Assessment of permeability and dry shrinkage in oil palm shell concrete enhanced by surface treatment with styrene acrylic emulsion

Abstract

This article introduces oil palm shell (OPS), an agricultural waste product, as a promising substitute for traditional aggregates in concrete. OPS exhibits lightweight characteristics, reducing the overall density of structures and aligning with sustainable building practices. Despite its advantages, OPS incorporation faces challenges, particularly in terms of durability and mechanical strength. The porous nature of OPS raises concerns about its long-term performance, susceptibility to environmental factors and impact on concrete workability. To address these challenges, this article proposes a surface treatment method using styrene acrylic emulsion (SAE) to modify the physical properties of OPS. This research aims to assess the influence of different SAE coatings on the durability properties, particularly in the permeability and dry shrinkage of OPS concrete. Results show that SAE treatment enhances particle density, reduces water absorption and improves adhesion between OPS and the cement matrix, which positively impacts concrete strength.

Alternate abstract:

Este estudio explora el potencial de la cáscara de palma aceitera (OPS), un residuo agrícola, como una alternativa sostenible a los áridos tradicionales en el hormigón ligero. A pesar de sus ventajas, la OPS presenta desafíos relacionados con la durabilidad, resistencia mecánica y trabajabilidad debido a su naturaleza porosa. Para abordar estos problemas, esta investigación investiga la aplicación de emulsión acrílica de estireno (SAE) como tratamiento superficial para la OPS. El estudio evalúa los efectos de diferentes recubrimientos SAE sobre la durabilidad del hormigón con OPS, centrándose en la permeabilidad y la retracción por seco. Los resultados indican que el tratamiento SAE mejora la densidad de las partículas de OPS, reduce la absorción de agua y mejora la unión entre la OPS y la matriz de cemento, lo que mejora la resistencia y durabilidad del hormigón.

1. Introduction ⌅

The increasing focus on sustainable practices in the construction industry has generated considerable interest in alternative materials for concrete production. Concrete, a composite building material, heavily depends on aggregates, constituting 70-80% of its total volume (1). The extraction of natural coarse aggregates through mining operations poses threats to ecosystems and landscapes (2), emphasising the need to explore eco-friendly alternatives. One such promising material is oil palm shell (OPS), an agricultural waste product derived from palm oil processing. Malaysia, the world’s second-largest palm oil producer after Indonesia, annually produces over 4 million tonnes of OPS (1). As concerns about resource depletion and environmental sustainability escalate, the exploration of viable alternatives to traditional aggregates becomes imperative and the utilisation of OPS as a partial replacement holds immense promise for sustainable construction practices.

A key advantage of incorporating OPS in concrete lies in its lightweight characteristics. The density of oil palm shell concrete (OPSC) is approximately 20-25% less than that of normal-weight concrete (NWC), with compressive strength typically ranging between 20 and 35 MPa (3). Studies by Mannan and Ganapathy (4), replacing normal weight aggregate with 20-25% oil palm shell (OPS) by volume, demonstrated comparable density and cube compressive strength for OPSC, falling within the range of 1890-1905 kg/m³ and 24-29 MPa, respectively. These findings align with the observations of Alengaram et al. (5), who report a compressive strength of approximately 38 MPa for OPSC with a saturated density of about 1960 kg/m³ after a 28-day curing period. Recent research (6, 7) has demonstrated that oil palm shells can serve as a lightweight aggregate, enabling the production of high-strength lightweight concrete. The achieved 28-day compressive strength reached as high as 54 MPa, accompanied by an oven-dry density ranging from 1870 to 1990 kg/m³. This collective evidence underscores the potential of OPS as a substitute for conventional aggregates, showcasing its ability to yield comparable strength properties while offering the added advantage of lightweight characteristics. As a lightweight aggregate, OPS reduces the overall density of concrete structures, providing advantages in terms of construction efficiency, transportation and seismic resistance. This attribute is particularly advantageous in applications where weight reduction is critical, such as in precast elements or structures with specific load-bearing requirements. Moreover, the use of OPS aligns with the principles of sustainable and green building practices, mitigating the environmental impact associated with waste disposal and potentially contributing to reduced carbon emissions from the concrete industry, given its lower energy requirement for production compared with traditional aggregates.

Despite the contributions to lightweight characteristics, the incorporation of OPS may lead to a reduction in mechanical strength, particularly in compressive and tensile strengths, limiting its applicability for certain structural applications (2, 8, 9). The integration of OPS into concrete, specifically for enhancing its durability, faces several critical challenges. One major concern revolves around the inherent properties of OPS that may impact the long-term performance of concrete structures. Durability concerns arise due to the porous nature of OPS (10-12), rendering resulting concrete more susceptible to environmental factors such as chloride penetration, sulphate attack and carbonation. These factors can contribute to the deterioration of the concrete matrix and the corrosion of steel reinforcements, ultimately compromising the structural integrity of the construction over time. Additionally, the porosity of OPS can lead to increased water absorption, presenting difficulties during the mixing process and adversely affecting concrete workability (10). Previous studies indicate that OPS absorbs more water than gravel aggregates, with a water absorption rate of 20-24%, 4-5 times higher than that of gravel aggregates (11). Shafigh et al. (13) also reported that the water absorption rate increases as the content of OPS in the concrete mix increases. The high porosity and water absorption of OPS lead to a decrease in effective water content for the hydration process, resulting in a loose interfacial transition zone (ITZ) between OPS and the cement mortar in hardened concrete (1, 10, 14). Furthermore, establishing a strong interface bonding between the cement matrix and the porous, irregular surface of OPS proves challenging, potentially resulting in diminished compressive and tensile strengths (8). The inherent variability in size, shape and density of oil palm shells introduce inhomogeneity within the concrete mix, impacting the uniformity of the mechanical properties and overall structural integrity (3, 9).

