Investigation of Self-Healing Mortars with and without Bagasse Ash at Pre- and Post-Crack Times

Round 1

Reviewer 1 Report

(1) This manuscript presents an experimental study of self-healing mortar based on the technology of microbiologically mediated calcite precipitation. The first section is somewhat disorganized and unclear, especially the late part about the presented study (Line 108~118); what is the main objective of this study, especially in relation to the literature review (what is the novelty/innovation beyond the published works)? What about mortar as opposed to concrete research reviewed?

In addition, what is the main idea of various selected materials investigated, e.g., why to use calcium lactate powder from discarded eggshells and lactic acid? why burnt bagasse ashes was used as a partial replacement? Admittedly some of the answer is hinted later in the main part of the manuscript, but it is very important to offer readers a general idea of what you want to achieve in the beginning.

There are also some poor writing of questionable choice of wording in this section, such as

Line 75~76, “a … difficult to cast concrete approach”;

Line 88~79, “has piqued the interest of many people”;

Line 89~90, “because to their endo spores' ability to withstand a hard environment”;

(2) Line 175~176, “then self-healing process was monitored after 7, 14, 175 and 28 days. The healed width of mortar samples was measured to determine the self- healing efficacy of mortar at the macro and micro levels.”

Are the detailed, quantified results about the healed width or self- healing efficacy available and shouldn't them be reported?

(3) Line 231~235, how the cracks are filled in the present study? is healing material injected or introduced to the crack to heal it (therefore it is more like a repair), or the mortar heals it on its own thanks to presence of bacterial and calcite lactate existing already in the mix (these two approaches are mentioned in the Introduction). I am left with an impression it should be the latter case, but it is never explicitly elaborated in the manuscript. This is very important detail and should be clearly presented without any ambiguity.

In the second case of self-healing, the curing condition is probably very important in order to facilitate the calcite precipitation, therefore should be elaborated.

(4) Line 248~249, “we found that the fracture healing effectiveness of mortars 248 reduced as BA replacement increased”

Any detailed results to support this finding?

(5) Line 268, “These precipitated calcium carbonate crystal particles … also caused fractures to develop on the mortar surface”

Why? Any explanation? Then is this a concern for such technology if new fractures develop?

Author Response

#   Comments and responses to reviewer 1

Reviewer 1: General Comments

Reviewer 1: Questions and suggestions

• This manuscript presents an experimental study of self-healing mortar based on the technology of microbiologically mediated calcite precipitation.

1. The first section is somewhat disorganized and unclear, especially the late part about the presented study (Line 108~118); what is the main objective of this study, especially in relation to the literature review (what is the novelty/innovation beyond the published works)?

• Authors’ response: Thank you so much for your comments: we modified and the late part is the manuscript as follows. “Our research focused on the investigation of self-healing mortars with & without the addition of bagasse ash at pre-and post-crack development times. We made Self-Healing Mortars (SHM) using harmless bacteria, Portland cement, and control burnt bagasse ash, fine sand, calcium lactate powder, and water. Control burnt bagasse ashes were used as a partial replacement for Ordinary Portland Cement (OPC) since bagasse ash has higher pozzolanic reactivity and also such replacement has economically benefit, as indicated in our earlier paper [30]. Research material, Calcium lactate powder, was also prepared from discarded eggshells and lactic acid to reduce the material cost of SHM.”
• The main objective of this study is to investigate the properties of bagasse ash blended SHM at pre- (before crack formation) and post-crack development times.
• At pre-crack development times (before crack formation in hardened mortars), both Self-Healing Mortars (SHM) & 5% bagasse ash blended SHM have higher compressive & flexural strengths than conventional mortals (without the addition of self-healing agents & bagasse ash). This is probably the pores/cracks inside of hardened self-healing mortars might be filled by the precipitated CaCO3 particles which do not happen in conventional mortars.
• Similarly, post-crack development times (after a crack developed/disintegrated of hardened mortars), both Self-Healing Mortars (SHM) & 5% bagasse ash blended SHM have higher compressive & flexural strengths than conventional mortals. This is due to the cracks on hardened SHM being filled by the precipitated CaCO3 particles which might not happen in conventional mortars. Disintegrated units of SHM mortars are also combined by the precipitated CaCO3 particles, unlike conventional mortars.
• We also tried to prepare and use some of the waste materials (BA & eggshells) as raw materials for the production of SHM samples to minimize materials cost and regarding environmental issues.

