Abstract
The consolidation of degraded carbonate stone used in ancient monuments is an important topic for European cultural heritage conservation. The products most frequently used as consolidants are based on tetraalkoxy- or alkylalkoxy-silanes (in particular tetraethyl-orthosilicate, TEOS), resulting in the formation of relatively stable amorphous silica or alkylated (hydrophobic) silica inside the stone pores. However, silica is not chemically compatible with carbonate stones; in this respect, nanocalcite may be a suitable alternative. The present work concerns the preparation of water suspensions of calcite nanoparticles (CCNPs) by controlled carbonation of slaked lime using a pilot-scale reactor. A simplified design of experiment was adopted for product optimization. Calcite nanoparticles of narrow size distribution averaging about 30 nm were successfully obtained, the concentration of the interfacial agent and the size of CaO being the most critical parameters. Primary nanoparticle aggregation causing flocculation could be substantially prevented by the addition of polymeric dispersants. Copolymer-based dispersants were produced in situ by controlled heterophase polymerisation mediated by an amphiphilic macro-RAFT (reversible addition-fragmentation transfer) agent. The stabilized CCNP aqueous dispersions were then applied on carbonate and silicate substrates; Scanning Electron Microscopy (SEM)analysis of cross-sections allowed the evaluation of pore penetration, interfacial binding, and bridging (gap-filling) properties of these novel consolidants.
Highlights
Designing new products suitable for conservation treatments of valuable cultural heritage objects and buildings is an open issue, due to the many requirements that such products should fulfil and the broad range of different materials to be treated
The general procedure for the synthesis of calcium carbonate nanoparticles (CCNPs) by carbonation of slaked lime was optimized by considering two compositional and two process (CO2 flow rate, stirring rate) parameters, which were varied in order to obtain small nanocalcite particles with narrow size distribution and uniform morphology, possibly resulting in aqueous dispersions with adequate colloidal stability
As a means to reduce the number of synthetic runs potentially required to exhaustively explore the multidimensional experimental space generated by the k experimental parameters and their respective variability range, a design of experiment (DoE) approach and ANOVA statistical analysis of the impact of the chosen process and formulation parameters was adopted
Summary
Designing new products suitable for conservation treatments of valuable cultural heritage objects and buildings is an open issue, due to the many requirements that such products should fulfil and the broad range of different materials to be treated. In the case of ancient stone materials, chemical and physical alteration due to natural weathering, often exacerbated by anthropogenic pollution, eventually results in deterioration phenomena lowering the stone cohesivity (microscopic) and mechanical properties (macroscopic). In such cases, the application of consolidants is a common practice. Polymeric, and hybrid materials that have been used in stone conservation, consolidant formulations based on tetraethoxysilane (or tetraethyl-orthosilicate, TEOS) are possibly the most common Their effectiveness is the result of good penetration within the porous stone network and the good film-forming properties of TEOS, leading to the formation of relatively stable amorphous silica [1,2]. It would be desirable to employ a carbonate-based consolidant for treating a carbonate stone
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