Synchrotron investigations of the nanolime reactivity on biocalcarenite stone surfaces

Highlights

• Synchrotron X-ray diffraction investigations to study the nanolime reactivity in air.

• The reactivity was analysed both in aqueous and in alcoholic nanolime suspensions.

• Nanolime reactivity investigated directly on Agrigento biocalcarenite stones.

• We defined a new parameter called nanolime carbonation rate.

• Study of the influence of several parameters on the carbonation rate.

Abstract

Synchrotron X-ray diffraction investigations were performed to study the reactivity of a nanolime produced by an innovative method based on an ion exchange process, both in aqueous and in alcoholic suspensions. Moreover, for the first time, the nanolime reactivity was investigated both on laboratory glass slides and directly on artistic relevant stones, the biocalcarenite of Agrigento. The results showed that only aqueous suspension exhibits a complete conversion into calcite. The transformation is favoured on porous stone surface, allowing the nanolime suspension to be absorbed and dispersed, increasing the CO₂ diffusion.

Introduction

Lime nanoparticles (Ca(OH)₂ NPs), commonly named nanolime, are used in many applications, ranging from environmental uses [1], [2], catalysis [3], [4], [5] to medicine [6]. However, thanks to the well-known conversion of lime into calcium carbonate (CaCO₃), nanolime is used most in Cultural Heritage conservation, such as in preservation of stuccos, frescoes and architectural surfaces as well as in deacidification of paper and wood [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. CaCO₃ is perfectly compatible with several artistic and architectonic substrates, which have calcite as the main crystalline compound. This makes nanolime particularly suitable for conservative treatments of natural stones and historic mortars. Currently, nanolime suspensions are produced according to several routes, such as by chemical precipitation in diols, water-in-oil micro-emulsions, in aqueous solutions with additives, by solvothermal reactions or by hydro plasma metal reaction [19], [20], [21], [22], [23]. In most cases, the direct product of the synthesis is not only Ca(OH)₂. Therefore, further washing or purification steps are necessary to eliminate secondary products and/or organic compounds. This leads to extended production times and low yields. We developed and patented an innovative one-step process allowing to produce pure and crystalline Ca(OH)₂ nanoparticles dispersed in water, in very reduced times (of the order of a few minutes), without intermediate steps, working at room temperature and with very high yields [24], [25]. The nanolime particles, once synthesized, can be dispersed in alcohol (e.g. ethanol or isopropanol), in water/alcohol mixtures, or in pure water, depending on the required use. Usually, commercial nanolime is applied in the form of alcoholic suspensions to enhance the suspension kinetic stability [19], [26], at relatively low concentration (5–10 g/l), for both pre-consolidation, consolidation and protection niche interventions [27], [28], [29], [30], [31]. According to literature [19], the kinetic stability of nanolime suspensions is defined as the ratio of the optical densities of the saturated Ca(OH)₂ solution respect to the original dispersion. Moreover, as reported in previous studies, at concentrations of 5 g/l, pure alcoholic nanolime suspensions can be generally stable within 16 h, while aqueous suspensions show a stability reduction of about 50% after 20 min [11], [18], [19].

The small particles dimension is at the basis of the potential advantages of nanolime, allowing to overcome the limits of the traditional lime-based treatments and giving rise to a complete conversion of calcium hydroxide into calcium carbonate. Moreover, the reduced particles dimension can guarantee a high penetration depth, avoiding free unreacted particles on the surfaces [32], [33]. However, the carbonation process of the Ca(OH)₂ nanoparticles can be affected by several parameters, such as the suspension concentration, the environmental conditions as well as the employed solvent [34], [35]. Typically, if exposed in the same conditions in air, diluted samples react better than concentrated samples [36], [37]. Both the time needed to complete the carbonation process and the stability of the CaCO₃ polymorphs that form strongly depend on the solvent and on the relative humidity (RH). It was found that, for alcoholic nanolimes, the carbonation process can require long time to complete even for diluted sample, up to 28 days or more [38], [39]. Only when exposed in moist conditions (RH ≥ 75%), the carbonation is completed in about 7 days, but the calcium carbonate crystallizes not only in form of calcite, but also in its polymorphs, vaterite and aragonite [30], [38], [40], [28], [41]. Therefore, the parameters affecting the carbonation process can negatively influence the treatment efficacy. This is not only due to a reduction of the newly formed carbonate network but also to the physical incompatibility with the original calcitic substrates. Recently, hydro-alcoholic or aqueous nanolime dispersions, characterized by a lower kinetic stability, result very promising, because they lead to a complete conversion into calcite and to an increased deposition in-depth. In addition, such dispersions present lower evaporation rates, favouring the CO₂ diffusion into the dispersion. This can promote the carbonation process also in dry conditions [17], [18], [26], [35].

The aim of this paper is to investigate the kinetic of the carbonation reaction and the phase evolution of nanolime suspensions by means of time-resolved synchrotron radiation X-Ray diffraction (SR-XRD). The carbonation rate (C_R) of the nanolime was estimated, taking into account several parameters such as the solvent, the CO₂ flux, the nanolime suspension concentration, the time necessary to complete the carbonation process, and the substrate. Finally, a simple formula was developed, useful to predict the minimum concentration that can completely convert into calcium carbonate in different environmental conditions. Since the investigation focuses on the kinetic process of the CaCO₃ formation, synchrotron X-ray radiation allowed performing reliable analyses in a very short time. The nanolime suspensions used for these measurements were synthesised in the laboratory and first characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). After that, they were prepared to investigate the carbonation reactivity by means of SR-XRD, either inside capillaries or on different substrates, such as non-porous substrates (laboratory glass slides) or natural porous stones. In particular, the nanolime carbonation process was followed, for the first time, also directly on the surface of natural stones in order to evaluate the influence of the substrate on the interaction between nanolime particles, carbon dioxide and solvent evaporation. For this investigation, a sample of biocalcarenite, which is interesting both for its artistic relevance and for the necessity to reduce their decay phenomena, was considered. This porous stone is found in monuments in the UNESCO site of “Valle dei Templi” in Agrigento (Sicily, Italy).