Compressive Strength and Resistance to Sulphate Attack of Ground Granulated Blast Furnace Slag, Lithium Slag, and Steel Slag Alkali-Activated Materials

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper studies the strength and sulfate resistance of ground granulated furnace slag-lithium slag-steel slag alkali-activated materials. The research of this paper has certain engineering value, but in order to meet the requirements of publication, the following parts need to be revised.

1. The abstract part needs to clearly state the purpose of the research and the methods used in this paper. The current abstract is not clear about the purpose of the research, and the research methods are not detailed enough. The main advantages of this paper are that the research object is alkali-activated materials containing three types of cementitious materials, and the mix ratio takes into account different material combinations, different alkali-activated material combinations, different water-binder ratios, and different exposure environments. At the same time, macro and micro comparative analysis is also carried out. The advantages of these material designs and experimental analysis need to be emphasized in the abstract.

2. In lines 84-89 of the introduction, the author's statement is inaccurate. At present, there are many papers on the resistance of alkali-activated materials to sulfate erosion. The original points of this paper are that the research object belongs to alkali-activated materials containing three types of cementitious materials, and most of the previous studies focused on slag. Second, the mix ratio of this paper takes into account various different materials and environmental exposure conditions. Third, the macroscopic mechanism is explained through microscopic analysis.

3. In the mix design table 2, three levels of water-binder ratio are considered, 0.38, 0.40, and 0.42. These three water-binder ratios are very close. Considering the influence of experimental error, is the choice of such a water-binder ratio reasonable?

4. As shown in Table 2, for alkali-activated materials, GLS13, GLS14, and GLS15, when exposed to sodium sulfate solution, can the sodium sulfate solution stimulate the continuous reaction of the cementitious material? If so, how does the author characterize it?

5. In Figure 9, in the XRD analysis, quartz appears. The author needs to explain the reason for the appearance of quartz. Generally speaking, crystalline quartz is not reactive. However, from the comparison of several XRD graphs, the corresponding peak of quartz appears to be reduced. What is the reason? The author needs to give a reasonable explanation.

6. In Figure 10, in the FTIR analysis, a table needs to be added to explain the chemical bonds corresponding to each peak, which can increase the readability of the article.

7. Generally speaking, the ability to resist sulfate erosion is closely related to the material's anti-permeability ability. The strength of concrete is one of the manifestations of anti-permeability ability. I suggest that the author conduct a correlation analysis between the ability to resist sulfate erosion and the compressive strength, so as to explain the relationship between different macroscopic results and enhance the persuasiveness of the article.

 

8. The English of this article needs extensive editing. The current English affects the reviewer's reading. In future writing, the author also needs to pay attention to English writing.

Comments on the Quality of English Language

Extensive editing of English language required.

Author Response

Comments 1: The abstract part needs to clearly state the purpose of the research and the methods used in this paper. The current abstract is not clear about the purpose of the research, and the research methods are not detailed enough. The main advantages of this paper are that the research object is alkali-activated materials containing three types of cementitious materials, and the mix ratio takes into account different material combinations, different alkali-activated material combinations, different water-binder ratios, and different exposure environments. At the same time, macro and micro comparative analysis is also carried out. The advantages of these material designs and experimental analysis need to be emphasized in the abstract.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the abstract to provide a clear statement of the aim and methodology of the study, as well as to highlight the strengths of the material design and experimental analysis. Page 1, paragraph 1, line 1.

 

Revised version: Alkali-activated materials (AAMs) are favoured for their low carbon emissions, excellent mechanical properties and excellent chemical resistance. In this paper, ternary alkali-activated cementitious materials are prepared from slag, steel slag and lithium slag to investigate strength and resistance to sulphate attack. A series of experiments were conducted using a variety of material combinations, alkali activator combinations, water-binder ratios and exposure environments. These experiments employed both macro and micro comparative analyses. The hydration reaction products, physical phase composition and microstructure of the ground granulated furnace slag-lithium slag-steel slag (GLS) ternary AAMs were analysed using x-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was experimentally demonstrated that the GLS ternary AAMs had excellent compressive strength, had good resistance to sodium sulphate erosion and the resistance to magnesium sulphate erosion decreases with time. This study contributes to the advancement of knowledge regarding the utilisation of lithium and steel slag, and offers new insights into the field of alkali-activated cementitious materials and their resistance to sulfate erosion.

 

Comments 2: In lines 84-89 of the introduction, the author's statement is inaccurate. At present, there are many papers on the resistance of alkali-activated materials to sulfate erosion. The original points of this paper are that the research object belongs to alkali-activated materials containing three types of cementitious materials, and most of the previous studies focused on slag. Second, the mix ratio of this paper takes into account various different materials and environmental exposure conditions. Third, the macroscopic mechanism is explained through microscopic analysis.

