Self-compacting mortar overlays using expanded polystyrene beads for thermal performance and energy efficiency in buildings

2.1 Materials

Samples were prepared from a mixture of ordinary Portland cement (OPC), EPSBs, polycarboxylate-ether based super-plasticizer (SP, Melflux 2651 F), and locally available organic Acacia Nilotica acting as a viscosity-enhancing agent, which is also a proven useful admixture [23], [24], [25]. OPC of Type-1 42.5 R [26] was used. Figure 1 shows the materials used in the formulations. The chemical formula of EPSBs is (C₈H₈) _n ) which is insoluble in water and soluble in benzene, carbon disulfide, and chloroform. The density, melting point, boiling point, and thermal conductivity of EPSBs were 8–9 kg/m³, ∼ 240 °C, 430 °C, and 0.033 W/(m·K), respectively.

Figure 1:

Materials used in the preparation of self-compacting mortar specimens.

Expanded Polystyrene Beads (EPSBs) used in this study exhibited low densities ranging from 8 to 9 kg/m³, with internal porosity exceeding 97 % across all sizes. Surface area decreased with increasing bead size, while water absorption remained low (0.09–0.15 %), consistent with their closed-cell structure and hydrophobic nature, confirmed by water contact angles exceeding 90°. Beads were near-spherical with slight shape deviation (aspect ratio 1.1–1.3), and their chemical composition consisted of >98 % pure expanded polystyrene with minimal additives. ANG (Acacia Nilotica Gum) was selected as a multifunctional admixture due to its moderate viscosity-enhancing capability, complete water solubility, and biodegradability. With a molecular weight of 250–580 kDa and thermal stability up to 80 °C, ANG offered suitable performance for SCMO. Compared to commercial VEAs such as welan gum, xanthan gum, and cellulose ethers, ANG demonstrated clear advantages in cost-efficiency ($2.5/kg vs $8–35/kg), environmental sustainability, and regional availability. Its dual function as a viscosity modifier and internal curing agent made it especially suitable for overlays, providing adequate rheological control without compromising strength development.

Compared to conventional viscosity-enhancing agents (VEAs), Acacia Nilotica Gum (ANG) offers a sustainable and cost-effective alternative with a dual mechanism: physical thickening and internal curing [23]. While commercial gums like welan and xanthan function via hydrogen bonding or polymer chain entanglement, they are 6–12 times more expensive and derived from synthetic fermentation. Cellulose ethers involve chemical modification, and clay-based VEAs, though natural, require much higher dosages (up to 2 %). In contrast, ANG operates effectively at a 0.33 % dosage by enhancing viscosity to suspend EPSBs and supplying internal curing through its hygroscopic nature. This is especially beneficial for thin overlays with limited external curing. FTIR analysis confirms that ANG does not chemically react with cementitious phases but interacts physically via hydrogen bonding with pore solution, maintaining hydration compatibility. Its biodegradable and renewable nature further strengthens its environmental appeal, making it a suitable choice for sustainable thermal mortars.

2.2 Mix proportions

In this research, a total of 10 SCMO formulations were studied with a view to finding a better energy-efficient system. The control sample (CS) had cement, sand, and a superplasticizer whereas other samples, i.e., S2 to S10, were prepared using a constant quantity of cement, sand, Acacia Nilotica gum (ANG) powder, and water-to-cement ratio along with varying ratios and sizes of polystyrene beads. The ratio of cement: sand was 1:1 by weight. Generally, the fresh density of SCMO using the stated mix proportions and w/c ratios lies within the range of 2–2.2 g/cm^3, which enables the quantities to be worked out for a given volume [25]. For all 10 formulations, the water to cement ratio was 0.4. The water-to-cement ratio of 0.4 was calculated as the mass ratio of mixing water to cement only (not including sand). With cement and sand taken in equal weights (1:1 ratio), the total water-to-binder ratio considering both cement and sand would be 0.2. However, for consistency with standard concrete terminology, the w/c ratio refers specifically to water-to-cement mass ratio. The ANG powder was taken as 0.33 % by weight of cement as recommended [25]. EPSBs and SP ratios were taken in terms of the weight of binder. SP dosage was determined by trials for a target flow of 30 ± 1 cm using Hagerman’s mini slump cone [27]. The laboratory temperature and relative humidity were 22 °C and 45 %, respectively, while the mixing water temperature was kept at 22 °C.

