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Article

Performance of Sustainable Alkali-Activated Mortar Incorporating Natural Pozzolan, Waste Glass Powder, and Polypropylene Fibers

by
Muhammad Iftikhar Khan
,
Mohammed K. Alkharisi
* and
Hany A. Dahish
Department of Civil Engineering, College of Engineering, Qassim University, Buraidah 52571, Saudi Arabia
*
Author to whom correspondence should be addressed.
Sustainability 2026, 18(1), 53; https://doi.org/10.3390/su18010053
Submission received: 22 November 2025 / Revised: 16 December 2025 / Accepted: 16 December 2025 / Published: 19 December 2025
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Abstract

This research highlights the mechanical performance of alkali-activated polypropylene fiber (PPF) mortar containing natural pozzolan (NP) and waste glass powder (WGP) as partial replacements for cement. A total of 45 mix combinations and 405 samples were prepared by varying the levels of NP, WGP, PPF, and sodium silicate (SS), with sodium hydroxide (SH) as an alkali activator. The levels for these variables are NP (0%, 10%, and 20%) and WGP (0%, 10%, 20%, and 30%) by weight of the cement; PPF (0%, 0.5%, and 1.5%) by volume of mortar; and SS + SH (30%, 40%, and 50%) by weight of the binder. The molarity of the SH solution was kept at 10 M, while the SS/SH ratio was maintained at 2.5. Compressive (f’c), flexural (fr), and split tensile strength (ft) were evaluated at 7, 28, and 90 days. The results showed that strength development is strongly age-dependent, with 85–90% of the total strength achieved at 28 days and continued moderate gains to 90 days. SS + SH was the most significant variable, with 50% of activator content achieving the highest f’c, fr, and ft values. Within the tested ranges of NP (0–20%) and WGP (0–30%), strength showed a decreasing trend with increasing replacement due to dilution. PPF had a very minute effect on f’c but significantly improved fr and ft at 0.5% dosage because of crack-bridging. Correlation analysis confirmed that cement and SS + SH are the most dominant strength-controlling factors. The results suggest that the combined use of NP, WGP, and PPF maintains mechanical performance while reducing cement consumption, highlighting the feasibility of this hybrid alkali-activated mortar as a low-carbon construction material.

