An Investigation into the Suitability of GGBS and OPC as Low Percentage Single-Component Binders for the Stabilisation and Solidification of Harbour Dredge Material Mildly Contaminated with Metals
Round 1
Reviewer 1 Report
The article deals with the use of OPC and GGBFS for stabilization and solidification of harbor dredge material. The topic is interesting but the article is not well structured and it is very unclear in some parts. In particular:
- The introduction is not exhaustive. It describes the use of OPC in S/S, but in this part the use of GGBFS in Stabilization and Solidification is missing.
- The reasons for choosing only two percentages of GGBS is not explained. Why did you use only 6 and 10% and not even 4 and 12% as for the samples made with OPC?
- It is not appropriate to write in an article the following sentence: “It was expected that the 6% of OPC mix could yield results but there was not sufficient quantity of the sample remaining to perform a direct comparison with the other OPC % mixes tested for strength” (Line 120-122)
- The limit of x axis in figure 2 is different from all the other figures
- The graphs are unclear. It is difficult to understand the behavior and the value of the different mixtures. It is not necessary to report the second decimal value in the y-axes.
-Figure 5B: it is not the better way to magnify a figure
-Figure 7: The x axis is cut at 14 days and 15-day data are missing.
-Figure 8: there are other clearer and more formal ways of representing the results (for example, scatter or line diagrams)
-The strengths of mixture made with GGBF or with percentages lower than 8% of OPC have never been reported in the article. Nevertheless, in the conclusions it is reported that "The strength of GGBS and the lower (<8%) OPC binder mixes would not generally be acceptable as suitable fill for earthworks".
- The discussion is not exhaustive: The reasons for the different behavior of mixtures made with different percentages of binder or with different binders against the Arsenic Leachate, Copper Leachate, Chromium Leachate, Mercury Leachate and Zinc Leachate are not described.
For the above mentioned reasons, I recommend to reject the paper.
Author Response
- The introduction is not exhaustive. It describes the use of OPC in S/S, but in this part the use of GGBFS in Stabilization and Solidification is missing. The introduction details several studies where GGBS is used as a component within a binder mix (Dublin Port, Port Gavle) however there has been little evidence of GGBS being used as the sole binder, hence this experiment. I have added another very recent publication comparing GGBS and OPC binder behaviour to expand the discussion on GGBS (Zhang et al, 2018)
- The reasons for choosing only two percentages of GGBS is not explained. Why did you use only 6 and 10% and not even 4 and 12% as for the samples made with OPC? The reasoning provided in Test Setup “The reasoning for two percentages of GGBS chosen are that binders of less than 6% GGBS are not considered due to lack of binding properties. GGBS mixes above 10% were not considered either. As OPC performs well above 10% as shown in the literature and it is typically slightly cheaper than GGBS, improved performance at higher GGBS mixes percentages would not be attractive from construction economic viewpoint, which is the focus of this study.
- It is not appropriate to write in an article the following sentence: “It was expected that the 6% of OPC mix could yield results but there was not sufficient quantity of the sample remaining to perform a direct comparison with the other OPC % mixes tested for strength” (Line 120-122) This sentence has been removed.
- The limit of x axis in figure 2 is different from all the other figures This has been corrected
- The graphs are unclear. It is difficult to understand the behavior and the value of the different mixtures. It is not necessary to report the second decimal value in the y-axes. The graphs have been refined to add clarity including adding gridlines.
-Figure 5B: it is not the better way to magnify a figure Graph has been refined, authors attempted to utilise primary and secondary y axis but results were not clear when doing so reverted to original format.
-Figure 7: The x axis is cut at 14 days and 15-day data are missing. This has been corrected
-Figure 8: there are other clearer and more formal ways of representing the results (for example, scatter or line diagrams) Figure 8 changes to a scatter diagram
-The strengths of mixture made with GGBF or with percentages lower than 8% of OPC have never been reported in the article. Nevertheless, in the conclusions it is reported that "The strength of GGBS and the lower (<8%) OPC binder mixes would not generally be acceptable as suitable fill for earthworks". It is accepted that the less binder added the lower the strength, the 8% OPC mix show 20KPA strength so it can be assumed that lower %OPC mixes would have lower strengths.
