Strength Performance and Stabilization Mechanism of Fine Sandy Soils Stabilized with Cement and Metakaolin

Round 1

Reviewer 1 Report

The manuscript is within the scope of this journal. In materials and methods section, it is very clumsy to understand the proportions of the components. How are the mix proportions fixed between the different components ratios? This needs proper explanation. Also the insightful of the tests is completely missing. All of this make the validation of all work and comparison with other authors (and discussion) impossible. It seems that a lot of work has been done but it is not well documented. Without the exact composition of the mixtures it is not possible to draw any conclusions. The specific comments are mentioned in the attached file.

Comments for author File: Comments.pdf

Author Response

First of all, we appreciate all reviewers’ precious comments on our manuscript. All the comments and suggestions are encouraging and constructive. We have carefully revised the manuscript following these suggestions. The detailed revisions are as follows.

Reviewer #1:

1. In Line 13-25: Two many times the same sentence. Perhaps it can be possible to insert an acronym. The same through all text.

• Thank you very much for your suggestions. We have used the abbreviation of CMSFSS for cement and metakaolin stabilized fine sandy soils in the whole text.

2. In Line 133: I couldn’t find anywhere in the text the conditions for these tests; loading rate, etc.

• We are very sorry for this issue. In Section 2.1.2, we just introduced basic parameters for the devices we used rather than the detailed testing parameters used in the experiments. Now we have given detailed statements after the introduction of these devices. They are:

To ensure the reliability and stability of testing results that might be affected by the size of specimens, the loading rate of unconfined compression tests was 1.0 mm/min.

In this study, SEM images of cement and metakaolin stabilized fine sandy soils (CMSFSS) at two magnifications were collected at an acceleration voltage range of 15kv.

3. In Line 156-157: Why are here the dosages of binder? it means that for all these mixtures the water-binder ratio was kept constant and equal to 0.6? Why is this ratio of 0.6 referred before (2) Tests for the water binder ratio. It is not understandable for the reader. The titles should be "tests with different cement-metakaolin ratio". The same for the other titles.

• We are very sorry for this confused expression. This study used metakaolin as the additive to replace cement partly in fine sandy soil stabilization since metakaolin could play a significant role in the hydration reaction of cement. However, the more dosage of metakaolin did not mean a better performance improvement. The optimal cement-metakaolin ratio was the primary condition for the other tests. Hence, a constant total dosage of cement and metakaolin should be adopted to determine the optimal cement-metakaolin ratio by single-factor experiment. To be easily expressed, this study also used the binder to denote the mixture of cement and metakaolin. So the dosage of the binder was the ratio of cement and metakaolin for soil stabilization. The water binder ratio equal to 0.6 in tests for the cement-metakaolin ratio was an assumed value and determined by trial and error, taking a sensual fluidity of soil mass. We have made a statement for this issue at the beginning of Section 2.2.

4. In Line 181-182: Please insert a table with the dosages of the components for all specimens and each test. For instance, curing time was only tested for 15 % of binder and not for the other ones? The same for SEM and MIP?

• Thank you very much for your suggestions. We have added a table for them, in which the cement-metakaolin ratio, the dosage of the binder, the water-binder ratio, and the curing age were all listed for each experimental design. Please see Table 2.

5. What does "four times" mean?

• We are very sorry for this wrong expression. The filling of samples was completed at one time. We have revised it.

6. In Line 204: But what is a standard curing environment? Please specify correctly the temperature and humidity.

• According to the Standard for Geotechnical Testing Method GB/T 50123-2019 issued by the Ministry of Housing and Urban-Rural Development of China, the temperature and humidity should be 20±3℃ and 90-95%, respectively. We followed these curing conditions in our experiments. We have made a statement in the manuscript now.

7. In Line 210: This description of specimen preparation is very incomplete. It can not be accepted like this. Why there are not photos of the process or of the specimens?

