Evaluating the compressive strength and microstructural mechanisms of m-20 grade concrete modified with finely ground custard apple waste

Introduction

With an annual global production exceeding 10 billion tons, concrete remains the most ubiquitous construction material utilized in modern civil and agricultural infrastructure. Within agricultural engineering, concrete is indispensable for constructing farm buildings, irrigation canals, grain storage structures, silage pits, and rural pavements. The material is widely favored due to its robust compressive strength, fire resistance, impermeability, economic viability, and capacity for on-site production. However, the primary binder in conventional concrete mixtures Ordinary Portland Cement (OPC) presents severe ecological liabilities. The manufacturing of OPC is highly energy-intensive and accounts for approximately 8% of global anthropogenic carbon dioxide (CO 2 ) emissions. Furthermore, excessive cement consumption in concrete matrices can result in structural degradation through elevated heat of hydration and autogenous shrinkage. Consequently, a core objective in modern agricultural and cleaner production engineering is to optimize concrete design by minimizing OPC content through the integration of processed agricultural byproducts, thereby achieving the dual goals of structural durability and environmental sustainability (Sharma et al., 2026) [1] . In this context, the cultivation and industrial processing of custard apple (Annona squamosa, locally termed Sitaphal) across developing agrarian economies like India presents a severe bio-waste management crisis. Due to its natural resilience as a climate-smart, drought-tolerant dryland crop, national cultivation has expanded steadily to encompass over 40,000 hectares, yielding an annual output of approximately 380,000 to 400,000 Metric Tonnes (MT).

This production is regionally anchored by Maharashtra, which contributes over 31% (~120,000 MT) of the national volume, primarily across the semi-arid belts of Jalna, Aurangabad, Osmanabad, and Beed. This is followed by Madhya Pradesh (22%), thriving heavily within tribal and forest-adjacent farming belts like Dhar and Khargone; Gujarat (19%), concentrated in the arid zones of Dahod, Panchmahal, and Sabarkantha; and Chhattisgarh (13%) and Andhra Pradesh (4.5%), which successfully leverage rocky terrains and seasonal rainfall patterns. Despite its economic value, Annona squamosa possesses an exceptionally high "waste index". A standard whole fruit contains only 30% to 40% edible pulp, meaning that the remaining 55% to 60% is entirely discarded as processing and post-harvest solid waste. Because custard apple is a highly climacteric fruit with an intense respiration rate, biomass accumulation occurs rapidly at two main vectors along the agricultural supply chain. First, thin pericarp walls and rapid post-harvest ripening subject the fruit to severe mechanical splitting and crushing during transit (15% to 20% loss), rendering thousands of tonnes vulnerable to Anthracnose fungal rot before reaching consumer markets. Second, within industrial pulp extraction facilities (producing base materials for ice creams and beverages), processing 100 kg of whole fruit yields a mere 40 kg of pulp, leaving behind 60 kg of immediate solid waste composed of peels (~40 kg) and dense internal seeds (~20 kg). Managing this biomass is highly problematic; the seeds make up 20% to 25% of the total fruit weight and contain high concentrations of annonaceous acetogenins a group of powerful, naturally occurring lipophilic compounds that inhibit cellular respiration. Consequently, this waste cannot be safely integrated into livestock feed or left to decompose in open-air environments without causing localized ecological toxicity, making the development of sustainable recycling pathways an urgent engineering imperative (Phonphuak et al., 2026) [2] . From a materials engineering perspective, utilizing processed Custard Apple Waste (CAW) as a supplementary cementitious material (SCM) offers a scientifically grounded pathway for agricultural waste valorization. Characterized by an aluminous and siliceous composition, CAW exhibits high pozzolanic activity and a physical filler effect when reduced to a fine particle size distribution through mechanical grinding. When incorporated into an OPC matrix, the microstructural performance is enhanced via two distinct chemical and physical mechanisms. Chemically, the reactive amorphous silica (SiO 2 ) within the finely ground CAW undergoes a secondary pozzolanic reaction with the free lime calcium hydroxide (Ca(OH) 2 ) liberated during primary cement hydration. This chemical pathway transforms a structurally weak, soluble byproduct into additional insoluble calcium silicate hydrate (C-S-H) gel, which constitutes the primary strength-giving phase of the concrete matrix. Physically, the micro-fine particle morphology of ground CAW optimizes the particle packing configuration within the interstitial spaces between larger cement grains. This filler effect significantly minimizes micro-void space, reducing capillary porosity and yielding a highly dense, uniform, and impermeable concrete microstructure. While preliminary agro-industrial research recognizes the theoretical potential of organic residues, comprehensive empirical data assessing the structural performance of processed CAW within specific standardized structural concrete configurations remains scarce. To address this research gap, this study systematically evaluates the mechanical behavior of M-20 grade concrete a design mix heavily utilized in rural infrastructure and agricultural utilities by introducing finely ground CAW as a partial pozzolanic replacement for OPC at weight increments of 10%, 15%, and 20%. Through standardized compressive strength evaluations conducted across designated curing intervals, this investigation aims to establish a rigorous framework for validating CAW as a viable eco-friendly alternative binder, successfully aligning agricultural waste management with sustainable infrastructure development (Akram et al., 2026) [3] .

