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Title: Superplasticizers for Calcined Clay Blended Cements

Author(s): Ran Li, Marlene Schmidt, Tongbo Sui, Johann Plank

Publication: Symposium Paper

Volume: 355


Appears on pages(s): 69-80

Keywords: Polycarboxylate; superplasticizer; composite cement; calcined clay; metakaolin; workability; slump retention

DOI: 10.14359/51736013

Date: 7/1/2022

In this study, the behavior of a calcined mixed clay (CMC) exhibiting a particularly high metakaolin content (~51 %) in composite cements (substitution rates 0–50 wt. %) was studied. It was found that CMC much decreases workability and substantially increases the water demand due to its higher fineness as compared to OPC. Furthermore, the water demand of pure calcined clays was investigated, and the order as follows was established: meta muscovite ≫ meta illite ≫ metakaolin > meta montmorillonite. Additionally, the dispersing effectiveness of a series of precast-type PCEs selected from the groups of MPEG, HPEG, and IPEG polymers was tested in blended cements holding 0–50 wt. % of the CMC. According to this, the HPEG PCE disperses these composite cements best, followed by the IPEG and the MPEG PCEs. Generally, the presence of CMC prompts significantly higher PCE dosages (up to 800 % more for the 50:50 OPC/CC blend). Furthermore, it was found that in OPC/CMC blended cements slump retention is much more difficult to achieve than in OPC. As such, an industrial ready-mix type HPEG PCE or its combination with sodium gluconate failed to provide flowability retention times which are commonly required by the ready-mix industry. Our study concludes that while such low carbon calcined mixed clay blended cements offer significant ecological advantages, they demand higher superplasticizer dosages which negatively affects their cost-effectiveness and at the same time poses significant technical challenges, particularly in ready-mix concrete applications. It should be mentioned that the problems pointed out here will be less severe for CMCs of lower metakaolin content.