Volcanic tuffs as natural building stones
Mineralogy, technical properties, deterioration and conservation strategies
by Christopher Pötzl
Date of Examination:2020-11-23
Date of issue:2021-11-19
Advisor:Prof. Dr. Siegfried Siegesmund
Referee:Prof. Dr. Siegfried Siegesmund
Referee:Prof. Dr. Rolf Snethlage
Persistent Address:
http://dx.doi.org/10.53846/goediss-8963
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Abstract
English
The present thesis deals with the deterioration and durability of volcanic tuff rocks used as building stones in historical architecture and presents a combined approach on identifying the main parameters responsible for the severe damages that can be observed. A comprehensive study on a dataset of over 500 tuffs from the literature regarding the technical parameters of tuff, in combination with an in-depth study of their petrographical properties, allows for the correlation of individual parameters and to identify key parameters, that influence the weathering behavior of tuff stones. A better evaluation of the weathering behavior of tuff stones can consequently allow for a more precise estimation of their durability. The results demonstrate that pore radii distributions are a good estimator for durability, since micropores (< 0.1 µm) have a particularly strong influence on the weathering behavior of tuffs. Therefore, the important influence of the micropores on salt weathering in tuff rocks is stressed. In addition, the effective porosity and bulk density are identified as reliable estimators to predict the strength (UCS, TS) and durability of tuff rocks (UCS reduction). The hydric expansion of tuff rocks can exceed values multiple times higher than other rock types and is considered to be an important factor for the deterioration of tuffs. It is typically associated with the presence of swellable clay minerals and two types of swelling mechanisms are generally discussed: stepwise intracrystalline swelling and continuous osmotic swelling. A mechanism that can cause expansion in the absence of swellable clay minerals, which is characterized by interaction of surface forces, is the disjoining pressure. The identification of the primary mode of swelling is important for understanding and finally preventing the swelling damage in tuff stones. The swelling experiments show, that intracrystalline swelling is the predominant mechanism for clay swelling in the investigated tuffs. The osmotic swelling on the other hand has only a minor influence on the clay swelling. Therefore, with a clay mineral analyses at hand, the swelling experiments proved to be a useful tool to differentiate between both clay swelling mechanisms in tuff rocks. Also, the importance of the location of the clay minerals in the tuff rock needs to be stressed. We could confirm that even small amounts of swellable clay minerals can cause significant expansion of the material if they are located in critical spots in the rock fabric. The role of the disjoining pressure is still unclear. New analytical techniques have to be developed to quantify its role in moisture expansion of tuff rocks. In addition, the effects of two consolidation agents, tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS), on a larger set of tuffs was evaluated by comparative analyses of petrophysical properties and weathering behavior before and after the treatments. The goal of this approach was to identify a general suitability of the consolidation treatments for different types of tuff. The application of tetramethoxysilane (TMOS) was also conducted with the aim of identifying if this consolidant can be absorbed more efficiently by tuffs with a high share of micropores, due to its smaller molecule size compared to TEOS. The investigation provided a variety of data that indicate, that TMOS may be a suitable candidate to overcome the bottlenecks in the pore space of tuffs, which limit the consolidation success of current products.
Keywords: Volcanic tuffs; Conservation; Material behavior; Building stones; Technical properties