On the other hand, the dry shrinkage phenomenon arises from volume changes induced by the evaporation of internal water from the matrix, driven by the disparity in humidity between the interior and exterior of cement-based materials (15). This intricate and time-dependent process unfolds in cementitious materials, resulting in volumetric contraction and the formation of cracks within concrete (16). The occurrence of cracks can compromise the strength and durability of the structure by enhancing concrete permeability, facilitating the ingress of aggressive agents and exposing vulnerabilities to corrosion and degradation of reinforcing bars, particularly in applications involving concrete structures. A pivotal factor influencing the drying shrinkage of concrete is the level of restraint imposed by the aggregate - the higher the elastic modulus of aggregates, the greater the restraint and volumetric proportion of the paste in the concrete mixture. The porous nature and low stiffness of OPS confer a lower modulus of elasticity, providing less restraint on the potential shrinkage of the cement paste (16, 17). Consequently, a substantial drying shrinkage strain is anticipated, primarily influenced by the properties and quantity of aggregates (16). In comparison with normal concrete, OPS concrete exhibits a potentially greater drying shrinkage. Mannan et al. (17) reported that lightweight aggregate (LWA) OPS concrete displays 6% and 14% higher drying shrinkage compared with control concrete at 28 and 90 days, respectively. Alengaram (18) observed OPS drying shrinkage at 90 days in the range of 540-1300 microstrain. Shafigh et al. (14) attributed the high drying shrinkage of OPS concrete to elevated cement and OPS content as coarse aggregate. Consequently, an effective strategy to control and mitigate drying shrinkage strain involves optimising the volume of coarse OPS aggregates in the concrete mixture. The substantial magnitude of the drying shrinkage in OPS underscores the significance of this phenomenon, particularly in concrete products and structures utilising OPS. Despite these challenges, the utilisation of OPS in concrete production remains particularly intriguing due to its economic and environmental benefits. Addressing these challenges requires extensive research focused on the durability aspects of OPS concrete. The heightened porosity and permeability characteristics of OPS render the resulting concrete more susceptible to environmental factors, enabling the ingress of aggressive substances such as chloride ions, sulphates and carbon dioxide through interconnected pores. This increased porosity poses a potential compromise to the long-term durability of the concrete, rendering it more susceptible to deterioration mechanisms.

Therefore, this research concentrates on investigating a surface treatment method using styrene acrylic emulsion (SAE) for surface-coated OPS as coarse aggregates in OPS concrete production. The aim of treating OPS with SAE is to modify and enhance its physical properties before incorporation into the concrete mixture. The expectation is that coating the surface of OPS with SAE has the potential to reduce porosity and permeability characteristics, thereby improving the resulting concrete properties. Specifically, the objectives of this research are as follows: (1) to assess the influence of SAE treatment on the physical properties of OPS aggregates, (2) to evaluate the impact of the SAE-treated OPS on the durability and mechanical properties of concrete, including compressive strength and drying shrinkage and (3) to investigate the microstructural changes in the treated OPS concrete through SEM analysis. Despite the lack of existing studies on the SAE-treated OPS, this research aims to provide a comprehensive analysis of how SAE treatment affects the physical properties, mechanical strength and durability of OPS concrete. The anticipated outcomes are expected to contribute to the development of sustainable concrete practices and promote the widespread use of OPS in the construction industry. By exploring the effects of SAE treatment on OPS, this research aims to enhance the durability and mechanical properties of OPS concrete, making it a more viable alternative to sustainable construction. The findings are expected to contribute valuable insights into the optimisation of OPS for concrete applications, promoting its wider adoption and supporting the development of environmentally friendly construction practices. This approach aligns with global sustainability goals and the drive towards reducing the environmental impact of construction materials.

2. Materials and methods ⌅ 2.1. Materials ⌅ 2.1.1. Cement ⌅

In this study, the primary binder employed was ordinary Portland cement Type I, conforming to ASTM Type 1 standards in accordance with BS 12:1991. It possesses a specific surface area of 1.043 m²/g and a specific gravity of 3.02.

2.1.2. Aggregates ⌅

This study employed local river sand as the designated fine aggregate, possessing particle density, fineness modulus, water absorption and a maximum grain size of 2.53 Mg/m³, 2.83, 0.98% and 2.36 mm, respectively, as detailed in Table 1. Figure 1 illustrates the grading curve for sand. For this investigation, the OPS aggregates were used as a complete substitute for conventional stone aggregates in concrete production. The coarse OPS aggregate was procured from a local palm oil factory. Table 2 provides the physical properties of the normal (untreated) OPS aggregate used for this study. To ensure a superior quality grade of OPS as a coarse aggregate, only those exposed to a tropical environment for over six months were chosen. These OPSs underwent an initial washing with detergent and a 24-hour water soak to eliminate any oil content. Subsequently, they were air-dried and stored. Utilising a sieve vibrator, the OPSs were graded into specific sizes to determine their size distribution. A sieve test, conducted in accordance with BS 812:103.1:1985, confirmed compliance with the grading requirements outlined in BS 882:1992 and determined their fineness modulus. The particle size distribution of OPS is tabulated in Table 1 and illustrated in Figure 2. According to Table 1 and Figure 1, the majority nominal size OPS aggregate is about 10 mm, which is often used for lightweight concrete.

Table 1. Sieve analysis of the fine and OPS aggregate.

	Aggregate passing (%) according to sieve size (mm)									Fine modulus
	0.15	0.3	0.6	1.18	2.36	5	10	14	20
Sand	0.4	6.9	28.5	58.3	90.3	98.5	100	100	100	2.83
OPS	0.0	0.0	0.0	0.0	0.4	2.0	50.8	80.4	100	5.67

Particle size distribution of sand.

Figure 1. Particle size distribution of sand.

Particle size distribution of OPS.

Figure 2. Particle size distribution of OPS.

Table 2. Physical properties of normal OPS aggregate.

Properties of Aggregate
Particle Density - Oven Dry (Mg/m3)	1.02
Particle Density - SSD (Mg/m3)	1.27
Apparent Particle Density (Mg/m3)	1.35
Water Absorption (%)	24.32
Bulk Density (kg/m3)	603.8

2.1.3. Mixing water and superplasticiser ⌅

The tap water used for mixing in this study complied with BS 3148:1980 standards and was considered suitable for concrete production. To maintain workability and improve the rheological properties of the concrete mix containing a high volume of OPS, the addition of a superplasticiser (SP) is required. The chosen SP, Conplast SP 2000, is a chloride-free admixture based on sulphonated naphthalene polymers, which comply with BS 5075-1. The SP molecules wrap around the cement particles, giving them a highly negative charge and enhancing the mixture’s flow characteristics.

2.1.4. Styrene acrylic emulsion ⌅

In this study, SAE was employed as a synthetic polymer solution to apply a coating to the surface of OPS. SAE is an acrylate-based copolymer emulsion modified with styrene, delivering enhanced water resistance, alkali resistance and coating film durability through the inclusion of styrene monomers. The selection of SAE for surface treatment was based on its superior hydrophobic characteristics, offering improved water resistance and moisture vapour transmission rate compared with all-acrylic polymers. Moreover, SAE stands out as an environmentally friendly coating with low levels of volatile organic compounds, contributing to the reduction of pollution and gas emissions [19]. Detailed information on the SAEs used for surface treatment in this research is provided in Table 3.

Table 3. Detailed specifications of the SAE.

