Influence of paper mill sludge ash on mechanical, microstructural and durability properties of metakaolin based geocrete

Abstract

This study investigates the influence of Paper Mill Sludge Ash (PMSA) on the mechanical, microstructural, and durability properties of metakaolin-based geocrete. Metakaolin (MK) was partially substituted with PMSA at varying levels (5%, 10%, 15%, and 20%), with mixes designed at alkali activator/binder (A/B) ratios of 0.40 and 0.45. The geocrete specimens were synthesized using 8 M sodium hydroxide under ambient curing (24 to 26 °C) conditions in the entire study. Mechanical properties were assessed through compressive, split tensile, and flexural strength tests, while microstructural analysis was conducted using scanning electron microscopy (SEM). Durability performance was examined through rapid chloride penetration (RCPT), water absorption, and porosity tests. The G6 mix, prepared with an A/B ratio of 0.45, consistently demonstrated higher mechanical strength than the G2 mix with an A/B ratio of 0.40, indicating that an increased A/B ratio accelerates the geopolymerization process, enhancing strength development. SEM analysis confirmed this strength improvement, which can be ascribed to the formation of a dense matrix in G6 mixes. The G2 and G6 mix exhibited superior durability, with RCPT values of 1521 and 1414 coulombs, water absorption rates of 2.8% and 2.5%, and porosity levels of 11.73% and 11.58%, respectively. The findings suggest that incorporating 10% PMSA in geocrete formulations provides a viable, low-carbon alternative to cement concrete.

