Effect of Incorporating Waste Limestone Powder into Solid Waste Cemented Paste Backfill Material
Round 1
Reviewer 1 Report
In abstract, authors described test results concerning the change in porosity, macropore proportion, average pore radius of the samples. However, it is not easy to understand whether these above parameters are significant findings in authors’ paper. If those are important, it should be mentioned in Conclusions.
In Table 1, each mineral composition of tailings A, B, or limestone powder is supposed to be summed 100%?
Line 74, Table 1 shows that SiO2 accounts for about 30.4%. Whereas in Line 282, the existence of SiO2 was assumed to be about 30.4%. Can readers estimate 30.4% from Table 1?
It is not sure that limestone powder is used as fillers for aggregate or supplementary materials for cement.
Line 103, 1<Cc<3, however, in Table 2 , all Cc values are below 1.00.
In Table 2, LSP for A1, A2, A3, A4 and B1, B2, B3, B4 should be 0, 1.4, 2.1, 2.8.
In Figure 3, what is the target compressive strength for this SWCPB samples?
Line 144, Figure 4 compares the pore distribution of groups A and B. Please rewrite it.
Line 147, what do the intensities mean? Compressive strength?
If the required strength was 1.3MPa for the SWCPB samples (see Q.7), authors need to explain whether strengths of B2 and B3 samples were satisfied.
Line 159, what does SPCTB mean?
In Figure 7, is there any reason not to show SEM images of B1 and B2?
Lines 222-224, “The porosity of B3 was significantly higher than that of B3, as shown in Figure 7b. The B3 had a large number of pores which was above 3mm in diameter as the number of particles was insufficient.” Can this be explained using Figure 5 or Table 3?
In Figure 8, is there any reason to compare A4 and B4 samples?
Line 243, “…A4 while for B1-à B4?
Line 247-248. The porosity was found to be slightly larger than the NMR porosity. However in Table 3, the calculated porosities of A1, A2, B3, B4 are larger than the porosity. Other 4 cases are reversed.
Any discussed summaries using test results should be re-emphasized in conclusions.
Author Response
Q1-(Reviewer1): In abstract, authors described test results concerning the change in porosity, macropore proportion, average pore radius of the samples. However, it is not easy to understand whether these above parameters are significant findings in authors’ paper. If those are important, it should be mentioned in Conclusions.
Response 1: thanks for your suggestions. The logic of conclusion is disordered. So I rewrote it. The revised abstract and conclusion are as follows:
To effectively reuse the waste limestone powder, which is a major solid waste around mines, we replaced the limestone powder into a part of cement in solid waste cemented paste backfill (SWCPB) and studied the parameters of pore structures. To optimize the pore microstructure characteristics of SWCPB in mines, two different components and grade tailings were selected. The samples were characterized by Scanning Electron Microscopy (SEM) and Nuclear Magnetic Resonance (NMR) to examine the pore properties and microstructure of SWCPB. The results showed that ① at the later curing stage, with the optimization of pore characteristics and microstructure through the limestone powder admixture, the strength of SWCFB can be still guaranteed at a 20% replacement degree of cement. ② the porosity, the macropore proportion, and the average pore radius all negatively correlate with limestone powder content, which reduce by 7.15%, 46.35%, and 16.37% respectively. ③ Limestone powder as crystal nucleus participated in the hydration reaction and embedded into the product to enhance the strength.
To effectively reuse the waste limestone powder, a part of cement in SWCPB was replaced by limestone powder into. This paper studied the parameters of its pore structures and strength characteristics. The main conclusions are as follows:
① the strengths of SWCPB have a negative correlation with limestone powder content in 3 and 7 curing days. However in 28 curing days, limestone powder content does not have a significant impact on the strength. Except groups B2 and B3, the strengths of other groups can meet the mines’ requirements.
② the porosity, the macropore proportion, and the average pore radius all negatively correlate with limestone powder content, which reduce by 7.15%, 46.35%, and 16.37% respectively. The limestone powder in backfill can reduce the number of pores and the values of average pore radius.
③ Limestone powder as crystal nucleus participated in the hydration reaction and embedded into the product to enhance the strength of SWCPB. Thereby the pore distribution of backfill was optimized. With the optimization of pore characteristics and microstructure through the limestone powder admixture, it can optimize the strength of SWCFB in a certain extent.
Q2-(Reviewer1): In Table 1, each mineral composition of tailings A, B, or limestone powder is supposed to be summed 100%?
Response 2: yes, Table 1 does not have the element content of oxygen. So it looks like not 100%. To be more precise, I added oxygen to the table. But a mistake also existed in the table 1. The original data of limestone powder did not been normalized. So the summed content is 82.5% before normalizing. I revised this mistake and the original data is at the end of this report.
Q3-(Reviewer1): Line 74, Table 1 shows that SiO2 accounts for about 30.4%. Whereas in Line 282, the existence of SiO2 was assumed to be about 30.4%. Can readers estimate 30.4% from Table 1?
