The influence of Ca-formate (C2H2CaO4) on the crystallization of the CSH-phase 11 angstrom tobermorite was investigated under hydrothermal conditions at 200 degrees C in a wide CaO/SiO2 range of 0.67-1.24. Besides clarifying questions on acceleration or retarding of CSH formation at high Ca-formate contents even a study on the effect of this salt addition on the crystal morphology was a main objective of the present work. All experiments were performed using fine grained SiO2 powder (P) or coarse grained quartz sand (S) as silicate source. The reactions were followed by XRD, SEM/EDX and FTIR. Except the reaction at lowest amount of formate (0.43 g), where only a very small effect on the crystallization rate was observed, higher salt concentrations were found to retard the reaction clearly. Here the reduction of the CSH-crystallization velocity and formation of tobermorite with a distinct character of the pre-phase CSH-I could be stated in each case. Its amount and crystal quality was not rigorously dependant on the inserted mass of Ca-formate in the 0.67-1.34 g range investigated here. A strong influence on the morphology of the CSH-crystals was further detected already in the whole range of salt concentration, i.e. even at the low value of 0.43g. examined in our experiments. Instead of the typical rod-like morphology of tobermorite formed without Ca-formate, a disturbed bent-needle-like morphology occurred at elevated formate concentrations. In contrast small compact crystals were observed at low amounts of Ca-formate. Syntheses with quartz powder and quartz sand in principle yielded to comparable results concerning the retarding effect and the Change of crystal morphology. As expected, the reaction rate of experiments with quartz sand was diminished in general, according to the lower specific surface of the sand grains, compared with the powder. A change of the pH-value with marked influence on the solubility rate of the educts and the further course of reaction can be excluded as cause for the observed behavior. Further reasons, based on crystal growth under influence of adsorption of formate ions at specific crystal faces were discussed. The improvement of knowledge on effects of additives on CSH crystallization will be of importance for future modifications of reaction processes of cement and concrete as well as steam hardened construction materials, where 11 angstrom tobermorite acts as the main binding agent. (C) 2014 Elsevier Ltd. All rights reserved.