Physical and chemical properties:
i. Appearance	Paste
ii. Colour	white
iii. pH	9
Technical information:
i. Tensile strength	~0.8 MPa
ii. Elongation at break	~260%
iii. Adhesion in peel	~1.2 N/mm2
iv. Vapour pressure	~ 23 hope
v. Density	~ 1.3 g/cm3 (20 °C [68 °F])

2.2. Experimental programme ⌅ 2.2.1. Treatment of OPS ⌅

The experiment focuses on assessing the effect of different concentrations of SAE as a surface treatment on the quality of OPS lightweight concrete. The treatment involves coating the exterior surface of OPS with SAE at various concentrations ranging from 5% to 25% by weight of OPS. Before the coating process, the OPS is thoroughly dried to prevent water retention and fungal growth, which could affect the performance of the concrete. The SAE solution is prepared by diluting SAE with distilled water to achieve the desired concentration and facilitate the coating process (solid content by weight ~65%). The SAE solution is then poured over the OPS particles in a proportion corresponding to the percentage of SAE used. The OPS and SAE mixture was mixed for 5 min or until the surface was evenly coated. Afterwards, the coated OPS aggregate was air dried for a specified period and further dried under sunlight at a temperature above 29 °C for 8 h to ensure complete dryness. The treated OPS was then adequately stored before being incorporated into the concrete mix.

2.2.2. Mix design ⌅

In this study, OPS aggregates were employed as a complete replacement for conventional coarse aggregates in the production of concrete. The concrete mix design featured a well-proportioned mix ratio of approximately 1:1.68:1.10 (cement:sand:OPS) by weight. This ratio was meticulously calculated to maintain a consistent, effective water-cement ratio of 0.4 across all the concrete mixtures. The aim was to achieve a target compressive strength of 20 MPa at the 28-day mark, along with a desired slump of 30-60 mm. To enhance the concrete mix, a superplasticiser (SP) was introduced at a dosage ranging from 1.5% to 1.7% by weight of cement. The mix proportions of this study encompassed various specimen types, each incorporating different concentrations of the treated OPS as the coarse aggregate, as outlined in Table 4. Throughout the investigation, the compressive strength, permeability and dry shrinkage properties of the treated OPS concrete specimens were systematically compared with those of the untreated OPS concrete, which served as the control specimen.

Table 4. Mix proportions of OPS concrete.

Mix	Types of OPS	OPS (kg/m3)	Cement (kg/m3)	Water (kg/m3)	Sand (kg/m3)	SP (%)
NT	Untreated OPS	498	450	180	760	1.5±0.2
S5	Treated with 5% SAE	498	450	180	760	1.5±0.2
S10	Treated with 10% SAE	498	450	180	760	1.5±0.2
S15	Treated with 15% SAE	498	450	180	760	1.5±0.2
S20	Treated with 20% SAE	498	450	180	760	1.5±0.2
S25	Treated with 25% SAE	498	450	180	760	1.5±0.2

2.2.3. Method of mixing and curing the concrete ⌅

The concrete mixing process detailed in the study adhered to the guidelines stipulated by BS1881-125:1986. A 75-litre drum mixer was utilised for the preparation of concrete mixes, following the sequence given below:

1. Dry mixing: Initially, all ordinary Portland cement and sand were placed in the drum mixer and dry-mixed for 30 s, ensuring a uniform distribution of aggregates. Adding water: Half of the required mixing water was introduced into the mixer, and the process continued for an additional 2 min. Adding cement: Cement was then added to the mixer, and mixing persisted for an additional 30 s. Remaining water and SP: The remaining half of the mixing water and the SP were incorporated into the mixer and the mixture was blended for approximately 2 min or until achieving a homogeneous consistency.

After completing the concrete mixing process, the fresh concrete mixture was poured into standard testing moulds, including a 100-mm cube and a 100 × 100 × 500 mm prism. The concrete mixture was layered in three equal parts within the moulds, compacted by vibrating each layer for approximately 10 s on a vibrating table. Excess concrete above the form’s upper edge was removed, and each layer was levelled using steel trowels.

To impede rapid water loss due to evaporation, the exposed surfaces of the concrete moulds were covered with polyethene sheets. The specimens were left to harden for 24 h in the moulds before removal. All the procedures, including mould preparation, filling, compaction, levelling and specimen placement, followed the standard outlined in BS EN 12390-2:2000. All hardened concrete specimens were cast under laboratory conditions, demoulded for 24 h after casting, and fully immersed in water at 25 ± 2 °C until the testing period.

2.3. Testing ⌅ 2.3.1. Determination of the properties of OPS ⌅

The experimental programme focused on evaluating and comparing the physical properties of the untreated and treated OPS. The objective was to understand the effect properties of OPS after surface treatment. Various parameters relating to the OPS properties were determined through experimental testing to ensure that OPS particle aggregates met the requirements for their application in concrete. Particle density, which is the mass-to-volume ratio of a solid material, was measured using laboratory procedures outlined in BS 812-2:1995. The testing included determining the particle density in three states: apparent, oven-dried basis and saturated surface-dried basis. The water absorption capacity of the aggregate was also determined, as it is an important factor in relation to concrete properties. The mass difference between the saturated and surface-dried samples after drying for 24 hours in an oven was used to calculate the aggregate’s water absorption capacity according to BS 812-2:1995.

2.3.2. Slump test ⌅

The slump test is a method used to assess the consistency or workability of fresh concrete. It follows the standard procedure outlined in BS EN 12350-2:2019. During the test, a sample of freshly mixed concrete is placed into a slump cone. The cone is then slowly lifted, allowing the concrete to slump or settle. The measurement is taken from the top of the cone to the highest point of the slumped concrete.

2.3.3. Density of hardened concrete ⌅

The density of the hardened concrete was determined using a water displacement method, following the guidelines of BS EN 12390-7:2019. The average density results were obtained from three types of specimens at curing ages of 7, 28, 56 and 90 days.

2.3.4. Mechanical strength ⌅

A compressive strength test was conducted to evaluate the ability of concrete to withstand stress and resist failure, as cracks are a common cause of concrete failure. The test followed the guidelines outlined in BS EN 12390-3:2009 and involved subjecting 100-mm concrete cubes to uniaxial compression. The compressive strength was determined by calculating the average strength of the three tested specimens. The average results of three specimen types were recorded for curing ages of 7, 28, 56 and 90 days.

