Improving the performance of PCE superplasticizers in early stiffening Portland cement

In the present study, the workability of two ordinary Portland cements, obtained from clinkers having similar phase composition and particle size distribution, but different orthorhombic C3A content, hydrated in the presence of four different PCEs, is investigated. Possible causes of the observed significant different levels of PCE compatibility, which are correlated with different amounts of orthorhombic C3A, are investigated by means of: (i) in situ X-ray diffraction, with the aim of assessing the time-dependent phase composition; (ii) imaging methods (scanning electron microscopy and X-ray microtomography) and gas adsorption, aimed at assessing the role of clinker porosity. Adopted strategies for mitigating PCE incompatibility included the addition of gypsum or bassanite, and the use of sacrificial agents for preventing any PCE consumption by early precipitating AFm phases. The results suggest that PCE incompatibility is mostly related to different specific surface areas and rates of early ettringite precipitation. The addition of a moderate amount of calcium sulfates improved PCE performance.