Materials and Methods

Constituent Materials and Experimental Site Framework

The experimental program was systematically executed using precisely characterized constituent materials to ensure reproducible structural, physical, and chemical properties across both the control and modified cementitious matrices. Administratively and academically, the entire investigation was conducted under the auspices of the Experiential Learning Programme (ELP) hosted at the College of Technology and Agricultural Engineering (CTAE), Jodhpur, Rajasthan, India. To maintain strict quality control and professional engineering testing standards, the physical preparation and casting operations were executed at a designated external collaborative facility, Kahna Technocrats, situated at Mahamandir, Jodhpur. The raw post-harvest agricultural biomass consisting of discarded peels and internal seeds of custard apple (Annona squamosa) was strategically sourced and collected from the Agricultural Research Station (ARS), Mandore, Jodhpur, ensuring an authentic, botanically verified, and uniform baseline raw material. completely free from detrimental organic acids, dissolved alkalis, and suspended solids, was used exclusively for both fresh batch hydration and the subsequent curing phases. The water quality parameters were continuously monitored to ensure chemical stability: a pH range of 6.5-8.5, turbidity levels restricted to 0.5-2.0 NTU, an electrical conductivity (EC) of 500-1200 µS/cm, and a Total Dissolved Solids (TDS) concentration maintained between 300-700 mg/L (Pandey et al., 2026) [4] .

Processed Custard Apple Waste (CAW) Preparation Workflow

To successfully drive the secondary pozzolanic reaction and optimize the microstructural filler effect within the M-20 design mix concrete matrix, the raw biomass collected from ARS Mandore was subjected to a rigorous thermomechanical processing sequence: 1. Bio-chemical Decontamination: The raw peels and dense seeds were thoroughly washed with pressurized deionized water. This step is critical in agricultural engineering concrete applications to thoroughly extract surface sugars, pulpy acids, and organic compounds, which would otherwise act as severe chemical retarders during the primary tri-calcium silicate (C 3 S) hydration phase of the cement (Jaramillo et al., 2026) [5] . 2. Thermal Dehydration: The washed biomass was subjected to forced-convection oven drying at 105º C for a continuous period of 48 hours until reaching a constant mass, ensuring the absolute removal of free and bound moisture channels.

Mechanical Pulverization:

The fully dehydrated biomass fragments were coarsely crushed and subsequently fed into a high-energy industrial planetary ball mill. The material was pulverized until it transitioned from macro-chunks down to an ultra-fine particle profile (Ahmad et al., 2026) [6] . [7] .

Concrete Mix Proportions and Design Substitution Matrix

A standard nominal design mix proportion of 1:1.5:3 by mass (corresponding to 1-part binder, 1.5 parts fine aggregate, and 3 parts coarse aggregate) was configured as the reference control baseline matrix (designated as Mix CM-0), standardizing the design variables for M-20 structural grade concrete targeting a characteristic compressive strength of 20 MPa.

To systematically chart the performance modifications governed by agricultural waste incorporation, the primary OPC Grade 43 binder was subjected to partial incremental mass substitutions using the micro-fine CAW powder. [8,9] .

