For several years functional organosilanes containing NH groups have been tested to improve the performance of TEOS-based consolidants for carbonate stones, however different efficacies are being reported and the role of side-effects in sol–gel routes is still unclear. The present study aims at improving the efficacy of a formulation for carbonate stone consolidation by adding N1-(3-trimethoxysilylpropyl)diethylenetriamine (SiDETA) to previously developed TEOS-based consolidants. While xerogels derived from reference sols were transparent, dense, and with smooth morphology, SiDETA-modified xerogels were translucent, coarser and composed of interconnected nanosized particles. It was concluded that SiDETA changed the mechanism of silica network formation by modifying the sol's pH, acting as a nucleation center, and by ionic interactions. Despite the changes in the silica network structure, SiDETA incorporation showed to cause additional strength increments on the treated stone.