Response 3: it is a difficult problem. The original data of limestone powder are the form of the compound concentration. At first I put the form of element content into manuscript by processing the original data. But this is not precise, so I put the original form into the manuscript and revised a mistake(Q2).
Q4-(Reviewer1): It is not sure that limestone powder is used as fillers for aggregate or supplementary materials for cement.
Line 103, 1<Cc<3, however, in Table 2, all Cc values are below 1.00.
Response 4: the Cc is the overall shape describing the distribution of the gradation cumulative curve. The smoother the curvature coefficient curve, the better the gradation, which means that the coarser intergranular voids are filled by the finer particles. So the compacting of backfill is good. That Cc values are below 1.00 means that coarse particles in raw materials of backfill are missing. However since filling slurry needs to be transported through the pipeline, raw materials cannot contain many coarse particles. So for many types of backfill used in many mines in China, their Cc values are below 1.00. This is one of defects of backfill and one of reasons for its low strength. Even in this study I added limestone powder to backfill, Cc values cannot be in a suitable interval.
I revised the manuscript to explain this problem.
Q5-(Reviewer1): In Table 2, LSP for A1, A2, A3, A4 and B1, B2, B3, B4 should be 0, 1.4, 2.1, 2.8.
Response 5: yes, I revised them.
Q6-(Reviewer1): In Figure 3, what is the target compressive strength for this SWCPB samples?
Response 6: the target compressive strength for these SWCPBs is 0.5 MPa for 7days and 1.3 MPa for 28days. It has been added in Figure 3.
Q7-(Reviewer1): Line 144, Figure 4 compares the pore distribution of groups A and B. Please rewrite it.
Response 7: I have revised this sentence.
Q8-(Reviewer1): Line 147, what do the intensities mean? Compressive strength?
Response 8: yes, I want to express UCS. This is a translation mistake. It has been revised.
Q9-(Reviewer1): If the required strength was 1.3MPa for the SWCPB samples (see Q.7), authors need to explain whether strengths of B2 and B3 samples were satisfied.
Response 9: strengths of group B2 and B3 do not meet mines’ requirements. I have explained it in the manuscript. The revised sentence is as follows:
Comparing the strength of groups A with B, all strengths of group A are higher than that of group B. In addition, according to mines’ experience, the target strength of 7 days is 0.5 MPa while for 28 days, the required strength is 1.3 MPa. As a consequence, 7-day strength of all groups meets the requirements, while 28-day strength of B2 and B3 does not meet mine's strength requirements of backfill.
Q10-(Reviewer1): Line 159, what does SPCTB mean?
Response 10: this is a mistake. It has been revised to SWCPB in the manuscript.
Q11-(Reviewer1): In Figure 7, is there any reason not to show SEM images of B1 and B2?
Response 11: I want to control the length of the article. Moreover, the pore characteristics of B1 and B2 are similar to those of B3 and B4. But now it seems that this is not wise. So I added the SEM images of B1 and B2 to the article. The corresponding analysis is also added in it.
The structure of group B was looser than that of group A. Without limestone powder particles in backfill, a lot of coarse particles can be observed and there was no particle that filled into gaps between tailings(Figure 7a). Obviously the structure of group B4 is denser than that of B2 and B3. Groups B2 and B3 had more macropores and a lot of macrpores combined together to form a complex pore structure as shown in Figure 7, which means that the pore structure of B4 was simpler than that of B2 and B3. The calculated and NMR porosities of B3 and B4 is almost the same(Table 3). However the average pore radiuses of B3 were higher than that of B4, which means that pores of B4 were small but the distribution was relatively uniform. In addition, group B3 had a large number of pores which was above 3 μm in diameter as the number of specific particles which can fill into the pores was insufficient.
Q12-(Reviewer1): Lines 222-224, “The porosity of B3 was significantly higher than that of B3, as shown in Figure 7b. The B3 had a large number of pores which was above 3mm in diameter as the number of particles was insufficient.” Can this be explained using Figure 5 or Table 3?
Response 12: yes, it can be explained using Table 3 and Figure 5. But the expression in the manuscript is not accurate. The revised sentences are as follows:
The structure of group B was looser than that of group A. Without limestone powder particles in backfill, a lot of coarse particles can be observed and there was no particle that filled into gaps between tailings(Figure 7a). Obviously the structure of group B4 is denser than that of B2 and B3. Groups B2 and B3 had more macropores and a lot of macrpores combined together to form a complex pore structure as shown in Figure 7, which means that the pore structure of B4 was simpler than that of B2 and B3. The calculated and NMR porosities of B3 and B4 is almost the same(Table 3). However the average pore radiuses of B3 were higher than that of B4, which means that pores of B4 were small but the distribution was relatively uniform. In addition, group B3 had a large number of pores which was above 3 μm in diameter as the number of specific particles which can fill into the pores was insufficient.