Introduction

Globally, the increasing demand for housing driven by population growth necessitates the adoption of alternative technology and materials in construction to fulfil these needs. Eco-friendliness and ecological impact are two critical considerations that must be addressed when exploring alternative technology and materials^1–3. In this context, Geocrete (Geopolymer Concrete) has gained prominence as an innovative and underexplored domain for investigators. Geocrete offers numerous benefits than cement concrete, including a reduced carbon footprint, lower production costs, and the capability to incorporate industrial and agricultural waste as binding materials^4,5. The formation of geopolymers involves the breakdown of alumino-silicate compounds in an activating solution, with industrial or agricultural waste by-products and an alkaline activation medium serving as the two fundamental components required for Geocrete. These raw materials must contain alumina-silicate content to facilitate the process^6–10. Based on previous studies, factors such as the chemical oxidation of both activator/binder (A/B), the SiO₂/Al₂O₃ ratio, the molarity ratio, and curing techniques significantly influence the efficiency of the geopolymerization (GP) process^11–15. GP refers to the reaction of monomer units combining to create interconnected 3-dimensional polymeric networks^16,17. Activated Binders are considered to demonstrate superior mechanical properties (MP) compared to cement concrete^18,19. Consequently, utilising geocrete not only aids in minimising industrial and agricultural waste but also facilitates rapid strength development, making it an ideal option for expedited construction projects²⁰. Advancements in geocrete research revealed that KOH and NaOH are commonly used as activator solutions. While KOH exhibits greater alkalinity, NaOH demonstrates superior efficiency in releasing monomers²¹. A notable higher in strength was noted within the concentration range of 4–10 M. However, a differing pattern emerged in the following range of 10–18 M, where a steady reduction in MP of geocrete was recorded^22,23. The SiO₂/Al₂O₃ ratio, which indicates the ratio of silica to alumina, plays a crucial role in determining the MP results of geocrete²⁴. The optimal MP of geocrete have typically been reported when the SiO₂/Al₂O₃ ratio is between 3.0 and 3.8, these ratio range promotes effective geopolymerization, leading to better matrix structure and MP in geocrete systems²⁵. The intricate interaction between alkali concentration and MP of geocrete highlights the need to precisely adjust this factor to achieve the intended performance attributes in geopolymeric binders^26,27. MP of geocrete cured at lower temperatures tends to exhibit significantly reduced, diminishing its capacity to endure exposure to aggressive periods. Curing temperatures ranging from 40 to 85 °C have been documented for optimal GP reactions. It has been found that elevated curing temperatures are beneficial for enhancing strength²⁸. Metakaolin, a calcined kaolin clay, forms a pure alumino-silicate polymer, offering greater strength and durability than cement concrete. The A/B ratio for MK and bottom ash-based geopolymer should range from 0.48 to 0.54. The study suggests that an A/B ratio of 0.5 is most suitable for achieving a significant improvement in MP of geocrete than other geocrete mixes^29,30. The MK and GGBS binders coexist, with the primary phase composed of fine calcium precipitates distributed within the binder at elevated alkali levels. The alkali level also influences the solubility of Ca²⁺, leading to instability in the system. MK, having a larger platelet surface area, absorbs more free water at the initial stages of the reaction and releases less in the later stages, resulting in a more compact structure than GGBS^31–33. Binders composed of 50% MK and 50% GGBS ash deliver the highest MP of geocrete at ambient temperature, the durability of solely GGBS-based geocrete mix with 2 M revealed better performance in terms of reduction in water absorption, VPV, chloride penetration and improved MP than compared to PC mix^34,35. The MP of fly ash-based geocrete enhanced by curing temperature. To address this limitation, research on GGBS is being conducted to increase the calcium content in the material source and accelerate the GP process³⁶. The elevated curing temperature effectively enhances the dissolution of Al and Si atoms from the inactive vitreous phases in fly ash. However, the effect of curing temperature on MK-based geocrete was less pronounced than its impact on fly ash-based geocrete³⁷. The inclusion of paper sludge-based geocrete influenced the MP of fly ash-based geocrete. Paper sludge-based geocrete incorporation enhanced strength under external exposure and ambient temperature conditions up to a 10% addition, after which strength diminished³⁸. The GGBS/dolomite geocrete (50/50 composition) demonstrated superior properties and early-age strength to cement concrete. It reached a compressive strength of 71.0 MPa, significantly higher than the 35.6 MPa of cement concrete³⁹. MK-based geocrete with strong MP requires high-molarity activators, particularly for low-Ca MK. Optimal dissolution occurs with 10–12 M NaOH, while 5 M shows minimal effect⁴⁰. The inclusion of slag expedites MK-based GP reactions, improving MP of geocrete and enabling the development of Ca-rich geopolymers with enhanced gel matrix. Sodium silicate-activated based MK geopolymers, supplemented with Ca(OH)₂, form hydrated (C-(A)-S-H) through interactions with free SiO₂⁴¹. India manufactures approximately 13 million tons of paper annually, accounting for 3.18% of global production. Pulp and paper mills produce 1.63 tons of lime sludge or paper mill sludge ash (PMSA) per ton of paper, positioning the industry as a significant source of pollution due to the large volume of waste generated⁴². PMSA can be reprocessed in the lime kiln for reuse; however, the energy-demanding process and elevated silica content (5–10%) in paper mills create difficulties, leading to buildup in the kiln. Consequently, paper industries typically opted to discard PMSA in landfills rather than reprocess it⁴³. The utilization of waste materials from diverse industries could help address the disparity between demand and availability in the construction field. Among these materials, PMSA offers considerable promise as a valuable building resource. The PMSA functions as a binding agent, and upon incineration, it can be employed in the cement and concrete sectors. It consists of approximately 20–50% non-crystalline silica and alumina^44,45. Although several review articles have been published on paper mill waste^46,47, none specifically address the application of MK-based PMSA in geocrete. This study explores the influence of PMSA on the mechanical, microstructural, and durability properties of metakaolin-based Geocrete.

Materials

Paper mill sludge ash (PMSA) and metakaolin (MK)

Paper Mill Sludge Ash (PMSA) and Metakaolin (MK) were the primary binding materials employed in this study. PMSA was produced by incinerating paper sludge from Seshasayee Paper Mill, Tamil Nadu, at 700 °C for a period of two hours. It exhibited a specific surface area of 3700 cm²/g and a specific gravity of 2.3. The oxide composition of PMSA revealed significant CaO content (38.31%), along with SiO₂ (35.22%) and Al₂O₃ (7.07%), highlighting its potential as a reactive supplementary binder. In contrast, MK, procured from Chennai, was obtained by calcining kaolin clay under controlled conditions. With a fineness of 20,000 cm²/g and a specific gravity of 2.5, MK demonstrated a high alumina (42.65%) and silica (50.23%) composition, making it a highly reactive alumino-silicate precursor for geopolymerization. The chemical contrast between PMSA and MK reveals a complementary balance: while MK provides silica and alumina essential for polymer chain development, PMSA contributes calcium, thereby enhancing the formation of secondary gel phases, such as C-A-S-H. Their combined use aimed to synergise these properties, producing a denser geopolymeric matrix. To assess this synergy, PMSA was used as a partial substitute for MK in various proportions (05–20%), enabling the examination of its impact on the mechanical, microstructural, and durability performance of MK-based geocrete. Both the PMSA and MK oxide compositions are detailed in Table 1.

Table 1.

