Correlation and matching

The rankings are correlated and show the “key parameters” for the selection of an appropriate replacement material. The ranking of the parameters in terms of material behavior and deterioration impact (Fig. 9.6) and their correlation, indicate the relevance for replacement criteria of potential building stones. The material behavior of the Drachenfels trachyte is determined by fabric and pore space parameters as well as moisture properties especially pore size distribution, porosity and matrix. In terms of deterioration besides the mentioned parameters also critical mineral content, i.e. clay mineral content, mesostasis, and capillary as well as water uptake by adsorption and moisture dilatation become more

pronounced. These eight parameters characterize the Drachenfels trachyte and are significant for the behavior of the stone in terms of extrinsic impact and decay. These are the key parameters a replacement stone for the Drachenfels trachyte should match and they should be within the mentioned limit values (see chapter 9.2).

The last step of recognition & measurement is to match the key parameters with those of the planned or historic replacement stone. In terms of restoration and conservation any import of potentially harmful substances by a new material, e.g. critical mineral components in a replacement stone has to be avoided. Furthermore, the optical properties of the replacement stone should be similar to the original material considering aging and patination. In respect to petrophysical criteria, the replacement stone for the Drachenfels trachyte should have a comparable PSD and porosity. A stone with a homogeneous matrix, well cemented without possible inherited “weak spots” and without critical mineral content would be preferable.

Moisture dilatation should not be pronounced and water uptake – capillary as well as by vapor adsorption – should be low. Generally the s-value should be less than 0.75. Although strength and thermal properties play a minor role in deterioration processes in the Drachenfels trachyte, the replacement stone should be in a range of 80-120% of the strength values (Snethlage 2005) and thermal dilatation should be less than the original stone.

For the matching those stones are of interest, which are placed in a masonry bond together with other stones, i.e. adjacent to other natural building stones. Within the present example of the Drachenfels trachyte at Cologne cathedral, these would be the Obernkirchen sandstone and the Krensheim Muschelkalk as well as Londorf basalt lava and Schlaitdorf sandstone. In many areas at the towers and the choir of Cologne cathedral the Drachenfels trachyte is employed in masonry bonds together with the Obernkirchen sandstone and the Krensheim Muschelkalk (Fig. 4.1 and 4.2).

In geological terms these three stones belong to different classes. The Obernkirchen sandstone with a concentration of 98 % monocrystalline quartz is a very deterioration resistant quartz arenite (Graue et al. 2011), thus implying no harmful components for adjacent stones. Comparing their patination, both stones tend to show a grey surface of similar brightness. Porosity of the Obernkirchen sandstone (18.6 %) is higher than that of Drachenfels trachyte (11.9 %). The Obernkirchen sandstone shows fewer micropores (Fig.

5.3) and thus is not as sensitive to water absorption, higher water vapor diffusion resistance and moisture dilatation (Tab. 5.1 and Tab. 5.2; Fig. 9.2 and 9.3). Capillary water uptake is slightly raised in the Obernkirchen sandstone (1.26 kg/m²√h) but the saturation coefficient (0.64) is significantly lower than in the Drachenfels trachyte (Fig. 9.1). Drying is less retarded in the Obernkirchen sandstone than in the Drachenfels trachyte (Kraus 1985a; Fig. 6.1).

Uniaxial compressive strength of the Obernkirchen sandstone is within the mentioned

%) are slightly higher (Fig. 9.4). Thermal dilatation is pronounced in the Obernkirchen sandstone, but without any residual strain (Tab. 5.2). In respect to the key parameters of the Drachenfels trachyte for a replacement stone, the Obernkirchen sandstone shows a relatively good matching (Fig. 9.7).

The Krensheim Muschelkalk itself is a relatively weathering resistant natural building stone.

In rain protected areas it tends to form massive gypsum crusts (Graue et al. 2013a) (Fig.