2.3.5. Durability properties ⌅

The water absorption test was performed in accordance with the guidelines of BS 1881-122:1993. Once the specified curing period was completed, the concrete samples underwent preconditioning in an oven at 110 °C until they reached a stable weight. The intrinsic air permeability of concrete was measured using a Leeds cell permeameter developed by Cabrera and Lynsdale (19) in accordance with their proposed method. The cylindrical concrete specimens (obtained by coring from the prisms of each concrete mixture), with dimensions of 45 mm in diameter and 44 mm in length, were used in the experiment. The value of intrinsic air permeability K was determined by the following Equation [1]:

K=2P2∙VL×1.76×10-16A(P12-P22)

[1]

where K is the intrinsic permeability (in units of m²), V is the flow rate (cm³/s), L is the length of the specimen (m), A is the cross-sectional area of the specimen (m²), P ₁ is the absolute applied pressure (usually 2 bars), and P ₂ is the pressure at which the flow rate is measured (atmosphere pressure), equal to 1 bar. The flow rate calculation is expressed as follows [2]:

V=(D2/4)πH/T

[2]

where D is the flowmeter diameter (mm), H is the length read on the flowmeter (mm), and T is the average time (s).

The vacuum saturation method, developed by RILEM (20), was used to assess the porosity of the concrete. In this study, similar cylindrical concrete specimens with an intrinsic air permeability were used for this testing. The porosity value of each concrete sample can be obtained by calculating with the following Equation [3]:

P=msat-mdrymsat-mwater×100

[3]

where P is the porosity (%), m _sat is the weight in air of the saturated sample, m _water is the weight in water of the saturated sample and m _dry is the weight of the oven-dried sample.

The capillary absorption test, based on the method developed by Chan and Ji (21), was performed to evaluate the water absorption characteristics of the concrete samples. The test involved immersing the concrete samples in water from a single side and observing the time-dependent increase in mass. After 28 days of curing, the samples were removed from water curing, dried and sealed on all sides with a waterproof sealer. The bottom face of the sample was submerged in a shallow tray of water, and, at specific time intervals, the mass gain due to water absorption was measured. The samples were periodically removed from the water, wiped, weighed and returned to the container. The sorptivity coefficient, indicating the rate of water absorption, was determined by analysing the relationship between the cumulative weight of water absorbed per unit area of the concrete surface and the square root of the elapsed time intervals. The capillary absorption coefficient is defined by the following relationship [4]:

Ca=Mx-MoA

[4]

where Mx is the mass of the specimen measured at maturity x (kg), Mo is the initial dry mass of the sample (kg), and A is a section of the immersed sample (m²).

The drying shrinkage test procedure follows the guidelines outlined in accordance with ASTM C 157, and the specimen dimensions adhere to ASTM C490, specifying concrete prisms measuring 40 × 40 × 160 mm. To measure shrinkage at 3, 7, 14, 21, 28, 56 and 90 days of curing, a length comparator apparatus incorporating a digital gauge capable of measuring the length accurately up to 0.005 mm was employed. The specimens were meticulously crafted, featuring a cast-in steel gauge stud for length change measurements. Utilising this apparatus, which was equipped with a reference bar, drying shrinkage was observed. The concrete specimen bar was then inserted into the instrument and rotated. Following this rotation, the minimum value on the meter was recorded. Consistency was maintained by placing the reference bar and test samples in the same position for each comparison reading. The length change at any given age was calculated using the Equation [5]:

L=Lx-LiG100

[5]

where L is the change in length at age x (%), L_x is the difference between the comparator reading of the specimens and reference bar at age x, L_i is the difference between the comparator reading of the specimens and reference bar when the initial reading was taken and G is the gauge length (250 mm for standard specimens).

2.3.6. Scanning electron microscopy ⌅

Scanning electron microscopy (SEM) analysis was conducted to examine the microstructure of the interfacial transition zone between the aggregate and the new cement matrix in order to deeply understand the fracture and bond strength mechanism of concrete with the inclusion of surface treatment on the coarse aggregates (OPS). Meanwhile, the interface bond between SAE and the matrix and crack pattern was analysed using this method. The samples for the SEM profile were obtained from the cored specimen from the concrete prism, which was carefully cut, polished and dried. A back-scattered electron image detector operating at approximately 15.0 and 20.0 kV was applied to view the cut and polished surfaces. The SEM of the samples was analysed under a Quanta FEG 650 microscope.

3. Results and discussion ⌅ 3.1. Physical properties of the treated OPS aggregates ⌅

The varying percentages of surface treatments on OPS used in this study might have various effects on the properties of OPS and concrete. Table 5 shows the results of the relevant tests on the physical properties determined for the aggregates of the untreated and treated OPS. The OPS density results showed that the untreated OPS exhibited considerably higher particle density than the treated OPS. Consequently, the untreated OPS has higher water absorption properties than the treated OPS, which ranges from approximately 1.4 to 2.2 times higher than the treated OPS. The low particle density and high water absorption of the untreated OPS were attributed to the characteristics of porosity and high void content. By contrast, considerable changes in the properties of density and water absorption can be observed in the treated OPS by using SAE. The OPS treated with SAE showed a considerable increase in particle density and a decrease in their water absorption characteristics. Compared with the untreated OPS, the values of the oven-dried density of OPS with 5%, 10%, 15%, 20%, and 25% SAE increased by approximately 5-13%. Meanwhile, the water absorption of the treated OPS exhibited a proportional decrease with an increasing percentage of SAE. The results showed that the water absorption rate of the treated OPS coated with 5%, 10%, 15%, 20%, and 25% SAE resulted in decreases of approximately 6%, 8%, 7%, 12% and 13%, respectively, in comparison with the untreated OPS.

Most of the untreated and treated OPS in this research were within the thickness range of 1.5-2.5 mm (Figure 3), and the largest grain size of OPS used in this investigation was 12 mm. Given the thin layer of SAE on the surface of the OPS, no remarkable difference in OPS thickness was observed. Compared with the untreated OPS, the texture surface of the treated OPS was much smoother. Figures 4(a), (b), (c), (d) and (e) represent the shape and surface texture of the untreated and treated OPS. As shown in these figures, the surface of the OPS was proportionally covered with increasing percentage concentrations of SAE. The surface of the OPS samples treated with 5% SAE was observed to have partially exposed uncovered SAE in comparison with the other treated OPS samples. By contrast, increasing to 25% SAE, the sample appeared to have been entirely covered with SAE particles, and the particles tended to clump together. In addition, the clumping effect resulted in the treated OPS with 25% SAE exhibiting less angularity in comparison with the others.

Table 5. Physical properties of the untreated and treated OPS.

Properties of aggregate	Untreated OPS (NT)	Treated with 5% SAE (S5)	Treated with 10% SAE (S10)	Treated with 15% SAE (S15)	Treated with 20% SAE (S20)	Treated with 25% SAE (S25)
Particle Density - Oven Dry (Mg/m3)	1.02	1.07	1.09	1.11	1.17	1.15
Particle Density - SSD (Mg/m3)	1.27	1.26	1.27	1.29	1.31	1.28
Apparent Particle Density (Mg/m3)	1.35	1.32	1.33	1.36	1.35	1.32
Water Absorption (%)	24.32	17.92	16.28	16.86	11.14	10.93

Depiction of the untreated OPS particles

Figure 3. Depiction of the untreated OPS particles

(a) OPS with 5% SAE; (b) OPS with 10% SAE; (c) OPS with 15% SAE; (d) OPS with 20% SAE; (e) OPS with 25% SAE.