- The discussion is not exhaustive: The reasons for the different behavior of mixtures made with different percentages of binder or with different binders against the Arsenic Leachate, Copper Leachate, Chromium Leachate, Mercury Leachate and Zinc Leachate are not described. The discussion has been expanded on the leachate behaviours with a focus on the atypical behaviours of GGBS leachate results. Comparison to recent studies on GGBS and OPC in aggregates is now provided
Reviewer 2 Report
Major comments and suggestions
1. 2. Nature of Contamination: Provide references for WAC, AA EQS, and MAC EQS.
2. 2. Nature of Contamination: No explanation on the procedure of the leaching test. Which standard method of leaching test the did author use to measure the WAC leachate concentration for raw samples? Was It the Tank Test EA NEN 7375:2004?
3. 2. Nature of Contamination: No explanation regarding a sample preparation. What’s the solid to liquid ratio for the leaching test? How long the test was performed? Did the author use deionized water or acid water?
4. Page 3, Lines 107 – 109: What’s the meaning of a percentage? Does it mean that a percentage with respect to the weight of the composite sample or the volume of the composite sample?
5. Page 3, Lines 107 – 109: What’s the water to cement ratio? Or What’s the water to solid ratio? Can the author provide a table of mixture proportion?
6. Page 5, Lines 142- 143: Please restate this sentence. This explanation does not make sense to the reviewer. “It could be deduced that the increase in arsenic is from leaching of the GGBS binder rather than the dredge material with levels of arsenic leachate peaking at almost three times the MAC EQS limit”.
7. Figures 2 and 3: By looking at the results of arsenic and copper leaching tests, the reviewer strongly recommends the author to characterize the components of GGBS. The leaching results are noticeably affected by the components of GGBS.
8. Figures 5A and 5B: If the author repeated the leaching test three times, please show the error bars in Figures 5A and 5B (all other Figures as well). Therefore, the reviewer and other readers can judge whether the mercury leachates of 10% and 12% cement are an outlier or human error.
9. Page 9, Lines 177 – 178: Why the GGBS reduces the pH of the composite sample?
Minor comments and suggestions
10. Page 2, Line 2: m3 -> m3
11. Figure 1: (c) 6%ggbs -> 6% GGBS
12. Figure 2: Keep the consistency of a label. Use either ggbs or GGBS throughout the manuscript.
Author Response
1. 2. Nature of Contamination: Provide references for WAC, AA EQS, and MAC EQS.
References provided
2. 2. Nature of Contamination: No explanation on the procedure of the leaching test. Which standard method of leaching test the did author use to measure the WAC leachate concentration for raw samples? Was It the Tank Test EA NEN 7375:2004?
Single stage compliance leaching test (BSI, 2002)
3. 2. Nature of Contamination: No explanation regarding a sample preparation. What’s the solid to liquid ratio for the leaching test? How long the test was performed? Did the author use deionized water or acid water?
Di ionized, liquid/solid (LS) ratio of 10 L/kg and agitated for 24 hours. The mixture is filtered and tested to give the aqueous concentrations leachable contaminant from which a leachable soil concentration is calculated.
4. Page 3, Lines 107 – 109: What’s the meaning of a percentage? Does it mean that a percentage with respect to the weight of the composite sample or the volume of the composite sample?
All mixes are mixed percentage by weight of composite sample
5. Page 3, Lines 107 – 109: What’s the water to cement ratio? Or What’s the water to solid ratio? Can the author provide a table of mixture proportion? For the leachate testing the samples are mixed without additional water, this is to represent industry practise. The raw material has a high moisture content >60% negating the need for additional water during mixing process.
6. Page 5, Lines 142- 143: Please restate this sentence. This explanation does not make sense to the reviewer. “It could be deduced that the increase in arsenic is from leaching of the GGBS binder rather than the dredge material with levels of arsenic leachate peaking at almost three times the MAC EQS limit”. This sentence has been rewritten “The behaviour of arsenic in GGBS binders requires further investigation with levels of arsenic leachate peaking at almost three times the MAC EQS limit. This represents an increased leaching compared with the leachate from the raw samples.”
7. Figures 2 and 3: By looking at the results of arsenic and copper leaching tests, the reviewer strongly recommends the author to characterize the components of GGBS. The leaching results are noticeably affected by the components of GGBS. The chemical constituents of GGBS and OPC are now provided in Table 2. Copper and Arsenic are not present in GGBS in noticeable quantities.