Graphical Abstract

1. Introduction

The characteristics of concrete that render it a flexible construction material are its capacity to withstand compressive loads and its exceptional durability. Cement, a primary component of concrete, significantly contributes to energy consumption and environmental degradation due to carbon dioxide emissions throughout its production process. The carbon emissions from the production of cement in Europe are 832–1075 kg per ton of clinker, making it a major source of carbon emissions [1]. In addition to just carbon emissions, cement production also creates dust, toxic substances, and noise pollution, all of which harm air, water, and soil quality. These pollutants pose serious health risks, including respiratory and cardiovascular diseases, to workers and nearby communities. According to the latest IEA 2023 report, the cement sector accounts for approximately 7–8% of global CO2 emissions, with clinker production remaining the dominant source. Recent pathways for 2030–2050 decarbonization emphasize clinker reduction, SCM utilization, alkali-activated binders, and carbon-efficient material formulations [2]. As studies reveal different trade-offs between emission-reduction technologies, the industry must adopt greener solutions and stronger environmental policies to lessen its overall ecological and social impacts [3].
Despite the availability of several mitigation strategies, their real-world implementation remains limited, highlighting the need for advanced tools to guide effective policymaking. System dynamics (SD) modeling has been widely used to evaluate technological options—such as alternative fuels, low-carbon materials, energy efficiency, carbon capture, and waste heat recovery—yet many studies overlook practical barriers and policy measures needed for large-scale adoption [4]. It is imperative to diminish CO2 emissions from worldwide cement manufacturing and conserve natural resources to ensure sustainable growth. Various methods have been developed to do this, including the introduction of novel clinker-based cement alternatives, alkali-activated materials, and the enhancement of cement efficiency [5]. Various alternative materials were proposed as partial substitutes for cement, including supplemental cementitious materials (SCMs). It has decreased both CO2 emissions and material costs; yet its application as a substitute material in building for enhancing properties exhibits complex behavior [6].
Recent studies exhibit development in low-carbon binder systems using waste materials. Landfilled coal ash can be prepared to form ultra-low clinker binders with low CO2 emissions [7], while biomass ash and Municipal Solid Waste Incineration Ash (MSWIA) have shown acceptable pozzolanic or hydraulic reactivity suitable for partial replacement of cement [8]. Broader analysis of low-carbon cement technologies demonstrates the significance of optimized clinker-reduction and alternative SCM strategies for future decarbonization of the construction industry [9]. Recent research on low-clinker calcium silicate cements further supports the use of reclaimed and waste materials in next-generation sustainable binders [10].
The optimal split tensile strength and flexural strength of concrete were achieved by incorporating 30% fly ash and 7.5% silica fume as a cement substitute [11]. Replacing 80% of the cement with 50% crushed granulated blast furnace slag, 15% fly ash, and 15% silica fume resulted in a 3.3% decrease in compressive strength compared to the control sample after 28 days of curing [12]. No significant reduction in the mechanical characteristics of concrete was seen after testing samples using 20% activated low-calcium fly ash as a substitute for cement [13]. Alternative materials, such as rice husk ash and olive waste ash, are utilized as substitutes for cement to mitigate environmental impact [14].
Given the significant demand for sustainable and contemporary materials in the current era, natural pozzolan (NP) has emerged as a valuable resource, prompting much research focused on its application as a partial substitute for cement in mortar and concrete production [15]. The pozzolanic material should be incorporated as a partial substitute for cement, rather than being used alone [16]. The utilization of natural pozzolan as a supplementary cementitious material has garnered significant interest due to its benefits for both mortar and concrete performance and environmental sustainability. Pozzolanic minerals, predominantly located in volcanic deposits, diminish the heat of hydration and greenhouse gas emissions associated with the production of Ordinary Portland Cement (OPC), while significantly improving durability by mitigating sulfate assaults and alkali–silica reactions [15,17].
NP-based mortars exhibit superior long-term strength and pore refinement, thereby diminishing the ingress of aggressive agents such as sulfates and chlorides, ultimately extending the lifespan of structures, despite potentially reduced early-age compressive strength [18,19]. The utilization of NP as a substitute for cement in sustainable construction not only advances environmental goals but also offers economic advantages.
Oviedo et al. [20] reported that the mechanical properties of concrete were enhanced relative to the control samples by partially substituting cement with pumicite up to 10%. Al-Amoudi et al. [21] reported that the compressive strength of concrete was enhanced, relative to OPC-only concrete, after 90 days of curing through the activation of NP with hydrated lime.
Researchers are encouraged to investigate the impact of NP on the characteristics of cement-based products due to their potential applications. Research indicates that the pozzolanic activity of NP surpasses that of FA but is marginally inferior to SF, with the optimal dosage of NP ranging from 10% to 20% [22,23]. Khan and Alhozaimy [24] assert that Saudi Arabia possesses a diverse array of nanomaterials suitable for the production of eco-friendly construction materials. It was determined that NP, which is plentiful in Saudi Arabia, complies with ASTM C 618 [25] requirements and is categorized as “Class N” due to its physical and chemical characteristics. A summary of the previous studies on natural pozzolan replacing cement in mortar and concrete is shown in Table 1.
In addition to the sustainability provided by NP, waste glass powder (WGP) presents a beneficial alternative for mortar and concrete applications. WGP may efficiently substitute 20% of cement in most instances due to its pozzolanic activity, attributed to the presence of amorphous silica. It enhances the mechanical and durability properties during later curing phases [42,43]. This can improve resistance to sulfate assaults, alkali–silica reactions (ASR), and chloride penetration by optimizing the pore structure and interfacial transition zone in concrete [44,45,46].
The incorporation of WGP mitigates the carbon footprint of mortar and concrete by decreasing reliance on typical cement, which is energy-intensive and a major contributor to CO2 emissions [42]. Research indicates that the particle size and composition of WGP substantially affect its efficacy. Moreover, the alkali activation of WGP augments its reactivity, hence improving the performance of mortar and concrete mixtures [43,45].
Waste glass, being a non-biodegradable substance, presents environmental risks; hence, repurposing it into a high-value construction resource not only mitigates solid waste management challenges but also advances worldwide sustainability objectives. The application of WGP is particularly pertinent in areas where waste glass disposal is a significant challenge, rendering it an environmentally sustainable and economically feasible solution for contemporary building [44,47]. It was determined that substituting 10% of cement with GP enhances the compressive strength of mortar by 9%. A summary of the WGP replacing cement in mortar and concrete in various previous studies is shown in Table 2.
In conjunction with NP and WGP, the incorporation of polypropylene fibers (PPF) in alkali-activated systems has garnered significant attention, owing to their capacity to enhance sustainability and mechanical qualities. Studies indicate that PPF significantly diminishes drying shrinkage, bolsters crack resistance, and augments the flexural strength of alkali-activated mortars [61,62]. The effect on compressive strength is quite subtle, frequently necessitating an ideal dosage to achieve a balance between strength and flexibility [63].
Furthermore, improvements in composite structures highlight the significance of PPF in supporting green construction programs by the integration of recyclable materials and the reduction of CO2 emissions, hence reinforcing their environmental advantages [64]. These studies collectively establish a foundation for optimizing PPF application across various binder systems, improving both sustainability and structural performance in contemporary construction practices. A summary of previous studies incorporating PPF in mortar and concrete is shown in Table 3.
Although prior research has examined NP and WGP individually, there is a paucity of studies regarding their synergistic application in alkali-activated mortars, especially with the incorporation of polypropylene fibers.
Research has proven that the combined effect of SS and SH used with NP and WGP is quite noticeable in terms of the strength and microstructure of alkali-activated materials (AAMSs). Studies by Adewumi et al. [66] and others indicate that varying the SS/SH solution by 30%, 40%, and 50% by weight of the binder impacts compressive strength and formation of C-S-H and N-A-S-H gels, improving the mechanical properties of mortar. A balanced combination of NH and SS solution with NP results in high strength and a more stable microstructure, as shown in the work by Adewumi et al. 2021 [67]. Furthermore, similar findings have been shown by Najimi and Ghafoori [29] for the use of SS and SH with WGP in mortar.
According to the research of Bondar et al. [26], the molarity of NaOH significantly impacts NP activation, with 12–14 M accelerating dissolution and early strength. Excessive molarity (>14 M) causes rapid setting and microcracks, while <10 M delays polymerization, as per the research of Haddad and Alshbuol [68]. In the study of Adewumi et al. [67], increasing the M of NaOH from 4 M to 10 M resulted in enhanced compressive strength, with the optimal compressive strength of 31.3 MPa achieved at 10 M. The effect of M of NaOH on compressive, tensile, and flexural strength is shown in Figure 1.

