**Andreja Padovnik \* and Violeta Bokan-Bosiljkov**

Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, SI-1000 Ljubljana, Slovenia; violeta.bokan-bosiljkov@fgg.uni-lj.si

**\*** Correspondence: andreja.padovnik@fgg.uni-lj.si

Received: 9 June 2020; Accepted: 27 July 2020; Published: 29 July 2020

**Abstract:** Injection-grout density is an important parameter when its additional weight leads to consolidated decorative plasters becoming damaged. This is especially evident in larger detached areas. In this study, thin-walled soda–lime–borosilicate glass microspheres were used as a density-reducing constituent in hydrated lime grout mixtures. The normal density grout composition—one volume part hydrated lime and three volume parts inert limestone filler with 0.5% of the polycarboxylate ether-based superplasticiser—was modified with partial substitution of the limestone filler with lightweight glass microspheres. The following volumetric proportions between limestone filler and glass microspheres were used: 100%:0%, 67%:33%, 50%:50%, 33%:67%, and 0%:100%. With the increase of the glass microspheres' volume, the density of the grout is gradually reduced. Furthermore, there is a decrease in the stability and injectability of fresh grout. In its hardened state, the grout's strength again reduces gradually, but there is no significant change in the grout's water absorption and water-vapour resistance. The resistance of the grout to freezing–thawing and heating–cooling cycles using distilled water or salt solution is highly improved when the microspheres are present.

**Keywords:** detached decorative plasters; injection grout; glass microspheres; reduced density; stability; strength; durability