 

Response 2: Thank you for pointing this out. We agree with this comment. We have realised that the statement in lines 84-89 of the Introduction about research on the resistance of alkali-inspired materials to sulphate attack is inaccurate. In fact, there have been many studies on the resistance of alkali-activated materials to sulfate erosion. We have corrected it in revised manuscript. Page 2, paragraph 5, line 1.

 

Revised version: This paper makes three original contributions to the field. Firstly, the study focuses on alkali-activated materials comprising three types of cementitious materials, which is a less common focus in previous studies, which have primarily concentrated on slag-based AAMs. While numerous studies have focused on the macroscopic results of sulphate erosion [33, 34], fewer have delved into the qualitative and quantitative analyses of the erosion products and the microscopic erosion mechanisms of AAMs after sulphate damage.Secondly, the research considers a variety of materials and environmental exposure conditions in its mix ratios. Thirdly, the macroscopic degradation mechanisms are elucidated through microscopic analysis. This study aims to fill these gaps to better understand the performance of AAMs in practical applications.

 

Comments 3: In the mix design table 2, three levels of water-binder ratio are considered, 0.38, 0.40, and 0.42. These three water-binder ratios are very close. Considering the influence of experimental error, is the choice of such a water-binder ratio reasonable?

 

Response 3: Thank you for your advice. In selecting the water-binder ratio, we took into account the potential impact of experimental error on the experimental design. To this end, we conducted three independent experiments, thereby ensuring the reliability and accuracy of the results. Three levels of water-binder were selected, 0.38, 0.40 and 0.42, despite the proximity of these values, in order to comprehensively assess the impact of varying water-gel ratios on the properties of the materials under investigation. The impact of experimental variability was mitigated by calculating an average value, and the findings indicated that the three ratios exhibited minimal influence on early strength. However, notable discrepancies were observed in flowability and late strength.

 

Comments 4: As shown in Table 2, for alkali-activated materials, GLS13, GLS14, and GLS15, when exposed to sodium sulfate solution, can the sodium sulfate solution stimulate the continuous reaction of the cementitious material? If so, how does the author characterize it?

 

Response 4:  Thank you for your insightful comment. Our study reveals that the compressive strength of GLS ternary AAMs increases with higher concentrations of sodium sulfate solution, showing a consistent trend after both 28 and 56 days of immersion. Initially, sodium sulfate inhibits early strength but enhances later strength due to its role as an external salt activator [1]. The formation of dense calcium sulfate precipitates on the specimen surface during immersion hinders ion exchange, leading to lower initial hydration and compressive strength compared to clear water immersion. After 56 days of erosion, the presence of Na⁺ in sodium sulphate solution has been observed to act as an alkaline exciter, thereby promoting the formation of hydration products, including silicates and aluminates, within steel and lithium slags. The sodium ions and the alkaline exciter work in concert over time to accelerate the hydration reaction within the material, forming additional gelling substances such as C-S-H and C-A-S-H gel . These hydration products fill the pore space within the material, resulting in a denser and stronger structure, which improves the compressive strength and durability of the material. Page 11, paragraph 3, line 14.

[1]. Mei, J., et al., Effect of sodium sulfate and nano-SiO₂ on hydration and microstructure of cementitious materials containing high volume fly ash under steam curing. Construction and Building Materials, 2018. 163: p. 812-825. http://doi.org/10.1016/j.conbuildmat.2017.12.159

 

Comments 5: In Figure 9, in the XRD analysis, quartz appears. The author needs to explain the reason for the appearance of quartz. Generally speaking, crystalline quartz is not reactive. However, from the comparison of several XRD graphs, the corresponding peak of quartz appears to be reduced. What is the reason? The author needs to give a reasonable explanation.

 

Response 5: Thank you for your question. We apologize for the inconvenience caused by us. With regard to the quartz peak displayed in the XRD analysis illustrated in Fig. 9, it should be noted that the initial labelling did not include the presence of spodumene. Indeed, the quartz peak in the XRD pattern is superimposed with the spodumene peak, indicating that the observed change in diffraction peak is primarily attributable to the continued hydration of spodumene. The continued excitation of spodumene by sodium ions in sodium sulphate resulted in the acceleration of the hydration reaction within the material. This led to the formation of additional gel substances, such as C-S-H and C-A-S-H gel, which enhanced the compressive strength and durability of the material. The figure has been modified and an accompanying explanation added to the manuscript in order to clarify this point. Page 13, paragraph 1, line 4.

