**3. The influence of fly ash on the rheology of backfill slurry**

When the backfill mining method was adopted, especially paste backfill, many materials treated at surface processing plants needed to be delivered to discharge sites via pipelines under gravitational force or pumping pressure ([21], p. 443).

Furthermore, it is advisable to assess the available gravitational force and the pumping capacity before the operation because insufficient power will not only fail to deliver the filling material to the specified location but also cause severe blockages in the pipeline. However, if too much redundancy is provided, energy wastage and wear on the pipes will increase ([22], p. 925). Given that, the rheology of backfill slurry becomes a concern of interest both for its influence on transportation expense and pipeline maintenance.

The rheology of a slurry is a mathematical description of its motion in response to shear stress. As illustrated in **Figure 7**, due to the different intrinsic properties of each slurry, they exhibit distinct differences in rheological behavior and are classified into different types. Moreover, the most common model is Newtonian, amongst those models describing the relation between shear rate and shear stress. This type of fluid is characterized by its viscosity being constant at a given temperature and not changing with the force applied to it. On the other hand, although the dense backfill slurry obeys different flow laws ([23], p. 1181), the commencement of its flow demands a force that exceeds the yield stress [24].

**Figure 8** demonstrates the relation between the shear rate and shear stress of a series of high concentration slurries with different dosages of fly ash. Generally, all the rheological curves depicted in this figure are inconsistent with Newtonian fluid. This kind of backfill slurry with a changing kinematic viscosity was identified as a Herschel-Bulkley fluid. In the **Figure 8**, with the increasing replacement of fly ash to cement, the shear stress declines rapidly, especially when the shear rate goes higher, which proves the significant positive effects of fly ash on the fluidity of the backfill slurry ([25], p. 223). Moreover, the root of this phenomenon has been attributed to the unique physical structure of fly ash. Fly ash has more spherical particles than the

**Figure 7.** *Rheological model.*

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

**Figure 8.** *Rheogram of slurry with different fly ash content.*

**Figure 9.** *Rheogram of slurry with a 10% fly ash addition.*

other binder in use. This feature can create a lubricating effect, known as the ballbearing phenomena, resulting in a frictionless flow in stowing pipelines ([26], p. 7).

In addition to the physical analysis of fly ash's role in changing the filling slurry's rheological properties, some tests were conducted to explain it from a chemical perspective (**Figure 9**). This figure illustrates that when the curing temperature is different, the rheological characteristics become distinctly different, although the fly ash content remains the same. It is generally accepted in the scientific community that the rate of hydration reaction of fly ash and the amount of polymer produced by hydration is dominated by temperature.

In practice, fly ash's effect on slurry rheology is directly reflected in the extraordinary pressure drop reduction during pipeline transport [27]. Pressure drop is produced mainly by the mechanical friction between the pipes and the transported slurry and the collision between solid particles dispersed in the slurry ([28], pp. 9–18).

It can be seen from **Figure 10** that the pressure drop per meter of paste slurry experiences a shape decrease with the increased fly ash-to-cement ratio. Therefore, the increasing ratio between fly ash and Portland cement means more cement was replaced by fly ash when preparing paste slurry. In the above narrative, we have mentioned the lubrication effects possessed by fly ash due to its spherical shape and stiffness, and that explanation applies here. Furthermore, the early hydration of fly ash is much slower than cement, so more cement is replaced by fly ash. These fewer hydration products possess cementing properties and can bond the aggregate together.

**Figure 10.** *Effect of fly ash on the backfill paste slurry through loop pipe.*

When the ratio of coal gangue to fly ash increases, the pressure drop value also increases dramatically. The lubrication theory can explain this phenomenon. Less fly ash in paste slurry means more collision between the large aggregate, such as coal gangue, and more friction between slurry and pipes.