Generally, the above points are some of the uniqueness of this study.

 What about mortar as opposed to concrete research reviewed?

è Authors’ response: Actually we did our investigation on mortars samples instead of concrete samples. However, the situation may not be different since concrete samples have additional course aggregates.

 

• In addition, what is the main idea of various selected materials investigated, e.g., why use calcium lactate powder from discarded eggshells and lactic acid?

è Authors’ response: As we explained in the manuscript, Self-healing mortars and concretes are expensive, mainly due to the calcium lactate powder (use as raw material for SHM production) and the production technique compare to conventional mortars. If we use waste eggshells for the production of calcium lactate powder, commercially available calcium lactate is costly, we can minimize SHM cost. Similarly, if we replace a certain percentage of cement with BA, the cost of SHM will be decreased and we can avoid landfilling of BA around sugar industries (It is one of the problems in Ethiopia sugar industries). Moreover, a certain amount of BA addition in mortar can improve its performance.

1. Why bagasse ashes burnt was used as a partial replacement? Admittedly some of the answers are hinted at later in the main part of the manuscript, but it is very important to offer readers a general idea of what you want to achieve in the beginning.

• Authors’ response: As we explained in the previous paper, the compressive strength of mortar with bagasse ash burnt at higher temperature (600 ^oC) is greater than mortar with bagasse ash burnt at a lower temperature (e.g. 300 ^oC). At a burning temperature of 300 °C/2hr, BA contains certain carbon and it harms the strength of BA blended mortars/concretes. Several investigations have also shown that compressive strength BA-blended mortars increase with decreased carbon content in the bagasse ash. Our previous experimental results have revealed that bagasse ash was produced more amorphous silica with very low carbon contents when it was burned at 600 °C/2hr. Thus, we used BA burned at 600 °C/2hr to make BA- blended self-healing mortar samples.

1. There are also some poor writing of questionable choice of wording in this section, such as

Line 75~76, “a … difficult to cast concrete approach”;

Line 88~79, “has piqued the interest of many people”;

Line 89~90, “because to their endo spores' ability to withstand a hard environment”;

• Authors’ response: Thank you so much for your comments: we changed as follow

Line 75~76, “…..and also the process of casting concrete is more difficult”;

Line 88~79, “has recently received great attention”;

    Line 89~90, “due to their endospores' ability withstand a harsh environment”

• Line 175~176, “then self-healing process was monitored after 7, 14 and 28 days. The healed width of mortar samples was measured to determine the self-healing efficacy of mortar at the macro and micro levels.” Are the detailed, quantified results about the healed width or self-healing efficacy available, and shouldn't they be reported?
• Authors’ response: Actually, we didn’t put all values of healed crack widths/self-healing efficacy of mortar at the macro and micro levels. We displayed some healed crack widths and explained self-healing efficiency relate to the healed crack widths at 7, 14 curing days.

• Line 231~235, how the cracks are filled in the present study? is healing material injected or introduced to the crack to heal it (therefore it is more like a repair), or the mortar heals it on its own thanks to the presence of bacterial and calcite lactate existing already in the mix (these two approaches are mentioned in the Introduction)? I am left with an impression it should be the latter case, but it is never explicitly elaborated in the manuscript. This is a very important detail and should be clearly presented without any ambiguity. In the second case of self-healing, the curing condition is probably very important in order to facilitate the calcite precipitation, therefore should be elaborated.

