Characterization of Ashes from Sewage Sludge–Limestone Incineration: Study of SSA Properties and Reactivity for SCM Use
, Philippe Grosseau 1, Hichem Krour 2, Laetitia Bessette 2, Gonzague Ziegler 3 and Anthony Serclerat 3Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe reviewer sent the comments and suggestions for the Authors in the attachment.
Comments for author File: 
Comments.pdf
Correct English, including formulated tasks.
Author Response
Reviewers Comments:
We first would like to thank the reviewer for the time he spent. In addition, we improved the English for a better understanding of the paper.
Reviewer 1:
Review of publication “Characterization of Ashes from Sewage Sludge-Limestone Incineration: A Study for Effective Use in Cement-Based Composites.” Authors: Danah Shehadeh, Alexandre Govin, Philippe Grosseau, Hichem Krour, Laetitia Bessette, Gonzague Ziegler and Anthony Serclerat
This manuscript is focused on research on characterizing (chemical, mineralogical, and mechanical) the sewage sludge ashes (SSA) produced by the fluidized bed calcination technique following the addition of limestone. In addition, SSA behavior in an aqueous solution through the electrical conductivity method. The objective of the work is relatively straightforward and clear. However, some issues should be addressed. Comments that should be taken into account by the authors before publication are presented below:
Comment 1:
Line 31- 71. The authors in the introduction limit themselves to presenting the general behavior of fly ash after the combustion of sewage sludge in a mineral binder. There are no literature reports on the behavior of the tested ash with limestone and no indication of its potential application directions. The reviewer suggests supplementing.
Response 1:
Thank you for highlighting this point. In fact, most of the incineration processes have a step of Ca-based additions to treat toxic gases, as mentioned in lines 73-74. Usually, these additives are injected within the fluidized bed. However, in the case of this study, the added additive, limestone, is injected directly into the dried sewage sludge. The following was added to the introduction:
“Calcium-based additives to the dewatered SS showed good reactivity and efficiency in decreasing sulfur release [17–19]. During carbonization, reactive CaO bonds with sulfur (S) to form CaSO4 [20]. Additionally, in the presence of a high Ca concentration and rich phosphorus (P) fuel mix, Ca-P compounds are observed to form (Ca3(PO4)2) [21,22].” (Lines 76-80).
This highlights the commonly formed compounds observed when introducing Ca-based additives.
Comment 2:
Line 143. The reviewer suggests supplementing the information with the type of calorimeter (isothermal or adiabatic), including parameters.
Response 2:
We appreciate your suggestion. We have now included detailed information regarding the calorimeter used (TAM air isothermal calorimeter) and the experimental parameters, including the temperature settings. These additions can be found in the revised manuscript on lines 157-158.
Comment 3:
Line 162: “The strength activity index (SAI) of concrete mortar specimens.” The reviewer suggests changing it to “mortar specimens or cement mortar.”
Response 3:
Thank you for the suggestion. We have updated the terminology to "cement mortars" for clarity and consistency. This change is reflected in line 182 of the manuscript.
Comment 4:
The order of table numbers given in the manuscript is incorrect.
Response 4:
We have reviewed and corrected the table numbering throughout the manuscript to ensure proper sequence and clarity.
Comment 5:
Line 198 -199. Table 1. The authors determined the specific surface area of the tested ashes using the BET method, where the given values may be unrealistic due to the saturation of the surface pores with the gas of sewage sludge ash samples. The presented results indicate a strongly developed specific surface area, which does not correspond to the results presented for the particle dimensions by the laser method. The reviewer suggests that the porosity value should be included in Table 1 or that the specific surface area of the tested ashes should be determined using another popular method, e.g., Blaine.
Response 5:
We have clarified the BET analysis process we have used. Before testing, we conducted a rigorous preconditioning phase, which involved degassing the samples under vacuum at high temperatures to remove contaminants and moisture. This step ensures accurate surface area measurement.
The following was added: “In addition, the specific surface area (BET method) was evaluated using ASAP 2020 (Micromeritics). Before analysis, approximately 1 gram was degassed for at least 24 hours at 100°C, to remove moisture and surface contamination. Following the degassing step, the analysis was done using the basis of nitrogen gas (N2) adsorption at -196.2 °C and a relative pressure range (p/p°) of 0.05 to 0.3.” (Lines 113-117).