• Yes, we have added more detailed information about the specimen preparation. Some important photos of the process and the specimens are presented in the manuscript now. They were:

It is worth noting that all mass mixing ratios in this study were based on the dry weight of the soil. As shown in Figure 3, when sampling, the dry fine sandy soils, cement, and me-takaolin were weighed following the designed experimental scheme and blended thoroughly first. Then the water was mixed into these mixtures several times until the sample was mixed uniformly. Their mixing time should be controlled for 10-20 mins at least according to practical experience. When the mixture was ready, it was filled into a cylindrical mold with a diameter of 50 mm and a height of 100 mm at one time. Each specimen in the mold would be tamped vertically 15 times from the edge to the center by a ramming bar in the spiral direction. The ramming bar must insert into the adjacent layer at least 5 mm. A scraper would also be inserted and extracted several times along the mold's inner surface to eliminate air and ensure the integrity of the specimen's surface. When these steps were completed, the final prepared specimens were placed in a standard curing environment for 24 hours and then de-molded and cured at the same conditions until the target curing age was reached. According to the Standard for Geotechnical Testing Method GB/T 50123-2019 issued by the Ministry of Housing and Urban-Rural Development of China, the temperature and humidity of a standard curing en-vironment were 20±3℃ and 90-95%, respectively. Since significant differences in the weight of specimens could appear after removing the mold, six specimens were prepared in the initial specimen preparation. Only three with the minimum weight error were selected as a parallel comparison for the final tests to ensure the experimental data's high reliability. For micro-structural observation, these cured specimens were cut into small pieces with sizes of 5 mm × 5 mm × 5 mm and 10 mm × 10 mm × 10 mm. Then they were polished and leveled carefully for SEM image collection and Mercury intrusion test. The small specimens for SEM tests would be sprayed with a gold layer and vacuumed to prevent the high-energy electron beam from being absorbed or scattered on the air molecules during the test process.

(a) Dry mixture  (b) Wet sample  (c) Cylindrical specimens

Figure 3 Specimen preparation

8. In Line 213: But what standard or protocol was follow for the tests?

• Yes, the tests followed the Standard for Geotechnical Testing Method GB/T 50123-2019 issued by the Ministry of Housing and Urban-Rural Development of China. We have made a statement in the manuscript.

9. In Line 214-215: Please emphasized here what is the water binding ratio of these specimens: 0.6? Was it kept constant? Does this mean that: a)10% of binder has 6% of water; b) 15 % of binder has 9% of water; and so on? And what about the soil? It is not clear whatsoever what was kept constant, (in weight?), the soil? The total components? In this last case each specimen has different dosages of soil? The table mentioned earlier in my comment is mandatory.

• We are sorry for our unclear expression. The water binder ratio was the ratio of water to cement and metakaolin in weight. Since the water binder ratio was unknown in the tests of the cement-metakaolin ratio, it was supposed to be 0.6. However, to investigate the acceptable value for sandy soil stabilization in this study, we also tested cement and metakaolin stabilized fine sandy soils (CMSFSS) with different water binder ratios. The results illustrated that the compressive strength of CMSFSS decreased nonlinearly with the water-binder ratio. The greater the water-binder ratio, the more significant the decrease. Considering the workability of cement and metakaolin stabilized fine sandy soils and the aim to achieve their compressive strength as high as possible, the water-binder ratio was suggested to be 0.6 for the subsequent tests. Namely, the water-binder ratio in tests of the dosage of binder and the curing age was 0.6. Here it should also be noted that the mass ratio defined all ratios used in this study. We have made a statement in Sections 2.2 and 3.1.

10. In Line 221: Not affected by the dosage of the binder? But it is 519 kPa for 10%, 1255 kpa for 15% and so on. Please clarify

• We are sorry for our inaccurate expression. Our original meaning was that the cement-metakaolin ratio (the mixing ratio of cement and metakaolin) was not affected by the total dosage of cement and metakaolin. We have revised this expression. They are: The maximum compressive strength of fine sandy soils with the same dosage of the binder was always obtained when the cement-metakaolin ratio was 5:1. Namely, the cement-metakaolin ratio did not change with the total dosage of cement and metakaolin. Further, the unconfined compressive strength of CMSFSS would not be affected by the dosage of the binder when their mixing ratio was keeping 5:1.

11. In Line 240: The four lines of the mean value must be in a unique graph for compairing. And it would be possible to see the maximum value for all at 5:1.

• Thanks very much for your suggestions. Now we have integrated these lines into one picture. Please see Figure 4. Figures 5-7 were also improved in the same way to keep the consistency of the picture format.

12. In Line 244: Please insert here that the dosage of the binder was 15%. And answer the same questions of before.

• Yes, we have added more detailed information about the experiment design for the readability of the results you suggested. They are: Here the water-binder ratios were 0.4, 0.6, 0.8, 1.0, and 1.2, the cement-metakaolin ratio was 5:1, the dosage of the binder was 15%, and the curing age was seven days. Similar issues have also been revised in the whole manuscript.

