Due to their low carbon dioxide emissions during calcination, thermally activated (calcined) clays represent a potential sustainable material to meet the future demand of SCMs by cement industry. Apart the side effect of a high water demand, substitution of ordinary Portland cement (OPC) by calcined phyllosilicates can create a very interesting rheological performance of blended cement. Investigations on the slump flow as well as measurements with a rotational rheometer reveal that the addition of meta muscovite in a range from 20 to 30 wt. {%} to OPC changes - a sufficient low water-to-cement ratio provided - the flow behavior from a shear thinning Bingham model with thixotropic properties to a time-dependent shear thickening rheology. This effect cannot be observed in the neat calcined clay paste admixed with synthetic cement pore solution (containing the ion composition of a typical OPC pore solution), but well in distilled water. Bonding and elimination of protons lead to a pH-dependent charge on the edges of the clay mineral crystals and consequently influence the overall surface charge of clay particles. Thus, ion exchange processes in the sheet structure of this phyllosilicate are seen as the main explanation for the variable rheological behavior in different media. Addition of a variety of anionic and amphoteric PCE-based superplasticizers to neat meta muscovite and blended cement increases the spread flow, showing the dependance of rheological parameters like yield point and plastic viscosity from the workability. Equivalent spread flow tests in mortar blended with thermally activated muscovite exhibit, that aggregates like sand or gravel weaken the shear thickening effect. The results allow to conclude that the mixing of OPC with calcined clays not only impacts the hydration kinetics and strength development but changes the (time-dependent) rheological properties of cement pastes, mortars and concrete. Summarized, the abovementioned extraordinary rheological characteristics of blended cement open a wide range of special applications, e. g. self-leveling underlayments.    «

Due to their low carbon dioxide emissions during calcination, thermally activated (calcined) clays represent a potential sustainable material to meet the future demand of SCMs by cement industry. Apart the side effect of a high water demand, substitution of ordinary Portland cement (OPC) by calcined phyllosilicates can create a very interesting rheological performance of blended cement. Investigations on the slump flow as well as measurements with a rotational rheometer reveal that the addi...    »