Superplasticizer Incompatibility in SuperSulfated Cement Containing BYF Clinker: Hydration, Microstructure, and Strength Development

Abstract

Low‐carbon supersulfated cements (SSC) are promising binders for reducing CO ₂ emissions because they contain high amounts of slag and only minor clinker. However, their practical use is still limited by slow early‐strength development and sensitivity to chemical admixtures. In this study, belite–ye'elimite–ferrite (BYF) clinker and CaO were used to enhance early hydration, while the influence of a polycarboxylate‐based superplasticizer (SP) on this activated SSC system was investigated. The binder contained 5% BYF clinker and 2% CaO, and mixtures with and without SP were characterized by TGA/DTG, XRD, selective dissolution, MIP, SEM, and compressive strength testing. SP caused a severe early‐strength reduction: the SP mortar could not be tested at 1 day and reached only 1.7 MPa at 2 days, compared with 12.9 MPa at 1 day without SP. At 7 days, strength remained lower with SP (10.5 vs. 30.0 MPa), while the difference decreased by 28 days (34.4 vs. 38.0 MPa). Hydration‐product analysis showed moderate differences: SP gave comparable or slightly higher ettringite, whereas the no‐SP mixture showed higher bound water and slag reaction. MIP and SEM revealed that SP increased pore volume and produced a more open, ettringite‐rich microstructure. Thus, the early strength loss was mainly caused by weaker hydrate connectivity and pore‐structure development rather than a lack of hydration products.