We have also included a discussion on the specific surface area of SSA and referenced literature that supports the high specific surface area observed. (Cyr, M., Coutand, M., & Clastres, P. (2007). Technological and environmental behavior of sewage sludge ash (SSA) in cement-based materials. Cement and Concrete Research, 37(8), 1278-1289.) (Lines 210-211).
In this study, we have chosen to work on BET as it does not require any assumption for the particle shape and provides direct measurements. Additionally, according to previous experimental work, a correlation exists between the BET and Blaine’s surface area.
Reference: Mantellato, S., Palacios, M., & Flatt, R. J. (2015). Reliable specific surface area measurements on anhydrous cements. Cement and Concrete Research, 67, 286-291.
Potgieter, J. H., & Strydom, C. A. (1996). An investigation into the correlation between different surface area determination techniques applied to various limestone-related compounds. Cement and concrete research, 26(11), 1613-1617.
Comment 6:
Line 220-221. Table 3. Are the heavy metal values given in terms of the dry weight of the samples? The reviewer suggests giving the values in mg/kg dry mass.
Response 6:
The heavy metal values in Table 3 were already in mg/kg dry mass, but the unit has been updated to mg/kg dry mass.
Comment 7:
Line 302-307. The reviewer agrees with the statement, but the Authors do not take into account that the introduction of ash with a high phosphorus and other elements content (Table 2) into this type may result in the formation of compounds that may disrupt the cement hydration process, including the precipitation of compounds that will affect mechanical strength. In turn, introducing ash with a high CaO content into cement may cause an increase in the Ca(OH)2 content, which in the case of exposure of building materials to a corrosive environment will result in a decrease in corrosion resistance, resulting from the fact that Ca(OH)2 will be washed out first, which is confirmed by the Authors' research (Figure 4).
Response 7:
Thank you for this insightful comment. We agree that the introduction of SSA may lead to the formation of new hydrates. However, our study (section 3.3.2) shows that only the concentration of leached Ca and S varies over time. For P and Fe elements, their concentration in the solution was detected to be very low. This indicates the limited availability in the solution and therefore might limit the formation of new hydrates. This is discussed in Lines 440-442.
Regarding Ca(OH)2 formation, our study focuses on the use of SSA as an SCM, with partial substitution of cement. Further research is needed to conclusively determine the impact of Ca(OH)2 levels in SSA-blended cement composites.
Comment 8:
Line 395-396. Table 3. No unit is specified (last column) in Table 3.
Response 8:
Thank you for pointing this out. The unit in the last column of Table 3 has been added and is now specified as mg/kg dry mass.
Comment 9:
Line 416. The authors focus on the behavior of Ca ions, not considering that the tested ashes contain significant amounts of phosphorus (from 10.3 to 12.7%), iron (from 4.9 to 15.4%), and SO3 (from 4.7 to 5.6%), where the presence of these elements may determine, for example, the value of the heat of hydration. The reviewer suggests supplementing the data with the correlation value between the heat of hydration and compressive strength (for example, after 7 days) to determine whether it exists.
Response 9:
We agree with your observation and have expanded our discussion to include the potential impact of phosphorus, iron, and SO3 on the heat of hydration. However, our analysis showed that components containing P and Fe have low solubility in alkaline solutions, suggesting they have a limited impact on hydration heat (lines 475-476).
We have supplemented the manuscript with preliminary correlation data between the heat of hydration and compressive strength (Lines 545-553).
Comment 10:
Line 425-426. The reviewer disagrees with the Authors' statement. It suggests that the tested ashes show a particle size smaller than the size of cement particles (Table 1). The smaller the grain size, the larger the specific surface area.
Response 10:
We acknowledge the reviewer’s point. Table 1 indicates that SSA particles are larger than OPC particles but exhibit a higher specific surface area due to high porosity. Our analysis focuses on the chemical and mineralogical composition of SSA and its effect on heat generation, rather than the physical characteristics, due to the current limitations in the data.
The R3 method test was done for the four ashes A1, A2, A3, and A4, to determine their reactivity, separately from the cement. This is why a comparison between the physical properties of SSA and cement was not discussed.
Comment 11:
Line 466-473. The authors state that replacing 25% of the weight of cement with another material reduces the cement content in the mortar, which results in less formation of C-S-H hydrate. On the other hand, the results presented in Fig.6 indicate that the value of pozzolanic activity after 28 days of curing is above 75%. How will they explain this fact?
Response 11:
Upon substitution, the formation of hydrates decreases, as a 25% reduction in cement content naturally results in 25% less C-S-H formation in the mortar. Consequently, the Strength Activity Index (SAI) is expected to be around 75% at 28 days.
