*Article* **The Influence of Water/Binder Ratio on the Mechanical Properties of Lime-Based Mortars with White Portland Cement**

**Dejan Vasovic \*, Jefto Terzovic, Ana Kontic, Ruza Okrajnov-Bajic and Nenad Sekularac**

Faculty of Architecture, University of Belgrade, 11000 Belgrade, Serbia; jefto@arh.bg.ac.rs (J.T.); an.ko@arh.bg.ac.rs (A.K.); ruza@arh.bg.ac.rs (R.O.-B.); nenad.sekularac@arh.bg.ac.rs (N.S.) **\*** Correspondence: d.vasovic@arh.bg.ac.rs; Tel.: +381-631-119-838

**Abstract:** Protecting the built cultural heritage is one of the most important tasks in architectural practice. The process of repair is time-consuming, weather-dependent, and sensitive to materials applied. Introducing new materials in historic building repair in order to decrease the time needed for repair, brings some risk in the preservation process. The most common material for masonry repair is lime mortar. Adding cement to lime mortar can improve the mechanical properties of mortar and speed up the repair process. The high amount of cement may increase the strength, but decrease ductility and permeability of mortar, causing damages to protected buildings. An increase in strength with the smallest amounts of cement demands optimization of water content in the mixture. Tests were performed to investigate the influence of the water/binder (w/b = water/(lime + cement) ratio on mortar strength and water permeability. An air-entraining agent (AEG) was introduced to improve permeability. Results confirmed that adding small amounts of cement to lime (20% by weight) and decreasing of w/b ratio, improves the strength, with almost negligible influence on water permeability. The addition of very small amounts of AEG did not decrease the strength, nor the permeability.

**Keywords:** lime-cement mortar; compressive strength; air-entrained agent; heritage conservation; reconstruction and restoration of historical buildings
