Page 187 - AJWEP-22-5

Use of coal gangue for mine backfill

Table 1. X‑ray fluorescence analysis results of coal was continued for 10 min using a mechanical mixer at
gangue and fly ash 200 r/min until homogeneous slurry was achieved. The

Molecular formula Coal gangue (%) Fly ash (%) well-mixed slurry was then promptly poured into standard
SiO 2 51.25 47.51 100 mm × 100 mm × 100 mm cubic molds and allowed
to cure for 7, 14, and 28 days for subsequent testing.
Al O 3 26.59 37.34
2
Fe O 3 5.78 5.31 2.2.2. Effect of coal gangue particle size matching on
2
SO 3 4.33 1.94 slurry performance
TiO 2 4.09 2.01 The particle grading design theory, first developed
K O 2.93 2.01 for concrete aggregates, uses mathematical models
2 to achieve optimal packing and dense stacking of
Na O 1.64 -
2 differently sized particles and thus enhance both
CaO 1.62 3.06 mechanical properties and workability. The classic
MgO 1.23 0.30 Fuller curve, an exponential function describing
P O 5 0.27 0.23 continuous gradation, is widely used; however, the
2
V O 5 0.09 - discontinuous size distribution that typically results from
2
ZrO 0.06 0.08 crushing coal gangue limits its applicability. To better
2 characterize and optimize gangue particle distributions,
SrO 0.05 0.06 this study adopts an improved variant of the “imethod.”
MnO 0.03 0.03 The variant introduces a dynamic grading coefficient,
ZnO 0.02 0.04 iₖ, to achieve non-geometric coordination among size
Cr O 3 0.02 - fractions, as expressed in Equation I.
2
Cl - 0.04 d i k
x
CeO 2 - 0.04 P 100 D (I)
x
PbO - 0.01
Note: “-” indicates content below detection limit. Where P represents the cumulative percentage
x
passing through the x sieve mesh, d is the sieve
th
x
aperture, D is the maximum particle size (5 mm), and i
k
is the grading coefficient for the k particle size fraction.
th
Laser particle size analysis revealed a distinct
boundary at 1 mm and 3 mm, allowing classification into
three fractions: Coarse (3 – 5 mm), medium (1 – 3 mm),
and fine (<1 mm). These fractions serve, respectively,
as skeleton support, filler, and lubricant/cementitious
components. Previous studies have shown that when the
fine particle content is below 70 wt%, the slurry is prone
to segregation with water separation rate exceeding 15%.
Conversely, when the coarse particle content is below
6%, the improvement in slurry flowability becomes
slow. Based on these findings, grading constraints were
imposed: The fine particles (<1 mm) must account for at
Figure 1. Particle size distribution of fly ash. The least 70 wt%, and the coarse particles (3 – 5 mm) must
graphs show both the differential volume distribution account for at least 6 wt%. Numerical optimization
curve (red curve, left axis) and the cumulative indicated that the optimal i values lie between 0.88 and
particle size distribution curve (blue curve, right 0.92. Five gradation levels (i = 0.88, 0.89, 0.90, 0.91,
k
axis) as functions of particle diameter and 0.92; Table 2) were therefore prepared, adjusting
k
For each formulation, the required amounts of coal the mass distribution ratios of the <1 mm, 1 – 3 mm,
gangue and fly ash were weighed and placed into a mixer and 3 – 5 mm fractions accordingly. The total solid mass
for thorough blending. Subsequently, the appropriate concentration for all samples was fixed at 72%, with the
amount of deionized water was added, and stirring solid components comprising 85% coal gangue and 15%

Volume 22 Issue 5 (2025) 181 doi: 10.36922/AJWEP025200154