Figure 4. (a) OPS with 5% SAE; (b) OPS with 10% SAE; (c) OPS with 15% SAE; (d) OPS with 20% SAE; (e) OPS with 25% SAE.

3.1.1. Workability of the OPS concrete ⌅

Figure 5 presents the workability of all the concrete mixes, assessed through slump test results, which indicate the mobility and placeability of the concrete. The target slump range for the concrete mix design was set between 30 and 60 mm, with SP adjustments ranging from 1.5% ± 0.2. The NT concrete (the untreated OPS) exhibited the lowest workability, with a slump value of 30 mm, compared with the OPS-treated specimens. The results indicate that the slump value of concrete improves slightly with the use of the treated OPS, with the SAE-treated OPS at 15% and 20%, showing a 40% increase in slump for lightweight concrete. The reduced slump of the NT concrete is attributed to the high absorbency of the untreated OPS due to its porous surface, which absorbs more water during mixing and, consequently, lowers workability. This investigation confirms that the slump variation between the untreated and treated OPS is significantly influenced by the OPS particle properties, as previously noted by Ikponmwosa et al. (22) and Maghfouri et al. (23).

Slump test results.

Figure 5. Slump test results.

3.1.2. Density of the hardened OPS concrete ⌅

Figure 6 depicts the densities of the hardened OPS concrete specimens subjected to curing periods of up to 90 days. The findings reveal a general trend of increased density for all the hardened concrete mixes as the curing days extend. This occurrence can be attributed to the accelerated hydration rate of the cement products when supplied with sufficient water, resulting in a denser microstructure formation within the concrete. In terms of specimen comparison, the untreated OPS concrete exhibited the lowest density, ranging from 1862 to 1899 kg/m³. Conversely, the results demonstrate that the density of all the treated OPS lightweight concrete surpasses that of the NT concrete, ranging from 1871 to 1940 kg/m³. Nevertheless, all the OPS concrete density values remain below 2000 kg/m³, which falls within the acceptable range for structural lightweight concrete, as specified in ACI 213-87:1999.

Density of the hardened OPS concrete.

Figure 6. Density of the hardened OPS concrete.

3.2. Mechanical strength properties of the OPS concrete ⌅

Figure 7 compares the overall compressive strength of all the tested samples based on the curing periods. Generally, the findings of this study reveal that the compressive strength of all the concrete specimens improves with the curing period. The figure shows that the surface treatment noticeably affects the compressive strength of the OPS lightweight concrete. Incorporating the treated OPS with 5%, 10%, 15% and 20% SAE in concrete produced better performance than the untreated OPS concrete. At 7 days, the strength of S5, S10, S15 and S20 is 35%, 22%, 7% and 18%, respectively, higher than that of the NT specimen and continuously higher after prolonged exposure to NW curing for up to 90 days. Amongst all the specimens, S10 and S15 were observed to reach the highest value of compressive strength, i.e., 24 MPa, at 90 days. Meanwhile, the effects of incorporating the treated OPS with OPS treated with 25% of SAE (S25) exhibited a lower compressive strength than the NT specimen and all the other treated OPS lightweight concrete across all testing days. Moreover, the compressive strength of S25 concrete does not achieve the design strength of 20 MPa at 28 days. The figure shows that at 7, 28, 56 and 90 days, the differences in the reduction in strength values of the prepared S25 mixture compared with the NT concrete were 24%, 2%, 16% and 30%, respectively.

Compressive strength of the tested specimens.

Figure 7. Compressive strength of the tested specimens.

Overall, the results obtained from this investigation showed that the inclusion of the fully untreated OPS in concrete mixtures leads to unfavourable results in terms of mechanical strength properties. The contributing factors to these changes may be related to the high absorption of the untreated OPS, which tends to accumulate the mixing water inside OPS particles, resulting in an internal bleeding effect. This phenomenon causes a lack of water use for cement hydration and, thus, creates pores along the ITZ between the cement paste and the OPS aggregate particles. Many authors (8, 22, 23) agree that the mechanical strength of the OPS lightweight concrete is severely limited by the bond between the OPS and the cementitious matrix. The enhancement of the compressive strength of the concrete is attributed to an improvement in adherence between the treated OPS and the cement matrix. The SAE developed a thin coating over the OPS surface, preventing water from infiltrating into the OPS. The cement matrix and the OPS particles exhibited solid adherence to the applied thin layer of SAE, producing a stronger surface contact at the interfacial zone between the cement paste and the OPS, which is important in reflecting the increase in concrete strength. However, the effect of increasing the concentration of SAE for the treated OPS has limitations, whereby mechanical strength test results obtained through this investigation show that surface treatment beyond 20% SAE with OPS in concrete leads to unfavourable results in mechanical strength properties. According to the study, compared with other OPS particles treated with various amounts of SAE, those coated with 25% SAE were more prone to clumping characteristics. The condition that the OPS particles clump together, and affect the adhesion between the OPS particles and the cement paste, deteriorates the mechanical strength of the concrete.

3.3. Durability properties of the OPS concrete ⌅ 3.3.1. Water absorption ⌅

Figure 8 illustrates varying water absorption rates for specimens preserved for up to 90 days of curing. Based on the data shown, the untreated concrete specimen has the highest water absorption rate value compared with the other specimens. The increased water absorption rate of the untreated specimens may be attributed to the more extensive porosity characteristic of the untreated OPS compared with the treated OPS. Mehdi Maghfouri et al. (8) agreed that OPS is also a porous material with high water-absorption capacity. The water absorption rate is effectively lowered when OPS is treated with SAE. This observation is confirmed by the fact that the water absorption rate obtained from specimens of the S-series for all the testing ages and curing conditions was considerably lower than that obtained from specimens of the NT concrete. The lower porosity in the treated OPS, as a result of surface treatment, is the cause of the lower water absorption rate of the treated OPS particles. The study reveals that specimen S20 had the lowest water absorption after a curing time of 28 days; however, during long-term exposure of up to 90 days, S15 exhibited the maximum decrease in water absorption content. When compared with the NT specimens, the reduction in water absorption that occurred between 28 and 90 days was in the range of 8% to 50% for all of the specimens. The correlation between the compressive strength and water absorption data from all the specimens is depicted in Figure 6. Notably, the highest observed R-square value of 0.67 signifies a robust correlation between these two parameters. The trend line graph within Figure 9 clearly demonstrates an inverse relationship between the aforementioned parameters, suggesting that its water absorption rate decreases as concrete’s compressive strength increases.