13. In Line 246: As asked before do the mixtures were prepare with the same amount of soil? What is the dosage of water+binder in the mixture? The same questions arise here after in this text.

• Yes, we have added more detailed information about the experiment design for the readability of the results you suggested. They are: Here the water-binder ratios were 0.4, 0.6, 0.8, 1.0, and 1.2, the cement-metakaolin ratio was 5:1, the dosage of the binder was 15%, and the curing age was seven days. Similar issues have also been revised in the whole manuscript.

14. In Line 273: But what mixtures were tested?

• Yes, we have added more detailed information about the experiment design for the readability of the results you suggested. They are: Here the curing ages were 3, 7, and 28 days, the cement-metakaolin ratio was 5:1, the water-binder ratio was 0.6, and the dosage of the binder was 15%. Similar issues have also been revised in the whole manuscript.

15. In line 290: Why are the figueres here before their citation?

• Very sorry for this typesetting error. We moved these figures after their citations. Some similar issues were also checked and revised carefully now.

16. In line 291: This is a comparaison? But it is not explained in the text what are the properties of the mixtures tested by the authors mentioned? The same comments for further comparisons

• Thanks for your questions. It was not a comparison. We just collected testing data on different soils to verify the effectiveness of the proposed formula. We have added more detailed information on these testing data in the manuscript.

17. In Line 311: But why these titles are not in the same order of 3.1, 3.2 ...

• We did not follow the same order as Section 3 because the mechanical performance improvement of fine sandy soils was significantly affected by the dosage of the binder, the water-binder ratio, and the curing age. Hence, the discussions on the relationship between compressive strength and these influence factors were further discussed in Section 4. Here we had made a statement in the manuscript.

Reviewer 2 Report

In this manuscript, a series of compression and microstructural observation tests on cement and metakaolin stabilized fine sandy soils were conducted with different cement-metakaolin ratios, water-binder ratios, dosages of binder, and curing ages. The influence of these factors on the mechanical performance of cement and metakaolin stabilized fine sandy soils was studied. The empirical relationships between compressive strength and these influence factors of cement and metakaolin stabilized fine sandy soils were discussed. Then the strengthening mechanism of cement and metakaolin stabilized fine sandy soils at different curing ages was investigated. This manuscript is well written and can be received after minor modifications:

(1)    In line 37-40: The fine sand distributed in regions along rivers and coasts not only has low bearing capacity, high settlement, and poor stability, but also has the characteristics of easily broken particles. The influence of particle breakage on the engineering mechanical properties of fine sand can not be ignored. The following literature is for reference:

Particle breakage behaviors of a foundation filling material on island-reefs in the South China Sea under impact loading. Bulletin of Engineering Geology and the Environment. 2022, 81 (9): 345. DOI: 10.1007/s10064-022-02844-3.

Particle breakage mechanism and particle shape evolution of calcareous sand under impact loading. Bulletin of Engineering Geology and the Environment. 2022, 81 (9): 372. DOI: 10.1007/s10064-022-02868-9.

(2)    In the introduction, what is the innovation of this article?

(3)    In line 215-217: Why were the unconfined compressive strength of cement and metakaolin stabilized fine sandy soils with dosages of cement and metakaolin of 10%, 15%, 20%, and 25%  all increasing first and then decreasing with the ratio of cement to metakaolin?

(4)    In section 3.2: Why the unconfined compressive strength of cement and metakaolin stabilized fine sandy soils decreased nonlinearly with the water-binder ratio?

(5)    In line 299: Whether the fitting parameters in Equation 1 have clear physical significance?

(6)    In section 5: What was the strengthening mechanism of cement and metakaolin stabilized fine sandy soils at different curing ages?

Author Response

First of all, we appreciate all reviewers’ precious comments on our manuscript. All the comments and suggestions are encouraging and constructive. We have carefully revised the manuscript following these suggestions. The detailed revisions are as follows.

1. In Line 37-40: The fine sand distributed in regions along rivers and coasts not only has low bearing capacity, high settlement, and poor stability, but also has the characteristics of easily broken particles. The influence of particle breakage on the engineering mechanical properties of fine sand can not be ignored. The following literature is for reference:

Particle breakage behaviors of a foundation filling material on island-reefs in the South China Sea under impact loading. Bulletin of Engineering Geology and the Environment. 2022, 81 (9): 345. DOI: 10.1007/s10064-022-02844-3.

