Mixing-induced control of C-S-H seed synthesis and its effects on accelerating cement hydration

Synthetic C-S-H seeds accelerate early cement hydration by increasing heterogeneous nucleation site density, but reported efficiencies vary substantially. This study investigates how mixing intensity, addition sequence, and precursor chemistry during seed synthesis jointly control particle size, phase composition, morphology, and acceleration performance in CEM I 52.5 R at w/c 0.4. Two precursor systems Ca(OH) 2 /SiO 2 ( CH ) and Ca(NO 3 ) 2 /Na 2 SiO 3 ( CN ) at C/S 0.8 and 1.4 were processed across six mixing protocols and evaluated by isothermal calorimetry, ultrasonic pulse velocity, and compressive strength testing. Seeding efficiency improves from manual shaking through ultrasonic (US) and 16,000 rpm treatment but deteriorates at 39,000 rpm due to mechanically stable secondary agglomerates, confirmed by SEM, dynamic image analysis, and pellet formation in ≈ 50% of paste samples, causing crumbling failure in ≈ 50% of compressive-strength specimens. Within the CH system at w/c 0.4, C/S 1.4 seeds outperform C/S 0.8 despite near-identical agglomerate sizes, reaching 1-d strengths of 132% of the reference versus 105%–115% for C/S 0.8. This is consistent with a working hypothesis of epitaxial compatibility with early calcium-rich C-S-H; the advantage disappears at w/c 0.5. TGA confirms higher salt loading for CN seeds at C/S 1.4 ( ≈ 47 wt% residual mass at 850 °C) versus C/S 0.8 ( ≈ 58 wt%), consistent with their selective late-age strength reduction. A sequential addition protocol (nHR) produces dendritic C-S-H with local C/S near 2.0, demonstrating that addition sequence controls phase composition independently of mixing intensity. CH seeds at C/S 1.4 processed by US or 16k represent the optimal combination for CEM I 52.5 R at w/c 0.4.