Oxide proportions of MK and PMSA.

Oxide proportions	PMSA (%)	MK (%)
SiO2	35.22	50.23
Al2O3	7.07	42.65
CaO	38.31	0.25
Fe2O3	3.14	0.64
MgO	15.01	0.12
SO3	0.69	0.78
LOI	0.61	7.35

M-sand and coarse aggregate

The manufactured sand (M-sand) and coarse aggregate used in this study were procured from Pallipalayam, Tamil Nadu, ensuring locally available and quality-controlled materials. Both aggregates were characterized as per IS 2386–3 (1963) [65] to ensure compliance with standard specifications. The M-sand exhibited a specific gravity of 2.7 and a fineness modulus of 2.45, indicating its suitability as a fine aggregate with desirable particle grading and packing properties. Coarse aggregate, on the other hand, displayed a specific gravity of 2.68 and a fineness modulus⁴⁸ of 6.71, reflecting its role in providing the necessary bulk, strength, and stability to the geocrete matrix. The proper grading of M-sand enhanced workability and reduced void content, while the well-graded coarse aggregate contributed to mechanical interlocking within the hardened geocrete. Together, these aggregates supported the geopolymerization process by creating a compact matrix that minimized porosity and improved mechanical strength. Their consistent quality ensured that variations in performance could be attributed to the experimental parameters, namely the incorporation of paper mill sludge ash (PMSA) in metakaolin-based geocrete. Thus, the selected M-sand and coarse aggregate formed a reliable base for producing sustainable, durable, and structurally sound geocrete mixes.

Alkaline agents

The alkaline agents used in the research are sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). Sodium silicate (Na₂SiO₃) was obtained by local dealer with combination of Na₂O − 8.5%, SiO₂ − 25% and H₂O − 63.5%. Sodium hydroxide was supplied in pellet form, white in colour with a specific gravity of 1.52, and dissolved in water to obtain the required molarity with molecular weight of 39.997 g/mol. In this study, an 8 M NaOH solution was prepared and used as one of the activators. Sodium silicate, provided in liquid form, appeared light yellow and had a specific gravity of 1.53. The combination of NaOH and Na₂SiO₃ served as the alkaline activator solution essential for initiating the geopolymerization process in metakaolin (MK) and paper mill sludge ash (PMSA)-based geocrete. After preparation, the solutions were rested for 24 h to ensure stability and uniformity before being mixed with binders. The synergistic role of these alkaline agents is crucial, as NaOH enhances the dissolution of silica and alumina from MK and PMSA, while sodium silicate supplies additional silica, facilitating the formation of polymeric N-A-S-H and C-A-S-H gel networks. These gels contribute to improved microstructural compactness, strength, and durability. The balanced use of NaOH and Na₂SiO₃ ensures effective activation of the binder materials, accelerating geopolymerization and resulting in a denser, stronger geocrete with reduced porosity and permeability, thereby enhancing mechanical and durability properties.

Mix proportions of MK-PMSA geocrete

The mixed proportions of MK-PMSA geocrete were designed to evaluate the role of Paper Mill Sludge Ash (PMSA) as a partial replacement for metakaolin (MK) under different activator/binder (A/B) ratios. A total of eight mixes were prepared, labelled G1 through G8, with PMSA replacing MK in increments of 5%, 10%, 15%, and 20%. For mixes G1–G4, the A/B ratio was set at 0.40, while for mixes G5–G8, the ratio was slightly higher at 0.45, allowing the study to examine the effect of activator concentration on geopolymerization. Sodium hydroxide of 8 M molarity, combined with sodium silicate, was used as the alkaline activator across all mixes. Both solutions were mixed and allowed to rest for 24 h prior to casting. Fine aggregates (M-sand) and coarse aggregates were used at fixed proportions of 570 kg/m³ and 1140 kg/m³, respectively, for all mixes, ensuring consistency in aggregate content. In the A/B = 0.40 series, G1 consisted of 95% MK and 5% PMSA, while G2, G3, and G4 contained 90% MK–10% PMSA, 85% MK–15% PMSA, and 80% MK–20% PMSA, respectively. Similarly, for the A/B = 0.45 series, G5–G8 were proportioned identically but with slightly higher activator content, increasing sodium hydroxide and sodium silicate to enhance reactivity. For instance, G1 used 43.43 kg/m³ NaOH and 108.57 kg/m³ Na₂SiO₃, while G5, with the same MK–PMSA blend, used 49.01 kg/m³ and 122.14 kg/m³, respectively, reflecting the elevated activator dosage. This systematic variation enabled the assessment of PMSA’s impact on strength, durability, and microstructural properties under various alkaline environments. Results indicated that 10% PMSA replacement (G2 and G6) was optimal, striking a balance between calcium-induced gel formation and aluminosilicate bonding. The mixed proportions of MK-PMSA geocrete are depicted in Table 2.