 

Revised version: When the immersion time is 28 days, the presence of spodumene can be detected in the XRD results, indicating the presence of lithium slag that has not undergone hydration. When the soaking time is further increased, it can be seen that the diffraction peaks of spodumene decrease significantly, the spodumene in the specimen can continue to participate in the hydration reaction with the increase of time, which is also the main reason for the gradual increase of the compressive strength of the specimen under the condition of sodium sulphate erosion.

 

Fig. 9. XRD pattern of GLS ternary AAMs

Comments 6: In Figure 10, in the FTIR analysis, a table needs to be added to explain the chemical bonds corresponding to each peak, which can increase the readability of the article.

 

Response 6: Thank you for your question. We agree with your comment and will add a table to the FTIR analysis to explain the chemical bonds corresponding to each peak.Page 14, Table 7.

 

Revised version: Table 7. Assignments of the bands in the FTIR spectra of GLS ternary AAMs.

Components	Wavenumber (cm-1)	Interatomic bonds	Groups
C-S-H	448-451	O-Si-O	SiO44-
	874-875	Si-O	SiO44-
	972-991	Si-O	SiO44-
	3451-3456	O-H	H₂O
	1648-1654	H-O-H	H₂O
C-A-S-H	713-714	Si-O-Al	AlO4
	874-875	Si-O	SiO44-
	972-991	Si-O	SiO44-
	3451-3456	O-H	H₂O
	1648-1654	H-O-H	H₂O
CaCO3	1418-1423	C-O	CO32-

 

Comments 7: Generally speaking, the ability to resist sulfate erosion is closely related to the material's anti-permeability ability. The strength of concrete is one of the manifestations of anti-permeability ability. I suggest that the author conduct a correlation analysis between the ability to resist sulfate erosion and the compressive strength, so as to explain the relationship between different macroscopic results and enhance the persuasiveness of the article.

 

Response 7: Thank you for your question. The compressive strength of GLS ternary AAMs was observed to increase with the concentration of sodium sulfate solution, reaching a higher value at 56 days than at 28 days for the same concentration. In contrast, the compressive strength of the magnesium sulphate solution exhibited a decline with the augmentation of its concentration and erosion time. GLS ternary AAMs demonstrated robust resilience to sodium sulphate erosion, as well as to magnesium sulphate during the initial stages. However, this resistance gradually diminished with the prolongation of erosion time. We concur with your recommendation and will undertake a correlation analysis between the resistance to sulphate erosion and compressive strength. This will facilitate a more comprehensive explanation of the relationship between the disparate macroscopic outcomes, thereby enhancing the persuasiveness of the article. The analysis has been completed and the results will be included in the revised manuscript. Page 11, paragraph 2, line 6. Page 12, paragraph 1, line 21.

 

Revised version: Fig. 8(a) shows the changes in compressive strength of GLS ternary AAMs immersed in different time and sodium sulphate solution concentration. It can be seen that the compressive strength of GLS ternary AAMs has the same trend of change after 28 days and 56 days of immersion, and the compressive strength increases with the increase of sodium sulfate solution concentration, and the compressive strength of 56 days is higher than that of 28 days at the same concentration. This indicates that GLS ternary AAMs display robust resistance to sodium sulfate erosion.Despite the initial inhibitory effect of sodium sulfate on specimen strength, it has been observed that this effect is reversed in the long term, resulting in enhanced strength.

 

The corrosion resistance coefficient of GLS ternary AAMs immersed in magnesium sulphate solution decreases with the increase of erosion solution concentration.GLS ternary AAMs have good resistance to magnesium sulphate erosion in the early stage, but the resistance of specimen to magnesium sulphate erosion deteriorates with the increase of erosion time.

 

Comments 8: The English of this article needs extensive editing. The current English affects the reviewer's reading. In future writing, the author also needs to pay attention to English writing.

 

Revised version:Thank you for your comment. We apologize for the inconvenience caused by us.We acknowledge the significance of your feedback and have taken the initiative to enhance the article's English version, with the objective of enhancing clarity and comprehension. Furthermore, greater attention will be paid to the use of English expression in future writing, with a view to enhancing the quality of the article.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors conducted a deep and fairly complete study of the strength characteristics, structure and corrosion resistance of the developed GLS ternary AAMs. The results obtained are interrelated and consistent.

Indexes in chemical compounds and units of measurement should be checked.

There are no serious comments or other recommendations for the article.

Author Response

Comments 1:

Indexes in chemical compounds and units of measurement should be checked.

