Time-Domain Characterization of the Pore Structure in Ultra-High Performance Concrete with Partial Substitution of Calcium Sulfoaluminate Cements

Moisture mobility is vital for the durability of ultra-high performance concrete (UHPC), yet the understanding of its evolution during hydration remains limited. This study applies time-domain ¹H NMR relaxometry, utilizing both solid echo (QE) and Carr-Purcell-Meiboom-Gill (CPMG) sequences, to quantitatively track the moisture dynamics in UHPC incorporating calcium sulfoaluminate (CSA) cement. This approach enables simultaneous quantification of mobile and chemically bound water, and provides nanoscale resolution of the evolving pore network. Solid echo measurements revealed a sharp increase in chemically combined water within the first hour, indicating rapid ettringite formation. The CPMG measurements showed a rapid increase in gel pores within the first hour, reflecting accelerated C₃S hydration. In CSA-UHPC, gel pores became the dominant pore type within just 4 hours, compared to 20 hours in conventional UHPC. An increase in interlayer pores was also observed over the same period, indicative of densification of the C–S–H structure. Furthermore, the addition of CSA cement led to a refined pore structure with reduced gel pore size, while maintaining comparable compressive strength. These trends, supported by isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, and mercury intrusion porosimetry, validate the findings from ¹H NMR. The integrated findings contribute to a better understanding of the changes in interlayer pore, gel pore, and chemically bound water, as well as the associated moisture dynamics and pore structure development in CSA-UHPC.