Water absorption of the tested specimens at various curing ages.

Figure 8. Water absorption of the tested specimens at various curing ages.

Relationship between the compressive strength and water absorption of the specimens.

Figure 9. Relationship between the compressive strength and water absorption of the specimens.

3.3.2. Intrinsic air permeability ⌅

The intrinsic air permeability results for all the specimens at various curing ages are shown in Figure 10. In general, regardless of the concrete type, the intrinsic air permeability of all the concrete specimens decreased with an increasing curing period. This finding is due to the continued cement hydration process, which gradually refines and minimises the capillary pores and voids of the cement matrix over time. The NT concrete specimen had the highest intrinsic air permeability rate concerning testing age for the entire curing age. By contrast, all the OPS concrete specimens incorporated with the treated OPS had lower intrinsic air permeability than the NT concrete. Hence, the incorporation of the untreated specimens leads to an increased number of pores in the concrete structure, thereby causing high permeability, that is, gases and fluids pass through the concrete structure easily. In addition, the angular shape of the OPS was responsible for the increased void content and permeability, which contributed to the higher rise in air permeability (24). As shown in Figure 7, at a curing age of 90 days, the intrinsic air permeability values of S15 and S20 concrete were, respectively, 12% and 13% lower than those of the NT concrete specimens.

However, while surface treatments with SAE above 20% adversely affected properties such as compressive strength, water absorption and porosity, especially at later ages (56 and 90 days), this negative trend was not observed in the air permeability results. It is important to note that intrinsic air permeability is governed by different mechanisms compared with other properties, such as water absorption and compressive strength. Air permeability is primarily influenced by the size, continuity and distribution of pores and voids within the concrete matrix (25). This can be explained by the formation of a thick polymer coating around the OPS aggregates at high SAE concentrations. This coating effectively blocks air movement through the concrete despite causing weaknesses in other properties. The dense barrier created by the higher SAE concentrations prevents gas flow, mitigating air permeability while, at the same time, the polymer layer disrupts the bond between the cement matrix and aggregates, leading to reduced mechanical performance and increased porosity observed in other tests.

This research focuses on exploring the connection between compressive strength and intrinsic air permeability across all the specimens, considering factors such as curing exposure and testing age. Figure 11 visually represents the relationship between these two variables through a graphical depiction and an empirical equation. Moreover, the study uses polynomial regression models to establish and validate this relationship. The analysis reveals a strong correlation between the parameters, with an R-square value of 0.88. Notably, the trend line graph in Figure 8 clearly demonstrates an inverse relationship between intrinsic air permeability and compressive strength. Consequently, an increase in compressive strength results in a decrease in the rate of intrinsic air permeability in concrete and vice versa.

Intrinsic air permeability of the tested specimens versus curing age.

Figure 10. Intrinsic air permeability of the tested specimens versus curing age.

Relationship between the compressive strength and intrinsic air permeability of the specimens

Figure 11. Relationship between the compressive strength and intrinsic air permeability of the specimens

3.3.3. Porosity ⌅

Variations in the porosities of all the concrete specimens with respect to testing age are presented in Figure 12. In general, the obtained total porosity rate of all the concrete mixtures follows a similar trend as the rate gain of intrinsic air permeability, where the gain in porosity values of all the concrete mixtures gradually decreases with the increase in the curing period. This result is related to the continued formation of hydration products over time, which refines the capillary pores and voids of the cement matrix (26). For all the testing ages, higher porosities were obtained from the untreated specimens. This phenomenon is attributed to the greater porosity of the untreated OPS; thus, it served as a conduit for water transport. By contrast, the use of the treated OPS considerably decreased the porosity values of concrete. The overall gain in the porosity rate of the S-series specimens was remarkably lower than that of the untreated specimens, especially in a long-term curing period. At 90 days, S10 and S15 concrete showed the lowest total porosity, which was 13% lower than that of the control specimens.

Figure 13 depicts the correlation between the total porosity and compressive strength of a sample of all the concrete specimens. In general, the compressive strength of concrete decreases proportionately with its porosity. Similar results have been reported by other researchers (26). According to the graph, the relationship between the porosity of all the evaluated specimens and compressive strength is inversely proportional, with R-squared values of approximately 0.73.

Total porosity of the tested specimens versus curing age.

Figure 12. Total porosity of the tested specimens versus curing age.

Correlation between the compressive strength and porosity.

Figure 13. Correlation between the compressive strength and porosity.

3.3.4. Capillary absorption ⌅

For various concrete specimens, Figure 14 depicts the capillary water quantity absorbed as a function of time per square unit. Figure 15 depicts the initial water absorption of concrete specimens via capillary and sorptivity. The initial water absorption and sorptivity rates of the untreated specimens were much higher than those of the other concrete specimens. This behaviour can be explained by the high absorption characteristic of the untreated OPS and the pore structure, which acts as a conduit for water movement. Furthermore, the formation of minute cracks in the concrete structure may cause changes in the pore-size distribution of the concrete and, as a result, allow capillary pores to reconnect with one another, leading to an increase in permeability. A similar finding is reported by Teo et al. (27). Most OPS-treated concrete specimens have lower initial water absorption and sorptivity rates than the control concrete. This finding implies that the reduction in the capillary absorption and sorptivity rates of the treated specimens is associated with the decreased porosity of OPS as a result of the SAE treatment. However, the results show that the percentage of SAE used during the OPS treatment has a considerable influence on the rate of capillary absorption and sorptivity of concrete. According to the findings of this study, S25 concrete appears to be more permeable than other treated OPS concrete specimens. This factor could be attributed to the clumping physical characteristics of OPS caused by the high concentration of SAE during treatment. As a result, the dispersion of OPS in concrete mixes is disrupted, and the possibility of voids being present in the concrete enables capillary pores to form, leading to a considerably increased permeability.

Amount of capillary water absorbed as a function of time per square unit.

Figure 14. Amount of capillary water absorbed as a function of time per square unit.

Initial absorption and sorptivity of the concrete mixes.

Figure 15. Initial absorption and sorptivity of the concrete mixes.