4.5f). In these areas the stone surface is black next to very white microkarst weathered surface areas. Thus, patination differs from that at the Drachenfels trachyte. Due to acid rain dissolution of the carbonate rock ion loaded waters may be transported from the Krensheim Muschelkalk to the silicate Drachenfels trachyte providing ions for salt formation and decreasing the pH, thus contributing to stronger decay of the Drachenfels trachyte (Graue et al. 2013a; see chapter 8.4.3). Porosity of the Krensheim Muschelkalk (16 %) is higher than that of the Drachenfels trachyte (11.9 %). The ratio of micropores is the same at the Krensheim Muschelkalk and the Drachenfels trachyte (Fig. 5.3). Krensheim Muschelkalk shows a medium capillary water uptake (1.3 kg/m²√h), very low water absorption (Fig. 9.2) and a low saturation coefficient (0.59) (Fig. 9.1). Water vapor diffusion resistance in the Krensheim Muschelkalk is high (Fig. 9.2) but drying is less retarded than in the Drachenfels trachyte (Kraus 1985a; Fig. 6.1). Moisture and thermal dilatation of the Krensheim Muschelkalk is neglectable (Tab. 5.2 and 9.3). Uniaxial compressive strength of the Krensheim Muschelkalk is less (69 %) than the mentioned constraints (Snethlage 2005) of 80-120 %, flexural strength (145 %) and tensile strength (129 %) are higher (Fig. 9.4). The matching of the key parameters of the Drachenfels trachyte with the parameters of the Krensheim Muschelkalk indicates a partly compliance in terms of constraints (Fig. 09.7). The aspect of calcium ion transport from the Krensheim Muschelkalk at an building exposition for sufficient water impact and a higher moisture import from the Krensheim Muschelkalk to the Drachenfels trachyte (see chapter 8.4.3) must be seen critical.

The current replacement stone for the Drachenfels trachyte at Cologne cathedral is the Montemerlo trachyte from Italy. If the two stones are compared in respect of the mentioned constraints, it is to ascertain that the mineralogical composition and optical properties match almost perfectly. The porosity of both is similar (Tab. 5.1); the pore size distribution shows a higher ratio of micropores in the Montemerlo trachyte (37%). In the Drachenfels trachyte the ratio of micro to capillary pores is 16:84 (Graue et al. 2011). Moisture dilatation is slightly pronounced (Tab. 5.2); capillary water uptake is higher but water absorption and saturation coefficients are lower (Tab. 5.1). In terms of strength properties the Montemerlo trachyte is a slightly stronger stone, on average 112%, which is in the range of constraints. Thermal dilatation of the Montemerlo trachyte is comparable to the Drachenfels stone (Tab. 5.2), as well is drying (Fig. 6.1).

DT key parameter OS KM MT

matrix + + +

pore size distribution + + -

porosity - - +

critical mineral content + - -

meso stasis + - +

capillary water uptake - - -

moisture expansion + + -

sorptive water uptake + + +

positive counts 6 of 8 4 of 8 4 of 8

Figure 9.7 Matching of the key parameters of the Drachenfels trachyte (DT) with those of Obernkirchen sandstone (OS), Krensheim Muschelkalk (KM) and Montemerlo trachyte (MT) within the mentioned constraints.

In general, the parameters of the Drachenfels and Montemerlo trachyte are in a close comparability to the key parameters of the Drachenfels trachyte. The higher ratio of micropores, the higher capillary water uptake and the slightly pronounced moisture dilatation can be critical. In resemblance to the observation by Lazzarini et al. (2008), the Montemerlo trachyte shows little resistance to salt deterioration experiments. It was the first of the eight investigated stones losing 50% of its weight after 19 cycles; Drachenfels trachyte is the second after 30 cycles (Fig. 6.7). However, Koch (2006) reported of possible clay mineral content. This would explain the higher moisture dilatation and the high cation exchange capacity observed in the leaching tests (Fig. 7.11i and j). These aspects might not necessarily imply a negative influence on the deterioration behavior of the Drachenfels trachyte, but should be looked at in terms of the deterioration behavior of the Montemerlo trachyte.