Effect of viscosity modifier admixture on Portland cement paste hydration and microstructure

Significant attention has been given to the development of new materials and techniques to be employed in the construction market. One of the techniques which has drawn noticeable attention is the additive manufacturing process (a.k.a. 3-dimensional printing (3D printing)). One of the approaches of this construction technique is the extrusion of cementitious composites to form contour of a desired geometry. To achieve high viscosity in cementitious materials, usually viscosity modifying admixtures (VMA) are employed. However, the consequences of using these admixtures at high dosages is still not fully understood. This study characterized the influence of different VMA dosages on Portland cement paste, through a microstructure analysis. Hydration development was assessed, and effect of the admixture was quantified at different curing ages. Techniques such as thermogravimetric analysis, optical and electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, micro computed tomography scan and nanoindentation were employed. Important negative side effects were found such as: VMA increasing the cement setting time, anomalous dispersion of hydration products in the bulk and increasing the void content. On the other hand, positive effects were also found such as: evidence of internal curing, higher degree of hydration and lack of undesired hydration products.