3.2.1. UCS Analysis of Different Type Backfill Samples
In order to deeply analyze the influence of SC, b/t ratio, and curing age on the uniaxial compressive strength (UCS) of backfill, the control variable method is adopted to change a single factor while keeping other factors unchanged.
Figure 7 shows the UCS results of different types of backfill.
Figure 7a shows the UCS result of CTB with different SC, different b/t ratio, and different curing age. Keeping the curing age and the b/t ratio constant, the UCS of CTB increases as the SC increases with a power function. Taking the curing age of 14d and the b/t ratio of 1:4 as an example, when the SC is 66%, the UCS of CTB is 0.622 MPa; when the SC increased to 68%, the UCS of CTB is 0.851 MPa, the growth rate is 36.8%; when the SC increased to 70% and 72%, respectively, the UCS of CTB is 1.254 and 1.663 MPa, respectively, and the growth rates are 101.6% and 167.4%, respectively. Keeping the curing age and the SC constant, the UCS of CTB decreases as the b/t ratio decreases with an exponential function. Taking the curing age of 14d and the SC of 68% as an example, when the b/t ratio is 1:4, the UCS of CTB is 1.999 MPa; when the b/t ratio reduced to 1:6, 1:8, 1:10, and 1:12, respectively, the UCS of CTB reduced to 1.235, 0.851, 0.612, and 0.401 MPa, respectively, the reduction rates are 38.2%, 57.4%, 69.4%, and 79.9%, respectively. Keeping the SC and the b/t ratio constant, the UCS of CTB increases as the curing age increases. Taking the SC of 68% and the b/t ratio of 1:8 as an example, when the curing age is 7d, the UCS of CTB is 0.348 MPa; when the curing age increased to 14 and 28d, respectively, the UCS of CTB increased to 0.851 and 1.315 MPa, respectively, and the growth rates are 144.5% and 277.9%, respectively.
Figure 7b shows the columnar relationship between the UCS of CFTB1-1 and SC, b/t ratio, and curing age. Keeping the curing age and the b/t ratio constant, the UCS of CFTB1-1 increases as the SC increases with a power function. Taking the curing age of 14d and the b/t ratio of 1:8 as an example, when SC increased from 66% to 72%, the UCS of CFTB1-1 reduced from 0.541 to 1.421 MPa, the reduction rate is 162.7%. Keeping the curing age and the SC constant, the UCS of CFTB1-1 decreases as the b/t ratio decreases with an exponential function. Taking the curing age of 14d and the SC of 68% as an example, when the b/t ratio reduced from 1:4 to 1:12, the UCS of CFTB1-1 reduced from 1.813 MPa to 0.392 MPa, the reduction rate is 78.4%. Keep the SC and the b/t ratio constant, the UCS of CFTB1-1 increases as the curing age increases. Taking the SC of 68% and the b/t ratio of 1:8 as an example, when the curing age increased from 7d to 28d, the UCS of CFTB1-1 increased from 0.131 to 1.022 MPa, and the growth rate reached 680.2%.
Figure 7c shows the columnar relationship between the UCS of CFTB1-2 and SC, b/t ratio, and the curing age. Keeping the curing age and b/t ratio constant, the UCS of CFTB1-2 increases as the SC increases with a power function. Taking the curing age of 14d and b/t ratio of 1:8 as an example, when SC increased from 66% to 72%, the UCS of CFTB1-2 increased from 0.433 to 1.222 MPa, the growth rate is 182.2%. Keep the curing age and the SC constant, the UCS of CFTB1-2 reduced as the b/t ratio reduced with an exponential function. Taking the curing age of 14d and SC of 68% as an example, when the b/t ratio decreased from 1:4 to 1:12, the UCS of CFTB1-2 decreased from 1.647 to 0.333 MPa, the reduction rate is 79.8%. Keep the SC and the b/t ratio constant, the UCS of CFTB1-2 increases as the curing age increases. Taking SC of 68% and b/t ratio of 1:8 as an example, when the curing age increased from 7d to 28d, the UCS of CFTB1-2 increased from 0.074 to 0.894 MPa, increases by about 11 times.
Figure 7d,f shows the UCS results of CFTB2-1 and CFTB2-2, respectively. Keeping the curing age and the b/t ratio constant, the UCS of CFTB2-1 and CFTB2-2 increases as the SC increases with a power function. Keep the curing age and the SC constant, the UCS of CFTB2-1 decreases as the b/t ratio decreases with a logarithmic function, the UCS of CFTB2-2 decreases as the b/t ratio decreases with an exponential function. Keep the SC and the b/t ratio constant, the UCS of CFTB2-1 and CFTB2-2 both increases as the curing age increases.
The comprehensive analysis of the above results shows that the UCS of the backfill increases with the increase of SC, decreases with the decrease of the b/t ratio, and increases with the increase of the curing age. As the SC increases, the solid particle content in the backfill slurry increases, porosity inside the backfill decreases after the hydration reaction, and the overall strength of the backfill increases; as the b/t ratio is reduced, the binder particle content in the backfill slurry is reduced, the C-S-H gel produced after the hydration reaction decreases, the overall strength of backfill decreases; as the curing age increases, the hydration reaction between particle and particle inside backfill is sufficient, more C-S-H gel was produced, and the overall strength of the backfill increases.
