How ions distribute in a drying porous medium: A simple model

Salt crystallization at surfaces is an important problem for buildings and monuments. We do not consider the formation of salt crystals as such, but focus on transport properties of ions in a drying porous medium. We deal with the first phase of the drying process, where the water is still uniformly distributed throughout the medium. An approximate model is presented, which accounts for both convection and diffusion. It is shown that the key parameter is the Peclet number at the evaporating surface, PehL/D, where h, L, , and D are the drying rate, sample size, porosity, and diffusion constant, respectively. When Pe1 (diffusion dominates over convection) the ions remain uniformly distributed throughout the system. Strong accumulation at the evaporating surface occurs for Pe1 (convection dominates over diffusion). Crossover behavior is found for Pe1. Therefore, it is likely that the first crystals will be formed both in the bulk and at the interfaces of the material when Pe1. For high values of Pe the density peak at the evaporating surface will reach the saturation concentration long before it is reached in the bulk of the material. As a consequence, the salt starts to crystallize at the interfaces

Moisture transport over the brick/mortar interface

The moisture transport in brick, mortar that was cured separately, and combined brick/mortar samples was studied using NMR. The experimental results show that the mortar is less permeable if it is cured bonded to the brick instead of cured separately. Models of the moisture transport are usually formulated on the basis of a diffusion equation. Preliminary simulations of the moisture transport over brick/mortar interfaces roughly correspond with measured moisture contents

New insights into autogenous self-healing with NMR tests

Concrete is a brittle composite cementitious material that easily fractures under tensile loading. Microcracks can appear throughout the concrete prior to application of any load because of temperature-induced strain and autogenous and drying shrinkage. There is no doubt that these cracks provide preferential access for aggressive agents to penetrate into the concrete, probably causing corrosion of reinforcement steel and degradation of concrete. As a result, the service life of reinforced concrete structures is shortened. Fortunately, concrete has the ability to heal the crack itself without manual efforts when water is present in cracks. This ability is defined as autogenous self-healing. However, the effect of migration of water from cracks into the bulk paste on autogenous self-healing is not clear yet. The aim of this study is to investigate the effect of water migration from cracks into the bulk paste on autogenous self-healing. Nuclear magnetic resonance (NMR) technique was utilized to monitor water migration from cracks into the bulk paste during the process of autogenous self-healing. NMR results show that in the beginning of autogenous self-healing the water in the crack migrates into the bulk paste and the water content of the bulk paste increases significantly. However, after 5-hour autogenous self-healing period the amount of non-chemically bound water in the bulk paste (adjacent to the crack surfaces) determined by NMR decreases instead. It indicates that some of the water coming from the crack was used by additional hydration of unhydrated cement particles in the bulk paste during the process of autogenous self-healing. NMR results reflect that most of the reaction products of additional hydration are formed in the bulk paste adjacent to the crack surfaces, rather than in the crack. Because of the additional hydration caused by the water from the crack, the capillary porosity of the bulk paste adjacent to the crack surfaces decreases significantly. Before this work the filling of cracks is the main concern in term of autogenous self-healing. The densification of the cement paste adjacent to the crack surfaces, which was observed for the first time in this study, will also decrease the ingress of aggressive agents into the bulk concrete matrix and prolong the service life of concrete structures. This observation provides a new insight into autogenous self-healing in cement paste

A review of salt transport in porous media : assessment methods and salt reduction treatments

It is an unpalatable fact that while objects can deteriorate through lack of care and attention, they can also deteriorate as a result of inappropriate and misguided interventions. This is particularly the case with regard to salt-related deterioration problems. A successful treatment outcome using aqueous salt reduction methods demands an understanding of the transport processes involved as well as detailed information regarding the characteristics and specific situation of the individual object. The use of poultice materials to reduce the salt content of salt deteriorated objects is a long established technique in conservation. However, due to the complex nature of salt problems within historic structures the result of such interventions can be variable and unpredictable. The amount and depth to which salts are mobilised, and where they are transported to, is dependent on the inter-relationship between the poultice and the substrate, the drying conditions and also the initial salt distribution. This paper examines the current scientific understanding of salt and moisture transport processes, and the extent to which this knowledge can feed back into the practical arena, to aid the conservator. Moreover, areas where further research is required are identified. In particular, the importance of pre- and post treatment investigations is highlighted, showing how, in combination with knowledge of salt and moisture transport mechanisms, these can give useful indications regarding treatment options. The role of selective salt extraction and the post treatment behaviour of residual salts are discussed

Autogenous healing in cementitious materials with superabsorbent polymers quantified by means of NMR

A recent advance in construction technology is the use of self-healing cementitious materials containing synthetic microfibers and superabsorbent polymers. By stimulating autogenous healing by means of superabsorbent polymers, cracks are closed and this will cause an increase in durability and service life. However, this improved healing capacity has not been quantified yet in terms of increased further hydration and volume of healing products. This is needed to model the material and to stimulate the practical application in constructions. This paper provides quantitative data, obtained by an NMR study. Addition of 1m% of selected superabsorbent polymer versus cement to a cementitious material, stimulated further hydration with nearly 40% in comparison with a traditional cementitious material, if 1h water contact per day was allowed. At 90% relative humidity, no healing was observed in reference samples. While the further hydration around a crack in specimens with superabsorbent polymers was still 68% of that of a reference system with cyclic water contact, due to the uptake of moisture by the superabsorbent polymers. As such, NMR results quantify the positive influence of superabsorbent polymers in terms of stimulated autogenous healing and substantiate their benefits for application in the construction area

Moisture transport in concrete during fire: an NMR study

The question how concrete responds to a fire is one of the main questions in fire safety. During a fire, a building material can suddenly be heated up to temperatures well above 1000 oC. At temperatures above 100 oC, water inside the pores will start to boil. Simultaneously, in concrete, but also for example in gypsum, chemically bound water will be released by dehydration of the porous matrix. If the concrete has a low permeability, the vapour pressure inside will increase which can give rise to a sudden (explosive) failure of a material. Numerous heat and mass transfer models have been used to predict the moisture transport and its consequences on the strength and permeability of the concrete. However, these models are only of use if they can be validated. For model validation, quantitative measurements of the evolution of moisture, temperature, and possibly pressure distributions in time are needed. For this purpose, we have developed an NMR setup to measure the moisture transport in heated building materials. This setup makes used of the 1.5 T magnet of a medical MRI scanner, while dedicated gradient coils are used to image the moisture profiles in 1D. The sample is heated with the help of four 100 W halogen lamps, capable of generating a heat flux of 12 kW/m2. The measured combined moisture content and temperature profiles give a unique insight in the moisture transport and dehydration kinetics inside concrete during fire. These measurements give the first quantitative proof for the build-up of a moisture peak due to the vapour pressure build-up. While many moisture transport models predicted the existence of such a peak, these predictions had never been validated with moisture profile measurements during fire tests