 

• Authors’ response: We elaborated on the information in the manuscript. “When fissures developed in hardened self-healing mortars, bacteria became active in the presence of moisture, causing calcite to precipitate and fill the cracks. This precipitated calcium carbonate also filled the pore/void spaces formed inside mortar samples.
• Line 248~249, “We found that the fracture healing effectiveness of mortars reduced as BA replacement increased” Any detailed results to support this finding?
• Authors’ response: Thank you for your suggestion. Our experimental results (figure 8) revealed that crack widths ≤ 13 mm on 5% BA blended SHM sample are healed. However, crack widths up to 0.6 mm on the SHM sample are healed as shown in figure 7. We also checked the healing efficiency of SHM with the addition of BA (10%, 15%, 20%). However, they tend to heal cracks almost none, as did the usage of BA burnt at 300 °C., We anticipated that a large amount of BA replacement might not be suitable for bacteria to precipitate calcite. This might be a reasonable hypothesis to lower the fracture healing efficacy of mortars with the addition of a higher amount of BA. This figure is not included in the manuscript.
• Line 268, “These precipitated calcium carbonate crystal particles … also caused fractures to develop on the mortar surface” Why? Any explanation? Then is this a concern for such technology if new fractures develop?
• Authors’ response: Sorry we made a mistake while we wrote this sentence. It is fact the precipitated calcium carbonate crystal particles never caused fractures to develop on the mortar surface. Figure 10 shows the production of calcite along with the fracture location, implying that the mortar's self-healing capacity is aided by bacterial activity via the generation of CaCO₃ Thus, we corrected as “These precipitated calcium carbonate crystal particles most likely filled some of the pores/voids inside of mortar samples and also filled the fractures on the mortar surfaces. As a result, we anticipated an increase in the strength of SHM samples as a result of this.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article is about some investigation of self-healing mortars with and without bagasse-ash at pre and post-cracks time. However, some issues must to be addressed:

1. Table 1 is useless, since the authors are presenting the same numbers for all batches.
2. 28 days as curing time is sufficient: the authors must to add results and for 56 and 72 days!! 3 or 14 days is not important …
3. Figure 2 and 3: the authors are investigating a calcium lactate … why is important and how is different in their case from others …?
4. Please enhance the contrast for figure 4 to be more readable.
5. Beyond the figure 9, the autors must to include a XRD analysis for the obtained samples, where they must to point out all phases.
6. Figure 11b: around 650 Celsius, there is another peak disconsidered by the authors; they must to explain why is appearing there and what was the importance since this seems to be a chemical (exo/endo) reaction …
7. Figure 12: the authors must to add results and for 56 and 72 days!! 3 or 14 days is not important …
8. Figure 13: 28 days ?!?! the authors must to add results and for 56 and 72 days!! 3 or 14 days is not important …
9. Please enhance the clarity for conclusion section!

Author Response

#   Comments and responses to reviewer 2

Reviewer 2: General Comments, Questions, and suggestions

The article is about some investigation of self-healing mortars with and without bagasse ash at pre and post-cracks time. However, some issues must be addressed:

• Table 1 is useless since the authors are presenting the same numbers for all batches.

è Authors’ response: Thank you for your suggestion. Actually, table 1 illustrates four batches that were prepared at different curing days (i.e. B1 only at 3 days, B2 only at 7 days, B3 only at 14 days, and B4 only at 28 days).  In addition, the compositions of three samples among each batch are not the same that is samples either contain healing agents (bacteria 10^-7cell/ml and calcium lactate powder) or 10^-9cell/ml, & calcium lactate powder or without adding healing agents. Using table 1 (mixing proportions), we made several mortar samples and then measure compressive strength as shown in the results in figure 12 (a). Thus, the authors put this table in the manuscript. However, the reviewer didn’t agree with it, we will delete table 1.

 

• 28 days as curing time is sufficient: the authors must add results and for 56 and 72 days!! 3 or 14 days is not important …

è Authors’ response: we appreciate your constructive comment. However, from the beginning, our aim was to investigate the properties of mortars samples at early curing ages (up to 28 days). Recently, many researchers also have given great attention to bio-concrete samples’ healing efficiency at early curing ages. We are also interested to examine the properties of self-healing mortars at pre- & post-crack development times after samples cured at 3, 7, 14, and 28 days. Actually, as you said, it is essential to check their properties for later ages (at 56, 72, 90day..) and we will study these for later curing ages (52, 72 days…) on our next work. However, at this time it is difficult to do further experiment works for 56 and 72 days and incorporate the results in the manuscript because of the time constraint. We have only given 10days to reply to the answers to reviewers’ questions

• Figure 2 and 3: the authors are investigating calcium lactate … why is important and how is different in their case from others …?

