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The desire to accelerate cement hydration for faster construction processes is almost as old as cement itself. In recent years, however, new requirements for the acceleration, and the precise control of the setting behavior and the early strength development of cementitious materials have emerged. This is due to the increasing proportion of cements with reduced clinker content and new technologies, such as additive manufacturing. As the use of accelerating admixtures offers high flexibility in controlling of early cement hydration, these admixtures might play a key role in developing fast-hydrating green cements and cementitious materials suitable for application in additive manufacturing. A detailed understanding of the mechanisms by which different accelerating admixtures influence cement hydration and their influence on setting and hardening is thus required. In this study, the most widely used accelerators for cast and sprayed concrete are reviewed, focusing on their influence on the aluminate and silicate reaction of cement hydration as well as the setting and hardening depending on the dosage of the admixture. [ABSTRACT FROM AUTHOR]

• 3D printable mortars containing different dosages of nanosilica (NS) were developed and evaluated. • NS accelerates significantly the setting and hardening of printable mortar, while reducing its open time. • Increment of yield stress, together with an increment in NS dosage, was found to have occurred. • Optimal NS dosage results in improvement of the compressive strength and pore structure. • Incorporation of NS in the mixture resulted in improved buildability and decrement in pore anisotropy. This study presents the experimental results of an investigation on the effects of nanosilica (NS) on the material characteristics of printable mortars used for additive manufacturing. Printable cement mortars based on Ordinary Portland Cement, limestone filler and silica sand were modified with different dosages of nanosilica (from 2% to 6% by weight of binder) and its influence on their hydration, rheological, mechanical and transport properties was assessed. The study showed that NS accelerates significantly the setting and hardening of printable mortar, while reducing its open time. Moreover, an increment of yield stress, together with an increment in NS dosage, was found to have occurred. The incorporation of an optimal NS dosage results in a noticeable increase in the compressive strength and alteration of the pore structure as determined by the MIP measurements. Moreover, transport properties of the produced mortar are significantly improved due to incorporation of NS. In addition to the microstructure refinement, Micro-CT and scanning electron microscopy (SEM) studies revealed that 3D printed mortars exhibit pore anisotropy in accordance with the printing direction. However, incorporation of NS in the mixture resulted in improved buildability, thus decreasing pore anisotropy. [ABSTRACT FROM AUTHOR]