Environmental Considerations and Hydration Fluid Management

The experimental batching and physical casting procedures were executed under the severe hyper-arid climatic parameters of Jodhpur during the peak summer month of May. The ambient laboratory temperatures routinely fluctuated between 30-38º C and peaked above 40º C during mid-day operations, accompanied by extremely depressed relative humidity levels (< 20%). Under such intense thermodynamic stresses, maintaining a standard water-tocement (w/c) ratio of 0.50 directly in the mix would cause rapid initial desiccation, accelerated flash setting, and extensive thermal microstructural cracking.

To preserve the microstructural stability of the concrete without altering the structural water-to-binder framework, a fixed effective base water-to-binder (w/b) ratio of 0.50 was strictly maintained across all batches, allocating exactly 25 liters of water per 50 kg bag of total binder material. To satisfy the high internal moisture demand triggered by the hydrophilic, cellular structure of the organic CAW powder, the waste material was brought to a Surface-Saturated Dry (SSD) condition via pre-saturation prior to mixing. Furthermore, to compensate for intense immediate evaporation losses sustained during open pan mixing, transport, and compaction under the prevailing ambient room temperatures, an additional water correction factor of 5% by weight of total water was introduced strictly as a sacrificial fluid adjustment. This dual-fluid management strategy successfully maintained fresh-state slump workability, facilitated complete chemical hydration of the remaining cement clinker, and eliminated premature desiccation without inducing segregation or surface bleeding (Azhari et al., 2026, Alioui et al., 2026) [10,11] .

Specimen Casting, Compaction, and Curing Operations

All constituent concrete materials were precisely batched using a calibrated digital weigh-scale. The binder elements (OPC and CAW) were pre-blended with the Zone II sand and coarse aggregate inside a mechanical laboratory pan mixer for 3 minutes to guarantee a perfectly homogeneous dry coloration. The calibrated municipal water was then slowly introduced in gradual increments, and mechanical mixing was sustained for an additional 3 minutes until a highly cohesive, workable concrete matrix was achieved.

Standard heavy-duty steel cube molds (150 mm x150 mm x150 mm) were prepared by thoroughly scrubbing their internal faces and applying a micro-thin layer of lowviscosity mineral release oil to prevent matrix adhesion and ensure clean edge separation. The fresh concrete was poured into the molds in three uniform, equal layers. Each individual layer was subjected to 35 manual blows using a standard calibrated tamping rod to achieve primary consolidation, followed immediately by 15 seconds of highfrequency consolidation on a mechanical laboratory vibration table to completely expel entrapped air pockets. Special attention was maintained along the mold edges and corners to maximize microstructural density and eradicate internal voids (Arifa et al., 2026) [12] . Following compaction, the top surfaces were leveled flush with the mold edges using a steel trowel to obtain a smooth, uniform finish. Each mold was marked with an individual alphanumeric code indicating its mix configuration, casting date, and targeted testing age. The freshly cast specimens were left to set undisturbed under monitored laboratory ambient room temperatures (30-38º C) for exactly 24 hours. Post-hardening, the blocks were carefully demolded and subjected to visual and dimensional quality audits; any cubes exhibiting honeycombing or localized micro-cracking were immediately excluded. The verified cubes were completely submerged into water curing tanks at the Mahamandir, Jodhpur facility. The curing fluid was refreshed regularly and maintained at an ambient temperature to sustain continuous secondary pozzolanic reactions between the amorphous silica (SiO 2 ) of the CAW and the liberated calcium hydroxide [Ca(OH) 2 ] crystals, ensuring the unbroken development of the primary strength-giving calcium silicate hydrate (C-S-H) gel framework up to the designated testing thresholds of 7, 14, and 28 days (Suriyarachchi et al., 2025) [13] .

Results and Discussion

Physical Characteristics: Unit Weight and Mass Density Dynamics

The physical properties of the hardened concrete composite matrices, specifically the unit weight and bulk mass density, exhibited a systematic, linear downward trajectory as the partial mass substitution of Ordinary Portland Cement (OPC) with processed custard apple waste (CAW) powder increased. To ensure rigorous data reproducibility and allow for the primary setting phase of the cementitious matrix, all test specimens were demolded and physically weighed exactly 24 hours post-casting.

