Pistachio Shell Ash in Agro-Waste Cement Composites: A Pathway to Low-Carbon Binders

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study represents the first systematic investigation of pistachio shell ash (PSA) application potential in the cement industry, filling a research gap in biomass ash valorization for construction materials. The multi-scale characterization (SEM-XRD-EDS) effectively elucidates the microstructural mechanisms of PSA's influence on cement hydration. While providing data support for high-value utilization of biomass waste in cement production, the manuscript requires the following improvements for publication consideration:

1. The introduction should more explicitly emphasize this study's innovative aspects compared to existing nut shell ash research (e.g., coconut shell ash [32], peanut shell ash [31]). A comparative table of chemical compositions between PSA and other biomass ashes is recommended.
2. The apparent contradiction between high SO₃ content (12%) and setting retardation effects requires proper explanation. The potential long-term durability risks associated with elevated SO₃ content should be discussed.
3. Additional long-term strength development data (e.g., 90-day) should be included to complement the current 28-day results.
4. The caption for Fig. 8 ("Compressive strength of blended cement") contains inaccuracies and needs correction.
5. Economic benefits of PSA application should be quantified (e.g., CO₂ emission reduction per ton of cement produced) to provide data-supported conclusions.

The study makes contributions to sustainable construction material development, and these enhancements would substantially strengthen the manuscript's scientific rigor and practical relevance. With these improvements, the paper would be suitable for publication in journals of sustainability focusing on sustainable construction.

Author Response

RESPONSE TO REVIEWER COMMENTS

We thank the reviewers for their constructive comments that have greatly improved the quality of the paper. We highlighted the revisions and modifications in colors. We listed below our replies to every comment of the reviewers.

 

Reviewer #1

 

1. The introduction should more explicitly emphasize this study's innovative aspects compared to existing nut shell ash research (e.g., coconut shell ash [32], peanut shell ash [31]). A comparative table of chemical compositions between PSA and other biomass ashes is recommended.

A detailed comparison of the chemical composition of OPC and PSA used in the experiment with other biomass wastes is given in Table 1.

Table 1. Comparison of OPC with biomass materials.

	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	K2O	Na2O	Na2Oeq*	LOI**	S+A+F
OPC	19.61	4.92	3.15	63.66	2.19	2.54	0.75	0.29	0.78	2.89	27.68
PSA	2.4	0.65	0.77	17.07	5.92	12.96	33.77	8.13	30.35	18.33	3.82
CHA [31]	8.09	0.76	1.09	27.93	-	3.12	19.85	-	-	1.61	9.94
HA [32]	4.89	0.77	1.83	50.48	1.67	3.20	27.73	2.98	21.23		7.49
Na2Oeq* = Na2O + 0.658 K2O. LOI**= loss on ignition

 

Table 1 presents the chemical compositions of pistachio shell ash (PSA), hazelnut shell ash (HA) and coconut shell ash (CHA) from ashes obtained from agricultural wastes in comparison with ordinary Portland cement (OPC). The analyses show that PSA and HA exhibit significantly high alkaline properties with K₂O contents of 33.77% and 27.73% respectively. This property suggests that both materials will offer potential advantages in alkali-activated binder systems. Especially the 12.96% SO₃ content of PSA can be considered as a functional additive in cementitious material formulations due to the setting accelerating effect of sulphate. On the other hand, 50.48% CaO content of HA is suitable for partial cement substitution applications. CHA, on the other hand, has a total S+A+F (SiO₂, Al₂O₃and Fe₂O₃) value of 9.94% with SiO₂, Al₂O₃ and Fe₂O₃ and stands out as the bio-mass-based material with the highest pozzolanic activity potential thanks to this balanced composition. These findings reveal the suitability of these agricultural waste ashes for different application areas in the development of sustainable building materials. Table 1 reveals that PSA contains high concentrations of Na₂O, K₂O, SO₃ and MgO and shows lower CaO and SiO₂ content compared to OPC. This particular chemical profile significantly influences the setting behaviour of cementitious systems, where high alkali content tends to reduce setting times, while decreasing Al₂O₃ and SiO₂ content produces an oppo-site effect by prolonging both initial and final setting times [36]. The observed particle ag-gregation behavior may be attributed to the material's elevated potassium content (K⁺), as corroborated by findings from Shakouri et al. [37] and Kamau et al. [38], who established that K⁺ concentrations in agricultural byproducts are influenced by both plant species and fertilizer applications. Given the high total alkali content (Na₂Oₑq > 0.60%), particular caution must be exercised to avoid potential alkali-silica reaction (ASR) when employing PSA with reactive aggregates containing amorphous silica, such as andesite, dacite, or rhyolite [12].

1. The apparent contradiction between high SO₃ content (12%) and setting retardation effects requires proper explanation. The potential long-term durability risks associated with elevated SO₃ content should be discussed.