The selection of a low EPSBs content range (0.1–0.2 % by binder mass, or 0.038–0.076 % by volume) was strategically made to meet the unique demands of overlay applications. Unlike high-volume EPSBs concretes, which often compromise compressive strength and structural integrity, this low content preserves bond strength and minimizes the risk of delamination or cracking. It also maintains self-compacting workability – crucial for overlays placed without vibration – as higher EPSBs levels (>0.3 %) were found to hinder flow even with increased superplasticizer dosage. Economically, the approach offers thermal benefits at lower cost and avoids the need for specialized mixing. Given the limited thickness of overlays (20–50 mm), optimizing thermal performance within this constraint is critical. Additionally, the selected content allows practical deployment using conventional tools and methods. Overall, this low-EPSBs strategy shifts the paradigm from bulk insulation to surface-level enhancement, delivering thermal efficiency without compromising structural or functional viability.

2.3 Mixing regime

The dry mixing of EPSBs, cement, sand, ANG powder, and powdered Melflux 2651 F was done for 1 min in the Hobart mixer of 5-L capacity, followed by the addition of 80 % of the total mixing water. The mixing was done for 1 min at a slow rotational speed of 145 rpm. The remaining 20 % of water was added and mixed at a high rotational mixing speed of 285 rpm for 2 min. Mixing procedure as per BS-EN 196–3 [28] and total mixing time was 4 min. The temperature of the mixing water and the lab was maintained at 22–23 °C. Meanwhile, relative humidity was maintained in the range of 40–45 %. After mixing at the fresh state, it is worth mentioning that no signs of bleeding, segregation, or clustering of EPSB was observed.

2.4 Casting of specimens

Prisms of dimensions (40 × 40 × 160) mm³, as prescribed by BS-EN 196–1 [29], were cast. The formulation details are given in Table 1.

After casting, the prisms were then covered with a polythene sheet for an initial 24 h such that the moisture present in the samples was not lost to the environment [30]. After demolding the samples, they were numbered, marked and weighed. Afterwards they were immersed in water at 22 °C in a curing tank until the testing age/(s) in saturated surface-dry (SSD) condition [31].

3.1 X-ray fluorescence test on OPC

The mineralogical composition of OPC was determined by the X-ray fluorescence test as shown in Table 2. Moreover, Table 2 shows the Bogue potential phases of OPC along with other oxides and gypsum modifications.

The chemical composition and Bogue potentials presented in Table 2 are essential for understanding the hydration kinetics and thermal behavior observed in subsequent sections. The high C₃S content (45.62 %) contributes to early strength development, while the C₃A content (8.22 %) influences the effectiveness of the superplasticizer and setting time modifications observed with EPSBs addition.

3.2 Superplasticizer demand of SCMO

For the determination of the SP demand (%) of SCMO, Hagerman’s mini slump cone of dimensions (6 × 7 × 10) cm³ was used. Figure 2 shows the results of the SP demand test for the target flow of various formulations using different EPSBs sizes. The test results reported represent average of three tested readings. It should be noted that by increasing ESPBs from 0.1-0.2 % for the same ESPB size 3, 6, or 9 mm), Melflux quantity remains the same because of the flexibility of the target flow (30 ± 1 cm) and internal friction within the matrix and at the interface starts increasing with the increase in size of beads and not with the small percentage difference of EPSBs.

Figure 2:

SP demand of samples as per varying sizes of EPSBs.

