Partial Wetting in Capillary Liquid Absorption by Limestones

Bioconsolidation of Damaged Construction Calcarenites and Evaluation of the Improvement in Their Petrophysical and Mechanical Properties

Bioconsolidation treatment using bacterial carbonatogenesis has been proposed as an environmentally friendly strategy for the efficient preservation of damaged stones, particularly suitable for carbonate stones. The study presented here deals with the evaluation of the performance of this treatment, applied to damaged carbonate stones in two historical buildings in Spain. The methodology applied in this research serves as a reference for future similar studies. Results showed significant improvement in the petrophysical and mechanical properties of the damaged stone following the treatment through the production of calcite and vaterite by the abundant carbonatogenic bacteria inhabiting the stone. These bacteria were able to effectively consolidate weathered areas if an adequate nutritional solution was employed, thereby augmenting the stone's resistance, as evidenced by the Drilling Resistance Measurement System (DRMS). FESEM images showed calcified bacteria and calcified exopolymeric substances (EPS) consolidating stone minerals without blocking their pores. In addition to consolidation, this biotreatment improves the stone's behavior against water absorption and increases the contact angle of water droplets without significant modifications in the pore size or diminishing vapor permeability. No color changes are observed. Overall, these results show that the application of the nutritional solution (M-3P) for in situ consolidation of different types of porous carbonate building stones is a highly effective conservation method, with no modification of the chemical composition of the treated materials.

Spontaneous Capillary Imbibition of Water and Nonaqueous Liquids into Dry Quarry Limestones

Rates of spontaneous imbibition of water and nonaqueous liquids into dry limestones have been measured at 25 °C. Thirteen English and French limestones were used, with eight liquids (water, decane, dodecane, sec-butanol, iso-propanol, tetrahydrofuran, perfluorodimethylcyclohexane, ethanediol). For the nonaqueous liquids, the measured sorptivity generally scales as (surface-tension/viscosity)$$^{1/2}$$ 1 / 2 (here called F-scaling). Water sorptivities deviate from F-scaling, indicating partial wetting. A wetting coefficient (wetting index) is derived. Data show that there is little difference in the Hirschwald saturation coefficient measured with the different liquids, although there is a large variation between stones. Results suggest that physicochemical alteration of exposed pore surfaces strongly (and unpredictably) influences the capillary absorption of water by limestones.

Alcohol dispersions of calcium hydroxide nanoparticles for stone conservation

Alcohol dispersions of Ca(OH)2 nanoparticles, the so-called nanolimes, are emerging as an effective conservation material for the consolidation of stone, mortars, and plasters present in old masonry and/or mural paintings. To better understand how this treatment operates, to optimize its performance and broaden its applications, here we study the nano and microstructural characteristics, carbonation behavior, and consolidation efficacy of colloidal alcohol dispersions of Ca(OH)2 nanoparticles produced by both homogeneous (commercial nanolime) and heterogeneous phase synthesis (aged slaked lime and carbide lime putties). We observe that the alcohol not only provides a high colloidal stability to Ca(OH)2 particles, but also affects the kinetics of carbonation and CaCO3 polymorph selection. This is due to the pseudomorphic replacement of Ca(OH)2 particles by calcium alkoxides upon reaction with ethanol or 2-propanol. The extent of this replacement reaction depends on Ca(OH)2 size and time. Hydrolysis of alkoxides speeds up the carbonation process and increases the CaCO3 yield. The higher degree of transformation into calcium alkoxide of both the commercial nanolime and the carbide lime fosters metastable vaterite formation, while calcite precipitation is promoted upon carbonation of the aged slaked lime due its lower reactivity, which limits calcium alkoxide formation. A higher consolidation efficacy in terms of strength gain of treated porous stone is achieved in the latter case, despite the fact that the carbonation is much faster and reaches a higher yield in the former ones. Formation of alkoxides, which has been neglected in previous studies, needs to be considered when applying nanolime treatments. These results show that the use Ca(OH)2 nanoparticle dispersions prepared with either aged slaked lime or carbide lime putties is an economical and effective conservation alternative to commercial nanolimes produced by homogeneous phase synthesis. Ultimately, this study contributes to show that nanotechnology can help saving the built and sculptural heritage.

