Chemical deterioration

In general, the impact of water, inorganic and organic acids as well as atmospheric gases (CO2 and O2) on rock-forming minerals is viewed as chemical weathering. Through the so called “carbonation weathering”, minerals are structurally changed or else dissolved completely (Press and Siever 2003). In comparison to the carbonation weathering of rocks and soils through CO₂ respectively, HCO₃^- diluted in water, the weathering within the built environment is accelerated by anthropogenic components. Through the combustion of fossil fuels, atmospheric gases such as SO₂ and NO_x components are released and enriched in the atmosphere. These react with rain water or the moisture content within the stones and form aggressive acids, which strongly contribute to the chemical weathering. This so called

“Rauchgasverwitterung“ – or weathering due to the atmospheric acidity (Camuffo 1992) - displays a specific chemical attack in building stones in the form of hydrolysis (Herscovici 1910; Kaiser 1910; Luckat 1984; Kraus 1988). Proton sources are the anthropogenic caused atmospheric gases, such as SOx and NOx, forming acids (White 2003). The annual H⁺ input from sulfuric and nitric/nitrate oxides in Germany is about 3.5 Kg/ha (Pleßow et al. 1997).

Furthermore, particulate matter plays an important role in chemical deterioration behavior.

Their function as catalysts or reaction nuclei is described (Charola and Ware 2002). Schäfer (1980) ascertained a four to eight times higher oxidation of SO2 to SO4 in urban areas in comparison with rural environments. In all these processes, water functions as a reactant as well as a transport medium for solutes and particles (Schlabach 2000). The pore space of the stone offers access for the chemical deterioration to the stone’s matrix and minerals. As mentioned before, when the pore space is enlarged through other weathering impacts, the reaction surface for chemical weathering increases.

The different rock-forming minerals react differently to the weathering attack. Minerals containing iron, manganese or sulfur in lower oxidation states produce higher oxidation states of these through the oxygen of the water (Kraus 1988). Carbonate rocks, i.e., minerals such as calcite and dolomite, are prone to carbonation weathering as well as to

“Rauchgasverwitterung”, and generally dissolve. Silicate minerals are less likely to be affected by “Rauchgasverwitterung” (Kraus 1988). The effective acids of the atmospheric deposition are buffered by acidification and hydrolysis reactions of silicates which change the silicate structure (White 2003). The resistivity of the minerals against hydrolysis differs significantly. Olivine shows little resistance, while augite, hornblende, biotite and the various feldspars are more resistant, while muscovite and quartz are barely degrading (Fig. 1.2).

Clay minerals are often formed as secondary products from the water containing silicon-oxide-relicts (Kraus 1988).

Chemical weathering of carbonate rocks

The weathering of carbonate rocks is so-called solution decomposition. Carbonate components i.e. calcite and dolomite, dissolve and are transported in their ionic form. Typical secondary reaction products are not formed, as it is with silicates (Colman and Dethier 1986). The solution of carbonate rocks in a CaCO3 – MgCO3-system is a stoichiometric solution (Wollast 1990). The presence of acidic components through the anthropogenic impact enhances the solution decomposition. Reaction (1) shows the dissolution of calcite within carbonic acid (Okrusch and Matthes 2009).

CaCO₃ + H₂ CO₃ ⇌ 2HCO₃^- + Ca²⁺ (1)

Chemical weathering of silicate rocks

Silicate minerals mainly weather by hydrolysis reactions that consume reactant species – i.e., primary minerals and protons. Weathering products are formed – i.e., solute species and secondary minerals (White 2003). The main representative of the rock-forming minerals of silicate rocks is the feldspar group. Feldspars deteriorate to water-bearing minerals (clay minerals), which may lead to textural impairments within the stone’s structure (Press and Siever 2003). Reaction (2) shows the hydrolysis of albite (White 2003).

2Na[AlSi₃O₈] + 2H⁺ + H₂O → Al₂[(OH)₄/Si₂O₅] + 4SiO₂ + 2Na⁺ (2)

kaolinite

Silicate weathering is commonly viewed as a ligand exchange process with the metal ions bonded in the mineral structure (Loughnan 1969; White 2003).

Kaiser (1910a) reported the kaolinization of plagioclase in the matrix of Drachenfels trachyte by a hydrolysis reaction through the “Rauchgasverwitterung”. In the experiments conducted with a gas mixture of 10% vol. SO₂, 10% vol. CO₂ und 80% vol. air, the formation of thenardite (Na2SO4) and gypsum (CaSO4 • 2 H2O) was detected, indicating the release of sodium and calcium.