Table 2.

Mix proportioning of MK-PMSA geocrete.

Mix ID	Metakolin (%)	PMSA (%)	F.A (kg/m3)	CA (kg/m3)	A/B ratio	SH (kg/m3)	SS (kg/m3)
G1	95	5	570	1140	0.40	43.43	108.57
G2	90	10	570	1140	0.40	43.43	108.57
G3	85	15	570	1140	0.40	43.43	108.57
G4	80	20	570	1140	0.40	43.43	108.57
G5	95	5	570	1140	0.45	49.01	122.14
G6	90	10	570	1140	0.45	49.01	122.14
G7	85	15	570	1140	0.45	49.01	122.14
G8	80	20	570	1140	0.45	49.01	122.14

Methodology

The mechanical and durability-related investigations of MK-PMSA geocrete were systematically conducted to establish the performance of the developed binder system under standard conditions. The mechanical properties were evaluated in accordance with IS 516 (1959)⁴⁹, which provides the procedures for testing hardened concrete. Compressive strength (150 mm x 150 mm x 150 mm) tests were performed at curing intervals of 7, 28, and 90 days for all mixes to determine the development of strength over time. Cylindrical specimens (150 mm x 300 mm) were prepared and cured under controlled conditions before testing. The results provided insights into the effect of mix design and curing age on the strength characteristics of MK-PMSA geocrete.

For microstructural characterization, scanning electron microscopy (SEM) analyses were performed to understand the morphology and bonding of reaction products. Specimens from mixes G1 and G2 with an activator-to-binder ratio (A/B) of 0.40, and G5 and G6 with A/B of 0.45, were selected for SEM analysis. The surfaces were examined using a VEGA 3 SBH (TESCAN) operating at an accelerating voltage between 10 and 30 kV. A low-pressure method was employed to preserve the fine features of the geopolymer matrix. These analyses helped in correlating the mechanical performance with the microstructural development and the distribution of geopolymeric gels.

Durability-related investigations included electrical resistivity, porosity, and water absorption. RCPT was to measure the level of electrical conductivity and resistance to the intrusion of chloride iron in concrete quickly. The cylinder discs of a diameter of 100 mm and a thickness of 50 mm were cast and tested after 28 days. On one end, the specimen contains a 0.3 M solution of NaOH and on the other, there is 3% solution of NaCl. At the same time, six hours shielding of a 60 V potential difference was done. The flow of current was applied to the specimens during the necessary period after every 30 min. The penetration of the chloride in the sample was given as the total charge that was transferred in coulombs per the time of the specified time. RCPT setup was illustrated in Fig. 1 of the manuscript. This test provided information on the pore connectivity and ionic transport properties of the geocrete.

Fig. 1.

RCPT test setup for MK-PMSA geocrete.

In addition, porosity and water absorption were determined for 28-day-cured specimens to quantify the permeable pore structure and water retention capacity of the geocrete. These parameters are crucial for assessing the long-term durability of cementitious composites, as they influence resistance to chemical ingress and thermal variations. Together, the combination of mechanical testing, microstructural examination, and durability evaluation provided a comprehensive methodology to assess the suitability of MK-PMSA geocrete as a sustainable construction material⁵⁰. The multi-level approach ensured that both strength-related and durability-related performance aspects were thoroughly investigated, supporting the practical applicability of the developed material in structural and environmental contexts.

