Impact of Polycarboxylate Superplasticizers on the Kinetics and Morphology of Diffusion‐Controlled Crystal Growth: A Comparative Study of Gypsum and Calcium Hydroxide

Abstract

The influence of a polycarboxylate ether (PCE)‐based superplasticizer (SP 120) on the diffusion‐controlled crystallization of gypsum and calcium hydroxide was systematically investigated using a coaxial diffusion setup combined with XRD and SEM analyses. SP 120 was added at dosages of 0.5, 1, and 2 wt.% to evaluate its effect on nucleation, growth kinetics, and crystal morphology. XRD confirmed that the additive did not alter the phase composition of either system, while significant changes in crystallization behavior and morphology were observed. In the Ca(OH) ₂ system, increasing SP dosage progressively delayed nucleation and promoted heterogeneous, branched, and dendritic structures, indicating a transition toward diffusion‐limited growth caused by restricted ion transport and local supersaturation gradients. In contrast, gypsum crystals exhibited suppression of their characteristic needle‐like morphology and evolved toward shorter, denser, lamellar aggregates due to selective adsorption of PCE molecules on specific crystallographic faces. The results demonstrate that the effect of PCE‐based superplasticizers strongly depends on the intrinsic growth mechanism and surface reactivity of each mineral system, providing new insight into additive‐controlled crystallization in gypsum‐based and cementitious materials.