The experiments performed by Correns and von Engelhardt (1938) showed that the K-feldspar (adularia) does not dilute stoichiometrically. At the beginning, a higher potassium concentration rather than aluminum and silicon concentrations is detected. Potassium is released more easily from the outer zone of the mineral. This leads to the formation of a K–

depleted “residual layer” on the surface of the adularia.

Chou and Wollast (1984) investigated the dissolution of albite in different pH. The detected concentrations of sodium, aluminum and silicon suggest the formation of a residual layer on the surface of the feldspar, enriched in Si and Al.

Efes and Lühr (1975) deduce the concentration decrease of SiO₂, CaO, Na₂O and K₂O to the dissolution for feldspars within the weathering horizons of the Drachenfels trachyte.

Mineral weather resistivity

The weather resistivity of rock-forming minerals is crucial in assessing the weather resistivity of natural stone in the anthropogenic environment. Goldich (1938) observed that the weathering sequence for common igneous rocks in the field was the reverse of Bowen’s reaction series, which ranked minerals in the order of crystallization from magma.

increasing weather resistivity

Calcite Olivine Anorthite Augite Pyroxene Amphibole Albite Biotite Orthoclase Muscovite Clay minerals Quartz

Figure 1.2 Mineral weather resistivity (after Press and Siever 2003)

At the lower end of the scale of weather resistivity is calcite, which shows dissolution in chemical weathering. Olivine shows little resistance, followed by anorthite and then Ca-plagioclase. Weather resistivity increases with augite, pyroxene, hornblende, albite – the Na-plagioclase – and biotite. The most weather resistant of the feldspars is the K-feldspar orthoclase. Muscovite and quartz are barely degrading (Fig. 1.2). For clay minerals, the scale of weather resistivity is not to define, since all conversion reactions are reversible. Clay mineral weathering takes place as a Me⁺ – H⁺ exchange towards montmorillonite and, if leaching is extensive, to kaolinite (Loughnan 1969, Snethlage 1984)

Solute composition, fluid flux, and secondary reaction products

In natural weathering, hydrolyses and other chemical reactions take place. These are mainly complexing or chelating reactions, especially in the presence of organics (White 2003). As mentioned earlier, weathering is influenced by intrinsic and extrinsic factors. In terms of mineral weathering rates, the solute composition has the most direct impact (White 2003).

Chemical weathering is ultimately dependent upon the concentration of reactants complexing and detaching the oxygen-bonded metal species from the silicate structure (Casey and Ludwig 1995). Principally, these are hydrogen ions, but complexing agents such as organic

anions can also participate in these processes. In contrast, some solute species, such as aluminum and sodium ions, inhibit experimental weathering rates by interfering and competing with the ligand exchange processes (Oelkers and Schott 1995; Stillings et al.

1996).

Within the pore space of natural building stones, weathering rates are controlled by the mechanisms of moisture transport. For structurally complex minerals undergoing incongruent or stepwise weathering in the natural environment, the relative rates become highly dependent upon specific reaction pathways (White 2003). White et al. (2001) have coupled the weathering rates of granite with the development of secondary permeability. At an initial state of the weathering of fresh granite, the weathering rate of plagioclases is mainly controlled by the low permeability; thus only a little water is transported, which constraints the fluid flux. Thus, the mass of feldspar that can be dissolved is restricted before thermodynamic equilibrium. Under such conditions, weathering is limited by the availability of water and not by the kinetic rate of feldspar weathering. Over time, this transport-limited weathering will lead to a mass loss from the granite with increasing porosity. White (2003) estimates a porosity increase of ~ 50% due to the conversion of plagioclase into kaolinite.

The increase of porosity produces higher pore-water flow – i.e., fluid fluxes – which accelerates saturation-limited weathering – “this coupled feedback accelerates plagioclase weathering, which gradually shifts from a transport limited to a kinetic limited reaction” (White 2003, 157). The increasing porosity might be impaired by a certain decrease of permeability due to secondary mineral formation. The rate of K-feldspar weathering shows a comparable transition from transport to kinetic control, but at significant higher flux ratios due to its lower solubility rather than slightly slower reaction kinetics. Concurrent plagioclase dissolution enhances this effect by producing solutes, principally silicon, which further suppress K-feldspar dissolution by increasing the saturation state (White 2003).

Surface reactivity may also be decreased by secondary coatings, i.e., the occlusion by secondary clays and iron and aluminum oxides, the formation of depleted leached layers and the adsorption of organic compounds (Banfield and Barker 1994; Nugent et al. 1998).

In general, clay minerals are the secondary reaction products of feldspar weathering. Very often, mixed-layer minerals are formed with layered structures of illite and montmorillonite with transition to pure swellable montmorillonite (Okrusch and Matthes 2009).