The influence of high volume cement replacement using a combination of slag and limestone, on the hydration, reaction products and pore structure, and strength of cementitious systems is reported in this paper. Total replacement levels vary from 20% to 50% by volume. Slag is blended with: (i) portland-limestone cement (PLC) that contains limestone interground with cement, or (ii) OPC and limestone of four different sizes in such a way that the resulting particle size distribution of the composite matches that of the corresponding PLC-based mixture. The hydration response of cement and cement-slag mixtures are found to be modified in the presence of limestone. It is observed from calorimetric and thermogravimetric analysis that a favorable slag-limestone synergy exists, that enables high volume replacement of cement without concomitant loss in properties. The early-age compressive strengths are beneficially impacted by the presence of limestone whereas the clinker factor does not play a significant role in later-age strengths in both the blended and interground systems. The study paves the way for development of multiple-material binders containing higher levels of cement replacement that demonstrate early and later age properties that are comparable to or better than that of traditional straight cement systems.

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Cementitious binders amenable to extrusion-based 3D printing are formulated by tailoring the fresh microstructure through the use of fine limestone powder or a combination of limestone powder and microsilica or metakaolin. Mixtures are proportioned with and without a superplasticizer to enable different particle packings at similar printability levels. A simple microstructural parameter, which implicitly accounts for the solid volume and inverse square dependence of particle size on yield stress can be used to select preliminary material combinations for printable binders. The influence of composition/microstructure on the response of pastes to extension or squeezing are also brought out. Extrusion rheology is used in conjunction with a phenomenological model to better understand the properties of significance in extrusion-based printing of cementitious materials. The extrusion yield stress and die wall slip shear stress extracted from the model enables an understanding of their relationships with the fresh paste microstructure, which are crucial in selecting binders, extrusion geometry, and processing parameters for 3D printing. K E Y W O R D S3D printing, extrusion, microstructure, rheology, wall shear stress, yield stress

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