**2. Types of backfill mining with fly ash admixture**

Backfill mining is a relatively newly emerged extraction method compared with traditional caving mining. Generally, this mining technology mixes aggregates and binders and then delivers the mixture by gravity force or pump to the underground stopes and goafs. The backfill mass in underground voids provides a variety of beneficial financial and environmental effects on mining activities, including increased sill pillar recovery, providing a secure working environment for mine operators ([9], p. 772).

Nevertheless, underground mining with backfill minimizes ore dilution, provides a working floor, controls subsidence, and facilitates subsequent excavation and ore removal nearby [10, 11]. Those numerous advantages of the backfill mining method make it a competitive alternative to traditional ways of excavation and also lead its subdivision into different subsystems to meet the diverse requirements of different mine sites.

The backfill subsystems currently in operation are solid backfill, cemented backfill, and paste backfill, and the role played by fly ash differs from backfill system to system.

#### **2.1 Solid backfill**

During excavation and afterwards processing, a large number of solid rejections are left over in mine sites. For instance, the volume of coal gangue in China is about 500 million tons and is still increasing at a speed of 70 million tons per year [12]. Therefore, how to dispose of those harmful residues in an eco-friendly way is a significant concern for the mining industry. Other than this, surface subsidence prevention is another vital driver for developing the solid backfill mining system. After coal or ore is mined out, the spaces left underground will lead to the overlying strata subsidence and further cause damage to surface buildings. The solid backfill method directly delivers dry solid wastes by scraper conveyor belts to underground caves, supporting the top strata, and surrounding rocks.

As illustrated in **Figure 4**, the solid backfill system is similar to longwall caving mining except for the tamping devices mounted at the rear part of the hydraulic supporter, and the tamping device is specially designed for the compaction of the filling materials. With the dumping of solid wastes from the conveyor, the tamping arm exerts pressure that can reduce the voids within the filling piles. Considering that the filling quality is mainly determined by the compressibility of the filling piles [11], the tamping arm is a genius initiative to improve the filling effect. However, the large size and poor particle gradation of the coal gangue dramatically drop its effectiveness. Therefore, fly ash is added to fill the voids between giant solid rock or particles to further enhance the backfill mass's compactness and minimize subsidence.

Due to no water added, chemical reactions between backfill materials are absent, i.e. no hydrates are generated, which can bind solid particles together and form a solidified filling mass. Hence, the function played by fly ash in the solid backfill mining method is to improve particle gradation physically.

*The Utilization of Fly Ash in the Mining Sector DOI: http://dx.doi.org/10.5772/intechopen.110846*

**Figure 4.** *Schematic of solid backfill.*

Compared with cemented backfill mining and paste backfill mining, solid backfill exhibits numerous noticeable advantages, and its most significant merit comes from a financial perspective. Firstly, the backfill system is easy to set up and requires fewer auxiliary facilities. For example, since the backfill mixtures do not include water, thus, the costs on building an underground reservoir and drainage pump for bleeding water can be avoided. Furthermore, solid backfill has a high tolerance to the size or type of backfill materials, so pre-treatment fees on backfill materials can be removed from the budget. Besides, filling material can be delivered to the design site in a very timely manner, as filling operations and mineral extraction can be carried out simultaneously, which is essential to reduce the amount of subsidence ([13], p. 2670).

#### **2.2 Cemented backfill mining**

The binding property of fly ash is utilized in cemented backfill mining, as opposed to the solid backfill that uses fly ash as a type of particle and can only fill the spaces between huge rocks. Backfill materials used for cemented backfill include large-size rocks and coal gangues from coal washing and processing, and those large-size aggregates work as the skeleton of the filling body.

Fly ash and Portland cement are mixed with water on the surface before the filling operation and then pump through pipelines to the designed sites. At the discharging point, the slurry contains fly ash and cement is poured on the aggregate pile delivered by belt or conveyor (see **Figure 5**). A dam must be built at the lower opening of the branch to store the filled materials from working areas and mining operations and to guarantee that the materials may fill the mined-out spaces. Once the coarse coal gangues, fine fly ash, and cement are put into the sealed branch, the filling materials are evenly mixed and stable.

**Figure 5.** *Discharging scene of the cemented backfill mining.*

The stable state creates a possible condition for the hydration reaction of the fly ash and cement. Compounds with cementing capacity resulting from hydration as a cementing agent hold all the filling together as a unit. Therefore, when overlying strata above the backfilled goaf continue to subside, it should break the friction force between particles and the bonding force of binders. The reasons mentioned above make the cemented backfill much more effective at preventing subsidence than the dry solid backfill method.

Although the hydration reaction of fly ash and cement results in the filling mixture's self-stabilizing capacity and the support strength's growth to a considerable degree, the excessive water washes more fly ash and cement down to the bottom. Therefore, in this backfill method, the cementitious properties of fly ash are not thoroughly exploited.