Particle breakage mechanism and particle shape evolution of calcareous sand under impact loading. Bulletin of Engineering Geology and the Environment. 2022, 81 (9): 372. DOI: 10.1007/s10064-022-02868-9.

• Thank you very much for your valuable suggestions. We have done a review on them carefully and cited them in the manuscript now.

2. In the introduction, what is the innovation of this article?

• Enhancing strength performance while reducing cement consumption for cement-stabilized soils is the key to improving the economic benefits of engineering construction projects like retaining structures of underground engineering, subgrade bases, and foundation reinforcement. This study employed metakaolin as the additive to replace cement partly in fine sandy soil stabilization since metakaolin could play a significant role in the hydration reaction of cement or have covalent polymerization due to the alkaline environment formed by calcium hydroxide. Both these two chemical reactions could produce gels filling the pores in fine sandy soils, improving the structural compactness of fine sandy soils and thereby enhancing their strength. For investigating the contribution of metakaolin on the mechanical performance improvement of fine sandy soils, a series of compression and microstructure observation tests, including Scanning Electronic Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) tests, were conducted following a well-designed experimental scheme. Now we have made a supplement to the innovation of this study at the beginning of the last paragraph in the introduction.

3. In Line 215-217: Why were the unconfined compressive strength of cement and metakaolin stabilized fine sandy soils with dosages of cement and metakaolin of 10%, 15%, 20%, and 25% all increasing first and then decreasing with the ratio of cement to metakaolin?

• The main reason for this outcome may be that when the ratio of cement to metakaolin was greater than 5:1, the calcium hydroxide formed by the hydration of cement could fully rehydrate with the silicon aluminum oxides in metakaolin or covalently polymerize the silicon aluminum oxides in metakaolin in an alkaline environment, more cementitious materials were formed to fill the pores in fine sandy soils, thereby effectively improving the impermeability of fine sandy soil. When the ratio of cement to metakaolin was less than 5:1, the dosage of metakaolin was relatively high. Namely, the dosage of cement was reduced relatively. The calcium hydroxide formed by the hydration of cement could not fully ensure to react with silicon-aluminum minerals in metakaolin forming CSH gel or covalently polymerize silicon-aluminum minerals to form geopolymer. Hence, the compressive strength of cement and metakaolin stabilized fine sandy soils decreased. This issue has been detailedly analyzed in Section 3.1.

4. In Section 3.2: Why the unconfined compressive strength of cement and metakaolin stabilized fine sandy soils decreased nonlinearly with the water-binder ratio?

• The reason for this nonlinear change may be that the high water-binder ratio would increase the residual pores after hardening in cement and metakaolin stabilized fine sandy soils significantly due to much free water evaporation; meanwhile, the specific surface area of metakaolin was small, and the excessive water may be detrimental to the silicon aluminum minerals in metakaolin reacting with calcium hydroxide. Therefore, their strength performance decreased with the water-binder ratio. This issue has been detailedly analyzed in Section 3.2.

5. In Line 299: Whether the fitting parameters in Equation 1 have clear physical significance?

• We wholeheartedly agree with your points on the fitting parameters used in the proposed empirical equation (1). Although we did not give a clear physical significance to them in the test, we found that the values of fitting parameters a and b should be 1.0×10³ and 0.5, which were obtained based on the fitting of the data from the experiment results in this study and verify with other cited studies. We have made a statement in the manuscript now.

6. In Section 5: What was the strengthening mechanism of cement and metakaolin stabilized fine sandy soils at different curing ages?

• The structural compactness of fine sandy soils determined their mechanical performance to some extent. In this study, the metakaolin with high pozzolanic activity could participate in the hydration reaction of cement in an alkaline environment or have covalent polymerization due to the alkaline environment formed by calcium hydroxide. Both these two chemical reactions could produce gels filling the pores in fine sandy soils, improving the structural compactness of fine sandy soils and thereby enhancing their strength. We have made a statement in the manuscript now.

Round 2

Reviewer 1 Report

    The english language must be checked.

Author Response

We appreciate your precious comments on our manuscript that the english language must be checked. Now we have tried our best to improve the English writting, including syntax errors, wrong expression, vague rhetoric，etc. Please see the attached revised manuscript. Thanks a lot for alll your help to our manuscript publication again.