The observed increase in SP demand with EPSBs size (from 0.75 % for 3 mm to 0.90 % for 9 mm beads) can be mechanistically explained through several interrelated factors. First, despite larger beads having a lower specific surface area, the total interfacial area with the cement matrix increases due to the irregular surface texture of EPSBs and complex packing arrangements arising from varied size distributions. Second, the hydrophobic nature of polystyrene surfaces leads to poor wetting with cement paste, necessitating higher SP dosages to overcome surface tension, improve dispersion of the hydrophobic inclusions, and prevent EPSBs agglomeration. Third, the shape factor plays a role as larger EPSBs tend to deviate more from spherical geometry, leading to increased inter-particle interlocking, elevated flow resistance during mixing, and consequently higher SP demand to achieve the required flowability. Statistical analysis supports these findings, with a coefficient of variation ranging between 2.8 % and 4.2 %, and a statistically significant effect of EPSBs size on SP demand confirmed by ANOVA (p < 0.001). A strong correlation (R² = 1.00) was observed between EPSBs size and SP requirement (Figure 2).

3.3 Flow of SCMO

Figure 3 shows the flow times of controlled and energy-efficient SCMO, measured using the standard Hagerman’s mini slump cone. The cone used conforms to the size of (6 × 7 × 10) cm³ as suggested by Rizwan et al. [25]. Two marks of 25 cm and 30 cm diameters were placed on the glass plate, and these times are a function of plastic viscosity and the yield stress. Figure 3 shows the T25 cm and T30 ± 1 cm flow times of various studied formulations. The test results reported represent average of three tested readings.

Figure 3:

Comparison of T25 cm and T30 cm flow time of SCMO samples having target flow with variable size and contents of EPSBs.

T25 cm time and T30 cm time tests are relative rheological indices of plastic viscosity and yield stress of a cementitious system. As the size of EPSBs in SCMO increases, the flow time increases (Figure 3). The formulation S10 with 0.2 % of 9 mm EPSBs showed the highest delay in flow time for both times as compared to the CS, probably due to higher internal friction and friction at the interface. The lower value of R² for plastic viscosity shows its sensitivity towards EPSBs size and its percent content. High internal friction has to be overcome during flow, requiring high SP dosages. From the above results, the T25 cm flow time is dominantly indicative of plastic viscosity [34], and it marginally increased with an increase in EPSBs size. However, T30 cm flow time is indicative of yield stress, appreciably increased with an increase in EPSBs size. Initial setting times increased with an increase in EPSBs size. However, for a given bead size, the final setting time remained almost similar, with larger beads giving increased flow time.

T25 cm and T30 ± 1 cm flow times are qualitative rheological indices. The authors developed a novel quantitative approach later [35].

3.4 Setting times of SCMO

Figure 4 shows the Vicat [29] setting times of 10 formulations used in this research for energy saving estimation. Setting times for energy-efficient samples and the control SCMO samples were estimated according to BS-EN 196–3 [28]. The test results reported represent average of three tested readings.

Figure 4:

Setting time of SCMO samples with respect to different content and size of EPSBs.

The use of EPSBs in SCMO in the presence of ANG powder stabilizes the systems and makes them internally cured as well [24]. The initial and final setting times are lowest for the CS as compared to the samples containing EPSBs and ANG powder, showing a maximum delay of 93 and 210 min (sample S10) in the initial and final setting times of the SCMO, respectively, compared to the CS (Figure 4). It was also observed that for all the samples with the same size EPSBs, there was a constant trend for an increase in initial and final setting times.

The observed setting time delay in SCMO can be attributed to combined effects of ANG and EPSBs. Acacia Nilotica Gum (ANG) retards hydration through calcium ion complexation via its carboxyl and hydroxyl groups, which temporarily reduce the availability of Ca²⁺ for C₃S and C₃A reactions. Additionally, ANG adsorbs onto cement particle surfaces, forming a diffusion barrier that slows water access, while also increasing pore solution viscosity, thereby limiting ion mobility and delaying key hydration processes. EPSBs further contribute to the delay by reducing heat dissipation during hydration, physically obstructing ionic transport, and slightly modifying local water availability through surface moisture interaction. Calorimetric analysis confirms a progressive delay in rate of hydration and peak heat evolution with an increase in EPSBs sizes, mirroring the setting time trends and reinforcing that the primary mechanism is rooted in hydration kinetics modulation.

3.5 Compressive strengths of SCMO

Figure 5 shows the compressive strength of SCMO at three distinct ages (1, 14, and 28 days) by taking average of three identical specimens. The compression test was performed in a compression testing machine at a loading rate of 2.4 kN/s until the fractures appeared.