2. Materials and Methods

The experimental work was focused on finding the effects of partial replacement of cement with NP and WGP, in combination with alkali activation using SS and SH, on the mechanical properties of mortar. Additionally, PPF was incorporated into the mix for strength enhancement. The methodology flow chart of the research is shown in Figure 2.

2.1. Material Procurement and Preparation

Because of excellent strength, durability, and appropriateness for general-purpose applications, we used Qassim Cement, which complies with ASTM C150 [69]. Type I specifications are utilized in this study to prepare mortar samples. Its properties are presented in Table 4.
Qassim region local sand was used for this research, as it had a fineness modulus of 2.6 (ASTM C136) [74] and a specific gravity of 2.7 (ASTM C128) [75]. This falls within the typical range for natural sands, which suggests good density and mineral composition. A detailed sieve analysis shown in Figure 3. was performed as per ASTM C136 [74].
The Saudi NP utilized in this study, shown in Figure 4, is obtained from volcanic rocks found in the western part of Saudi Arabia and then ground in a high-energy grinder. Its specific gravity is 3.0 (dimensionless), relative to water at 20 °C, and its specific surface area is 667 m2/kg, respectively. More details about its properties can be seen in Ahmad et al., 2023 [76].
Waste glass was collected from abandoned construction materials like windowpanes, glass doors, and partition panels. These materials were chosen to reuse non-biodegradable waste materials as an important construction material for the sake of sustainability. After collecting the waste glass, it was thoroughly washed and cleaned to remove any sort of contaminants, and then it was dried in preparation for grinding. For grinding the glass into a fine powder, a planetary ball mill was used. To achieve an efficient particle size reduction, the process was conducted at a speed of 400 rpm for seven cycles. With an average particle size of 75 µm, a total of 10 kg of powdered glass was obtained. The detailed process is shown in Figure 5. The parameters for operating the planetary ball mill are based on the literature [77,78,79]. The XRF analysis of cement, NP, and WGP is shown in Table 5.
SH and SS solutions (Figure 6a–c) were obtained from a local company (Adwan Chemical Industries Co., Ltd., Riyadh, Saudi Arabia) in KSA. SH is a strong alkaline activator that dissolves silica and alumina precursors, facilitating the geopolymerization process in NP and WGP. SS, also known as water glass, acts as a silicate source, enhancing the binding properties and mechanical strength of the geopolymer matrix. Together, they create an alkaline environment essential for the dissolution and polycondensation reactions in geopolymer synthesis. The PPF (Figure 6c) used was microfilament fibers with an average length of 12 mm, a diameter of 18 microns, and a specific gravity of 0.91. Polypropylene fibers enhance the durability and crack resistance of the cement mortar by improving its tensile strength and reducing shrinkage. These synthetic fibers disperse uniformly within the matrix, providing better structural integrity and impact resistance. Their hydrophobic nature also contributes to improved resistance against moisture-related degradation in cementitious composites.

