Issue 78

C. S. Karadumpa et alii, Fracture and Structural Integrity, 78 (2026) 146-167; DOI: 10.3221/IGF-ESIS.78.09

Mix ID

Slump (mm) Inference

Mix 1 (Control) Mix 2 (GO 0.01%) Mix 3 (GO 0.03%) Mix 4 (GO 0.05%) Mix 5 (CNT 0.1%) Mix 6 (CNT 0.2%) Mix 7 (CNT 0.3%) Mix 8 (Hybrid)

95

Acceptable, cohesive Highly workable Highly workable, stable Slight stiffening observed Reduced flow, cohesive Noticeable viscosity increase Significant reduction in flow

165 165 158 150 140 125 160

Balanced workability Table 5: Slump test results for concrete mixes.

M ECHANICAL PROPERTIES

M

echanical performance was evaluated through compressive, split tensile, and flexural strength testing at 7, 28, and 56 days for the concrete samples with varying dosages of GO and CNTs.

Compressive strength Compressive strength development exhibited distinct patterns across curing ages, reflecting both nanomaterial nucleation effects and ongoing pozzolanic reactions. The control concrete achieved 12.5 MPa at 7 days, 28.0 MPa at 28 days (meeting the M30 grade requirement), and 35.1 MPa at 56 days, demonstrating slower early-age development characteristic of GGBS/Fly Ash composite cements together with continued strength gain through extended pozzolanic reactions (Tab. 6). Nano-modified concretes showed dramatic early-age acceleration: GO 0.03% reached 21.13 MPa at 7 days (+69.0%), 34.5 MPa at 28 days (+23.2%, the highest among all formulations), and 39.09 MPa at 56 days (+11.4%, maintaining the highest absolute strength). Similarly, CNT 0.1% achieved 22.06 MPa at 7 days (+76.5%, the highest early-age strength), 33.53 MPa at 28 days (+19.8%), and 36.5 MPa at 56 days (+4.0%). The dosage-strength relationship for GO (Fig. 2) reveals a clear optimum at 0.03%, with strength increasing from 0.01% to 0.03% before declining at 0.05% across all curing ages—28-day strength improved from control (28.0 MPa) to GO 0.01% (32.8 MPa) and peaked at GO 0.03% (34.5 MPa), before dropping to 24.96 MPa at GO 0.05%, below even the control baseline. Similarly, CNT dosage optimization (Fig. 3) demonstrates peak performance at 0.1% with exceptional early-age acceleration (22.06 MPa, +76.5%), followed by plateaued performance at 0.2% (21.13 MPa) and 0.3% (21.86 MPa), indicating diminishing returns beyond the optimal dosage. The hybrid formulation demonstrated balanced performance, achieving 21.2 MPa at 7 days (+69.6%), 33.9 MPa at 28 days (+21.1%), and 36.5 MPa at 56 days (+4.0%). Its compressive strength remained intermediate between those obtained for the optimum GO-only and CNT-only mixtures across all curing ages. The substantial early-age strength enhancement observed in the hybrid mixture is consistent with the known ability of GO and CNTs to influence hydration and microstructural development in cementitious systems. SEM observations revealed a denser matrix and improved interfacial transition zone (ITZ), while EDAX and XRD analyses indicated the formation of well-developed hydration products. These observations suggest that the incorporation of GO and CNTs contributed to matrix refinement and improved mechanical performance. However, the specific interactions between GO and CNTs and their individual contributions to strength development were not directly investigated in the present study and therefore cannot be conclusively established. Further studies employing advanced nanoscale characterization techniques are required to verify the underlying mechanisms governing the behavior of hybrid nano-modified concrete systems. Critically, excessive dosages degraded performance across all curing ages, as clearly illustrated by the downward trends beyond optimal points in Figs. 2 and 3: GO 0.05% yielded only 15.73 MPa at 7 days (+25.8%), 24.96 MPa at 28 days ( − 10.9%), and 31.47 MPa at 56 days ( − 10.3%), substantially lower than GO 0.03% despite higher nanomaterial content. This finding suggests that excessive GO content may promote agglomeration and the formation of localized defects. The steep decline in Fig. 2 from 0.03% to 0.05% particularly emphasizes the critical importance of avoiding excessive GO dosage. Similarly, CNT 0.2% (21.13 MPa at 7 days, 32.1 MPa at 28 days, 33.2 MPa at 56 days) and CNT 0.3% (21.86 MPa at 7 days, 32.8 MPa at 28 days, 35.2 MPa at 56 days) plateaued below CNT 0.1%, with CNT 0.2% showing actual reduction at 56 days ( − 5.4%), confirming agglomeration at higher contents impairs effectiveness. The relatively flat performance across CNT dosages of 0.1-0.3% in Fig. 3, contrasting with the sharp peak-and-decline pattern for GO in Fig. 2, suggests different agglomeration sensitivities between the two nanomaterials. Complete strength data for all eight mix designs are presented in Tab. 6, showing consistent trends across the all curing ages.

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