Figure 5:

Development of compressive strength of SCMO formulations with respect to age, different contents and size of EPSBs.

Compressive strength at 14 and 28 days decreases with an increase in EPSBs sizes, with respect to the CS, which is due to increased porosity and lighter weight of EPSBs acting as a kind of void. The water absorption in SSD conditions for SCMO increased with time. This may be due to higher pore sizes having possibly connected porosity with the increase in EPSBs sizes. The compressive strength of SCMO continued to decrease with the introduction of EPSBs in the mixture. A similar trend of decreasing compressive strength by the addition of EPSBs was observed by Ning Liu et al. in their study [37]. The compressive strength in this research for the CS at 28 days was 50.5 MPa, whereas it reduced to 37.4 MPa for 0.2 % of 3 mm beads SCMO (Figure 5, sample S4). With the increase in size of EPSBs to 6 and 9 mm with a 0.2 % content, the compressive strength was further reduced to 31.8 MPa and 21.9 MPa, respectively (Figure 5, samples S7 and S10). This is due to the presence of EPSBs in SCMO, which possibly act like engineered air voids, because EPSBs contain approximately 98 % air within themselves.

The strength-to-density ratios and efficiency indices further quantify this trend, with normalized strength decreasing from 1.00 in the control (CS) to 0.43 in S10. Efficiency indices, which account for both structural and thermal performance, followed a similar pattern. The strength reduction was also size-dependent, with an estimated loss of 13.1 MPa, 18.7 MPa, and 28.6 MPa per 0.1 % EPSBs size of 3 mm, 6 mm, and 9 mm, respectively. Despite these reductions, all mixes except S10 surpassed the minimum compressive strength requirement of 17–21 MPa outlined by ACI 546.3 R for structural repair mortars, confirming their suitability for overlay applications under moderate loading conditions while offering improved thermal performance.

The strength reduction with EPSBs inclusion can be attributed to several mechanisms: (1) EPSBs-matrix interface weakness due to the hydrophobic nature of polystyrene surfaces, which creates poor bonding with the hydrophilic cement matrix; (2) increased porosity and void content as EPSBs act as lightweight inclusions; and (3) stress concentration effects around EPSBs under loading. Literature indicates that EPSBs-cement interfaces typically exhibit weak bonding due to the non-polar nature of polystyrene, which cannot form chemical bonds with cement hydration products. The 98 % air content within EPSBs effectively creates engineered voids that reduce the load-bearing cross-sectional area of the matrix.

3.6 Water absorption of SCMO

Figure 6 shows the water absorption for SCMO at two different ages, namely 14 days and 28 days showing the discontinuity of pore networks, if any, and indirect pore refinement. Water absorption was calculated by the conventional method by taking average of three identical specimens.

Figure 6:

Water absorption of SCMO samples with respect to age, different contents, and sizes of EPSBs.

Water absorption of the SCMO increased by adding EPSBs and ANG powder. With an increase in EPSBs size, the water absorption increased by a maximum of 23.4 % for 14 days and 31.4 % for 28 days, with respect to water absorption in the CS. It was also observed that for all the samples with the same EPSBs size, there was a constant trend of increase in water absorption from 14 to 28 days. Also, since EPSBs are non-absorbent of water, this increase of water absorption (water uptake) may be due to the presence of air or internal porosity. ANG powder might have created voids while acting as a kind of internal curing agent in the SCMO with increased hydration. Nadhim Hamah et al. [38] also found the increasing water absorption as the number of EPSBs replaced increased.

Water absorption results, shown in Figure 6, demonstrate a clear increase with EPSBs size, rising from 1.91 ± 0.10 % in the control mix (CS) to 2.51 ± 0.12 % in S10, an 31 % increase. This trend is attributed to microstructural changes induced by both the EPSBs inclusion and the ANG-modified matrix. Poor bonding at the hydrophobic EPSBs surfaces leads to interfacial transition zones (ITZs), which serve as preferential pathways for water ingress. Larger EPSBs generate more extensive ITZ networks, enhancing water absorption. Simultaneously, ANG’s hygroscopic nature introduces microscopic water retention sites that evolve into accessible capillary pores during drying, further contributing to absorption. The stepwise increase in absorption across formulations S4, S7, and S10 indicates a progressive development of pore connectivity. Notably, this enhanced pore network correlates with improved thermal insulation, as the formulations with the highest absorption also exhibited the lowest thermal conductivity. This suggests that the microstructural features responsible for increased water uptake also facilitate thermal resistance, linking porosity evolution to multifunctional performance in EPSBs-modified SCMO.

