbability of the cement composite occurs with the increase in RCM calcination temperature. In case of using RCM calcined at a temperature 650 ◦C or higher, water absorbability comparable to the absorbability of the control mortar was achieved. However, extended calcination time had a relatively negative influence on this property. It should be noted that the observed changes in water absorbability were relatively small and ranged from 8.7 to 9.6%. This is probably due to the fact that the RCM has a developed specific surface area similar to the cement used. The lowest water absorbability was recorded from the composite with RCM calcined at temperature 712 ◦C (series 6, Figure 20), which is also related to the test results of other parameters such as compressive strength or flexural strength.

#### **6. Conclusions**

Cementitious supplementary material used in cement composites was obtained as a result of thermal and mechanical treatment of concrete rubble as part of comprehensive recycling of reinforced concrete structures.

The statistical analysis of test results of compressive strength, flexural strength and water absorbability of mortars with RCM made it possible to determine the optimal conditions for production of cementitious supplementary material. It was found that the calcination temperature of concrete rubble had the most significant effect on the analyzed parameters of cement composites. The effect of calcination time was statistically less significant. The regression equations can be useful for estimation of the physical properties of composites with RCM considering the conditions of thermal treatment of concrete rubble.

The calcination of concrete rubble at a temperature of about 650 ◦C caused partial dehydration of cement hydration products, mainly the disintegration of portlandite (Ca(OH)2) into CaO and H2O. This treatment partially removed the hydration reactivity of old cement mortar, which resulted in improved physical properties of cement composites with RCM.

The results of extensive microstructural analysis, including X-ray diffractometry (XRD), differential thermal analysis (DTA), thermogravimetry analysis (TG) and scanning electron microscopy, confirmed the presence of non-hydrated cement, calcium hydroxide (CH), calcium oxide (CaO) and dicalcium silicate (C2S) in RCM, which are capable of hydration and creation of rehydration products. The influence of RCM treatment temperature on its rehydration reactivity properties was assessed based on the analysis of heat of hydration.

The proposed highly ecological solution for the management of waste generated in the concrete recycling process supports the idea of sustainable development by limiting the consumption of natural resources and reducing CO2 emissions generated during the cement production process. The test results showed that appropriate treatment of concrete rubble allows to obtain high-quality fine fraction which may be successfully used as a cement substitute or as pozzolanic additive for cement composites.

**Author Contributions:** Conceptualization, K.K.-W., M.K.-K. and E.P.; methodology, E.P.; software, M.K.-K.; validation, M.K.K, E.P. and K.K.-W.; investigation K.K.-W., M.K.-K., E.P. resources K.K.-W., E.P.; data verification, K.K.-W.; writing—original draft preparation, K.K.-W. and M.K.-K.; writing—review and editing, M.K.-K., K.K.-W., E.P.; visualization, K.K.-W., M.K.-K., E.P. supervision, M.K.-K.; project administration, M.K.-K., K.K.-W., E.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The study was performed under the research project number S/WBiIS´/2/2017 funded by the Polish Ministry of Science and Higher Education.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*