On the role of organic adlayers in the anomalous water sorptivity of Lépine limestone

Sorptivity data are reported for the capillary absorption of water, ethanol, propan-2-ol, and n-heptane by the calcitic limestone Lépine (Lavoux à grain). The data confirm that the water sorptivity is anomalously low, an indication of partial wetting by water. Results are expressed in terms of a wetting index. The water sorptivity increases after heat treatment and chemical oxidation by hydrogen peroxide bleaching, while the sorptivity with organic liquids is unchanged. These treatments, therefore, increase the water wetting index. The results provide strong evidence that the presence of a natural organic adlayer is responsible for the anomalously low water sorptivity of this particular limestone. This natural water repellency effect may be exploited in developing chemical treatments to modify the water transport properties of stone.

Towards a better understanding of tetrachloroethene entry pressure in the matrix of Permo-Triassic sandstones

The pressure required for a chlorinated solvent to enter a geological medium can be calculated given knowledge of the characteristic pore size of the medium and the interfacial tension (IFT) and contact angle of the solvent-water-rock system. Using a centrifuge-based method, capillary pressure-saturation curves have been determined for 30 water-saturated samples of Permo-Triassic sandstones for the solvent tetrachloroethene (PCE). These curves have been successfully fitted using the van Genuchten function to determine PCE entry pressure for each of the sandstone samples. A plot of PCE entry pressures against average pore diameter shows a linear relationship in log-log space; however, observed values for PCE entry pressure are significantly lower than would be expected theoretically for a sandstone-PCE-water system. This may be explained either by a decrease in the IFT or an increase in the contact angle. The IFT may decrease during contact with sandstones due to hysteresis effects during imbibition and drainage of fluids, but this is unlikely to be sufficient to account for the low entry pressures observed. Therefore, it is inferred that the low observed PCE entry pressures are due to higher than expected PCE contact angles, and that the average pore-throat surface of the sandstones is more solvent wetting than would be expected. A weak acid extraction indicates the presence of calcite and dolomite in the sandstone cores, and a correlation is observed between carbonate content per unit porosity and a reduction in PCE entry pressure. It is suggested that these mineral phases are responsible for observed wettability changes and a conceptual model is proposed. One consequence of the lower observed entry pressures is that solvents are likely to penetrate deeper into the matrix of water-saturated sandstones than previously expected.

Effect of Capillary Element Aspect Ratio on the Dynamic Imbibition within Porous Networks

The Washburn equation is widely accepted for describing capillary imbibition. It has, however, been shown to be insufficient at very short times due partly to the lack of inertial terms. Bosanquet (C. H. Bosanquet, Philos. Mag. ser. 645, 525 (1923)) applied an inertial term via momentum, Szekely et al. (J. Szekely, A. W. Neumann, and Y. K. Chang, J. Colloid Interface Sci.35, 273 (1971)) examined single capillaries based on a revised boundary-condition model, and Sorbie et al. (K. S. Sorbie, Y. Z. Wu, and S. R. McDougall, J. Colloid Interface Sci. 289 (1995)) reviewed and applied Szekely's work to examine the effects of comparative imbibition into a parallel pore doublet. The study here extends the work of Sorbie et al. by applying the equation of Bosanquet to a three-dimensional network model, Pore-Cor. All authors agree that, with the inclusion of inertial terms at short times, smaller radius capillaries will initially fill faster than larger radius capillaries which disagrees with the Washburn equation. It is shown that the aspect ratio of a capillary, defined as its length divided by its radius, plays an important role, in combination with the capillary radii themselves, in determining the filling rate of individual elements. The distribution of this ratio associated with the capillary throat elements within a network structure is investigated. The result is that a preferred pathway of permeation is observed under supersource imbibition conditions in the case where a broad size distribution of capillary elements occurs within a network structure.