Issue 78

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

elemental composition, and crystalline phase evolution with observed mechanical performance. Specifically, this multi technique characterization investigates how GO's nucleation mechanism may compensate for the slower of SCM hydration to achieve 69-77% early-age strength acceleration, and how CNT nano-bridging produces 196.8% early-age tensile enhancement—phenomena inadequately explained by mechanical testing alone.

Mix ID

Mix Description

7 Days (MPa) % vs Control 28 Days (MPa) % vs Control 56 Days (MPa) % vs Control

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Mix 7

Control (0%)

6.60 8.40 9.15 7.80 9.60 9.12 9.39

—

7.50

—

12.00 13.80 16.12 12.30 16.20 14.30 15.10

—

GO 0.01% GO 0.03% GO 0.05% CNT 0.1% CNT 0.2% CNT 0.3%

+27.3% +38.6% +18.2% +45.5% +38.2% +42.3%

13.20 15.60

+76.0% +108.0%

+15.0% +34.3% +2.5% +35.0% +19.2% +25.8%

8.10

+8.0%

15.80 13.20 14.35

+110.7% +76.0% +91.3%

Hybrid (GO 0.03% + CNT 0.1%)

Mix 8

9.21

+39.5%

14.92

+98.9%

15.60

+28.5%

Table 8: Flexural strength test results.

Scanning Electron Microscopy (SEM) SEM revealed distinct morphological differences between control and nano-modified concretes, providing mechanistic understanding of observed mechanical enhancements. Fig. 10 and Fig. 11 present comparative SEM micrographs at 7 and 28 days for all critical formulation SEM revealed distinct morphological differences consistent with the mechanical performance trends. The control concrete exhibited irregular loosely-packed C-S-H with abundant capillary pores (1-10 μ m), a relatively weak ITZ (20-50 μ m thick), and significant porosity at 28 days, explaining modest strength values (28.0 MPa compressive, 2.45 MPa tensile). Optimal nanomaterial dosages produced dramatic refinement: GO 0.03% appears to exhibit best-developed microstructure with highly dense fibrillar C-S-H network, minimal porosity, refined ITZ as shown in Fig. 10, and GO sheets fully embedded as thin layers filling nano-pores—directly explaining the 23.2% compressive strength enhancement and 69.0% early-age acceleration through accelerated hydration compensating slower GGBS/Fly Ash reactions. However, these results are qualitative/semi qualitative and the quantitative verification of pore refinement and ITZ modification would require additional characterization techniques such as image-based quantitative SEM analysis, mercury intrusion porosimetry (MIP), X-ray micro-computed tomography (micro-CT), or statistically rigorous ITZ thickness measurements. These investigations were outside the scope of the present study and are recommended for future research. CNT 0.1% displayed individual nanotubes and small bundles (2-5 tubes) visibly bridging 1-5 μ m distances across pores and microcracks with intimate paste bonding, minimal porosity, and uniform three-dimensional reinforcement. This observed nano-bridging architecture directly validates the substantial tensile strength enhancement (196.8% at early-age and 31.8% at 28-day) through nano-scale crack arrest, fundamentally different from GO's matrix densification mechanism. Critically, excessive dosages revealed visible agglomeration: GO 0.05% showed dark layered clusters (5-15 μ m) and CNT 0.2-0.3% exhibited large bundles (>20 tubes, 10-50 μ m) acting as macro-defects creating interfacial voids, heterogeneous C-S-H distribution, and localized weak zones.— These observations provide microstructural evidence consistent with the observed reduction in mechanical performance (GO 0.05%: 24.96 MPa vs GO 0.03%: 34.5 MPa; CNT 0.2%: 32.1 MPa vs CNT 0.1%: 33.53 MPa), demonstrating proper dispersion at moderate dosages outweighs quantity. The hybrid formulation confirmed successful co-dispersion with both GO sheets and CNT fibers integrated creating dual reinforcement architecture—GO-densified matrix combined with CNT fibrillar bridging directly accounting for the balanced mechanical performance across all loading modes (compressive +21.1%, tensile +11.0%, flexural +98.9%) suitable for multi-axial applications.

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