Combining nanolime and calcium carbonate polymorphs in the consolidation of porous stone: a small angle neutron scattering and synchrotron X-ray computed microtomography study

Sound Maastricht limestone, a calcarenite stone with high porosity, mostly employed as construction material in Belgium and Netherlands, was chosen for consolidation treatments with novel agents containing nanolime suspension and polymorphs of calcium carbonate (CaCO3) particles. The alteration of microstructure after consolidation treatment was investigated, for the first time, using the combination of small angle neutron scattering (SANS) and synchrotron X-ray computed microtomography (μ-CT). Good compatibility between nanolime, CaCO3 particles and limestone matrix was observed under the scanning electron microscope. SANS measurement exhibited two scattering regimes, corresponding to pores with smooth surface and rough fractal surface. After the consolidation treatment, mainly pores in region ranging from 30 to 300 nm were reduced and complexity of the pore surface was increased. μ-CT results revealed a high number of pores up to 400 μm highly connected, and many fossils with mostly closed pores (see Fig. 1). The decrease in total porosity after the consolidation treatment was around 3 % - quite a high decrease considering that the amount of consolidating material that was applied was very low; the external volume of the samples (usually the one with an accumulation of consolidating agent) was excluded from the calculation of porosity, and the μ-CT technique detected both open (as in case of mercury intrusion porosimetry) and closed porosity, that cannot be affected with consolidation treatment. The tested system, represents a new possibility of consolidation treatment that preserves the chemical compatibility of the agent with the limestone and performs better than the same treatments with nanolime alone. The advantage of the usage of SANS and μ-CT is their non-invasive approach and possibility of detecting nano-/microstructural changes in very small specimens, which are usually needed when real samples from cultural heritage objects are tested.