è Authors’ response: according to us, it is important to have the results of Figures 2 and 3 in the manuscript. Since we have prepared calcium lactate powder from discarded eggshells and lactic acid to reduce the material cost of self-healing mortars, we should be checked whether the synthesized powder is calcium lactate powder or not. To confirm it, we used XRD and FTIR analysis and cross-checked with the reference calcium lactate powder. Figure 2 shows the XRD peaks of the synthesized powder are well-matched with standard/reference calcium lactate powder. Similarly, the functional groups (or chemical bonds) of calcium lactate powder were also confirmed using Fourier-transform infrared spectroscopy (Figure 3).

• We didn’t use directly reference/standard calcium lactate powder for the production of mortar specimens in our experimental works due to its cost being high in the market instead we prepared calcium lactate powder in the laboratory (using discarded eggshells and lactic acid) to lower the cost and used it for preparation of self-healing mortar samples.
• Please enhance the contrast for figure 4 to be more readable.

è Authors’ response: thank you for your comment. We enhanced the contrast for figure 4, and now the scale bar is also more readable.

• Beyond figure 9, the authors must to include an XRD analysis for the obtained samples, where they must point out all phases.

è Authors’ response: Thank you for your suggestion. We accepted your suggestion and incorporated the information in figure 9. X-ray diffraction patterns (figure 9) revealed several diffraction peaks at a 2θ angle for both reference calcite and precipitated white powders. The precipitated white powders’ peaks are well-matched with reference CaCO₃ material, calcite crystal, (JCPDS NO. 00-047-1743). Thus, the precipitated powder along the fracture surface of mortar is predominantly phased for calcite (CaCO₃), indicating that the bacillus bacteria successfully produced calcite to repair the mortar break.

• Figure 11b: around 650 Celsius, there is another peak is considered by the authors; they must explain why is appearing there and what was the importance since this seems to be a chemical (Exo/endo) reaction …

è Authors’ response: We incorporate the explanation in the manuscript. Figure 11 (a and b) illustrate the weight loss of the samples above 550 °C is most likely due to the decomposition of calcite CaCO₃ to free CaO and CO₂(g), causing volatile matters to escape (decarbonization). Decarbonization reaction is highly endothermic. However, around 650 °C, small exothermic peaks have also appeared; a reasonable hypothesis is that the formation phase by reaction of free CaO and oxides in cement. [35] Hewlett, P.C. Lea’s Chemistry of Cement and Concrete, 4th ed.; Elsevier: Burlington, NC, USA, 1998.

 

• and (8) Figure 12: the authors must add results for 56 and 72 days!! 3 or 14 days is not important …and Figure 13: 28 days ?!?! The authors must add results for 56 and 72 days!! 3 or 14 days is not important …

è Authors’ response: We agree with you and it is also good to investigate the mortar samples properties at 56 and 72 days. Figure 12 shows the compressive strength results of mortars for early curing ages (up to 28 days). We used these curing ages (3, 7, 14, 28 days) because 3CaO.SiO2 phase in OPC cement reacts relatively quickly with water (around 70% of the C3S typically reacts up to 28days) [35]. This phase is the abundant & most important phase for strength development at ages up to 28 days in cement concrete. Thus, our investigation focused on the strength of mortars up to 28 days of curing ages. As we said before, we couldn’t check the compressive strength and water absorption of mortar samples at 56 and 72 days due to time constrain.  In the future, mortar samples will be studied for later curing ages including 52, 72 & 90 days.

