**2.3 Paste backfill**

The use of paste as an underground backfill started in 1979 when it was used at Preussag's Bad Grund Mine in Germany [10]. Paste backfill is mixing grounded coal gangue powder or classified tailings with fly ash and cement, making a homogenous paste slurry, and then transporting it through a pipeline to gob, providing confinement to the surrounding rocks and roof.

Two reasons mainly drive the commencement of paste backfill. One is that the primary purpose of using fly ash in the mining sector has changed from disposing of harmful residues to beneficially applied as scientists and engineers become more knowledgeable about fly ash's physical and chemical properties. Another contributing factor is subsidence control in some areas calls for a better filling solution. The main components of paste backfill are dewatered mine tailings and grounded coal gangue (70–85% solids by weight), binders (3–7% by dry paste weight), and mixing water (fresh or processed) ([14], p. 2). The most commonly used binders are fly ash, cement or a mixture of those two ([15], p. 4). The solid fraction in the paste slurry is considerably high, generally in the range of 65–75%, in some practical cases, it can

**Figure 6.** *Paste backfills.*

reach 80%, and the properties of paste flurry change significantly even if the solid fraction is slightly changed [16]. The high solid concentration in paste slurry increases the number of particle-particle contacts, thereby increasing the gel formation rate ([4], p. 10). However, it also contributes to water retention, thus eliminating the need for drainage (**Figure 6**).

Therefore, to maintain that all the solid particles in the slurry are in a suspension state, ensuring the highly dense backfill slurry can be delivered through the pipeline without clogging or segregation, strict standards for particle size and grading are applied ([17], p. 116871; [18], p. 1443). With its physical and chemical characteristics, fly ash has become an indispensable additive for paste filling. For example, its large specific surface makes the fly ash particles more easily grafted by the surfactant, making it more suitable to be modified. In addition, the high specific surface area of fly ash will also result in a robust adsorptive capacity of fly ash to surfactant suspensions.

As the setting time of backfilled paste is mainly associated with the fineness of the binder ([19], p. 101), the addition of fly ash paste can provide support to adjacent rocks very soon. Furthermore, due to their pozzolanic reaction, the fly ash blended pastes have a lower early reactivity than cement. Thus, it raises the amount of C-S-H and C-A-S-H and creates a compact microstructure free of cracks, boosting the material's long-term strength and durability ([19], p. 105).

In addition to filling efficiency, filling costs are a significant consideration for the coal sector. In terms of filling effectiveness, paste filling has a marked advantage over the previously described mining methods, but the complexity of the operation and high cost makes it less attractive. The cost of filling materials with paste backfills is 10 and 20%, whilst cement accounts for up to 75% of that cost ([20], p. 284). So, maximizing inexpensive fly ash as a replacement for cement in paste filling is key to further reducing filling costs and promoting more widespread use of paste backfill mining.
