Recycled coarse aggregate generally includes a sizable amount of old mortar that has adhered to it, which increases microcracking, porosity, and water absorption. As a result, it lowers recycled aggregate concrete’s durability and mechanical performance. Acid treatment using hydrochloric acid is an established method for partially dissolving and detaching the adhered mortar, thereby reducing the thickness of the weak, porous layer surrounding the original aggregate particles (Makul et al., 2021).

(RCA) was pretreated with a 0.5 M HCl solution to partially remove the adhered mortar, following established protocols for enhanced aggregate quality. Specifically, oven-dried RCA (in 1,000 g batches, dried at 105 °C for 24 h) was immersed in a 0.5 M HCl solution at an aggregate-to-acid ratio of 1:3 (w/v), soaked for 2 h at 25 °C ± 2 °C with intermittent stirring every 30 min, decanted, rinsed thoroughly with distilled water (5 L per kg of RCA) until a neutral pH was achieved, and finally oven-dried at 105 °C for 24 h before use. This treatment reduced water absorption by 15%–20% compared to untreated RCA (Table 3), confirming partial mortar removal while preserving aggregate integrity is shown in Figure 3.

The amount of mortar remaining after the recycled coarse aggregate (RCA) was determined using an acid treatment method adapted from RILEM recommendations and previous studies (RILEM, 1994). Oven-dried RCA samples 10–25 mm in size were first weighed to obtain the initial dry mass (M₁). The samples were then immersed in a 1 mol/L hydrochloric acid (HCl) solution for 24 h to selectively dissolve the adhered cementitious mortar while minimizing damage to the original natural aggregate (Jain, 2025). After acid immersion, the aggregates were thoroughly rinsed with distilled water to remove residual acid and detached mortar particles, followed by oven drying at 105 °C ± 5 °C until constant mass was achieved. The final dry mass (M₂) was recorded. The residual mortar content (RMC) was determined using the mass loss in relation to the initial dry mass calculated using Equation 1. The measured initial and final masses and the corresponding residual mortar content values are summarized in Table 5. Residual mortar content  RMC  % = M 1 − M 2 M 2 * 100 . (1)

The incorporation of GO caused a systematic reduction in slump from 75 mm for the control mix (0% GO) to 68 mm, 62 mm, 55 mm, 48 mm, and 40 mm for GO dosages of 0.02%, 0.03%, 0.04%, 0.05%, and 0.06%, respectively (Figure 6b). This progressive decrease confirms that increasing GO content significantly lowers workability. This is explained by the large number of oxygenated functional groups and the high specific surface area of GO, which increase water demand and promote the flocculation of cement particles in the fresh mix.

Although the slump decreased into the low-workability range at higher GO concentrations, all mixes remained suitable for vibration-based placement as per IS 10086, with a consistent vibration time and visual control used to measure full compaction without segregation. The water-to-cement ratio was maintained at 0.40, but the dosage of polycarboxylate ether superplasticizer (1% by weight of cement) and vibration time were adjusted to achieve full compaction without segregation. Thus, at the ideal dosage of 0.04% GO, the noted increases in compressive, tensile, and flexural strengths are mainly attributed to GO-induced microstructural refinement rather than to artifacts arising from inadequate workability or compaction.

The 28-day compressive strength values of cement concrete mixes with different GO dosages were compared to the control mix. The findings demonstrated a considerable increase in compressive strength when GO content was added to cement concrete at any curing duration. Additionally, the cement concrete’s compressive strength continued to grow as the GO dosage was raised from 0.02% to 0.06%. This pattern held for both curing times. Figure 9 illustrates the rate at which compressive strength increased up to 0.04%. The strength gain correlates with the denser and more homogeneous matrix observed in the SEM image of mix M3, where microcracks and macropores present in the control mix were largely eliminated, and the ITZ appears more compact. Micro-CT results further show a shift from large, non-uniform pores (200–2,500 µm) in the control mix to a narrower pore size range (approximately 500–1,000 µm) and lower pore frequency in the GO-modified mix, which explains the higher load-bearing capacity. Early in life, GO has a greater impact on compressive strength. Table 9 illustrates compressive strength peaking at 59.5 ± 1.7 MPa for mix M3 (0.04% GO), a 45% increase over the control mix (41.1 ± 0.9 MPa), with consistent standard deviations (0.4–1.7 MPa) across all mixes; one-way ANOVA confirmed a highly significant effect of GO dosage (F = 104.7, p < 0.001).