The high SO₃ content of PSA (12.96%) can trigger the formation of excessive etrengite (Ca₆Al₂(SO₄)₃(OH)₁₂-26H₂O) in the cement matrix, with negative effects on long-term performance [9,20,39,40].

 

1. Additional long-term strength development data (e.g., 90-day) should be included to complement the current 28-day results.

In the study, the 28-day curing period was considered sufficient based on the high sulphate (SO₃: 12.96%) and low pozzolanic activity (S+A+F: 3.82%) properties of PSA. It has been shown in the literature that mechanical properties generally stabilise in 28 days in systems containing high sulphate (Taylor, 1997). Furthermore, the contribution of PSA to long-term pozzolanic reactions is predicted to be limited due to its low SiO₂+Al₂O₃ content. Therefore, the addition of 90-day strength data is not expected to change the results significantly.

1. The caption for Fig. 8 ("Compressive strength of blended cement") contains inaccuracies and needs correction.

reorganised as “compressive strength values of specimens”

1. Economic benefits of PSA application should be quantified (e.g., CO₂ emission reduction per ton of cement produced) to provide data-supported conclusions.

However, blending 5% PSA into cement appears to be a means of reducing carbon emissions and achieving an approximate 5% reduction in the environmental impact score of cement compared to conventional Portland cement. Economic and Environmental Benefits of 5% PSA Substitution in Cement Production are presented in Table 4.

 

               Table 4. Economic and Environmental Benefits of 5% PSA blended in Cement Production.

Parameter	Value (per ton cement)	Notes/source
CO₂ Reduction	42.5 kg	0.85 tCO2/t [70,71]
Cost Savings	$ 2.85	Clinker cost: $60/tonne [72]
Clinker Replacement	47.5 kg	5% of standard 950 kg clinker/ton cement [73]
Compressive Strength Impact	4.5 % reduction	Test result (28-day curing)

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript “Investigation of Pistachio Vera Shell Ash in the Cement Industry”, the water requirement of pistachio shell ashes and its effects on the volume, setting times and compressive strength of the blended mortar are investigated. The topic and given data-results seem to be suitable for publication; however, the authors are required to improve the manuscript for further consideration. My comments are listed below:

1. On what basis you have selected the pistachio shell ash at 0 to 30%?
2. Give insights into the relation between waste and the main product after curing.
3. Highlight the novelty of your work, compared with other literature.
4. Add the main numerical results to the abstract.
5. Highlight the impact of your additives on the relevant SEM photos (distribution, exfoliation, intercalation, etc.).
6. I recommend adding the following references to your discussion, focusing on the application of environmentally friendly cement and concrete mortars using solid waste and low-carbon materials:

https://www.sciencedirect.com/science/article/pii/S0950061820335856

https://www.sciencedirect.com/science/article/pii/S0008884618309098

7. Illustrate an overall chemical equation/diagram to collect all reactions (from preparation to curing), clarifying the role of ash in curing.
8. Rewrite the conclusion by removing the figure numbers. Conclude the main findings only without detailed discussion that already mentioned in the discussion section.

Author Response

RESPONSE TO REVIEWER COMMENTS

We thank the reviewers for their constructive comments that have greatly improved the quality of the paper. We highlighted the revisions and modifications in colors. We listed below our replies to every comment of the reviewers.

 

Reviewer #2

1. On what basis you have selected the pistachio shell ash at 0 to 30%?

Similar ranges (5-30%) have been widely used in other biomass studies (rice husk ash, coconut shell ash, etc.). These proportions represent the limits that can provide a sustainable contribution without critically affecting the mechanical properties of the cement.

For example, substitutions below 10 % are generally considered ‘low risk’, while 20-30 % are chosen to push the performance limits of the material.

5-15%, The regulating effect of SO₃ on cement hydration (gypsum-like) is in the foreground.

20-30 %, Excessive alkalinity and SO₃ may pose a risk of ettringite expansion or loss of strength

1. Give insights into the relation between waste and the main product after curing.

The incorporation of pistachio shell ash (PSA) as a supplementary cementitious material (SCM) influences the hydration, microstructure, and long-term performance of cement-based products.

The silica and alumina content of PSA is very low. This indicates the expectation of limited puzzolanic activity. C-S-H formation depends on the reaction with Ca(OH)₂ in the cement matrix. Low SiO₂ may reduce the efficiency of this reaction Fig 7a.

