Rheology and early age evaluation of 3D printable cement-limestone filler pastes with nanoclays and methylcellulose

Abstract

Nanoclays (NC) are known to increase the structural build-up of cement-based materials (CBM) while methylcellulose (MC) is used to increase open time and workability, helping to prevent cold joints in 3D Printing applications. The combination and compatibility of both NC and MC could lead to the design of new 3D printable CBM able to tackle all the manufacturing process requirements. The aim of the study is to evaluate the effect of sepiolite nanoclay and MC on rheology and early age (EA) parameters of cement-limestone filler pastes for 3D printing applications, identifying fresh structural build up and setting mechanisms. The effect of two cement types with coarser and finer particle size distribution (CEM I 42.5-R and CEM I 52.5-R, respectively) was considered. Pastes rheology was characterized by Cone Penetration Test (CPT) and Dynamic Shear Rheometer (DSR). EA was monitored by P- and S-waves Ultrasonic Pulse Velocity (UPV), internal temperature, capillary pressure and drying shrinkage. Fresh Shear Yield Stress (τ0) and shear elastic modulus (G), effective thixotropy (Athix), critical strain (γcrit) and their evolution over time were calculated form CPT and DSR experimental results. A limit of usability was defined for τ0 up to 20 kPa for CPT with a 30º tip cone. Although pastes with both NC and MC showed thixotropy, NC reduced γcrit while MC strongly increased it. NC significantly increased Athix and ΔG with coarser particle size cement when compared to the finer cement. MC delayed hydration reaction and setting, while NC did not affect early age stages. On the other hand, combining NC and MC increased superplasticiser demand and reduced effective thixotropy (Athix). Finally, an integrated overview of the physical and chemical mechanisms involved in rheology and early age properties of cement-based systems and the key experimental parameters for their identification was proposed.