PCE Superplasticizers for Calcined Clay Blended Cements – Challenges and Solutions

A high content of metakaolin in calcined clay blended cement has a positive effect on the early strength development, however, it also leads to a poor workability. Moreover, slump retention is more difficult to achieve in this system than in OPC or other composite cements. In this study, several approaches were investigated to achieve fluidizing ability of mortars prepared from composite cements holding 30 wt. % of calcined clay with varying amounts of metakaolin (∼ 23 wt.%, ∼ 51 wt.%). First, it was found that the slump retaining performance of a common industrial ready‐mix type HPEG PCE rapidly decreases when the content of metakaolin increases. Furthermore, a combination of the ready‐mix HPEG PCE and a retarder (sodium gluconate) which is commonly applied in ready‐mix concrete, also fails to improve fluidity retention in calcined clay blended cement with a high level of metakaolin, hence highlighting the difficulty of slump retention for such cements. To mitigate this problem, a unique admixture based on the combination of a precast type HPEG PCE and a novel PCE‐LDH nanocomposite is introduced. Mortar tests reveal that the high water‐reducing (precast) type HPEG PCE and the PCE‐LDH nanocomposite enables to achieve a remarkable improvement in fluidity retention of such calcined clay blended cements.