Q13-(Reviewer1): In Figure 8, is there any reason to compare A4 and B4 samples?
Response 13: Figure 8 shows two samples that contrasted original SEM images and its binarized images. The purpose of Figure 8 is just to show the changes and differences of SEM and binarized images. If all SEM and binarized images were put in the manuscript, the length cannot be controlled. So I just put A4 and B4.
Q14-(Reviewer1): Line 243, “…A4 while for B1-à B4?
Response 14: it has been revised.
Q15-(Reviewer1): Line 247-248. The porosity was found to be slightly larger than the NMR porosity. However in Table 3, the calculated porosities of A1, A2, B3, B4 are larger than the porosity. Other 4 cases are reversed.
Response 15: yes, it is not accurate. So I revised the sentence, which is as follows:
Table 3 showed that the calculated porosity was calculated by using the image processing method of Figure 8. With the help of stereology principles, the calculated porosities of groups A1, A2, B3 and B4 were slightly larger than the NMR porosity. Other 4 cases are reversed. Although there were differences between the calculated porosity and the NMR porosity, the values were almost the same.
Q16-(Reviewer1): Any discussed summaries using test results should be re-emphasized in conclusions.
Response 16: thanks for your suggestions. I rewrote the conclusions, which is as follows:
To effectively reuse the waste limestone powder, a part of cement in SWCPB was replaced by limestone powder into. This paper studied the parameters of its pore structures and strength characteristics. The main conclusions are as follows:
① the strengths of SWCPB have a negative correlation with limestone powder content in 3 and 7 curing days. However in 28 curing days, limestone powder content does not have a significant impact on the strength. Except groups B2 and B3, the strengths of other groups can meet the mines’ requirements.
② the porosity, the macropore proportion, and the average pore radius all negatively correlate with limestone powder content, which reduce by 7.15%, 46.35%, and 16.37% respectively. The limestone powder in backfill can reduce the number of pores and the values of average pore radius.
③ Limestone powder as crystal nucleus participated in the hydration reaction and embedded into the product to enhance the strength of SWCPB. Thereby the pore distribution of backfill was optimized. With the optimization of pore characteristics and microstructure through the limestone powder admixture, it can optimize the strength of SWCFB in a certain extent.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This paper deals with the substitution of cement by limestone powder in solid waste cemented paste backfill which is a common solution in concrete solution. The paper is interesting and well written even if some typos are remaining (in citations format, surry instead of slurry in the title of table 2…). The paper can be accepted in the present state but I have some remarks that should be addressed before final acceptance.
1°) Please add the PSD of cement in figure 1. Also, the PSD of final mixes (A1, A4, B1 and B4) can be also added to show the difference in Cu and Cc
2°) How the authors explain the apparent retardation in strength development due to cement substitution?
3°) Is the limestone addition change the rheology of the materials? If the material gets more fluid, it can explain the macropore decrease.
Author Response
Q1-(Reviewer1): Please add the PSD of cement in figure 1. Also, the PSD of final mixes (A1, A4, B1 and B4) can be also added to show the difference in Cu and Cc
Response 1: thanks for your suggestions. I added the PSD of cement in Figure 1. And the PSD of final mixes is a good idea to show the difference in Cu and Cc. But as shown in the next Figure, the differences from A1 to A4 and from B1 to B4 are so small that we cannot identify them. So I did not add this Figure in the manuscript.
Q2-(Reviewer1): How the authors explain the apparent retardation in strength development due to cement substitution?
Response 2: thanks for your suggestions. The reason is that the hydration processes of stone powder cement tailings backfill can be divided into following four stages: dissolution period, condensation period, infiltration period and hardening period. At the early curing stage, limestone powders play a role of crystal nucleus, while at the later curing stage, hydration products can react with SiO2. This can improve hydration reactions. The detailed explanation can be found in reference[10] of the manuscript.
I also added this explanation into the manuscript.
Q3-(Reviewer1): Is the limestone addition change the rheology of the materials? If the material gets more fluid, it can explain the macropore decrease.
Response 3: thanks for your suggestion. Yes according to another test, larger the amount of stone powder, longer it takes the slurry to reach equilibrium state, whereas smaller are the values of equilibrium shear stress and equilibrium viscosity. That means limestone powder can reduce the dynamic viscosity of slurry and enhances the slurry conveying performance.
According to your suggestion, I revised the manuscript, which is as follows:
According to another constant shear test results, larger the amount of stone powder, longer it takes the slurry to reach equilibrium state, whereas smaller are the values of equilibrium shear stress and equilibrium viscosity. That means limestone powder can reduce the dynamic viscosity of slurry and enhances the slurry conveying performance. As a consequence, less macropores will be formed in backfill after slurries solidify[25]. The strength of samples with cement substitution remained high even after replacing the limestone powder because the proportion of macropores in group A was reduced and the microstructure in presence of limestone powder admixtures was optimized.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
It was revised well by responding reviewer's comments.