In order to understand the influence of the coupling factors of the SC and b/t ratio on the UCS of backfill, keep the curing age unchanged, and change the SC and b/t ratio to obtain the three-dimensional scattered diagram between the SC, b/t ratio, and UCS, as shown in
Figure 8. It can be seen more intuitively from
Figure 8 that the UCS of backfill increases with increasing SC and decreases with decreasing b/t ratio. A three-dimensional surface fitting is carried out for the three-dimensional scatter diagram, and the fitting results are shown in
Table 6. It can be seen from
Table 6 that there is a good quadratic polynomial function relationship between the SC, b/t ratio, and UCS of the backfill. The fitting multiple correlation coefficient R
2 is greater than 0.97, and the average multiple correlation coefficient R
2 is as high as 0.9871.
3.2.2. Comparative Analysis of UCS of Different Types of Backfill Samples
In order to further understand the influence of FA type and dosage on the UCS of backfill, the difference between the UCS of backfill without FA and the UCS of backfill with FA is obtained. The results are shown in
Figure 9.
Figure 9a shows the decrement between the UCS of backfill without FA and the UCS of backfill with an FA1 dosage of 20%. As a whole, keeping the curing age and the b/t ratio constant, the UCS decrement increases as the SC increases. Taking the curing age of 14d and the b/t ratio of 1:8 as an example, when the SC is 66%, the UCS decrement is 0.081 MPa; when the SC increased to 68%, the UCS decrement increased to 0.038 MPa; when the SC increased to 70% and 72%, respectively, the UCS decrement increased to 0.232 and 0.242 MPa, respectively. Keeping the curing age and the SC constant, the UCS decrement decreases as the b/t ratio decreases. Taking the curing age of 14d and the SC of 68% as an example, when the b/t ratio is 1:4, the UCS decrement is 0.186 MPa; when the b/t ratio reduced to 1:6, 1:8, 1:10, and 1:12, respectively, the UCS decrement reduced to 0.125, 0.038, 0.098, and 0.009 MPa, respectively. Keeping the SC and the b/t ratio constant, as the curing age increases, the UCS decrement first decreases and then increases, and the general distribution pattern is the shape of a ‘V’. Taking the SC of 68% and the b/t ratio of 1:8 as an example, when the curing age is 7d, the UCS decrement is 0.217 MPa; when the curing age increased to 14d, the UCS decrement reduced to 0.038 MPa; when the curing age increased to 28d, the UCS decrement increased to 0.293 MPa.
Figure 9b shows the UCS decrement between the UCS of backfill without FA and the UCS of backfill with the FA1 dosage of 50%. As a whole, keep the curing age and the b/t ratio constant, the UCS decrement increases as the SC increases. Taking the curing age of 28d and the b/t ratio of 1:4 as an example, when the SC is 66%, the UCS decrement is 0.581 MPa; when the SC increased to 68%, 70%, and 72%, respectively, the UCS decrement increased to 0.806, 1.238, and 1.781 MPa, respectively. Keeping the curing age and the SC constant, the UCS decrement decreases as the b/t ratio decreases. Taking the curing age of 28d and the SC of 70% as an example, when the b/t ratio is 1:4, the UCS decrement is 1.781 MPa; when the b/t ratio reduced to 1:6, 1:8, 1:10, and 1:12, respectively, the UCS decrement reduced to 1.465, 0.890, 0.658, and 0.526 MPa, respectively. Keep the SC and the b/t ratio constant, as the curing age increases, the UCS decrement first decreases and then increases, the distribution pattern is the shape of a ‘V’. Taking the SC of 68% and the b/t ratio of 1:4 as an example, when the curing age is 7d, the UCS decrement is 0.561 MPa; when the curing age increased to 14d, the UCS decrement reduced to 0.352 MPa; when the curing age increased to 28d, the UCS decrement increased to 0.806 MPa.
Figure 9c shows the UCS decrement between the UCS of backfill without FA and the UCS of backfill with FA2 dosage of 20%. As a whole, keep the curing age and the b/t ratio constant, the UCS decrement increases as the SC increases. Taking the curing age of 28d and the b/t ratio of 1:4 as an example, when the SC is 66%, the UCS decrement is 0.380 MPa; when the SC increased to 68%, 70%, and 72%, respectively, the UCS decrement increased to 0.607, 1.102, and 1.561 MPa, respectively. Keep the curing age and the SC constant, the UCS decrement decreases as the b/t ratio decreases. Taking the curing age of 7d and the SC of 70% as an example, when the b/t ratio is 1:4, 1:6, 1:8, 1:10, and 1:12, respectively, the corresponding UCS decrement is 0.938, 0.715, 0.627, 0.456, and 0.343 MPa, respectively. Keeping the SC and the b/t ratio constant, as the curing age increases, the UCS decrement first decreases and then increases, the distribution pattern is the shape of a ‘V’. Taking the SC of 72% and the b/t ratio of 1:4 as an example, when the curing age is 7, 14, and 28, respectively, corresponding UCS decrement is 0.938, 0.757, and 1.561, respectively.