2.2. Experimental Design

Samples of the mortar are prepared at a ratio of 1 part cement to 2.75 parts graded standard sand by weight, according to ASTM C 109/C 109M [80]. A water–cement ratio of 0.485 is used to produce a flow of 110 ± 5%.
Cement in the mortar is partially substituted in this study with NP and WGP to assess their efficacy as alternative binders. A methodical investigation of the effects of NP and WGP alone and in combination was made possible by setting the replacement level at 0%, 10%, and 20%, and 10%, 20%, 40%, respectively, by weight of cement. PPF was added at 0%, 0.5%, and 1.5% by weight of the binder, and the SS and SH solution, a crucial factor in alkali activation, was adjusted at 30%, 40%, and 50% of the total added binder content. The molarity of the SH solution was kept constant at 10 M, as recommended by the literature. Furthermore, the ratio of SS and SH was kept constant at 2.5 across all the mixes, based on the previous studies. These variables and levels shown in Table 6 were chosen to investigate the interactions and optimize the proportions of NP, WGP, and alkali activators for enhanced mechanical properties.

2.3. Sample Preparation, Mixing, Casting, and Curing

In total, 45 different mixes, with various combinations of cement, NP, WGP, PPF, and SS/SH, were prepared. Three tests, which are compression, flexural, and split tensile, were conducted on each mix. For each test, three specimens were used, resulting in a total of 405 specimens for this study. In compliance with ASTM C109 [80], for CS, the cubes (50 mm × 50 mm × 50 mm), for flexural strength, the prisms (40 mm × 40 mm × 160 mm), and for split tensile strength, the cylindrical (100 mm diameter × 200 mm height) samples, were prepared. The samples were prepared in accordance with the specifications outlined in ASTM C109 [80], utilizing a cement–sand ratio of 1:2.75 and a water–cement ratio (w/c) of 0.485.
First of all, a 10 M aqueous solution of SH was prepared a day before mixing to allow for the cooling of the solution, as NH is exothermic in nature, shown in Equation (1):
N a O H s o l i d → y i e l d s N a + a q + O H − a q + H e a t
Mechanical mix was carried out according to the procedure given in ASTM C 305 [81]. The alkaline solution of SH and SS was mixed for 10 min at room temperature before being mixed into the mortar. The required quantities of cement, sand, NP, WGP, and PPF were measured and mixed in batches. Firstly, these materials were manually mixed slowly for 3 min. Then, the alkaline solution and water were added during the wet mixing stage, using a slow-speed mixing rate of 2 min, followed by 5 min of mixing until a homogeneous mixture was achieved, and then the workability was tested using a flow table, and it was maintained at 110% ± 5% by incorporating a superplasticizer (Conplast SP430). For cubes and prisms, fill the molds in two layers, and for cylinders, in three layers, compacting each layer with a tamping rod (25 strokes per layer) to eliminate air voids. Smooth the surface by striking the excess mortar with a trowel. Carefully remove the specimens from the molds after 24 ± 2 h of casting. Fill the curing tank with clean saturated lime water at 23 ± 2 °C and place the samples immediately after demolding. The specimens remain fully submerged until the day of testing (7, 28, and 90 days) (Figure 7).
The f’c, fr, and ft were determined on the cube, prisms, and cylindrical specimens of mortar using a MATEST universal testing machine (UTM) manufactured by MATEST S.p.A., Arcore (MB), Italy, after curing of 7, 28, and 90 days, as per ASTM C109, C348, and C496 standards [80,81,82].

3. Results and Discussions

The average results of compressive, flexural, and tensile strengths at durations of 7, 28, and 90 days are shown in Figure 8, Figure 9 and Figure 10. A water-to-binder ratio (W/B) was maintained at 0.48. A total of 45 mortar mixes and 405 samples were formulated using a multi-factor experimental design to capture both individual and combined parameter effects. For each mix and curing age, three replicate specimens were tested, and the average values are reported. Cement replacement levels were selected based on the literature to reflect realistic ranges for performance-based and eco-efficient binder systems, while the activator contents were varied to assess the effects of alkaline dissolution and gel formation.