3.7 Early linear shrinkage of SCMO

Early shrinkage measurements were performed on SCMO and are shown in Figure 7 for first 24 h using the modified German Schwindrinne apparatus with (4 × 6 × 25) cm³ channels. The test results reported represent average of three identical specimens. Maximum 24 h shrinkage was observed for the control mix, as anticipated, and reduced shrinkage was observed with an increase in EPSBs size. This reduction in shrinkage may be attributed to increased porosity, which possibly acts as a cushion for accommodating shrinkage strains. The shrinkage looks to be predominantly chemical shrinkage during the first 24 h [39]. The ASTM prescribed measurement of drying shrinkage using a dilatometer after 24 h certainly misses this early age shrinkage, which is quite important [40], 41].

Figure 7:

Comparison of the early 24 h shrinkage of SCMO having different sizes of EPSBs 0.2 % content.

This trend is linked to several mechanisms: EPSBs serve as internal cushioning zones that dissipate shrinkage-induced stresses; their inclusion also increases porosity, providing void space for strain accommodation; and the reduced hydration efficiency associated with EPSBs limits overall shrinkage potential. These factors collectively result in a longer early expansion period and significantly lower shrinkage rates. From a practical perspective, this behavior is advantageous for overlay applications by minimizing early-age cracking, enhancing dimensional stability, and improving substrate adhesion. Although long-term shrinkage testing per ASTM C157 is pending, the observed early-age trends suggest a potential 20–40 % reduction in 28-day drying shrinkage, offering promising implications for durable overlay systems.

3.8 Calorimetric response of SCMO

The calorimetry of SCMO was performed on a semi-adiabatic calorimeter FCAL 8100, USA and temperature log details were plotted for 24 h as shown in Figure 8.

Figure 8:

Calorimetric response of SCMO having different sizes of EPSBs of 0.2 % content. Note: 1 °F (degree fahrenheit) is −17.222 °C (degree celsius).

When shrinkage and calorimetric responses are seen together, it becomes obvious that the rate of hydration slows down with an increase in EPSBs sizes. Calorimetric studies also showed that the evolution of heat of hydration decreased with the increase in EPSBs content in SCMO formulations, possibly due to increased porosity.

3.9 X-ray diffraction of SCMO

X-ray diffraction analysis was conducted at 60 days to ensure near-complete hydration and stable crystalline phase development (Figure 9). This analysis helps understand how EPSBs incorporation affects cement hydration products, which directly influences the thermal conductivity and mechanical properties reported in previous sections. The crystalline phases present affect heat transfer pathways and matrix bonding characteristics. XRD results provides acceptable information about the most probable phases and is considered as a sensitive technique [42].

Figure 9:

X-ray diffractogram of control sample.

The peaks and classification show the prominent crystalline hydration products. From the XRD analysis of the CS, it may be seen that the dominant crystalline peaks included albite, grossite, portlandite, quartz, and calcite and were the major crystalline hydration products. Whereas Figure 10 shows the diffractogram obtained when the EPSBs, ANG and SP are within a typical SCMO formulation. Very low Portlandite (CH) occurs at 2θ values of around 18 and 34°. This means that in the presence of EPSBs, the mullite, grossite, gismondine, quartz, calcite, and portlandite are significant.

Figure 10:

X-ray diffractogram of self-compacting energy-efficient SCMO with 6 mm EPSBs size of 0.2 % content.

3.10 Thermal conductivity of control and other SCMO with EPSBs

Thermal conductivity response was measured according to ASTM C518 on a sample of (50 × 50 × 50) mm³. Figure 11 shows the thermal conductivity response based on the EPSBs size. The test results reported represent average of three identical specimens.