• Please enhance the clarity of the conclusion section!
• Authors’ response: we rewrite again the conclusion section as your suggestion as follow

4. Conclusions

We successfully made conventional mortar samples, self-healing mortar samples, self-healing mortar samples with bagasse ash. Surface fractures up to 0.6 mm widths were entirely filled by the precipitated calcite in self-healing mortar samples. The healing efficiency of mortar was improved with the addition of bacteria 10^–7 /10^–9 cells/ml and curing days. Cracks on mortar samples having bacteria concentrations of 10^–7 cells/ml as well as10^-9 cells/ml were filled by calcite after 7 and 14 days of curing ages. However, fracture healing effectiveness of mortars was reduced as control burnt bagasse ash replacement increased. Self-healing mortar samples are thermally stable in the same way as conventional mortars. However, the weight loss of Self-Healing Mortar (SHM) is greater than that of conventional mortar at higher burning temperatures (up to 750 °C). The extra precipitated calcite in SHM samples started to decompose at a temperature greater than 600 °C, causing weight loss (15.41 percent) of the SHM sample is greater than the conventional mortal sample (5.54 percent). The compressive strength of SHM samples with bacteria 10^-7cell/ml and with bacteria 10^-9 cells/ml were higher than the compressive strength of regular mortar at 3 days of curing ages. Similarly, at 28 days of curing, the compressive strength of SHM samples containing bacteria 10^-7 cells/ml (higher 9.1 percent) and 10^-9 cells/ml (higher 6.9 percent) compared to conventional mortar. At pre- and post-cracks development times, the compressive strength of self-healing mortars is higher than that of conventional mortars. Flexural strength of 5% bagasse-ash blended self-healing mortar sample is higher than conventional mortar before (pre-) cracks develop in the samples. After samples disintegrated (at post-crack development time), SHM containing 5% bagasse-ash was healed it is self the broken pieces, and exhibited advanced flexural strength (100 kPa)) compared to conventional mortars (zero kPa) at 28 days of curing time. This is attributed to the precipitated calcium carbonate filling the pore spaces and fractures in self-healing mortars, whereas cracks in traditional mortars were not repaired by themselves.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

1. Please delete table 1 and replace it with a sentence: is enough!
2. As I said and you understand, it is very important to check the properties at later ages; if 10 days is not enough you can resubmit article when you have the research ready to be published.

Author Response

Reviewer 2: Comments and suggestions

• Please delete table 1 and replace it with a sentence: is enough!

 

Authors Response: Thank you for the suggestion. Based on your suggestion, we deleted table 1 and replaced it with the sentence as follows.

“Self-healing mortar (SHM) samples with composition (%) of Ordinary Portland Cement (100-x)%, Calcium Lactate powder (x)%, Bacillus subtilus Bacteria (10^-7 or 10^-9 cells/ml), Sand and Water (W) were prepared by the conventional method. The composition of Calcium Lactate powder (x%) was = 0 or 1; W/(Cement + x) ≈ 0.53 and Cement to Sand ratio = 1:3. These prepared mortar bricks samples were cured for various curing ages i.e. at 3 days, 7 days, 14 days, and 28 days, separately.”

 

• As I said and you understand, it is very important to check the properties at later ages; if 10 days is not enough you can resubmit the article when you have the research ready to be published

 

Authors Response: Thank you again for the suggestion and you consider it positively for the future.

For this experiment works, we used Ordinary Portland Cement, which has high early strength (3, 7, 14, 28 days), for preparation of mortars; we didn’t use Portland Pozzolana Cement (PPC) which has high later ages (56, 72, 90 days….) strength.

Thus, it is out of scope of the present status of our manuscript if we check of the properties OPC-mortars for later age’s strength experimentally. The reason is that at early curing ages, ordinary Portland cement reacts relatively quickly react with water (around 70% of the 3CaO.SiO₂ phase typically reacts up to 28days). This phase is the abundant & most important phase for strength development at ages up to 28 days in cement mortars.

Therefore, we concentrated to investigate the properties of mortars samples only at early curing ages (up to 28 days). From the observed trends in Figure 12, it has been observed that the strength of OPC/SHM mortar samples will be little increase under the curing time of 56, 72, 90... days.