The efficacy of cementitious grouts in micro-crack remediation is contingent on a delicate balance of rheological, mechanical, and volumetric properties. This study critically investigates the multifunctional role of nano-CaCO3 [n-CC] in a high-volume fly ash [HVFA] [29 % by binder weight] cementitious system. By systematically varying n-CC dosage from 0% to 4.88 %, we quantify the complex performance trade-offs inherent to nano-modification. Microstructural analyses, including scanning electron microscopy, reveal that n-CC acts as a nucleation catalyst, accelerating hydration and densifying the calcium-silicate-hydrate [C-S-H] gel structure. While this mechanism enhances mechanical performance - culminating in a 19 % increase in 28-day compressive strength at 2.23 % n-CC - it concurrently impairs workability and volumetric stability. Grout fluidity diminished with n-CC content above 0.20 %, and drying shrinkage increased by up to 108.5 %, a consequence of pore structure refinement explained by the Kelvin-Laplace principle. Critically, the optimal dosage for strength [2.23 %] did not align with that for toughness, where the flexural-to-compressive [F/C] strength ratio was maximized at a 1.33% dosage. Through a multi-criteria performance analysis, we identify a 1.33 % n-CC dosage as the optimal content, offering a superior balance of enhanced toughness and manageable shrinkage. This work elucidates the intrinsic performance conflicts in nano-modified, HVFA systems and provides a mechanism-driven framework for designing high-performance grouting materials for precision engineering applications.