# Use of Mineral Waste for the Production of Artificial Aggregates
## 5 Potential Uses of Waste
### Aggregates Based on Coal Mining Waste  
The popularity of aggregates derived from mining waste can be attributed to their availability in regions where mineral resources are extracted. This demand is especially prevalent in areas where transport infrastructure is being significantly expanded because the road construction industry is the primary consumer of these materials. The largest quantity of waste materials is provided by the coal mining industry, with up to 30 million tons of waste produced annually [87]. Considering the varying degrees of direct or indirect usefulness of mining waste (commonly referred to as coal shale) and the methods and conditions of its use in road surfaces, the following types of waste can be distinguished:  
* unburnt coal shale (raw) taken directly from the coal washing plant;
* unburnt coal shale stored in a heap for at least one year;
* decarburized shale from the secondary mechanical processing of fresh unburnt shale;
* decarburized shale stored in a heap for at least one year;* burnt coal shale (red shale);
* waste containing rocky and non-rocky soil from preparatory works (overburden from open-pit mining). Coal shale, particularly the unburned variety, is characterized by poor mechanical properties, moderate physical properties in terms of durability (e.g., absorbability), and a significant organic content [88]. This material can be used for large-scale land levelling and earthworks (embankments), provided that it is embedded outside the frost zone. Burnt coal shale (after thermal transformation) exhibits enhanced mechanical properties. The most important characteristic in this group is resistance to fragmentation, as per standard EN 1097-2:2020-09 [89; 90], \"Tests for mechanical and physical properties of aggregates\", \"Part 2: Methods for the determination of resistance to fragmentation\". Aggregate from burnt shale has an LA coefficient in the range of 35-50%. Aggregate from weak rocks falls within a similar range, which makes it suitable for use in subgrade improvement layers. Fully burnt material is typically similar to debris from well-fired ceramic brick, with grains displaying a uniform brick-red color on both the surface and fracture. Well-burnt shale is a relatively lightweight material and sufficiently resistant to water and frost, which is particularly important when constructing embankments on weak, wet soils, or when building embankments and protective layers exposed to frost. Following thermal transformation, burned-out shales display enhanced mechanical properties, rendering them a viable option for incorporation into enhanced ground and road layers. With regard to both types of shale, the most significant geometric property is the sand index, which is determined after subjecting the material to five cycles of compaction in accordance with the Proctor method. This characteristic is of significant importance in determining the susceptibility to the formation of heaves and, as a consequence, the extent to which shale can be used in road construction. The physical weathering process of coarse rock material collected in the landfill, which progresses over time, leads to its fragmentation and an increased moisture content, making its compaction easier during earthworks. The shredding of waste may result in an excessive increase in fine dust and clay fractions, giving the material properties that are similar to lining material, which in turn may limit its applicability in structures or areas exposed to water and frost. In Poland, the assessment of the technical properties of shale is essentially limited to evaluating its suitability for earthworks according to the PN-S-02205:1998 standard, \"Roads, Earthworks, Specifications and Testing\" [87]. Therefore, shale serves as an alternative not so much to natural aggregates, but rather to native natural soils, particularly in road construction projects where there is an unfavorable earthworks balance, i.e., when the volume of embankments exceeds the volume of excavations for a given investment.