Lime mortar consolidation with nanostructured calcium hydroxide dispersions: the efficacy of different consolidating products for heritage conservation

This paper analyses the efficacy of dispersions of nanoparticles of calcium hydroxide for consolidating lime mortars, according to three variables: the type of dispersion (three consolidating compounds—CaLoSil ® , Nanorestore ® and Merck ® —with different-sized calcium hydroxide particles); the concentration of the dispersion (5 and 25 g/L of Ca(OH) 2 in isopropyl alcohol); and the state of the sample (comparison of saturated and non-saturated samples). The outcome of the consolidation process was studied in terms of improved carbonation of the mortar, mineralogy (by means of X-ray diffraction and thermogravimetric analyses), texture (study of the porosity by mercury intrusion porosimetry) and compactness (measurement of ultrasonic velocity propagation through samples). To ensure that the treatment had no negative effects on the physical characteristics of the mortars, we performed microstructural (phase morphology studied by means of scanning electron microscopy) and aesthetic (colour and lightness measured by spectrophotometry) analyses. Of the different dispersions, CaLoSil ® at 5 g/L produced the most significant improvement in the degree of carbonation and in the compactness of the mortar, thanks to the precipitation of small crystals of calcite and aragonite in the pores located between the matrix and the aggregate grains. This product also caused the least significant chromatic changes (slight decrease in lightness and yellowing) and the greatest increase in ultrasonic propagation velocity through the mortar samples. This research has specific application in restoration work that involves consolidation of lime mortars, especially in tropical climates or in confined environments with high humidity levels (such as deep hypogea).