Sustainable ultra‐high performance concretes achieved by reducing the clinker factor using calcined clays (Metaclay‐UHPC) – rheological properties and compressive strength

Abstract

Reducing the clinker factor of cement is a key strategy for lowering the CO ₂ ‐emissions of concrete. This is also particularly significant for ultrahigh performance concrete (UHPC), which contains approx. 800 kg/m ³ of cement. Calcined clays are a promising option that has already produced excellent performance and sustainability results. So far, studies have focused exclusively on highly kaolinitic clays. Research into the influence of 2:1 phyllosilicates (metaclay), which are available in Germany and Central Europe, however, is limited. The objective of this study was to replace up to 40 % of clinker with calcined metaclay in order to significantly enhance sustainability. Unlike standard concrete, UHPC is based on a w/b ratio of less than 0.25, optimised packing density, and reactive and inert fine filler additives, all of which contribute to strength development. The influence of the w/b ratio, clay type, superplasticiser and silica fume content on workability and compressive strength development at various substitution levels was investigated. The results demonstrate that workable concretes with strengths of up to 130 MPa after 28 days can be consistently produced. Using metaclay enables the production of more sustainable, clinker‐efficient UHPC and reduces CO ₂ emissions by up to 40 %.