3.3.5. Drying Shrinkage ⌅

Drying shrinkage refers to the volumetric reduction experienced by concrete as it undergoes the drying process and loses moisture. This phenomenon is particularly pronounced during the hardening phase and is a key contributor to the formation of cracks, especially in restrained concrete members (15, 16). The aggregate component, constituting a substantial proportion of concrete, significantly influences its properties. Critical factors such as aggregate size, shape, surface texture and moisture absorption capacity play pivotal roles in shaping the overall behaviour of concrete shrinkage. Figure 16 depicts the typical variation of shrinkage development that occurs for all the concrete mixes exposed to the atmosphere’s conditions. It is observed in this study that the magnitude of drying shrinkage values was relatively similar, which shows limited change during the early ages (up to 28 days), especially on specimens of the S-series. This is due to a large amount of free water in the pores evaporating at the beginning stages, and, as a consequence, limited drying shrinkage is observed. It significantly affects the drying shrinkage at the later stage.

At 90 days of curing, the drying shrinkage values varied across the different concrete mixtures, ranging from 0.58% to 1.2%. Among these, the untreated OPS (NT) specimens exhibited the highest drying shrinkage of 1.2%. This elevated shrinkage is attributed to the high porosity and water absorption capacity of the untreated OPS aggregates, which leads to a more rapid and significant loss of water through the pore system. As a result, the untreated OPS concrete experiences greater shrinkage compared with the treated mixtures. Notably, the NT concrete exhibited approximately 50% higher shrinkage strain compared with the SAE-treated concretes at 90 days.

In comparison, SAE 5% (S5) treatment resulted in a drying shrinkage of 0.82%. This reduction indicates that the SAE treatment improves the surface properties of the OPS aggregates, reducing their moisture absorption and mitigating shrinkage. SAE 10% (S10) treatment achieved the lowest drying shrinkage at 0.59%, highlighting the effective reduction of porosity and moisture absorption by the SAE coating. The SAE 15% (S15) treatment showed a drying shrinkage of 0.68%, demonstrating a continued reduction in shrinkage but not as pronounced as S10. The SAE 20% (S20) and SAE 25% (S25) treatments resulted in drying shrinkages of 0.77% and 0.83%, respectively, indicating that while SAE treatment remains beneficial, higher concentrations do not yield proportionally greater reductions and might introduce other issues such as increased brittleness of the aggregate coating.

The higher magnitude exhibited by the untreated OPS concrete can be attributed to a higher amount of water loss during concrete drying. As reported earlier, the untreated OPS has a higher level of porosity and water absorption than the treated OPS. Therefore, water is eliminated more rapidly and in more significant amounts through the pore system, consequently leading to increased shrinkage. In addition, the irregular shape, as well as the low stiffness of the OPS aggregates, is another factor that promotes a higher shrinkage strain for the concrete (15).

Dry shrinkage of all the specimens subjected to a period of exposure.

Figure 16. Dry shrinkage of all the specimens subjected to a period of exposure.

3.3.6. Microstructure of the OPS concrete ⌅

This study used SEM to examine the contrasting microstructure development in the untreated and treated OPS concrete. SEM analysis focused on the OPS aggregate, cement paste matrix and the quality of the paste-aggregate interfacial zone. The results of the SEM observations showed that, after 28 days, the untreated OPS concrete displayed a loose and porous ITZ with a width ranging from 157 to 165 nm, as depicted in Figure 17. Additionally, Figure 18 illustrates a deteriorated bond interface between the untreated OPS and cement paste after 90 days, resulting in an increased ITZ width. The SEM analysis further revealed the presence of porous layers and cracks in the cement matrix of the untreated OPS concrete, which hindered the development of crystalline calcium silicate hydrate gel formation in the ITZ. Consequently, a loosely packed cement matrix surrounded the OPS aggregate, adversely affecting its fracture properties, strength and durability. The untreated OPS exhibited a higher water absorption capacity, which was attributed to the presence of pores. Consequently, during the mixing of concrete materials, a considerable amount of free water was absorbed by the OPS particles, leading to a greater flow of water from the cement matrix to the OPS. As a result, a reduced amount of water was available for cement hydration in the NT concrete. This condition could result in premature formation within the concrete microstructure and leave substantial quantities of large calcium hydroxide (CH) crystal particles in the pores of the ITZ. Meanwhile, water evaporated from the bleeding OPS, which was not involved in cement hydration, thereby creating voids. These factors collectively contributed to a lower mechanical strength performance in the untreated OPS concrete compared with the treated concrete. In addition, the loose and porous ITZ of the untreated OPS concrete made it vulnerable to the penetration of gases, liquids or aggressive ions, thereby reducing its long-term durability.

SEM of ITZ between the untreated ops and cement paste at 28 days.

Figure 17. SEM of ITZ between the untreated ops and cement paste at 28 days.

SEM of ITZ between the untreated OPS and cement paste at 90 days.

Figure 18. SEM of ITZ between the untreated OPS and cement paste at 90 days.

SEM of ITZ between the treated OPS and cement paste at 28 days.

Figure 19. SEM of ITZ between the treated OPS and cement paste at 28 days.

SEM of ITZ between the treated OPS and cement paste at 90 days.

Figure 20. SEM of ITZ between the treated OPS and cement paste at 90 days.

By contrast, the SEM findings demonstrated that the ITZ structure development in the treated OPS (S10) was superior to that in the untreated OPS concrete. The application of surface-applied SAE narrowed the thickness of the ITZ. Figure 19 illustrates S10 concrete specimens, which exhibited decreasing pore diameters within the ITZ, ranging from 56 to 152 nm after 28 days, in comparison with the NT concrete. The S10 specimens treated with SAE displayed a more uniform distribution of the hydrated cement paste phase and exhibited no remarkable shrinkage around the treated OPS in the concrete. Furthermore, at 90 curing ages, the S10 specimens (Figure 20) showcased a considerably narrower ITZ and fewer pore voids compared with the untreated concrete. This indicated that the cement hydration process occurred on the surface between the treated OPS and the cement matrix.

The energy dispersion X-ray (EDX) analysis was specifically conducted to identify hydration products in the interfacial transition zone (ITZ) region between the untreated (NT) and treated (S10) specimens. Tables 6 and 7 outline the weight percentages of major elements (C, Al, Si and Ca) obtained from the EDX analysis of both the 28-day untreated OPS lightweight concrete and the treated OPS lightweight concrete. The analysis was meticulously performed directly for the ITZ region, as indicated. The results highlight the discernible impact of the surface treatment on OPS using SAE, revealing a slight increase in calcium silicate hydrate (C-S-H) and a reduction in the formation of calcium hydroxide (Ca(OH)₂) within the ITZ region. This is corroborated by the Ca/Si ratio at the ITZ of the S10 specimen, which is lower than that of the NT specimen, as evidenced in Tables 5 and 6. Furthermore, the analysis reveals that the cement matrix in the ITZ region of the untreated OPS had a low quantity of silicate (SiO₂), indicating a significant percentage of Ca(OH)₂. In contrast, the ITZ region of S10 exhibited a higher concentration of SiO₂, suggesting that the presence of the SAE coating prevents internal bleeding, inhibiting the production of Ca(OH)₂ crystals. This contributes to enhanced adhesion, resulting in a substantial decrease in Ca(OH)₂ crystals around the OPS surface and improved adherence between the cement paste and aggregate. Consequently, the surface-treated OPS significantly strengthens and enhances the ITZ between the aggregate and cement paste. Additionally, the improvements in the ITZ of S10 may compensate for the reduction in the penetration of gas, fluid or ions into the concrete caused by the inherent porosity of OPS. Meanwhile, in line with Krishnamurthy and Vandanapu (28), the higher water absorption rate of OPS is attributed to the elevated presence of the formation element Al₂O₃ in the ITZ region. The study findings underscore that the value of Al₂O₃ in the NT is greater than that in specimen S10, clearly demonstrating that this surface treatment effectively reduces excess water absorption. This reduction, in turn, enhances the strength and durability of the concrete by mitigating the absorption of excess water into the material.