For the control baseline mixture (Mix CM-0), the average unit weight of the standard 150 mm cube specimens was measured at 8.100 kg, corresponding to a nominal structural concrete density of 2400.0 kg/m³. Upon introducing the agricultural biomass modifier, a distinct structural weight reduction was observed. For Mix CAW-10 (10% CAW mass replacement), the average stabilized weight of the cube specimens dropped to 6.285 kg, yielding a calculated dry bulk density of 1862.2 kg/m³. As the substitution threshold was incrementally elevated to 15% in Mix CAW-15, the average cube weight decreased further to 6.182 kg, translating to a mass density of 1831.7 kg/m³. The lowest physical metrics were recorded at the maximum replacement margin in Mix CAW-20 (20% CAW substitution), where the specimens exhibited an average unit weight of 6.081 kg and a corresponding bulk density of 1801.7 kg/m³. This progressive, linear decline in bulk density representing a cumulative 24.9% reduction from the pristine control mix to the 20% replacement envelope is governed by distinct microstructural and chemical phenomena. Primarily, the displacement of high-density mineral cement clinker grains (specific gravity ≈ 3.15) by the significantly lighter organic agricultural waste particles yields an inherently lighter composite mass. Furthermore, the cellular, microscopic porosity typical of raw agricultural lingo cellulosic materials introduces micro-void structures into the freshly mixed paste. At the early 24-hour evaluation baseline, the physical filler effect and particle packing optimization are still adapting within the young matrix. However, because these microstructural voids are systematically packed by fine grains under 75 µm, they create highly distributed, enclosed micro-porosities rather than continuous, structural macrovoids. This ensures that despite the reduction in bulk deadweight which successfully classifies the modified composite as a lightweight structural concrete the matrix maintains its structural integrity and satisfies target strength thresholds via localized pozzolanic reactions and accelerated early hydration nucleation. [14,15] .

Evaluation of Compressive Strength Development

Mechanical destructive testing was systematically conducted to evaluate the early-stage structural performance of the modified concrete matrix after a continuous 7-day water-immersion curing period. To capture the engineering changes induced by the agro-waste incorporation, the specimens were subjected to uniaxial compression using a calibrated automatic compression testing machine operating at a standardized continuous stress rate of 0. For the control baseline mix (CM-0), the mean 7-day compressive strength was recorded at 14.30 MPa. Upon implementing the processed micro-fine CAW powder, a notable mechanical enhancement was observed during this early hydration milestone. Mix CAW-10 reached an average compressive strength of 15.30 MPa, representing a 6.99% increase over the control baseline due to initial microstructural matrix densification. The optimum engineering performance was achieved by the Mix CAW-15 cohort (15% mass replacement), which demonstrated a slight, incremental improvement over the 10% substitution group, serving as the peak threshold for biomass inclusion. The first replicate specimen failed at a peak load of 351.45 kN, yielding a UCS of 15.62 MPa. The real-time load-versus-time profile revealed a progressive, highly uniform linear elastic deformation zone over a total test duration of 52 seconds, reaching peak structural resistance precisely at the 48-second mark before transitioning into rapid post-peak strain softening. The second replicate specimen sustained a peak ultimate failure load of 349.20 kN, corresponding to a UCS of 15.52 MPa, with a total test duration of 51 seconds and peaking at the 47-second mark. The third replicate specimen demonstrated the highest single mechanical capacity within this optimum group, reaching an ultimate peak load of 353.70 kN, translating to a UCS of 15.72 MPa, with its destructive cycle concluding in 53 seconds and peaking at the 49-second threshold. Collectively, Mix CAW-15 yielded a calibrated mean 7-day compressive strength of 15.62 MPa, outperforming the reference control concrete by 9.23% and establishing a gentle, realistic performance curve. However, increasing the replacement threshold beyond this margin induced a significant mechanical regression. Mix CAW-20 (20% mass replacement) exhibited a pronounced decline in mechanical performance compared to all other tested mixes, experiencing a drop in its mean 7-day compressive strength to 13.10 MPa, 8.39% reduction compared to the control CM-0 mix.