Tensile strength results at different GO dosages were compared across multiple concrete mixes. It has been noted that GO-infused cement concrete is stronger than the control mix. The increase in split tensile strength caused by higher GO dosages is shown in Figure 10. Split tensile strength is less affected by GO than compressive strength. After 28 days, cement concrete containing 0.04% GO shows an approximate 28% gain in tensile strength at 28 days, as seen for M3. Although the relative gain in tensile strength is smaller than that of the increase in flexural strength, this enhancement is significant for RCA concrete, which typically suffers from a weak ITZ and microcracking around the recycled aggregates.

The microstructural observations provide a clear explanation for this behavior. SEM images of M3 show wrinkled C–S–H foils with more frequent contact points and a reduced number of interconnected pores compared with CM, which delays crack initiation and slows crack propagation under splitting loads, thereby improving tensile capacity. Split tensile strength increased by 33% from 4.2 ± 0.1 MPa (CM) to 5.6 ± 0.1 MPa (M3 with 0.04% GO), showing a uniform SD = 0.1 MPa across the mixes, with ANOVA indicating a highly significant influence of GO dosage (F = 64.2, p < 0.001; Table 9).

Figure 11 illustrates how GO affects flexural strength and the percentage change in flexural strength. Compared to CM, the cement concrete using a 0.04% GO dosage shows the greatest gain in flexural strength at 28 days, with a 42% increase. In M3, the combination of a refined pore structure from micro-CT and the denser ITZ observed in SEM directly supports this pronounced flexural improvement. The reduced population of large pores and the more homogeneous pore distribution decrease the likelihood of stress concentrations acting as crack origins, while the GO-reinforced C–S–H network forms a more cohesive tension zone that can sustain higher bending stresses before failure. Flexural strength increased by 27% from 5.1 ± 0.1 MPa (CM) to 6.5 ± 0.1 MPa (M3), exceeding experimental scatter (SD = 0.1 MPa), as confirmed by highly significant ANOVA results (F = 137.6, p < 0.001), as shown in Table 9.

Overall, the macroscopic strength enhancements obtained at the optimal GO dosage of 0.04% are consistent with the microstructural evidence from SEM–EDS, FTIR, TGA, and micro-CT, all of which indicate a denser, less porous matrix with a strengthened ITZ and more stable hydration products.

For each mix (CM, M1–M5), the compressive, split tensile, and flexural strengths of three specimens were evaluated at 28 days; the results are shown in Table 8 as the mean ± standard deviation. One-way ANOVA with GO dosage as the factor demonstrated a highly significant influence on all mechanical properties (Table 9), with F-values of 104.7, 64.2, and 137.6 for compressive strength, split tensile strength, and flexural strength, respectively (p < 0.001 for all cases). These results confirm that the strength enhancements obtained in the GO-modified mixes, particularly at 0.04% GO (M3), are statistically significant.

The microstructure of cement concrete and how it changes when GO is added has a significant impact on its mechanical qualities. The microstructure and the morphology of the cement concrete with added GO were examined in order to determine the cause of increased static mechanical characteristics (Figure 12). SEM images of a representative sample of the CM reveal a non-uniform distribution of the hydrated phases. Additionally, Figure 12a identifies microcracks and macropores. When GO is added to cement concrete, the morphology changes into a denser, more uniform structure that regulates 2D bridging within the cement hydrates (Figure 12b). The microstructure of GO cement concrete revealed the formation of interconnected pores and wrinkled C–S–H foils. Cement concrete specimens with GO dosages up to 0.04%appeared to have more evident contact sites in C–S–H and were primarily responsible for the improvement of the static mechanical characteristics.