Result and discussion

Compressive strength of MK-PMSA geocrete

The compressive strength of MK-PMSA geocrete with (A/B = 0.40 and 0.45) as depicted in Fig. 2. At 7 days, the compressive strength of MK-PMSA geocrete mixes with an A/B ratio of 0.40 varied from 20.47 MPa for G1 to 23.35 MPa for G2, showing a 14.2% increase in strength for G2 than G1. G3 recorded a slight reduction of 4.1% to G1, while G4 exhibited a significant decrease of 49.2% relative to G1. The G1 to G4 mix series demonstrated early (7-day) compressive strength development, primarily due to the accelerated rate of strength development facilitated by the rapid GP process, a trend similarly observed in studies by Saeli et al.⁶³. At 28 days, G2 continued to show improvement, with a 6.31% increase over G1 (34.51 MPa), reaching a maximum of 36.71 MPa. Meanwhile, G3 demonstrated an 11.73% reduction over G1. G4 displayed a substantial decrease of 55.83% to G1, achieving only 15.25 MPa. By 90 days, G2 demonstrated a 5.73% increase over G1 (37.26 MPa), reaching a maximum of 39.41 MPa. In contrast, G3 and G4 showed continued reductions of 10.0% and 48.01%, respectively, with G3 at 33.63 MPa and G4 at 19.37 MPa. At 28 and 90 days, strength enhancement was most evident with 10% PMSA, reflecting the increased chemical activity of the calcium component. However, replacing more than 10% of PMSA led to a decline in compressive strength due to incomplete GP, attributed to the higher PMSA content with lower SiO₂ and Al₂O₃ proportions.

Fig. 2.

The compressive strength of MK-PMSA geocrete with A/B ratios of 0.40 and 0.45.

At 7 days, for the mixes with an A/B ratio of 0.45, G6 exhibited a 13.2% increase over G5 (22.27 MPa), reaching a compressive strength of 25.38 MPa. G7 showed a slight reduction of 3.0%, with a compressive strength of 21.62 MPa, while G8 demonstrated a significant decrease of 42.4%, achieving a strength of 12.83 MPa, which is lower than that of G5. Similarly, at 28 days, G6 showed a 7.77% increase over G5 (37.72 MPa), reaching 40.65 MPa, while G7 demonstrated a 19.19% reduction, and G8 showed a notable decrease of 54.06% over G5. The strength progression in MK-PMSA geocrete was largely attributed to the higher concentration of CaO relative to SiO2 in the oxide composition of PMSA. This leads to a binding system based on Si + Ca rather than Si + Al. Calcium reacts with soluble Si and Al species to form C-S-H and C-S-A-H gel phases, which coexist with geopolymer phases, thereby contributing to the development of strength in MK-PMSA geocrete⁶⁴. By 90 days, G6 continued to outperform G5 (40.43 MPa), with a 10.3% increase in strength, reaching 44.52 MPa, while G7 showed a slight reduction of 4.7%, and G8 demonstrated a 43.5% decrease from G5. The results indicate that specimens with an A/B ratio of 0.45 exhibited higher strength than those with an A/B ratio of 0.40. This variation can be attributed to the influence of the solution and the pore ratio present in the geocrete^51,52.

Split tensile strength of MK-PMSA geocrete

The split tensile strength of MK-PMSA geocrete provides valuable insight into its performance under tensile stresses, which are critical for assessing durability and structural integrity. The study compared mixes with different activator-to-binder (A/B) ratios of 0.40 and 0.45 across 7, 28, and 90 days, highlighting the influence of PMSA replacement levels and mix proportions (Fig. 3).

Fig. 3.

The split tensile strength of MK-PMSA geocrete with A/B 0.40 and 0.45.

For an A/B ratio of 0.40, the results showed that the addition of PMSA up to 10% significantly improved the tensile strength. At 7 days, G2 achieved a 20.6% increase over G1, while G3 and G4 experienced reductions of 5.9% and 28.4%, respectively, indicating that optimal PMSA addition benefits early strength, but higher dosages hinder strength development. At 28 days, G2 maintained superiority, recording a pressure of 2.58 MPa, which was 10.7% higher than that of G1. However, G3 decreased by 5.2%, and G4 dropped drastically by 57.9%, reflecting a reduced geopolymerization efficiency at higher PMSA levels. By 90 days, G2 further improved to 2.75 MPa (14.1% above G1), while G3 stabilized around G1, and G4 remained significantly weaker.

For an A/B ratio of 0.45, a similar trend was observed, but with overall higher strengths across all age groups. At 7 days, G6 surpassed G5 by 25.6%, achieving 1.57 MPa, while G7 and G8 showed reductions of 3.2% and 22.4%, respectively. By 28 days, G6 again outperformed G5 by 9.6%, while G7 slightly decreased, and G8 dropped significantly (54.6%). At 90 days, G6 achieved 2.97 MPa, an 11.7% improvement over G5, while G7 nearly matched G5, and G8 recorded a 50.4% reduction. These findings confirm that increasing PMSA up to 10% enhances strength; however, excessive replacement leads to incomplete geopolymerization (GP), thereby reducing structural performance.

