5.2.5. California Bearing Ratio (CBR)

CBR values of tested materials vary within wide limits, from 7% for ash-slag mixture TENT A to 57% for fly ash KOS FA. With the use of binders, for all tested samples, there is a clear trend of CBR increase over time (Figure 12). Except for ash-slag mixture TENT A, there is no need for stabilization of tested materials for road subgrade construction purposes [40].

**California Bearing Ratio (CBR) [%]**

#### <sup>39</sup> <sup>46</sup> <sup>64</sup> <sup>72</sup> <sup>76</sup> <sup>67</sup> <sup>92</sup> <sup>81</sup> <sup>89</sup> **TENT A+Cem 2% TENT A+Cem 4% TENT A+Lime 5% TENT A+Lime 7% KOS AB+Cem 2% KOS AB+Cem 4% KOS AB+Lime 5% KOS AB+Lime 7% California Bearing Ratio (CBR) [%] ASH-SLAG MIXTURES** W GD\ W GD\V W GD\V 7(17\$ .26\$%

**Figure 12.** The California bearing ratio of waste materials with binders.

#### **6. Conclusions**

Thermal power plants have multiple negative effects on the environment: They pollute air with harmful gases and fly ash; landfills of ash and slag occupy large areas of mainly agriculture land; deposited ash can potentially pollute land and water due to the presence of trace elements and radionuclides. The amount of deposited ash and slag can be significantly reduced by use in the construction industry.

In order to assess the applicability of fly ash and ash-slag mixtures for subsoil stabilization and embankments construction, the laboratory tests on different mixtures of soil, ash, slag and binders were performed. The soil stabilization efficiency of a non-self-cementing class F fly ash without a binder was tested, as well as the effects of adding a binder as the cementation agent. The characteristics of fly ashes and ash-slag mixtures as the construction material were also investigated.

Considering the results of CH clay stabilization, fly ashes from power plants Kolubara and Kostolac could be successfully used as an additive that improves all mechanical characteristics of the soil required for the subsoil. Due to improved mechanical parameters, the stabilized soil has better bearing capacity and low compressibility. The increase in CBR values and elimination of swell potential make tested soil usable for road construction. With the addition of binders, all tested engineering properties were significantly improved. The addition of lime yields more significant stabilization results compared to cement.

Regarding the embankments and road subgrade design purposes, fly ash and ash-slag mixtures from Serbian thermal power plants have comparable mechanical properties with sands. The use of binder contributes to the substantial increase of shear strength parameters, compressibility modulus and stiffness of tested materials. Achieved high shear strength of waste materials ensures higher bearing capacity and slope stability. Low compressibility also makes waste materials suitable for embankment construction. The use of ashes and ash-slag mixtures as construction material provides multiple benefits: Reduced amount of ash on landfills, preservation of natural resources, lower price of embankment construction, lower energy consumption and CO2 emission.

**Author Contributions:** Conceptualization, Data Curation, Funding Acquisition, Supervision, M.V.; Investigation, Validation, Writing (Review & Editing), M.V., M.M., V.P. and S.J.; Methodology, M.V., M.M. and V.P.; Visualization S.J.; Writing (Original Draft), M.V., M.M. and S.J.

**Funding:** This research was funded by Public Company "Elektroprivreda Srbije" (Electric Power Industry of Serbia) via studies "Use of fly ash of thermal power plants for soil stabilization, self-compacting and rolling (RCC) concrete with emphasis on durability of cement mortar and fine grained concrete" (2014) and "The use of fly ash and slag from "Elektroprivreda Srbije" thermal power plants in railways" (2015). This research is partially sponsored by Serbian Government, Ministry of Education, Science and Technological Development via Project TR 36046: Research on the effects of vibration on people and objects for the sustainable development of cities.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