Response 1: Thank you for your insightful comments. It is a very good suggestion. In response to your suggestion, We have double-checked and corrected the indexes and units of measure in the compounds.All relevant sections have been updated for accuracy.Your understanding is greatly appreciated. Page 2.

 Revised version: Table 2. Mix proportion (by wt.%).

Table 1. Chemical and physical properties of raw materials .

Materials	GGBFS	LS	SS
CaO (wt. %)	50.22	54.86	44.40
SiO2 (wt. %)	25.62	22.39	15.70
Al2O3 (wt. %)	12.07	13.72	5.45
Fe2O3 (wt. %)	0.31	1.27	20.56
SO3 (wt. %)	2.41	6.05	0.85
MgO (wt. %)	5.18	0.32	4.64
K2O (wt. %)	0.30	0.60	0.46
SSA(cm²·g-1)	2525	7623	10128
D10(μm)	2.46	1.50	2.03
D50(μm)	10.56	6.368	25.73
D90(μm)	33.52	35.94	88.14

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

- General remark: It would be better if the author placed the figures after the discussion of each section, as this is the common practice in the articles.

- Line 53 and 54: Ensure there is a space after each period at the end of the sentence.

- Effect of water/binder ratio on strength: The strength was similar for the three samples on the 7th curing day. Does this indicate that the ratio does not significantly affect the geopolymerization reaction at an early age? Please provide more explanation.

- The samples were activated using a combination of sodium silicate and sodium hydroxide. However, in the discussion, it is mentioned that only CASH and CSH gels were produced, with no NASH gel formation. Could you please explain why this is the case?

Author Response

Comments 1: It would be better if the author placed the figures after the discussion of each section, as this is the common practice in the articles.

 

Response 1: Thank you for your insightful comments. It is a very good suggestion. We have rearranged the relevant figures and tables that follow the discussion in each chapter to conform to the common format and conventions of the article, based on your suggestions.

 

Comments 2: Line 53 and 54: Ensure there is a space after each period at the end of the sentence.

Response 2: Thank you for pointing out this mistake.Following your suggestion, we have double-checked and made sure that there is a space after each period in lines 53 and 54.

Comments 3: Effect of water/binder ratio on strength: The strength was similar for the three samples on the 7th curing day. Does this indicate that the ratio does not significantly affect the geopolymerization reaction at an early age? Please provide more explanation.

Response 3: Thank you for your question. We apologize for the inconvenience caused by us. The strength results you mention for the 7th day of conditioning show similar strengths for all three samples, suggesting that the effect of the w/b ratio on the reaction is not significant in the early stages (e.g. days 3 and 7). The reason for this is that the early reaction rate is slow, the moisture mainly promotes contact between the reactants, the pore structure has not yet changed significantly and the generation of hydration products is low. Therefore, the effect of different w/b ratios on early strength is small. However, over time the change in water/bender ratio has a significant effect on late strength and fluidity. Page 7, paragraph 3, line 9.

Revised version: Furthermore, it was determined that during the initial stages (specifically on days 3 and 7), the influence of the w/b ratio on the material's strength is not statistically significant. This is due to the fact that the early reaction rate is slow, the water mainly facilitates the contact of reactants, the pore structure has not yet undergone a notable alteration, and the generation of hydration products is minimal. Consequently, the impact of varying w/b ratios on the early strength is insignificant. However, as time progresses, the modification of the w/b ratio exerts a considerable influence on the late strength and fluidity.

Comments 4: The samples were activated using a combination of sodium silicate and sodium hydroxide. However, in the discussion, it is mentioned that only CASH and CSH gels were produced, with no NASH gel formation. Could you please explain why this is the case?

Response 4: Thank you for your insightful comment. Although the samples were activated by sodium silicate and sodium hydroxide, and the discussion only mentions the generation of CASH and CSH gels, the presence of NASH gels was also confirmed. The presence of a high sodium (Na⁺) content in the hydration product can be confirmed through energy-dispersive spectroscopy (EDS) analysis. Furthermore, X-ray diffraction (XRD) analysis indicates that Na⁺ is not involved in the crystal formation, suggesting that it is present in the gel. Nevertheless, the

NASH gel was not identified in the XRD analysis, as it was challenging to discern directly in the XRD pattern. Page 18, paragraph 3, line 8.

Revised version: The combination of XRD analysis demonstrated that Na+ was not involved in the formation of crystals, indicating that it was present in the gel. Although XRD analysis did not identify the presence of the N-A-S-H gel, this does not preclude its possibility. The high sodium content and lack of involvement in crystal formation indicate that the N-A-S-H gel may be present in the sample.

Author Response File: Author Response.pdf