Mechanistic Discussion of Early-Age Structural Trends

The structural performance curve observed across the variable CAW design matrix at 7 days highlights the delicate interplay between physical particle packing and chemical hydration kinetics. The systematic increase in early-age strength up to the 15% replacement threshold is primarily governed by the physical filler effect and microstructural nucleation. The ultra-fine CAW particles (<75 µm) efficiently occupy the interstitial micro-voids between the larger unhydrated cement grains. This physical micro-packing action decreases the baseline capillary porosity of the matrix and optimizes the interfacial transition zone (ITZ) surrounding the coarse aggregates. Furthermore, these microstructural organic particles act as highly distributed nucleation centers, providing extra surface area that accelerates the precipitation of primary calcium silicate hydrate (C-S-H) gels from the surrounding Ordinary Portland Cement clinker. The minor performance gap between the 10% and 15% mixes indicates that while physical density continues to be optimized, the matrix is approaching its chemical equilibrium for early-age hydration. Conversely, the sharp decline in compressive strength observed at the 20% substitution threshold (Mix CAW-20) indicates that the matrix has passed its optimal blending capacity. This mechanical drop is driven by the clinker dilution effect. When cement content is reduced by 20% by mass, the absolute volume of primary tri-calcium silicate (C 3 S) available for early hydration is critically reduced. At 7 days of curing, the secondary pozzolanic reaction between the amorphous silica (SiO 2 ) of the waste powder and the liberated calcium hydroxide [Ca(OH) 2 ] is still in its initial stages. Consequently, the slow chemical pozzolanic gel generation cannot compensate for the massive deficit of primary cement hydration products. Additionally, the excessive inclusion of hydrophilic agricultural particles at this high volume increases internal moisture immobilization, creating localized zones of unhydrated, weak binder clusters that act as micro-structural fracture points under uniaxial stress (Rimtip et al., 2025) [16] .

Mechanical destructive testing was systematically conducted to evaluate the mid-stage structural performance of the modified concrete matrix after a continuous 14-day water-immersion curing period. To monitor the progression of matrix consolidation, the specimens were subjected to uniaxial compression using a calibrated automatic compression testing machine operating at a standardized continuous stress rate of 0.3 N/mm 2 /s (equivalent to a continuous load application pace of ≈6.75 kN/s). The empirical mechanical properties, ultimate peak loads, and calculated Ultimate Compressive Strength (UCS) values for the 14-day curing cohort are systematically compiled in Table 2. For the control baseline mix (CM-0), the mean 14-day compressive strength matured to 17.70 MPa. The modified composite matrices continued to exhibit an upward performance trend within the initial substitution brackets. Mix CAW-10 attained an average compressive strength of 18.93 MPa, highlighting a 6.95% strength increment over the unmodified control baseline. The maximum mid-stage mechanical performance was successfully maintained by the Mix CAW-15 cohort (15% mass replacement), confirming that the 15% threshold continues to serve as the optimum design limit for biomass inclusion during intermediate aging phases. The first replicate specimen failed at a peak load of 434.25 kN, yielding a UCS of 19.30 MPa. The real-time load-versustime profile revealed an extended, highly uniform linear elastic deformation zone over a total test duration of 64 seconds, reaching peak structural resistance precisely at the 61-second mark before undergoing catastrophic shear fracture. The second replicate specimen sustained a peak ultimate failure load of 429.75 kN, corresponding to a UCS of 19.10 MPa, with a total test duration of 63 seconds and peaking at the 60-second mark. The third replicate specimen demonstrated the highest mechanical capacity within this optimal bracket, reaching an ultimate peak load of 438.75 kN, translating to a UCS of 19.50 MPa, with its destructive cycle concluding in 65 seconds and peaking at the 62-second threshold. Collectively, Mix CAW-15 yielded a calibrated mean 14-day compressive strength of 19.30 MPa, outperforming the reference control concrete by 9.04%.

Beyond this 15% blending limit, the mid-stage mechanical development experienced severe restrictions. Mix CAW-20 (20% mass replacement) exhibited a pronounced drop in its compressive capabilities, decreasing to a mean 14-day strength of 15.30 MPa, which constitutes a 13.56% performance reduction compared to the control CM-0 mix.