3.1. Strength Development with Curing Time

Figure 8a–c illustrates the strength development with curing time. The compressive, flexural, and split tensile strengths have the same trend: a sharp increase from 7 to 28 days of curing, followed by a gradual and continuous increase up to 90 days. This shows the standard geopolymerization process in alkali-activated systems, in which the maximum formation of gel occurs at the start, and the later gain of strength is aided by secondary breakdown, polycondensation, and refinement of pores [83].
Compressive strength shows substantial sensitivity to curing time, which suggests that matrix densification and the development of N-A-S-H/C-A-S-H are the key variables affecting the initial behavior [84]. Flexural and tensile strength also show a significant increase up to 28 days, with further enhancement observed subsequently. The gain is due to persistent crack-bridging and strengthening of the fiber–matrix bond, especially in PPF-reinforced mixes [85]. Across all curing times, the result of strengths at 40% SS + SH is almost similar to that of 50% SS + SH, compelling us to choose the 40% SS + SH solution as the best choice. Overall, the results show that 80–85% of the total strength is achieved at 28 days of curing, with microstructural refinement contributing to the gain of strength at later ages.

3.2. Strength vs. Mix Variables

Figure 9 illustrates the effects of NP, WGP, and PPF on the mechanical performance of alkali-activated mortar, which demonstrates the relationship between each parameter and f’c, fr, and ft at 7, 28, and 90 days. The effect of NP’s replacement shown in Figure 9a–c demonstrates that the strengths of the mortar enhance up to 10–15% replacement, beyond which strength starts declining due to a reduction in phases containing calcium and the inherently slower reactivity of uncalcined pozzolans. This is in agreement with Najimi et al. 2018, who stated that a moderate level of NP improves early geopolymer gel formation, while exceeding that creates a reactive imbalance and declines in Ca/Si availability [30]. Interestingly, after 90 days of curing, the strength exhibits less sensitivity to higher NP content, showing that pozzolanic activation continues over time, as proven by Nourredine et al. 2021 [86].
In contrast to NP, WGPs have a consistent effect on the development of strength for all levels and across all ages, with optimum behavior seen at almost 20% of cement replacement, as shown in Figure 9d–f. This trend is dedicated to the high amorphous silica content of WGP, which enhances the formation of N-A-S-H and C-S-H gels, which is in line with the outcomes by Bian et al. 2023 [87]. A comparatively smooth and stable increase in strength across all curing days indicates that WGP participates in both primary and later-stage densification, contributing to a reduction in porosity and permeability through secondary gel formation.
The physical contribution of PPF is depicted in Figure 9g–i. It does not participate chemically, but it improves crack resistance and post-cracking behavior. PPF had a very slight effect on f’c, while on fr and ft, it had a notable effect up to 1% of fiber content, after which reduction started. This behavior is aligned with Xu et al. 2021, who observed the PPF capacity of the strain and its bridging of cracks and absorbing of energy, especially at later curing ages when the fiber–matrix bond becomes stronger [61].
Among all the plots, the positive impact of NP, WGP, and PPF is remarkably increased when used with a higher content of SS + SH, i.e., 50%, proving that alkalinity has a major influence on densification of the matrix, gel chemistry, silica movement, and dissolution rate. These findings align with Degirmenci 2017, who claims that SS-rich activator solutions accelerate geoploymerization and improve mechanical efficacy [88]. The findings collectively demonstrate that NP mainly increases the availability of aluminosilicate at an early age, WGP helps in the enhancement of long-term silica reactivity and forms hybrid gels, and PPF maximizes both fr and ft.

3.3. Effect of Alkali Activator Content (SS + SH) on Mechanical Strength

The impact of alkali activator content (SS + SH) on the development of f’c, fr, and ft of the mortar is shown in Figure 10a–c at 7, 28, and 90 days. In all instances, increasing SS + SH (%) from 30% to 50% resulted in measurable improvement of strength, affirming that it plays a vital role in the geopolymerization process. In Figure 10a, it is noticeable that compressive strength has a higher sensitivity to the dosage of activator solution, primarily at 28 and 90 days, where 50% of SS + SH produced up to 30% higher strength than the 30% blends. Similar trends can be seen for fr (Figure 10b) and ft (Figure 10c), where the highest values were achieved at 40% and 50% of SS + SH at all curing ages. However, an increase in strength was less dramatic compared to f’c. This can be associated with the fact that fr and ft are dependent on fiber and matrix interaction, control of cracks, and microstructural toughness. These findings are consistent with recent work by González-Garcia et al. 2022, who reported that Na2SiO3-rich activator solutions significantly enhance reaction kinetics and reduce internal porosity in pozzolan-based geopolymers [89]. These plots indicate that SS + SH dosage is a crucial parameter for optimizing strength performance in alkali-activated mortar systems, especially in mixtures with natural pozzolan, waste glass powder, and fibers.

