Alkali activation of Mercia mudstone offers a pathway to developing low-cement-content construction materials, but early-age reaction pathways and strength development are highly sensitive to water availability, calcium supply, and carbonation. The present study explored whether press mud, a Ca/Fe-bearing by-product of sugar refining, can be used as a co-activatable precursor to guide these processes deliberately. Clay was blended with 10–45 wt.% press mud and activated at an activator-to-press-mud (A/P) mass ratio of 0.22–0.28, then cured either sealed (non-carbonated) or under intermittent CO₂ exposure (carbonated) for 3, 7, and 21 days. Time-resolved diffuse reflectance infrared Fourier transform spectroscopy, X-ray diffraction, and scanning electron microscopy were used to track phase evolution and relate it to unconfined compression behavior at elastic, peak, and post-peak strain states. Early curing was governed by surface silication and Fe(III) hydrolysis introduced by press mud; increasing A/P raised alkalinity and dissolved silicate, delayed goethite crystallization, and stabilized ferrihydrite skins, extending the alkaline window for early calcium silicate hydrate and sodium aluminosilicate hydrate formation. At intermediate press mud content (~25 wt.%), Fe-oxyhydroxide pellets disrupted Si-O-Al and Si-O-Si networks, producing a transient reduction in strength. Single-point total suction values, inferred from chilled-mirror dew point psychrometry provided direct water-budget evidence. This showed a suction maximum at 25 wt.% press mud at comparable water contents. Observations were consistent with transient sequestration of pore water by hydrophilic organo-Ca(II) complexes and zeolitic phases and explained the observed strength minimum near ~25 wt.% press mud. Carbonation generally increased strength; however, baseline-corrected carbonate formation was consistently lower at high press mud contents, which gives direct evidence for slowed carbonation kinetics when siliceous and Fe-rich surface coatings limited CO₂ access.
