Moisture and ion transport in layered porous building materials : a nuclear magnetic resonance study

Moisture and salt decay processes are amongst the most recurrent causes of damage of built objects and monuments. Although salt damage has been intensively investigated for several decades, the mechanisms and factors that control salt transport, accumulation, and crystallization in porous media and the development of damage are poorly understood. Knowledge about the transport of water and ions and salt crystallization in masonry (in the thesis referred to as substrate) and plasters is needed to explain salt damage and to develop durable materials. In chapter 2 the moisture and ion transport in porous media is reviewed brie°y. The drying behavior of a material is determined by the capillary pressure, and therefore by pore-size distribution. As a consequence the largest pores are emptied ¯rst. In layered systems of di®erent materials the material with larger pores should dry ¯rst. Ions are transported via di®usion and advection with the °ow of the water. These processes are described by a di®usion-advection equation. For high drying rates advection dominates the ion transport, whereas for low drying rates di®usion dominates the ion transport. Chapter 3 reviews the Nuclear Magnetic Resonance (NMR) technique and the ex- perimental set-up, that is used for the investigations described in this thesis. NMR is a non-destructive technique, suitable for probing the transport of water and dissolved ions in porous materials. The time evolution of the hydrogen pro¯les and the pro¯les of dissolved sodium can be measured quasi-simultaneously, which enables monitoring the moisture and salt transport during drying of (layered) porous media. The spatial resolu- tion of the NMR measurements is of the order of one millimeter, which is very accurate compared to conventional destructive techniques (hygroscopic moisture content analysis, chemical methods). It is outlined that using a stimulated echo NMR pulse sequence the self-di®usion of water in porous materials can be measured up to di®usion times exceeding 1 s. In chapter 4 the moisture and salt transport in a two-layer plaster/substrate system is discussed. It is shown experimentally that moisture and salt transport in the plaster layer is in°uenced by the pore-size distribution of the underlying substrate. The same plaster was applied on a Bentheimer sandstone substrate and a calcium-silicate brick sub- strate. In case of the plaster/Bentheimer sandstone system the plaster has pores that are an order of magnitude smaller than those of the substrate. On the other hand, in case of the the plaster/calcium-silicate brick system the plaster has pores that are an order of magnitude larger than the smallest pores of the substrate. The moisture and salt transport and the salt accumulation di®ers signi¯cantly for these two systems. Dur- ing the drying process, air invades ¯rst the layer with the largest pores. Therefore, in the plaster/Bentheimer sandstone system the substrate dries before the plaster, while in the plaster/calcium-silicate brick system the substrate stays wet longer than the plaster. This has important implications for the transport and accumulation of salt. In the plas- ter/Bentheimer sandstone system (the pores of the plaster are smaller than those of the substrate) all salt is removed from the substrate and accumulates in the plaster, because this layer remains wet for a longer time than the substrate. In the plaster/calcium-silicate brick system (the substrate has smaller pores than the plaster) some salt crystallizes in the plaster layer, but a signi¯cant amount of salt crystallizes within the substrate itself. The e±ciency of the salt transport from substrate to plaster is quanti¯ed in terms of an e±ciency number ², which can be estimated in di®erent ways. It is a simple, practical number, useful for the choice of a compatible plaster for a particular substrate. Based on the results of this study, we have discussed the salt transport for various combinations of plasters and substrates, of which the pore-size distributions have been reported in the literature. In chapter 5 the question is addressed whether a plaster/substrate system which has two plaster layers with di®erent pore sizes, can act as an accumulating system, i.e., a system in which salt crystallizes in the plaster layer and not in the substrate or at the external surface. To this end we have studied the drying behavior of such plasters applied on two di®erent substrates: ¯red-clay brick and Bentheimer sandstone. On the substrate we applied two plaster layers, of which the ¯rst (base) plaster layer has pores that are signi¯cantly smaller than the pores of the second (external) plaster layer. These systems were either saturated with pure water or a NaCl solution. We also studied non-uniformly salt loaded systems, in which the two plaster layers were saturated with pure water, and the substrate with a NaCl solution. The moisture and salt transport during drying were investigated. Among the investigated systems, it is found that only a system consisting of an external gypsum layer and a base lime-cement plaster layer on a Bentheimer sandstone substrate behaves as a salt accumulating system when it is non-uniformly salt loaded. We conclude that it is di±cult to make an accumulating system only based on di®erences in the pore-size distributions of the individual layers, because in practice it is di±cult to prepare plasters with exactly speci¯ed pore-size distributions, in particular, plasters with pores which are larger than the pores of the substrate. Chapter 6 describes NMR measurements of the self-di®usion of water in porous ma- terials containing a large amount of magnetic impurities. These impurities (Fe), which are often present in building materials, hinder the di®usivity measurements by NMR. To investigate the self-di®usion of water in these materials, a stimulated echo NMR technique is applied. A new analytical equation for the long-time stimulated-echo decay in the pres- ence of spatially varying internal ¯eld gradients is derived and experimentally con¯rmed. This equation is used to interpret our NMR di®usion measurements on three materi- als with di®erent amounts of magnetic impurities. It is shown that, using this method, the self-di®usion constant of water in these materials can be determined correctly. The method presented in this chapter may be useful for di®usivity measurements in many classes of materials in which internal magnetic ¯elds interfere with the applied gradients (i.e. susceptibility induced ¯elds in natural stones, inorganic and biologic materials). Verwering ten gevolge van vocht en zout is een van de meest voorkomende oorzaken van schade aan bouwconstructies en monumenten. Hoewel al tientallen jaren lang veel onderzoek is verricht naar zoutschade, zijn de mechanismen en factoren die van invloed zijn op zouttransport, ophoping van zout en kristallisatie nog steeds slecht begrepen. Inzicht in het transport van water en ionen en zoutkistallisatie in metselwerk (in dit proefschrift