Comparatively, mixes with an A/B ratio of 0.45 consistently outperformed those with a 0.40 ratio across all curing ages, suggesting that a higher activator content enhances the dissolution of precursors and promotes better gel formation⁵³. This higher reactivity accelerates early-age strength gain and sustains long-term strength. Importantly, the observed tensile strength trends align with the compressive strength behaviour of MK-PMSA geocrete, reinforcing the correlation between microstructural development and mechanical performance.

Overall, the study highlights that PMSA incorporation up to 10% optimizes tensile strength, while higher replacement diminishes performance due to unreacted phases. Additionally, adopting a higher A/B ratio (0.45) is more beneficial, as it fosters stronger geopolymer networks and provides enhanced mechanical performance at both early and later ages, thereby demonstrating the potential of MK-PMSA geocrete in sustainable construction applications.

Flexural strength of MK-PMSA geocrete

The flexural strength performance of MK-PMSA geocrete with A/B ratios of 0.40 and 0.45 demonstrates the influence of alkali concentration and PMSA inclusion on mechanical behaviour over different curing periods (Fig. 4). For an A/B ratio of 0.40, results indicated that at 7 days, G2 achieved the highest strength (1.23 MPa), outperforming G1 by 20.6%, while G3 and G4 showed declines of 5.9% and 28.4%, respectively. At 28 days, the positive effect of PMSA was sustained in G2 (2.58 MPa), which exceeded G1 by 10.7%. In contrast, G3 exhibited only a slight reduction (2.21 MPa), while G4 drastically underperformed at 0.98 MPa, reflecting a 57.9% loss. After 90 days, G2 again surpassed G1 by 14.1%, achieving 2.75 MPa, with G3 stabilizing near G1 and G4 continuing to display a marked deficiency, retaining only 1.13 MPa.

Fig. 4.

The flexural strength of MK-PMSA geocrete with A/B 0.40 and 0.45.

For an A/B ratio of 0.45, similar trends were noted but with more pronounced improvements. At 7 days, G6 achieved 1.57 MPa, a 25.6% improvement over G5, while G7 experienced a minor drop (1.21 MPa) and G8 declined by 22.4% (0.97 MPa). By 28 days, G6 again exhibited superior performance, recording 2.75 MPa (9.6% higher than G5), whereas G7 fell slightly to 2.42 MPa, and G8 dropped by 54.6% to just 1.14 MPa. At 90 days, G6 maintained its advantage, attaining 2.97 MPa compared to G5’s 2.66 MPa (an 11.7% improvement), while G7 decreased marginally (2.63 MPa) and G8 lagged significantly with a 50.4% reduction (1.32 MPa).

These findings highlight that increasing the alkali content from 0.40 to 0.45 enhances the reactivity of the MK-PMSA system, fostering improved gel formation and strengthening. The superior performance of G2 and G6 demonstrates that PMSA addition promotes the formation of secondary C–S–H and C–A–S–H gel phases, resulting in denser microstructures and enhanced load-bearing capacity. Conversely, G4 and G8 consistently underperformed, possibly due to excessive PMSA content or suboptimal particle packing, which limited effective geopolymeric gel development. Overall, the incorporation of PMSA at balanced proportions with higher alkali content proves beneficial in enhancing flexural strength, particularly under ambient curing conditions, aligning with earlier findings by Pham et al.⁵⁴.

Microstructural analysis

The microstructural analysis of geocrete samples (G6, G2, G8, and G4) with A/B ratios of 0.40 and 0.45 reveals distinct morphological and phase-related characteristics that directly influence the material’s performance (Fig. 5). SEM images show that all mixes developed well-defined crystalline structures, indicating effective geopolymerization. The dominant phases observed were N-A-S-H and C-A-S-H gels, which are critical in determining the strength and durability of geocrete.

Fig. 5.

SEM image of MK-PMSA geocrete samples (a) G6; (b) G2; (c) G8; (d) G4.

In G6 and G2 mixes, C-A-S-H gels emerged as the predominant binding phase. This can be attributed to the higher calcium content present, which enhances reactivity with soluble silica and alumina species. The formation of these gels significantly contributes to strength development, as they not only bind particles together but also act as efficient pore-fillers. By infiltrating pore spaces, they reduce overall porosity, densify the microstructure, and improve load-bearing capacity. The enhanced C-A-S-H formation in G6 and G2 was strongly associated with the increased PMSA content and the relatively low A/B ratios, which promote efficient dissolution and gel growth.

