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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 effect of triethanolamine (TEA) at various dosages on the early performance of cement paste was systematically evaluated through the techniques of rheological measurements, penetration tests, and ultrasonic pulse velocity. The correlation of early performance to the chemical hydration process was analyzed by calorimetry, zeta potential, in situ XRD, and pore solution analysis. It is found that the effect of TEA on the early performance of cement paste is strongly dependent on its dosage. With the TEA dosage below 0.1 wt%, the setting and microstructural development of cement paste are retarded. Meanwhile, the yield stress of fresh paste is decreased due to the increasing zeta potential of cement grains. The promoted formation of ettringite (AFt) and monosulfate (AFm) caused by TEA decreases the rheological retention ability. At dosages ≥0.2 wt%, the reaction of aluminate‐containing phases is greatly accelerated and a flash setting is observed. Besides, the importance of ferric phase on the reaction of cement with TEA is highlighted. At a low dosage, TEA prefers to accelerate the dissolution of tetracalcium aluminoferrite (C4AF) first and increases the [Fe] in the pore solution of cement paste. In cement without C4AF, the retardation of TEA on silicate phase hydration is significantly alleviated. [ABSTRACT FROM AUTHOR]

Extrusion-based additive manufacturing imposes high requirements on the material stability right after the extrusion. Therefore, a thorough understanding of the chemical reactions that determine the early reduction in processability is necessary. Accelerators are especially considered here, which have a major influence on the early reaction. This study contributes to these issues by analyzing the influence of 0.1 wt% TEA (triethanolamine) and 2.0 wt% Ca(NO3)2 on the hydration of two CEM I 52.5 R. The hydration was analyzed by isothermal heat flow calorimetry and in-situ X-ray diffraction. Vicat needle penetration, a penetrometer of own design, and ultrasonic P-wave velocity development were used to monitor the early change in workability. The obtained results indicate that ettringite formation is the main factor influencing workability during the first 60 min of hydration. Afterwards, the influence of ettringite is exceeded by the formation of C–S–H. Ca(NO3)2 was shown to enhance C–S–H formation and had no significant effect on the workability during the first 60 min of hydration while rapidly decreasing workability during the induction period. TEA was shown to increase brownmillerite dissolution and ettringite formation and, consequently, promote the workability loss during the initial hydration period. The time of initial and final setting determined by the Vicat needle test depended mainly on the formation of C–S–H phases. [ABSTRACT FROM AUTHOR]