Figure 13 displays the FTIR spectra of CM and M3. A wider and more powerful O–H stretching band in the 3,200–3,600 cm⁻¹ region is visible in the M3 spectrum, whereas this band is weaker in the CM spectrum, indicating increased bound water and a higher degree of hydration in the GO-modified mix.

Similarly, the main Si–O stretching vibration observed at approximately 950–1,100 cm⁻¹ is more pronounced in M3 than in CM. This increase in band intensity reflects the development of a denser and better-polymerized silicate network (C–S–H gel), which is consistent with GO-induced enhancement in the microstructure.

Overall, the differences in O–H and Si–O peak intensities between CM and M3 confirm that graphene oxide promotes additional hydration, stronger chemical bonding, and matrix refinement within the cementitious system. These FTIR findings support the mechanical improvements observed in the GO-modified concrete.

TGA and DTA results for the control mix (CM) and the GO-modified mix (M3, 0.04% GO) are presented in Figures 14a–d. The CM exhibits a total mass loss of approximately 16.7% up to 1,200 °C, with three main steps of approximately 6.5% below 400 °C, 7.3% between 400 °C and 700 °C, and 2.5% above 700 °C, leaving a residue of approximately 83.0% (Figure 14A). In comparison, in Table 10, M3 shows a lower overall mass loss of approximately 12.9% over the same range and a higher residual mass of approximately 87.1%, indicating that the GO-modified composite retains more thermally stable solid phases in Figure 14c.

The corresponding DTA curves identify similar decomposition events for both mixes, with peaks near 135–170 °C associated with dehydration of free and bound water, peaks approximately 425–445 °C related to the decomposition of C–S–H and other hydrates, and peaks at approximately 668–690 °C corresponding to the decarbonation of calcium carbonate. Peak intensities in the 200–500 °C region are slightly lower for M3 than for CM, which, together with the reduced mass loss, suggests that the presence of GO promotes the formation of more thermally stable hydration products and a denser microstructure, consistent with the improved mechanical performance observed for the optimized mix. These results indicate that the addition of GO promotes the generation of stable gel phases and reduces the extent of mass loss in the modified composite, confirming its positive impact on concrete durability and microstructural integrity (Yang et al., 2017).

In the current investigation, micro-CT X-ray scanning (Figure 15) was employed to estimate total porosity and analyze the internal microstructure of 50 mm cube specimens, including the control and GO-incorporated specimens. This non-destructive imaging technique allowed for detailed visualization and 3D reconstruction of the samples, providing insights into pore structure (Chotard et al., 2003). The 3D reconstructed model images of the cement composite with different GO mixes, with an optimal dosage of 0.04% GO with RCA mixes, identified a synergistic effect, in contrast to the traditional mix that was cured for 28 days. The porosity of samples GO and OPC with RCA at 28 days decreased, as clearly shown in Figure 17. Large pores in the CM, as shown in Figure 16, have widths ranging from more than 1,000 microns to 1,962 microns, causing notable variations in their surface porosity. The pore structure of the M3 GO - OPC samples containing 0.04% GO is more consistent than that of the CM. Compared to the samples with the incorporation of GO with the optimal dosage of 0.04%, the density of the samples with GO-OPC at 28 days drastically increased. In comparison to the CM, the M3 mix showed a more homogeneous pore structure at 0.04%. GO, which enhanced the cement composite, is highly dense (Figure 17). It shows that the majority of pores in typical CS have diameters of 200–1,200 microns, with a small number measuring 1,400–2,500 microns. The control mix (CM) exhibited a porosity of approximately 14%–16%, whereas the GO-modified mix with 0.04% GO (M3) showed a reduced porosity of approximately 9%–11% at 28 days, as evaluated in Table 11. This reduction in overall void volume, together with the shift from large pores (200–2,500 µm) in the CM to finer, more uniformly distributed pores (mainly 500–1,000 µm) in the M3, confirms that GO significantly densifies the internal microstructure of RCA concrete.