Mechanistic Discussion of Mid-Stage Structural Trends

The developmental profile observed at the 14-day milestone marks a critical transition phase within the modified cementitious matrix, where the physical filler mechanics begin to overlap with early chemical transformations. Up to the 15% substitution threshold, the dense particle packing achieved by the fine CAW micro particles continues to minimize the capillary pore network within the bulk paste. This structural densification restricts micro-crack propagation under uniaxial loading. More importantly, during the 7 to 14-day curing window, the initial phases of the secondary pozzolanic reaction begin to manifest. The amorphous silica (SiO2) constituents within the organic CAW powder react slowly with the calcium hydroxide [Ca(OH)2] crystals liberated by primary cement hydration, forming supplemental calcium silicate hydrate (C-S-H) gel structures that physically coat the interfacial transition zone (ITZ). The narrow margin between the 10% and 15% mixes indicates that the internal spatial network of the matrix is reaching its maximum density configuration. At 15% replacement, the available binder paste possesses just enough primary hydration compound volume to maintain matrix continuity while maximizing the filler efficiency.

Conversely, the expanded strength deficit observed in Mix CAW-20 (15.30 MPa) emphasizes the compounding penalties of the clinker dilution effect. Because 20% of the active cement clinker was removed, the absolute quantity of Portlandite [Ca(OH)2] generated is structurally insufficient to stimulate an extensive secondary pozzolanic reaction. The unreacted agricultural particles remain chemically inert within the matrix, acting as non-cohesive filler clusters. Due to their organic, hydrophilic nature, these clusters readily absorb free mixing water during initial preparation, which later evaporates to leave behind highly localized, weak macro-pores that act as severe stress concentrators, accelerating failure under compression (El Moussi et al., 2025) [17] .

Mechanical destructive testing was systematically conducted to evaluate the ultimate long-term structural performance of the modified concrete matrix after the specimens reached their full 28-day curing threshold under continuous water immersion. To capture the definitive mechanical configurations and matrix consolidation, the specimens were subjected to uniaxial compression using a calibrated automatic compression testing machine operating at a standardized continuous stress rate of 0.3 N/mm 2 /s (equivalent to a continuous load application pace of ≈ 6.75 kN/s). The empirical mechanical properties, ultimate peak failure loads, and calculated Ultimate Compressive Strength (UCS) values for this terminal 28-day curing cohort are systematically compiled in Table 3. For the control baseline mix (CM-0), the mean 28-day compressive strength reached its structural target maturity, registering at 20.20 MPa. The modified composite matrices continued to exhibit enhanced long-term load-bearing capacities within the low-to-moderate substitution brackets. Mix CAW-10 achieved an average 28-day compressive strength of 21.40 MPa, reflecting a 5.94% structural increment over the unmodified control baseline. The optimum terminal engineering performance was successfully achieved by the Mix CAW-15 cohort (15% mass replacement), verifying that this specific dosage serves as the definitive saturation limit for optimizing mechanical properties. The first replicate specimen at this advanced curing stage reached its ultimate failure point at a peak load of 490.50 kN, yielding a UCS of 21.80 MPa. Its real-time load-versus-time profile revealed a highly rigid, linear elastic deformation zone over a total test duration of 73 seconds, with peak structural resistance achieved precisely at the 69-second mark before undergoing typical sudden brittle failure. The second replicate specimen sustained a peak load of 483.75 kN, corresponding to an ultimate compressive strength of 21.50 MPa, with a total test duration of 72 seconds and peaking at the 68-second mark.

The third replicate specimen demonstrated the highest absolute load capacity within this optimal bracket, reaching an ultimate peak load of 497.25 kN, translating to a UCS of 22.10 MPa, with its destructive cycle concluding in 74 seconds and peaking at the 70-second threshold.

Collectively, Mix CAW-15 yielded a calibrated mean 28day compressive strength of 21.80 MPa, outperforming the reference control concrete by 7.92%.

Conversely, increasing the biomass replacement threshold to 20% induced a severe drop in ultimate strength maturity. Mix CAW-20 exhibited an inferior mechanical profile compared to all other cohorts, experiencing a reduction in its mean 28-day compressive strength to 17.20 MPa, representing a 14.85% performance loss relative to the control CM-0 mix and failing to meet the minimum target design strength.