Figure 11:

Comparison of thermal conductivity response between control and SCMO containing different sizes of EPSBs of 0.2 % content.

The use of EPSBs has been found to render SCMO energy-efficient and make them sustainable solutions. The EPSBs provide a reasonable reduction in the thermal conductivity of the SCMO. As shown in Figure 11, thermal conductivity decreased from 0.1589 to 0.0615 W/m.K when EPSBs of different sizes were introduced in the mixture of SCMO. According to energy analysis, this decreased the thermal conductivity values with the inclusion of insulating EPSBs in SCMO. Another reason may be the presence of the ANG powder which creates internal water entrainment. Conductive heat transfer through SCMO using EPSBs was measured using the Fourier’s Law (Equation (1)) [43].

The heat transfer per unit area is given by equation (2).

Where,

k = Thermal conductivity of material (W/m.K or W/m.C, Btu/(h ft °F))

s = Material thickness (m, ft).

A = Heat transfer area (m², ft²).

dT = t ₁-t₂ = temperature gradient across the material (^oC/K).

In this study, it was considered that heat transfers through 0.0381 m (1.5 inch or 4 cm) thick layer of SCMO. The K values were determined earlier for control SCMO sample and EPSBs modified SCMO mortars with three sizes of polystyrene bead (3, 6, and 9 mm) as per ASTM C518. The typical temperature on the outer side of the SCM overlayed wall was assumed to be 42 °C and 25 °C on the indoor side, as per the local conditions in the summer season with a temperature difference of 17 °C (62.6 K). Figure 12 shows the conductive heat transfer through a 0.0381 m thick SCM overlayed wall of 1.0 m² area with temperature difference of 17 °C (62.6 K), whereas the thermal conductivity for each sample type is given in Figure 11.

Figure 12:

Comparison of conductivity heat transfer between control and SCMO having EPSBs of 0.2 % content.

Figure 12 shows that the maximum heat per unit area transferred by the control SCMO sample was 97.9 W/m² for temperature difference of 17 °C across the two sides of SCMO whereas heat transfer through SCMO using 3, 6, and 9 mm EPSBs was 62.35, 52.06, and 37.89 W/m², respectively, for the same temperature difference of 17 °C (the margin of error was less than 5 %). It is clear that the use of SCMO using EPSBs topping reduces energy consumption by 36.3 %, 46.8 %, 61.3 % for 3, 6, and 9 mm EPSBs, respectively, thus demonstrating an energy-efficient solution in buildings. Earlier, it was shown in a side investigation that [44] EPSBs of 6 mm size gave the optimized response in both fresh and hardened states. The present investigation also seems to supplement the earlier results.

3.11 Scanning Electron Microscopy of SCMO with EPSBs

Figure 13 shows an SEM image of the S6 SCMO formulation incorporating 6 mm diameter EPSBs with 0.15 % content. Due to the presence of EPSBs, porosity of the structure has been seen in the SCMO formulation. Hydration products like CSH gel, ettringites can also be seen in Figure 13.

Figure 13:

Scanning electron microscope image of SCMO formulation S6 using EPSBs.

3.12 BET-nitrogen sorptivity of SCMO with EPSBs

V-Sorb 2800 P surface area and porosimetry was used for SCMO S6 formulation to look at qualitative analysis of pore structure in the matrix. This size was investigated because the earlier study [44] had shown that this size of EPSBs gave the optimized response in both the fresh and hardened state properties of SCMO formulations incorporating EPSBs. It is shown that with the incorporation of EPSBs, the porosity of SCMO formulations increases. Figure 14 shows BET nitrogen adsorption and desorption curves. The hysteresis phenomenon, especially between relative pressures of 0.6–0.9 indicates that the system has a porous structure. Beyond relative pressure of 0.9, up to 1, significant gas adsorption and desorption take place, indicating a connected porosity.

Figure 14:

BET-nitrogen porosimetry of the SCMO formulation S6 with EPSBs.

The results shown in Figures 14 and 13 are in agreement and support the properties of SCMO formulations investigated in fresh and hardened states. The specific surface area = 8.61 m²/g, volume of pores was 0.073002 cm³/g, and porosity was 30.62 %.