Figure 9d shows the UCS decrement between the UCS of backfill without FA and the UCS of backfill with FA2 dosage of 50%. Keep the curing age and the b/t ratio constant, the UCS decrement increases as the SC increases. Keeping the curing age and the SC constant, the UCS decrement decreases as the b/t ratio decreases. Keeping the curing age and the b/t ratio constant, when the SC is 66%, 68%, and 70%, the UCS decrement increases as the curing age increases; when the SC is 72%, as the curing age increases, the UCS decrement first decreases and then increases.
A comprehensive analysis of
Figure 9a–d shows that compared to the UCS of CTB, the UCS decrement of backfill with FA increases as the SC increases, decreases as the b/t ratio decreases, first increases and then decreases as the curing age increases. An analysis of the reasons shows that as the SC increases, the porosity inside the backfill decreases, the UCS of CTB and the UCS of backfill with FA both increase, then due to the addition of FA, the UCS increment of the backfill with FA decreases, and causes the UCS decrement to increase. As the b/t ratio decreases, the content of binder inside the backfill decreases, the C-S-H gel produced by the hydration reaction decreases, the UCS of CTB and the UCS of backfill with FA both decreases, the smaller the b/t ratio, the smaller the difference of the C-S-H gel produced by hydration reaction inside the backfill, so the UCS decrement decreases. In the initial curing phase (CT = 7d), the binder inside the backfill rapidly hydrates with water and tailings particles, which results in the strength increasing rapidly, the strength of the backfill varies greatly with different types; in the curing interim (CT = 14d), the hydration reaction process inside the backfill is almost complete, the strength of backfill reaches a high level with different types, and the strength difference decreases; in the curing later period (28d), the hydration reaction inside the backfill is basically over, the strength of the backfill no longer changes significantly with the curing age, and the strength of different types of backfill is close to its peak strength, leading to a strength difference increase again.
Figure 10 shows the strength difference results between the backfill sample with FA of different SC, different b/t ratio, and different curing ages.
Figure 10a shows the strength difference results between the backfill with an FA1 dosage of 20% and the backfill with an FA1 dosage of 50%. It can be seen from
Figure 10a that all of the results are positive, and it shows that the UCS of backfill decreases as the FA1 dosage increases. Keeping the curing age and the b/t ratio constant, the UCS decrement increases as the SC increases, taking the b/t ratio of 1:4 as an example, when the SC increased from 66% to 72% at the curing age of 7d, the UCS decrement increased from 0.133 to 0.322 MPa; when the SC increased from 66% to 72% at the curing age of 14d, the UCS decrement increased from 0.043 to 0.245 MPa; when the curing age was 28d, the corresponding UCS decrement increased from 0.355 to 0.900 MPa. Keeping the curing age and the SC constant, the UCS decrement decreases as the b/t ratio decreases, taking the SC of 68% as an example, when the b/t ratio decreased from 1:4 to 1:12 at the curing age of 7d, the UCS decrement decreased from 0.168 to 0.041 MPa; when the curing age was 14 and 28d, respectively, the corresponding UCS decrement decreased from 0.166 to 0.059 MPa and decreased from 0.294 to 0.149 MPa, respectively. Keeping the SC and the b/t ratio constant, as the curing age increases, the UCS decrement increases slowly first and then quickly. Taking the SC of 68% and the b/t ratio of 1:12 as an example, when the curing age is 7d, the UCS decrement is 0.041 MPa; when the curing age is 14d, the UCS decrement is increased to 0.059 MPa; when the curing age is 28d, the UCS decrement is increased to 0.149 MPa.
Figure 10b shows the strength difference results between the backfill with an FA2 dosage of 20% and the backfill with an FA2 dosage of 50%. It can be seen from
Figure 10b that all of the results are positive, and it shows the UCS of backfill decreases as the dosage of FA2 increases. When the SC is 66%, 68%, and 70%, as the curing age increases, the UCS decrement shows a trend of increase first and then a decrease; when the SC is 72%, as the curing age increases, the UCS decrement increases slowly first and then quickly. Under the same curing age and SC condition, the UCS decrement decreases as the b/t ratio decreases. Under the same curing age and b/t ratio condition, the UCS decrement increases as the SC increases.
Figure 10c,d shows the strength difference results between the backfill with FA1 and the backfill with FA2 when the dosage of FA is 20% and 50%, respectively. All of the results in the figure are positive, and it shows under the same FA dosage condition that the strength of backfill with FA1 is bigger than backfill with FA2. The results shown in
Figure 10c,d show no obvious pattern, but on the whole, under the same curing age and b/t ratio condition, the UCS decrement increases as the SC increases; under the same curing age and SC condition, the UCS decrement decreases as the b/t ratio decreases; under the same SC and b/t ratio condition, as the curing age increases, the UCS decrement first increases and then decreases.