Determination of the Salt Distribution in the Lime-Based Mortar Samples Using XRF and SEM-EDX Characterization

Abstract

Although, the salt crystallization is one of the most common causes of the deterioration of lime-based mortars, testing of their resistance to the soluble salt action has not yet been standardized. The problems following the development of the globally accepted testing method are, among others: defining the type of mortar samples, ways of samples’ contamination, the type and the concentration of the salt solutions used, environmental conditions during testing, determination of the damage development and the durability assessment. Another task of the testing method is to explain and connect the processes developing in the materials when they are applied in laboratory and real conditions. In this paper, soluble salt resistance testing of lime mortars on the composite samples is presented. The main focus of the paper is on the determination of the salt distribution in this type of samples after the five wetting and drying cycles. Samples consisted of two lime rendering layers (inner – 1/3 and outer – 1/1), both prepared according to the experiences found in the literature for these types of lime mortars when applied on historical structures, placed on the natural stone bases. They were cured in laboratory conditions for 90 days, before drying and exposing to soluble salts action. Two types of 10% salt solutions were used for the test: sodium-chloride and sodium-sulfate. Salt contamination was performed by capillary action only in the first cycle, while in the other cycles samples were wetted by deionized water. After the finalization of the cycles, the detached pieces of mortar and efflorescence were removed from the samples. One of the samples from both groups were then cut in two halves, from which one was used for X-Ray Fluorescence (XRF) and another for Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM – EDX) characterization. For the XRF analysis samples were divided into four layers, and then crushed and sieved through 0.5 mm sieve before testing. For the SEM-EDX analysis the polished thick cross sections were prepared. The paper presents the results of these two analyses, and discusses the advantages and disadvantages of their application for this purpose. Mineralogical analysis of the samples was performed using XRD analysis. It was shown that XRF analysis provides more precise quantification of the elements within one sample, while SEM-EDX analysis gives possibilities for testing of layers with smaller depth within one cross – section.