Lines 517-518 in the text: “Generally, a 75% SAI belongs to 25% of OPC substitution by an inert material”.
Wang, Y., Burris, L., Shearer, C. R., Hooton, D., & Suraneni, P. (2021). Strength activity index and bulk resistivity index modifications that differentiate inert and reactive materials. Cement and Concrete Composites, 124, 104240.
However, in our study, the observed SAI values exceeded 75% at 28 days. This indicates that the SSA enhanced the mortar's properties, compared to inert materials. This improvement may be attributed to the slow pozzolanic activity of SSA, which precipitated additional C-S-H due to the pozzolanic reaction, contributing to the development of mechanical strength at 28 days (Lines 530-533).
Comment 12:
The conclusions contained in the manuscript do not raise any objections. The title itself raises doubts. The authors conducted a detailed characterization of fly ash after sewage sludge combustion. However, the studies do not concern their behavior after introducing cement composites. The authors presented the characteristics (chemical, mineralogical, and physical) of the tested ashes, the leachability of heavy metals from the tested samples, and the heat of hydration of samples based on the RILEM TC 267-TRM standard using XRD and DTG methods. To determine the behavior of the tested ashes, it would be necessary to conduct tests based on cement mortars, where their properties may be influenced by the type of cement, w/c ratio, amount of SSA, and its role (cement or sand substitute). For example, when introduced into cement composites with such an amount of CaO, the tested type of fly ash may show a high susceptibility to corrosive aggression. The reviewer suggests changing the title of the manuscript. Based on the presented research results, can the Authors state that the tested ash can partially replace Portland cement as Supplementary Cementitious Materials (SCM)?
Response 12:
We appreciate the reviewer's feedback and have revised the title to better reflect the study’s scope: " Characterization of Ash from Sewage Sludge-Limestone Incineration: Study of Properties and Reactivity for SCM use."
Based on our findings, SSA shows promising mechanical properties, one of the key factors in the construction sector, indicating its potential as a Supplementary Cementitious Material (SCM). However, further research is required to fully assess its behavior when used in cement composites. The following was added in the conclusion: “Based on our findings, the application of SSA in cement composites showed promising mechanical properties, indicating that the tested ash can partially replace Portland cement SCM. Further investigations are to be considered regarding the impact of SSA on cement mortar properties, as well as exploring activation mechanisms to improve SSA reactivity.” (Lines 573-577).
Reviewer 2 Report
Comments and Suggestions for Authors1. The physical and chemical properties of the material in the results section should be included in the material-method section of the article. In the results section, only the experimental results related to the final samples should be included.
2. Section 3.2 is supported by the literature.
3. According to which standard was the strength activity index of SSA determined? How many days' strengths were evaluated?
4. There are some grammatical errors in the paper. These should be corrected.
5. The Introduction section of the paper should be added to the recent publications on roller compacted concretes.
Comments on the Quality of English Language-There are some grammatical errors in the paper. These should be corrected.
Author Response
Reviewers Comments:
We first would like to thank the reviewer for the time he spent. In addition, we improved the English for a better understanding of the paper.
Reviewer 2:
Comment 1:
The physical and chemical properties of the material in the results section should be included in the material-method section of the article. In the results section, only the experimental results related to the final samples should be included.
Response 1:
Thank you for your insightful comment. We appreciate your suggestion. We have merged Tables 1 and 2 into a single table (now Table 1 Line 119-120), which includes the physical properties and oxide composition of the raw materials. This table has been relocated to the Materials and Methods section as requested (section 2.2)
Comment 2:
Section 3.2 is supported by the literature
Response 2:
We appreciate your positive assessment of Section 3.2. We have ensured that this section is well-supported by the relevant literature to maintain the scientific rigor of our study.
Comment 3:
According to which standard was the strength activity index of SSA determined? How many days' strengths were evaluated?
Response 3:
Thank you for raising this point. The strength activity index (SAI) of SSA was determined according to the French standard EN450-1, with a cement replacement of 25%, as highlighted in lines 182-184.
We evaluated the strengths for up to 28 days, which we have now clarified in the manuscript in lines 187-188.
Comment 4:
There are some grammatical errors in the paper. These should be corrected
Response 4:
We have thoroughly reviewed the manuscript and corrected the grammatical errors identified (highlighted throughout the manuscript). We hope this enhances the clarity and readability of our paper.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article has been corrected according to the comments. I recommend that the article be accepted.