8. Figures 5A and 5B: If the author repeated the leaching test three times, please show the error bars in Figures 5A and 5B (all other Figures as well). Therefore, the reviewer and other readers can judge whether the mercury leachates of 10% and 12% cement are an outlier or human error.
The Raw material leachates were repeated 3 times but the mixed samples were only tested once per time period, leachate. However all testing was undertaken in a UKAS certified Laboratory.
9. Page 9, Lines 177 – 178: Why the GGBS reduces the pH of the composite sample?
The pH of GGBS is lower than OPC in its raw state therefore by replacing GGBS with OPC the composite pH is also reduced.
Minor comments and suggestions
10. Page 2, Line 2: m3 -> m3
Corrected
11. Figure 1: (c) 6%ggbs -> 6% GGBS
Corrected
12. Figure 2: Keep the consistency of a label. Use either ggbs or GGBS throughout the manuscript.
Corrected
Round 2
Reviewer 1 Report
The authors modified the paper according to the indications of the previous review.
Author Response
Thank you for your review of the paper.
Reviewer 2 Report
Major comments and suggestions
1. Table 2: The author added Table 2. However, the reviewer recommended the author to characterize the components of the actual GGBS which was used in the test, Not the typical components of GGBS, since the data show that the addition of GGBS affects the leaching results.
2. Figures 5A and 5B: The author responded that the mixed samples were only tested once per time period, leachate.
Based on the reviewer’s experience, the leaching test results are greatly affected by many factors not only the source but also the L/S ratio, particle size and shacking time, etc. The reviewer is skeptical whether the leaching results in the manuscript have the representativeness unless the author shows more results so that the reviewer can check a trend. The result might be an outlier.
3. Figure 7: For the pH of 6% and 10 GGBS, the author responded that the pH of GGBS is lower than OPC in its raw state therefore by replacing GGBS with OPC the composite pH is also reduced.
The reviewer knows that the addition of Portland cement increases the pH. By adding more Portland cement, the pH of a mixture will increase. However, there is no pH trend for the addition of GGBSs. For the 6% GGBS, it’s not a replacement of 6% Portland cement. It’s a mixture of the contaminated dredge material and 6% GGBS addition. And, 6% GGBS increases the pH of the mixture (6% GGBS). However, the addition of 10% GGBS decreases the pH of the mixture (10% GGBS) in comparison with 6% GGBS. In other words, the addition of 6% GGBS to the raw material increases pH up to around 9.7 but the addition of 10% GGBS to the raw materials increase pH only up to 8.5. Does the addition of GGBS increase or decrease the pH of the raw material? How would the author explain this result? There is no pH trend for the mixture of raw material and GGBS.
4. The entire manuscript and figures: Keep the consistency of an abbreviation. Use either ggbs or GGBS throughout the manuscript.
Author Response
1. The Table 2 gives the components of GGBS used in the mixing as tested by the supplier of the GGBS - ECOCEM.
2. We have updated the discussion on mercury summarised below:
Based on this the high concentrations of mercury in the OPC could be due to localised pockets of contamination in the samples used for the 4% and 12% OPC sample.
Mercury is the only metal that is soluble in water so the aqueous mercury concentrations would develop in the sample during the mixing stage and readily diffuse out from the porewater.
Another key point is that the impact of the structural integrity of the GGBS samples on leaching from the monoliths. The monolith tests are designed to assess surface diffusion – once the samples structure fails the transport processes will likely be affected. So the rising trends observed for the GGBS samples could be a direct consequence of the structural changes. It should be noted that the focus of the manuscript is on discussing the suitability of different binders for mildly contaminated dredged materials and the impact on cost. We haven’t introduced this paper as study into the chemical processes with the different binders nonetheless further discussion on partition coefficients is provided for each contaminant.
3. The discussion on pH has been expanded to include the following
"Conversely, the pH of the GGBS mixes reduces as the percentage of GGBS binder increases, as the pH for the 6% sample exceeds the pH of the raw dredged material and decreasing the 10% GGBS sample. It is likely that this is due to variation in the pH of the dredged material used in the 10% GGBS sample rather than the increase in the GGBS binder."
4. The graphs have been amended with GGBS and OPC.
Round 3
Reviewer 2 Report
The paper has been revised by authors and its quality has an improving.
