Triethanolamine (TEA) and Triisopropanolamine (TIPA) are used in small amounts as grinding aids in the cement grinding process. TIPA is particularly known to enhance mechanical strength of mortars at 7 and 28 days while TEA does not. A mechanism based on the formation of a soluble TIPA-iron hydroxide complex which could increase the degree of hydration of the cement and so could improve the mechanical properties, has been proposed for the TIPA. The aim of this work is to explain why addition of TIPA or TEA which have close molecular structure lead to different results on the mechanical properties of mortars. The physico-chemical evolution of a cement's hydration was first followed by coupling isothermal calorimetry and ionic concentrations measurements. Then, mechanical compressive tests were carried out on mortars (limestone aggregate). Tetracalcium aluminoferrite (C4AF) hydration is modified in presence of both additive because of the formation of a soluble complex between trialkanolamine and iron III. An adsorption of TEA on the Portlandite surface is significant during the silicate phase hydration, while TIPA does not adsorb. In the case of TEA, the molecule affinity for the Portlandite surface is stronger than that of the formation of the soluble complex. These results could explain differences obtained on mechanical compressive tests of mortars characterising by an increase of the mechanical strength in presence of TIPA.