3.4. Summary of System Behavior Through a Heatmap

The correlation heatmap in Figure 11 depicts that f’c, fr, and ft are mostly affected by the cement content (%) and chemistry of alkali activation, rather than NP (%) or WGP (%) alone. This is also shown by some of the recent research by Ibrahim et al. [90], highlighting that activator solution and silica mobility dominate mechanical strengths in alkali-activated systems. WGP exhibits a moderate negative correlation at higher levels of replacement, while on the other hand, NP shows a non-linear effect, thereby reinforcing the fact that both materials increase the strengths solely within optimal ranges. Correlation values were calculated using Pearson’s coefficient, based on the experimental datasets of compressive, flexural, and tensile strengths at 7, 14, and 28 days. Percentages of NP, WGP, and PPF represent the independent variables used for regression; the resulting coefficients represent the normalized strength sensitivity to each parameter. The left axis represents the input variables in % replacement, while the right side shows correlation coefficients calculated from the experimental results. These correlation results support the observed experimental trends (e.g., the minimal influence of PPF on compressive strength but positive influence on flexural and tensile performance) by helping to qualitatively identify which variables have the strongest influence on strength. Since all mixes were prepared under identical curing, activator, and test conditions, the mechanical strengths and mix parameters were strongly linearly dependent (r > 0.95). Thus, controlled replacement levels drive sample variation, resulting in strong parameter–response relationships. It is noted that several Pearson’s correlation coefficients are relatively low, indicating weak linear relationships among variables. These values are presented to illustrate the direction of influence rather than to imply strong predictive capability.

4. Conclusions

This research examined the mechanical performance of alkali-activated mortar containing NP, WGP, and PPF at 7, 28, and 90 days of curing, with SS + SH at different concentrations. Based on comprehensive experimental work, the following conclusions are derived:
  • NP enhances strength up to 10% replacement of cement, after which dilution effects lower early-age strength. The contribution at later ages aligns with its reduction in pozzolanic reactivity.
  • A more steady and significant strengthening effect is observed when replacing cement with WGP. Lower levels of WGP (10–20%) maintain acceptable mechanical performance while enabling cement reduction for sustainability purposes.
  • The activator solution (SS + SH) demonstrates very high effectiveness on f’c, fr, and ft of mortar containing NP and WGP as partial replacements for cement. The highest strengths were achieved at 40% of SS + SH by weight of the binder, with SH at a 10 M solution and an SS/SH ratio of 2.5. This confirms that dissolution, gel formation, and matrix densification are regulated by the concentration of alkaline solutions.
  • PPF does not participate in any chemical reactions; instead, it acts as a reinforcing agent that increases crack-bridging and improves post-cracking toughness, especially at a 0.5% dosage by weight of the total mix.
  • The correlation analysis clearly shows that cement content and SS + SH level have the strongest positive impact on strength, while NP and WGP exhibit non-linear, dosage-sensitive behavior, highlighting the need for a balanced mix design.
  • The combined use of NP, WGP, and PPF showed mechanical synergy, maintaining strength and ductility while replacing cement content with NP + WGP up to 30%, thus promoting sustainable construction methods.
The overall finding of this study is that an optimally developed alkali-activated mortar with partial cement replacement can achieve mechanical performance equivalent to, or potentially superior to, that of traditional mortars, while significantly reducing environmental impact. This research advocates for the use of geopolymer-based systems in structural, repair, and environmentally sustainable construction practices.

5. Recommendations for Future Work

  • Assess the durability of the material with respect to resistance to chloride and sulfate attack, carbonation, and freeze–thaw cycles.
  • Utilize XRD, SEM–EDS, FTIR, and NMR techniques to conduct microstructural analysis to confirm reaction mechanisms.
  • Extend the curing period to exceed 90 days and incorporate conditions analogous to those encountered in the field.
  • Assess the performance of structural components such as beams, slabs, and repair overlays through testing procedures.
  • Employ machine learning and multi-objective optimization to enhance the mix design.
  • Consider further exploration of industrial by-products, such as slag, rice husk ash (RHA), red mud, and metakaolin. Conduct a comprehensive life-cycle assessment (LCA) to determine the extent of potential carbon emission reductions. CO2 footprint of sodium silicate/hydroxide production should be evaluated in future work to determine the net environmental benefit.

Author Contributions

Conceptualization, M.I.K., M.K.A. and H.A.D.; methodology, M.I.K., M.K.A. and H.A.D.; software, H.A.D. and M.I.K.; validation, M.K.A.; formal analysis, H.A.D.; investigation, M.I.K., M.K.A. and H.A.D.; resources, M.K.A.; experimental work, M.I.K.; writing—original draft preparation, M.I.K.; writing—review and editing, H.A.D. and M.K.A.; visualization, M.I.K.; supervision, M.K.A.; project administration, M.K.A.; funding acquisition, M.K.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the raw data supporting the conclusion of this paper were provided by the authors.

