Do PCEs Adsorb Preferentially on OPC or Calcined Clay?

ABSTRACT In multi‐component binders, admixtures such as polycarboxylates (PCEs) may exhibit preferential adsorption onto one component. Hence, for the first time PCE adsorption on ordinary Portland cement (OPC) and calcined clay (CC) in an OPC/CC blend was quantified by separating the OPC and CC particles from the hydrated cement employing a sophisticated filtration method. Two different CCs (a calcined common clay and a smectite–illite‐rich clay) and two methallyl ether (HPEG) PCE samples were investigated. Total organic carbon (TOC) analysis was utilized to quantify PCE adsorption. By weight of each binder , both PCEs adsorb in almost comparable amounts on OPC and CC at the saturation point. However, by surface area of each binder , PCEs adsorb by far more on cement (90%) and much less on CC. The low PCE affinity to CCs is explained by their initially negative surface charge, which requires uptake of Ca 2+ to facilitate PCE adsorption. Thus, in OPC/CC blends, PCEs preferentially adsorb onto OPC, while CC surfaces remain only marginally occupied. This explains why CCs are much more difficult to fluidize than OPC. Our approach opens up new insights into the behavior of PCE superplasticizers in blended low carbon cements holding supplementary cementitious materials.