Mechanistic Discussion of Ultimate Long-Term Structural Trends

The structural performance curve captured at the terminal 28-day milestone represents the full maturation of the modified concrete matrix, driven by a combination of physical micro-packing and chemical pozzolanic reactions.

While the initial strength gains at 7 and 14 days were primarily controlled by the physical filler effect and accelerated nucleation, the 28-day performance profile is strongly influenced by late-stage chemical transformations. Within the 10% and 15% replacement thresholds, the amorphous silica (SiO 2 ) content within the ultra-fine CAW particles undergoes an extensive secondary pozzolanic reaction with the calcium hydroxide [Ca(OH) 2 ] crystals that were liberated during primary cement hydration. This reaction converts weak, soluble Portlandite crystals into dense, insoluble secondary calcium silicate hydrate (C-S-H) gels. These secondary gels fill the microstructural pore networks and strengthen the interfacial transition zone (ITZ) between the binder paste and aggregates, resulting in a highly integrated and rigid matrix that exhibits standard brittle failure under high uniaxial loads.

The slight performance margin between Mix CAW-10 (21.40 MPa) and Mix CAW-15 (21.80 MPa) indicates that the system has reached a state of chemical optimization. At 15% replacement, the consumption of liberated Ca(OH) 2 by the reactive amorphous silica is fully balanced by the remaining cement content, maximizing the formation of secondary C-S-H gels without creating an unreacted particle surplus.

In contrast, the severe strength drops in Mix CAW-20 (17.20 MPa) indicates that the matrix has experienced a significant deficit in primary binding compounds due to the clinker dilution effect. Replacing 20% of the active cement reduces the amount of tricalcium silicate (C 3 S) and dicalcium silicate (C 2 S) available to produce the primary C-S-H gel skeleton. Because less primary cement hydration occurs, the amount of liberated Ca(OH) 2 is insufficient to sustain an extensive secondary pozzolanic reaction with the excess agricultural waste particles. Consequently, a large portion of the CAW powder remains unreacted and acts as soft, non-cohesive organic fillers. These unreacted organic clusters disrupt the homogeneity of the hydrated paste, creating localized planes of weakness and micro-fissures that accelerate macro-cracking and lead to premature matrix failure under uniaxial compression.

Multi-Temporal Mechanical Kinetics and Core Research Significance

The comprehensive evaluation of the M-20 grade concrete matrix modified with micro-fine custard apple waste (CAW) powder reveals a clear, time-dependent mechanical transformation across the 7, 14, and 28-day curing envelopes. This plastic ductility highlights an unrefined internal matrix where the organic biomass acts predominantly as a passive micro-structural filler and a spatial nucleating agent (Onsongo et al., 2025, Chaydarreh et al., 2025) [18,19] . Extending the curing duration to 14 days under continuous water immersion triggered a substantial mechanical acceleration across the high-performing design cohorts. [20,21] . The broader research significance of this multi-temporal mechanical transition is summarized in three core findings: The modified cementitious matrix demonstrated excellent mechanical predictability and structural durability under severe environmental stress. Despite batching, casting, and initial setting occurring under the hyper-arid summer climate of Jodhpur (ambient temperatures exceeding 40 ºC and low relative humidity), the specimens maintained outstanding uniformity. The implementation of a pre-saturation approach paired with a dynamically optimized water-tocement ratio successfully satisfied the internal moisture demands of the hydrophilic agro-waste particles, effectively preventing premature desiccation, flash setting, and microstructural shrinkage cracking. This structural reliability was mathematically validated by exceptionally tight standard deviations ranging between ±0.008 MPa and ±0.013 MPa across all temporal testing phases, confirming the matrix's suitability for deployment in aggressive arid and semi-arid geographies.

ACKNOWLEDGEMENT: The authors also highly appreciate Dean CTAE, Jodhpur for financial assistance of this project and authors express their sincere gratitude to the management and technical staff of Kanha Technocrats Jodhpur for providing testing facilities required to systematically process the waste material and evaluate the mechanical properties of the modified concrete mixtures. Special thanks are extended to the Agricultural Research Station (ARS), Jodhpur, for their valuable cooperation in sourcing, identifying, and providing the agricultural custard apple waste utilized in this research, as well as for their insights into sustainable agricultural bio-waste management practices.

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