**1. Introduction**

Consolidation of detached plasters with historic value is an irreversible conservation treatment for re-establishing adhesion between delaminated decorative layers. Injection grouts, used for stabilisation of decorative plasters, have to be compatible with original historical materials. Furthermore, they should provide adequate flowability, injectability, and stability in a fresh state. Requirements regarding the grouts' hardened state are very often given in relation to the properties of detached historical plaster; they should exhibit similar dry porosity, water-vapour permeability, capillary water absorption, and mechanical strength. An example of such requirements summarised by Padovnik et al. [1] is given in Table 1.

Additionally, specific properties, such as a low density of the grout, its increased durability, etc., are sometimes required. For larger detachments, a high quantity of grout is used to re-establish adhesion between the delaminated layers; low density of the grout is a key property that must be prescribed to prevent the formation of new damage and/or falling of the plaster from wall or ceiling, during or after consolidation by grout injection. Said density can be considerably reduced by the incorporation of lightweight filler to the grout mixture. The types of fillers used for such purpose are mineral materials with high porosity and water absorption, such as pumice, perlite and expanded glass or clay aggregate [2,3], and glass microspheres [4,5]. Very often, however, a composite filler—a mixture of pumice and glass microspheres—is used in conservation practice [6–8].

The glass microspheres—also known as glass bubbles—are nonabsorbent and have a significant advantage over porous fillers, since they can preserve extremely low wet and dry densities. Their spherical morphology, regular surface texture and extremely small particle size can help achieve and maintain suitable viscosity and stability, as well as improved injectability of the grout [4,7]. Zajadacz and Simon [4] studied the grout composition with Scotchlite Bubbles and Silcosil (fine-ground silica). They attributed the improved injectability to the small particle size of the glass microspheres and silica filler. The glass microspheres were well dispersed inside the grout mixture and tended not to segregate from other constituents, resulting in improved stability. The viscosity of this grout, however, was higher than usual. Rickerby et al. [7] used a high proportion of glass microspheres in a grout based on mud and pumice. They found that the spherical morphology of the microspheres contributed to poor packing ability and reduced internal cohesion of the grout mixture. Furthermore, Rousset et al. [9] reported that injection grout, prepared using a hydrated lime binder and glass microspheres as filler, possessed lower shrinkage and good adhesion. In Pasian et al. [8], microstructural analyses of the grout, based on slaked lime, pumice powder, and glass microspheres, showed the presence of hydraulic components on the microspheres' surface; a result of the pozzolanic reaction. Additionally, it was evident from the backscattered electron images of the cross sections and broken sections that some glass microspheres in the studied grouts were broken.


**Table 1.** Requirements for nonstructural lime-based grout [1].

When using the glass microspheres as a weight-reducing filler in a hydrated lime grout composition, the viscosity of the paste must be high enough to prevent the segregation of solid particles. To provide both an increased viscosity and adequate injectability of the hydrated lime grout, superplasticiser (SP; polymeric admixture) can be incorporated into the grout mixture [13,14]. Subsequently, much lower water content is needed for adequate workability and injectability. Among three SP groups frequently used in the lime-based grouts—poly-naphthalene sulfonates (PNS), lignosulfonates (LS) and polycarboxylate ethers (PCE)—the PCE products seem to be the most efficient solution [1,13].

The objective of this study was to develop a low-density hydrated lime injection grout, possessing adequate properties in both its fresh and hardened state in order to be used for the consolidation of detached lime plasters with historical value. Additionally, the grout was to be resistant to freezing–thawing and heating–cooling cycles, if possible, also in the presence of salts. The reduced density of the grout was obtained by using glass microspheres in the composite filler, which also contained fine-ground limestone particles. The required viscosity of the lime paste—to prevent segregation of filler particles and provide adequate injectability of the grout—was achieved using a combination of reduced water content and PCE superplasticiser. A parametric study was carried out as a means of determining the optimal composition of the lightweight grout, which would, in turn, ensure its improved durability.