Table 6. Element at EDX spot (NT) at 90 days.

Element	Weight %	Atomic %	Formula
C	29.53	41.14	CO2
Al	1.30	0.80	Al2O3
Si	5.20	3.10	SiO2
Ca	18.72	7.82	CaO
Mg	0.42	0.33	MgO

Table 7. Element at EDX spot (S10) at 90 days.

Element	Weight %	Atomic %	Formula
C	14.66	23.43	CO2
Al	1.20	0.90	Al2O3
Si	8.84	6.04	SiO2
Ca	28.42	13.61	CaO
Mg	0.71	4.69	MgO

However, the SAE concentrations above 20% led to unfavourable microstructural outcomes. The SEM analysis revealed that, while SAE treatments at 10% and 15% improved the interfacial transition zone and enhanced the overall microstructure, concentrations exceeding 20% led to the formation of a thick, impermeable polymer layer on the OPS aggregate. This excessive coating disrupted the normal bonding process between the aggregate and the cement matrix, resulting in the increased prevalence of voids and microcracks within the ITZ, which weakened the bond strength. The thicker polymer coating at higher SAE concentrations not only impeded mechanical interlock between the OPS and cement paste but also prevented proper integration of the aggregates into the cementitious matrix. This weaker interfacial bond reduced the mechanical performance of the concrete, notably affecting its compressive strength and long-term durability. The SEM images of S25 specimens (Figure 21) with higher SAE concentrations revealed a noticeable increase in microcracking and void formation, which significantly undermined the concrete’s microstructural integrity. These microstructural deficiencies accounted for the observed decline in mechanical properties, including compressive strength and drying shrinkage, in concrete with SAE concentrations exceeding 20%. These findings emphasise the critical need to optimise SAE treatment levels to ensure a balance between enhancing the microstructure and maintaining the mechanical performance of the OPS-based concrete.

SEM of the treated OPS (S25) at 90 days.

Figure 21. SEM of the treated OPS (S25) at 90 days.

4. Conclusions ⌅

According to the results acquired throughout the experiment, the effects of surface treatment by using SAE on the OPS properties and the performance of the OPS lightweight concrete can be summarised as follows:

1. The physical properties of the treated OPS aggregates were remarkably influenced by the percentage of surface treatment with SAE. The untreated OPS exhibited lower particle density and higher water absorption due to its porosity and high void content. However, the application of SAE resulted in an increase in particle density and a decrease in the water absorption characteristics of OPS. The incorporation of the treated OPS in concrete improved the compressive strength properties due to improved adherence between the treated OPS and the cement matrix. The thin coating of SAE on the OPS particles prevented water infiltration, enhanced surface contact at the interfacial zone and contributed to increased concrete strength. However, increasing the concentration of SAE for the treated OPS beyond 20% led to unfavourable results in terms of the compressive strength properties. As shown in this study, the OPS coated with 25% SAE tended to clump together, affecting the adhesion between the OPS and cement paste and deteriorating the compressive strength of the concrete. The presence of the untreated OPS led to an increase in the number of pores within the concrete structure. Consequently, the untreated OPS specimens exhibited higher levels of water absorption, intrinsic air permeability, total porosity and capillary absorption rate compared with the treated OPS specimens. By contrast, the lower absorption rate observed in the treated OPS specimens can be explained by the reduced porosity resulting from the pre-treatment process, which involves a coating with SAE. Consequently, the inclusion of the treated OPS in the concrete mixture led to a decrease in the number of pores within the concrete structure, resulting in lower permeability. This result implies that gases and fluids can pass through the concrete structure with less ease, contributing to the refinement of capillary voids and, ultimately, reducing the overall permeability of the OPS concrete. Notably, the untreated OPS specimens demonstrated the highest drying shrinkage values at both early and later ages, estimated to be approximately 50% higher than the treated concrete. The increased drying shrinkage in the untreated OPS concrete is primarily due to a higher water loss during drying, as the untreated OPS has greater porosity and water absorption compared with the treated OPS. The SEM observations demonstrated that the effect of coating with SAE on the OPS surface resulted in enhanced microstructural performance of the concrete through a reduction in the width of the ITZ and improved adhesion between the cement paste and OPS aggregate. Conversely, the untreated OPS concrete displayed a loose and porous ITZ, leading to detrimental effects on the mechanical strength and long-term durability.

In conclusion, the integration of the treated OPS in concrete offers significant improvements in material properties and performance. The surface treatment with SAE not only enhances the mechanical strength and durability of the OPS concrete but also reduces its porosity and permeability, making it a more robust and sustainable construction material. The observed benefits, including improved adherence and reduced drying shrinkage, highlight the potential of the treated OPS as a valuable alternative to conventional aggregates. Practically, OPS concrete can be effectively used in various construction applications such as residential, commercial and infrastructure projects. Its reduced weight offers advantages in reducing structural load and transportation costs, while its enhanced properties contribute to longer-lasting and more energy-efficient structures. The adoption of the OPS concrete supports sustainability goals and provides a cost-effective, high-performance option for modern construction practice.

Acknowledgements ⌅

The authors would like to thank the Universiti Teknologi MARA (UiTM) for providing the supports for this study. Appreciation is also directed to the parties involved in conducting the experiments at the UiTM Perak laboratory.

Authorship contribution statement ⌅

Nurhasyimah Ahmad Zamri: Conceptualization; Data cleansing; Formal analysis; Fund raising; Research; Methodology.

Sallehan Ismail: Project administration; Resources; Software; Supervision; Validation; Visualization.

Azamuddin Husin: Write-up - original draft; Write-up - review & editing.

Declaration of competing interest ⌅

The authors of this article declare that they have no financial, professional or personal conflicts of in-terest that could have inappropriately influenced this work.

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