In contrast, G8 and G4 exhibited a more distinct pore structure, suggesting reduced gel densification. This phenomenon is linked to addition of PMSA, resulting in dense geopolymer gels^55,56. Such changes resulted in less compact matrices compared to G6 and G2. These findings align with mechanical performance results, where mixes with stronger C-A-S-H gel development demonstrated superior strength.

Additionally, microcracks were detected across all samples. These cracks are not necessarily indicative of poor material performance but are likely artefacts from sample preparation, where cutting and handling induced localized damage. Nonetheless, their presence emphasizes the importance of careful specimen preparation in microstructural studies.

Overall, the SEM observations indicate a dense, compact, crystalline phase, micro cracks, pores associated with the incorporation of PMSA in geocrete for G6, G2, G8 and G4. While G6 and G2 benefited from robust C-A-S-H gel growth and pore refinement, G8 and G4 reflected more porous structures, correlating with slightly reduced strength. These insights demonstrate the pivotal role of calcium-rich additives and optimized A/B ratios in achieving denser, stronger geocrete microstructures, ultimately validating the microstructural–mechanical property relationship.

Rapid chloride penetration test (RCPT) of MK-PMSA geocrete

The Rapid Chloride Penetration Test (RCPT) is a crucial assessment for evaluating the durability of cementitious and geopolymer systems against chloride ion ingress, a primary cause of steel reinforcement corrosion in concrete structures. In the study of MK-PMSA Geocrete, RCPT was performed at 28 days for mixes with A/B ratios of 0.40 (G1–G4) and 0.45 (G5–G8), as depicted in Fig. 6. The results demonstrated significant variations in chloride ion permeability influenced by both mix proportions and liquid-to-binder (A/B) ratios.

Fig. 6.

RCPT of MK-PMSA geocrete with A/B ratios of 0.40 and 0.45.

For the A/B ratio of 0.40, mixes G1 and G2 showed lower total charge passed values of 1742 and 1521 coulombs, respectively, which fall within the “Low” permeability range (1000–2000 C). These values suggest that G1 and G2 developed a dense and refined microstructure, restricting chloride ion ingress. In contrast, mixes G3 and G4 exhibited higher values of 2679 and 4455 coulombs, with G4 falling in the “High” permeability category (> 4000 C). The elevated permeability in G3 and G4 is likely due to the presence of excess hydroxyl ions in the pore solution, which disrupts geopolymer network formation and reduces structural compactness.

For the A/B ratio of 0.45, G5 and G6 displayed improved resistance, with charge passed values of 1628 and 1414 C, respectively, also within the “Low” range. The finer particles of metakaolin (MK) and palm oil mill sludge ash (PMSA) contributed to pore refinement, densification, and enhanced chloride resistance. Conversely, G7 and G8 recorded higher values of 2511 and 4204 coulombs, with G8 categorized as “High” permeability. This increased chloride transport may be attributed to a lowered Si: Al precursor ratio resulting from PMSA incorporation, which weakens the geopolymeric gel structure⁵⁷.

Among all mixes, G2 and G6 emerged as the most resistant to chloride ion penetration in their respective A and B groups. This is linked to the effective consumption of OH⁻ ions during geopolymerization, which favoured the formation of durable N-A-S-H and C-A-S-H gel phases. These gels efficiently filled pores, reducing permeability and strengthening the microstructure⁵⁸. In contrast, mixes G3, G4, G7, and G8 showed higher permeability, indicating porous microstructures with less effective gel development.

The RCPT results correlate closely with both mechanical performance and microstructural observations. Mixes with denser gel formation not only showed improved chloride resistance but also superior strength characteristics. In contrast, mixes with disrupted gel networks exhibited higher porosity, greater chloride ingress, and weaker structural performance.

Water absorption and porosity test of MK-PMSA geocrete

The water absorption and porosity behaviour (Figs. 7 and 8) of MK-PMSA geocrete mixes at 28 days provides significant insights into the influence of varying PMSA contents and A/B ratios on the microstructural development and durability of the material. For the mixes with an A/B ratio of 0.40 (G1–G4), G1 recorded a water absorption value of 3.1%. A slight improvement was observed in G2 (2.8%), attributed to enhanced geopolymerization that promoted a denser microstructure with reduced permeability. However, G3 and G4 exhibited increased absorption values of 3.5% and 4.1%, respectively. This negative trend resulted from the excessive addition of PMSA at 15% and 20%, which disrupted the optimal Si/Al ratio. Such an imbalance hindered the effective dissolution of silicon and aluminium ions, thereby slowing polycondensation and leaving an incomplete aluminosilicate network, consistent with earlier findings⁵⁹.

Fig. 7.