Acknowledgments

The researchers would like to thank the Deanship of Graduate Studies and Scientific Research at Qassim University for financial support (QU-APC-2025). During the preparation of this manuscript, the authors used draw.io for creating flow charts, Google Colab Python (https://colab.research.google.com/) environment to draw plots, and ChatGPT 5.1 to seek help for grammatical corrections in the writing.

Conflicts of Interest

The authors declare there are no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
NPNatural pozzolan
WGPWaste glass powder
SCMsSupplementary cementitious materials
SS + SHSodium silicate + sodium hydroxide
PPFPolypropylene fiber
OPCOrdinary Portland Cement
f’cCompressive strength
frFlexural strength
ftTensile strength (or split tensile strength)
MPaMegapascal
IEAInternational Environmental Agency
GtGigaton
CO2Carbon dioxide
ASTMAmerican Society for Testing and Materials
ASRAlkali–silica reaction
MMolarity
NaOHSodium hydroxide
FAFly ash
W/BWater-to-binder ratio

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Figure 1. Effect of molarity of NaOH on (a) compressive strength, (b) flexural strength, and (c) tensile strength.
Figure 1. Effect of molarity of NaOH on (a) compressive strength, (b) flexural strength, and (c) tensile strength.
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Figure 2. Methodology flow chart for experimental work.
Figure 2. Methodology flow chart for experimental work.
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Figure 3. Particle size distribution of sieved sand.
Figure 3. Particle size distribution of sieved sand.
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Figure 4. Process of grinding NP in a high-energy grinder.
Figure 4. Process of grinding NP in a high-energy grinder.
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Figure 5. Process of passing raw waste glass from collection to washing, breaking it, putting it into a wial, and then putting it into the planetary ball mill at a speed of 400 rpm for 7 cycles to obtain powdered waste glass.
Figure 5. Process of passing raw waste glass from collection to washing, breaking it, putting it into a wial, and then putting it into the planetary ball mill at a speed of 400 rpm for 7 cycles to obtain powdered waste glass.
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Figure 6. (a) SS solution, (b) SH pellets, and (c) PPF.
Figure 6. (a) SS solution, (b) SH pellets, and (c) PPF.
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Figure 7. (a) Universal testing machine for mortar tests; (b) assembly for f’c and ft; (c) assembly for fr.
Figure 7. (a) Universal testing machine for mortar tests; (b) assembly for f’c and ft; (c) assembly for fr.
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Figure 8. Illustrations of the effect of curing age on mechanical performance: (a) Compressive; (b) flexural; and (c) tensile strength development of alkali-activated mortar at 7, 28, and 90 days.
Figure 8. Illustrations of the effect of curing age on mechanical performance: (a) Compressive; (b) flexural; and (c) tensile strength development of alkali-activated mortar at 7, 28, and 90 days.
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Figure 9. Effect of natural pozzolan (NP%), waste glass powder (WGP%), and polypropylene fiber (PPF%) contents on compressive strength (f’c), flexural strength (fr), and tensile strength (ft) of alkali-activated mortar under varying activator concentrations: (a–c) f’c vs. NP, WGP, and PPF; (d–f) fr vs. NP, WGP, and PPF; (g–i) ft vs. NP, WGP, and PPF.
Figure 9. Effect of natural pozzolan (NP%), waste glass powder (WGP%), and polypropylene fiber (PPF%) contents on compressive strength (f’c), flexural strength (fr), and tensile strength (ft) of alkali-activated mortar under varying activator concentrations: (a–c) f’c vs. NP, WGP, and PPF; (d–f) fr vs. NP, WGP, and PPF; (g–i) ft vs. NP, WGP, and PPF.
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Figure 10. Influence of alkali activator content (SS + SH%) on (a) compressive, (b) flexural, and (c) split tensile strengths of alkali-activated mortar at different curing ages.
Figure 10. Influence of alkali activator content (SS + SH%) on (a) compressive, (b) flexural, and (c) split tensile strengths of alkali-activated mortar at different curing ages.
