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Precast concrete, cold weather concreting, and the emerging technique of concrete additive manufacturing are applications in which the acceleration of cement hydration plays a critical role. To allow precise control of early cement hydration in these applications, a thorough understanding of the working mechanisms of cement hydration accelerators is required. This study contributes to the understanding of the mechanism by which calcium nitrate (Ca(NO3)2) influences early cement hydration. The influence of Ca(NO3)2 on the hydration of an ordinary Portland cement has been followed by isothermal calorimetry, in situ X‐ray diffraction (XRD), quantitative XRD, compressive strength testing, and the analysis of the pore solution composition. Further, the initial pore solution, the initial phase composition, and the phase composition in the fully hydrated cement have been estimated by thermodynamic calculations to corroborate the experimentally obtained results. The results indicate that Ca(NO3)2, especially at the highest analyzed dosage of 5 wt.%, enhances the formation of ettringite and a nitrate‐containing AFm phase. Furthermore, Ca(NO3)2 accelerates alite hydration. Besides the increased Ca concentration in solution, it has been found that a reduction of the Al concentration in the initial pore solution by Ca(NO3)2 possibly contributes to the accelerating effect of Ca(NO3)2 on alite hydration. [ABSTRACT FROM AUTHOR]

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]

[Display omitted] • Natural and calcined Halloysite clay additives were characterised. • Halloysite clay additives were incorporated to modify AAMs' printing properties. • Enhanced compatibility of calcined Halloysite clay additives in the AAMs was verified. • Buildability and mechanical properties were improved in CH-modified AAMs. This study investigates the effects of natural and calcined halloysite clay minerals ("NH" and "CH", respectively) on the performance of 3D printed alkali-activated materials (AAMs). Halloysite clay minerals are selected as they are low-cost and abundantly available. At first, different characterisation techniques were employed to characterise the NH and CH additives. Mechanical performance, extrusion window, and shape stability of several AAM formulations containing various dosages (0.5 wt% to 5 wt%) of the NH and CH additives were evaluated. The best-performing mixtures in terms of fresh and hardened properties namely, NH-1.5 and CH-1.5 mixtures (containing 1.5 wt% of NH and CH additives, respectively) were then selected for 3D printing. The results showed that the CH-1.5 mixture exhibited enhanced shape stability, buildability, and mechanical properties as compared to the control mixture. The flexural and compressive strengths of 3D printed CH-1.5 samples were 88% and 40%, respectively higher than those of the printed control samples. Using the CH-1.5 mixture, a twisted column with an intricate shape was printed to verify the suitability of the developed CH-modified AAM for the construction of complex structures. This study establishes the use of halloysite clay minerals as low-cost additives for enhancing the mechanical properties and printing performance of AAMs. [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]