Water absorption result of MK-PMSA geocrete with A/B 0.40 and 0.45.

Fig. 8.

Porosity result of MK-PMSA geocrete with A/B 0.40 and 0.45.

For the mixes with a higher A/B ratio of 0.45 (G5–G8), water absorption initially decreased with moderate PMSA additions. G5 showed 2.9%, while G6 exhibited the lowest absorption (2.5%), marking a 19.4% reduction compared to G5. This confirmed that 10% PMSA facilitated a more compact geopolymer structure, effectively minimizing permeability. In contrast, further increases in PMSA content had an adverse impact on performance. G7 and G8 registered absorption values of 3.3% and 4.4%, respectively, reinforcing the detrimental effect of excess PMSA.

The porosity results mirrored these absorption trends. G1 and G2 displayed relatively lower porosity, reflecting dense matrices with fewer voids. G3 and G4, however, demonstrated increased porosity, corroborating the presence of an incomplete geopolymer network resulting from a silica–alumina imbalance. Similarly, G5 and G6 exhibited reduced porosity, with G6 yielding the densest microstructure. Conversely, the G7 and G8 revealed higher porosity, indicating a weaker matrix that is prone to permeability. These findings are consistent with Liu et al.⁶⁰, who highlighted that excessive precursor addition can compromise network compactness.

Supporting this, sorptivity results also followed the same pattern, with mixes containing 15–20% PMSA showing higher values compared to those with 5–10% PMSA^61,62. The elevated sorptivity indicated reduced resistance to fluid ingress, directly linked to incomplete geopolymerization and porous microstructures.

Conclusion

The following insights were derived from the experimental evaluation of MK-PMSA-based geocrete.

• The study highlights that the geocrete mix with an A/B ratio of 0.40, containing 90% MK and 10% PMSA (G2), achieved higher mechanical properties at all testing ages than other mixes (G1, G3 and G4). These improvements were linked to the additional calcium hydration promoted by the inclusion of PMSA. Similar strength enhancements were observed in mixes with an A/B ratio of 0.45, indicating a consistent trend of superior performance, although slightly higher than that of the 0.40 ratio mixes. Overall, the inclusion of 10% PMSA significantly contributed to the strength development in both mix ratios.

• The SEM images of the G6 and G2 geocrete samples demonstrated enhanced microstructural integrity due to an efficient polymer synthesis process under ambient curing conditions. Notably, these samples exhibited a denser matrix with increased development of C-A-S-H gel phases and a lower porosity structure than G8 and G4 geocrete samples.

• The RCPT results confirmed that MK-PMSA geocrete with 10% PMSA content (G2 and G6 with A/B = 0.40 and A/B = 0.45) exhibited lower chloride ion permeability due to the development of dense N-A-S-H and C-A-S-H gel phases, reducing porosity and enhancing microstructure. Mixes with higher PMSA content, such as G4 and G8, exhibited increased permeability due to the presence of excessive OH⁻ ions and a lower Si: Al ratio, which impacted the geopolymer structure.

• The water absorption and porosity tests revealed that MK-PMSA geocrete mixes with 10% PMSA content (G2 and G6 with A/B = 0.40 and A/B = 0.45) exhibited the lowest water absorption and porosity, indicating a denser microstructure. These improvements were attributed to an optimal polymer synthesis process and the development of N-A-S-H and C-A-S-H gel phases, reducing voids. In contrast, mixes with 15% and 20% PMSA (G3 and G4 for A/B = 0.40, G7 and G8 for A/B = 0.45) exhibited increased water absorption and porosity due to their lower Si: Al ratio, resulting in an incomplete polymerization process.

The future prospects of this work involve the sustainable advancement of MK-PMSA-based geocrete for practical applications in various construction fields.

Author contributions

K. Yuvaraj: Conceptualization, Data curation, Formal analysis, Investigation, Writing - original draft. G. K. Arunvivek: Conceptualization, Writing - original draft. Pramod Kumar: Project administration, Data curation, Formal analysis, Writing – original draft, Writing – review and editing. Keshav Kumar Sharma: Writing – original draft, Writing – review & editing. Regasa Yadeta Sembeta: Writing – original draft, Writing – review and editing.

Funding

This research received no external funding.

Data availability

The data used in the study are presented in the manuscript.

Competing interests

The authors declare no competing interests.

Ethical statement

This study did not involve human participants or animals, and therefore, no ethical approval was required. All research procedures adhered to relevant ethical guidelines and best practices for non-human and non-animal research.