Principal factor of the sustainability of historic monuments is usage of compatible materials. The aim of this research was the investigation of the chemical composition of several mortars from local historic buildings (Vingrotaju Str. 1 (Riga), Daugavpils fortress, banks of river Aleksupite (Kuldiga) etc. (total of 13 samples)) with a purpose of elaboration of restoration strategy, including the choice of restoration materials. Most of the examinated mortars are lime-based hydraulic mortars, characteristic to architecture of 19th/20th century. In order to examine the changes of hydraulicity of perspective restoration binders, a series of blended lime–cement mixtures were synthesized with growing content of cement. Pure aerial lime binders show reduced compatibility with historic materials, that is why lime binders with pozzolan additive (cement) are one of most appropriate choice for restoration.