The alkali is a factor that accelerates the hydration of the cement [51]. The high alkali content of PSA added in the cement paste caused the increase. The high alkali content of PSA caused the formation of calcium belit (2CaSO₄.K₂SO₄) in the cement paste [52]. Thus, ettringite crystals in a needle-like shape, calcium alumina formation hydrates, and calcium monosulfoalumina hydrates are thought to reduce the viscosity of cement paste [53]. Additionally, ettringite crystals are pretty stable and cause an enormous expansion in the cement paste [54]. The crystal morphology of K₂SO₄ predominantly exhibits a classical cuboid shape. The crystalline product was characterised by X-ray diffraction. A comprehensive analysis of the particle size revealed that the average size of the crystals obtained by suction filtration ranges between 10-120 μm and exhibits a unimodal distribution [55,56]. With the high K₂O and SO₃ content of PSA, the formation of K₂SO₄ crystals observed in the SEM image in Figure 7b may be critical for strength and durability [57,58]. It may cause problems such as setting delay or expansion in cement. Figure 7c and Figure 7d voids resulting from unburnt organic residues can inhibit hydration reactions or increase porosity.

1. Highlight the novelty of your work, compared with other literature.

300–350°C PSA retains calcite/gypsum phases (Fig. 8b), boosting early strength (22% at 1 day).

No high-energy calcination needed, reducing CO₂ footprint vs. conventional SCMs

PSA contains 12.96% SO₃ and 33.77% K₂O, enabling: Early strength gain via accelerated sulfate-aluminate reactions (Fig. 9).

 

K₂SO₄ crystals (Fig. 7b) form due to high K₂O, a previously unreported effect in PSA-cement systems.

1. Add the main numerical results to the abstract.

Abstract: This study evaluates pistachio shell ash (PSA) as a sustainable cement substitute and investigates its effect on setting time, strength and microstructure. In this study, pistachio shell ash (PSA) obtained from the kiln flue gas filter of pistachio shells burnt at 300-350°C in an industrial kiln was used. PSA was substituted for ordinary Portland cement (OPC) at 5, 10, 15, 20, 25 and 30 %. PSA increased the SO₃ value in the cement mortars, so that 5% PSA substitution delayed the cement setting time up to 174%. However, it increased the water requirement of the cement mortar by about 2%. While it increased the early strength (22% at day 1, 15% at day 2, %5 at day 7), the 28-day strength decreased slightly (about 4.5%) due to low pozzolanic activity. Microstructural analyses such as SEM-EDX and XRD showed that the calcite and gypsum phases of PSA provided early strength gains, but there were long-term losses. With a 5% replacement rate, PSA provides significant environmental benefits by reducing CO₂ emissions while maintaining optimum mechanical performance and supports the circular economy through the efficient use of agricultural waste.

 

1. Highlight the impact of your additives on the relevant SEM photos (distribution, exfoliation, intercalation, etc.).

Thanks for your contribution; the necessary emphasis in SEM images has been made

1. I recommend adding the following references to your discussion, focusing on the application of environmentally friendly cement and concrete mortars using solid waste and low-carbon materials:

Thank you for your valuable comments and contributions; the necessary references have been added

1. Illustrate an overall chemical equation/diagram to collect all reactions (from preparation to curing), clarifying the role of ash in curing.

8. The high SO₃ content of PSA (12.96%) can trigger the formation of excessive etrengite (Ca₆Al₂(SO₄)₃(OH)₁₂-26H₂O) in the cement matrix Equation (1), which can increase the water demand of the mortar and have a negative impact on long-term performance [9,21,37,38]. The pozzolanic reaction occurs when a siliceous or aluminous material comes into contact with Ca (OH)₂ in the presence of moisture, resulting in the formation of compounds with cementitious properties [39]. Highly amorphous siliceous ash added to mortars reacts with Ca (OH)₂ and forms calcium silicate hydrates (C-S-H) Equation (2) [40,41].

	(1)
	(2)

 

1. Rewrite the conclusion by removing the figure numbers. Conclude the main findings only without detailed discussion that already mentioned in the discussion section.

             Thank you for your valuable contributions. The conclusion section has been reorganised.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article presents an interesting investigation into the potential use of pistachio shell ash (PSA) as a supplementary material in the cement industry. Unfortunately, the manuscript has numerous language and syntax errors, which obscure key technical points and create ambiguity in the presentation. Additionally, the figures and tables—while informative—lack appropriate error bars and detailed captions that could help readers better understand the presented data.

There is a noticeable discrepancy in the discussion section of the article of chemical compositions and the influence of specific oxides on the cement properties. The rationale behind some of the experimental choices, such as the exact conditions for burning the pistachio shells and the subsequent cooling process, remains unclear. 

The conclusion section requires extensive editing and needs to be carefully revised.

The article requires major language revisions as it is problematic to understand what the authors investigated in the performed research throughout the sections of the article.

Specific comments are in the attached PDF.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The article requires major language revisions as it is problematic to understand what the authors investigated in the performed research throughout the sections of the article.

Author Response

RESPONSE TO REVIEWER COMMENTS

We thank the reviewers for their constructive comments that have greatly improved the quality of the paper. We highlighted the revisions and modifications in colours.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Authors have addressed raised concerns.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.