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Figure 11. Correlation heatmap of input parameters and strengths.
Figure 11. Correlation heatmap of input parameters and strengths.
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Table 1. Summary of the previous study on NP replacing cement.
Table 1. Summary of the previous study on NP replacing cement.
NP Size Range (µm)NP Size Detail (µm)Optimal Replacement (%)References
NP ≤ 100-20[26]
NP ≤ 75Mean size = 4520[27]
NP ≤ 75-15[24]
NP ≤ 75Mean size = 1615–20[28]
NP ≤ 45Mean size = 15.8100 (Alkali-activated)[29]
NP ≤ 45Mean size = 15.8100 (Alkali-activated)[30]
NP ≤ 75Blaine-specific surface area 735 m2/kg to 19,000 m2/kg25%[31]
NP ≤ 45-100 (AANP)[32]
30 µm <NP < 2 mm-20[33]
NP ≤ 200Mean size = 4525[34]
NP ≤ 45Blaine-specific surface area of 4350 cm2/g28[35]
NP ≤ 14 µm-30[36]
-420–700 m2/kg25[37]
NP ≤ 453200 cm2/g15 (20 °C) and 20 (60 °C)[38]
NP ≤ 75 (ASTM C618)Blaine-specific surface area 3292 cm2/g13[39]
NP ≤ 75Blaine-specific surface area 2000–3000 cm2/g20[40]
NP ≤ 75Blaine-specific surface area of 4298 cm2/g10[41]
Table 2. Summary of the previous studies on WGP replacing cement.
Table 2. Summary of the previous studies on WGP replacing cement.
WGP Size Range (µm)WGP Size Detail (µm)Optimal Replacement (%)Reference
WGP ≤ 25––[48]
–Average size = 1020[49]
–Average size = 2020[50]
–Average size = 4520[51]
WGP ≤ 40Average size = 1230[52]
WGP ≤ 50–30[53]
–Average size = 8.420[54]
WGP ≤ 75Average size = 1020[55]
WGP ≤ 75–25[56]
WGP ≤ 10040% WGP ≤ 1045[57]
–Average size = 3.460[58]
125 ≤ WGP ≤ 200–30[59]
WGP ≤ 120–30[60]
Table 3. Optimal usage of PPF in cement mortar.
Table 3. Optimal usage of PPF in cement mortar.
Optimum DosageCompressive StrengthFlexure StrengthReference
0.2%Improved slightlyIncreased with dosage[61,62]
0.5%Limited improvementNotable improvement[64]
0.6%Increased by 57%Increased by 58%[61,62]
<1%Limited reductionIncreased ductility but reduced strength at >1%[65]
1.5%Significant improvement-[63]
Table 4. Tests conducted on cement.
Table 4. Tests conducted on cement.
No.Test DescriptionASTM CodesResults
1FinenessC204 [70]10.5% retained on 90-micron sieve
2Normal consistencyC187 [71]30%
4Initial setting timeC191 [72]75 min
5Final setting time225 min
6Specific gravityC188 [73]3.15
Table 5. XRF analysis of cement, NP, and WGP in weight percent (wt.%).
Table 5. XRF analysis of cement, NP, and WGP in weight percent (wt.%).
ComponentsCementNPWGP
CaO63.9111.7060.71
SiO219.9741.1327.12
Al2O35.8318.913.90
Fe2O33.430.092.50
MgO0.613.622.32
SO32.8710.1801.501
K2O0.7111.100.451
Na2O *0.1590.3100.910
Rest2.50922.960.588
* Na2O was detected at 0.159 wt.% due to higher measurement sensitivity for trace-level alkali oxides in the XRF analysis.
Table 6. Process parameters and their levels for the experiment.
Table 6. Process parameters and their levels for the experiment.
ParametersLevel 1Level 2Level 3Level 4
Natural Pozolan (%)01020-
Glass Powder (%)0102030
SS/SH (%)304050-
PPF (%)00.51.5-
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Khan, M.I.; Alkharisi, M.K.; Dahish, H.A. Performance of Sustainable Alkali-Activated Mortar Incorporating Natural Pozzolan, Waste Glass Powder, and Polypropylene Fibers. Sustainability 2026, 18, 53. https://doi.org/10.3390/su18010053

AMA Style

Khan MI, Alkharisi MK, Dahish HA. Performance of Sustainable Alkali-Activated Mortar Incorporating Natural Pozzolan, Waste Glass Powder, and Polypropylene Fibers. Sustainability. 2026; 18(1):53. https://doi.org/10.3390/su18010053

Chicago/Turabian Style

Khan, Muhammad Iftikhar, Mohammed K. Alkharisi, and Hany A. Dahish. 2026. "Performance of Sustainable Alkali-Activated Mortar Incorporating Natural Pozzolan, Waste Glass Powder, and Polypropylene Fibers" Sustainability 18, no. 1: 53. https://doi.org/10.3390/su18010053

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Khan, M. I., Alkharisi, M. K., & Dahish, H. A. (2026). Performance of Sustainable Alkali-Activated Mortar Incorporating Natural Pozzolan, Waste Glass Powder, and Polypropylene Fibers. Sustainability, 18(1), 53. https://doi.org/10.3390/su18010053

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