Time-dependent rheology and pore-structure evolution of ultra-low clinker cement based foamed pastes

This study investigates the time-dependent rheology, in-situ foaming behavior, pore-structure evolution and mechanical performance of coal-ash-based ultra-low clinker cement pastes. Sodium sulfate, sodium citrate and their combined use were compared as activators, while polycarboxylate superplasticizer and H 2 O 2 dosages were varied to clarify the coupled effects of chemical activation, particle dispersion and gas generation. For non-foamed pastes, the combined sodium sulfate–sodium citrate activator produced the most gradual viscosity evolution, which was consistent with an apparent balance between sulfate-related early structuration and citrate-related dispersion/retardation. Increasing superplasticizer dosage reduced the dynamic yield stress from 53.35 Pa at 1 wt% SP to 21.99 Pa at 3 wt% SP and suppressed structural rebuilding. The short-time flow curves were satisfactorily fitted by the Bingham and Herschel–Bulkley models. The HB flow index remained close to 0.8, suggesting that PCE mainly weakened the apparent flocculated-network strength rather than changing the fundamental shear-thinning mechanism. During in-situ H 2 O 2 foaming, the rheometer-recorded apparent stress exhibited a reproducible three-stage evolution, interpreted as being consistent with early gas generation and structural perturbation, an intermediate crossover between gas-related weakening and matrix build-up, and an increasing contribution of matrix structuration at later times. Increasing H 2 O 2 dosage coarsened the optically resolved macropores and increased the MIP-accessible pore volume, reducing density from 1.01 to 0.70 g/cm³ and compressive strength from 6.88 to 2.2 MPa. The two-dimensional macropore fractal dimension ranged from 1.75 to 1.85 and provided supplementary information on macropore geometric irregularity. A conceptual framework is proposed to interpret the common and distinct structural-evolution features of non-foamed and foamed pastes.