Caracterização micro-nanoestrutural e química de argamassas de cimento Portland

Abstract

This work is the result of an experimental research project that aimed to characterize the microstructure, nanostructure and chemical composition of Portland cement mortars and the evaluation of adhesion tensile strength. For the characterization all the available research tools have been used to try to explain some micro and nanostructural aspects of the cementitious material. Among these tools, much emphasis was given in the use of high-resolution scanning electron microscopy (SEM-FEG) with the support of chemical microanalysis by energy-dispersive X-ray spectroscopy (EDS). This approach has enabled a significant research and microstructural characterization of Portland cement mortars to the nanoscale, thus being an essential part to identify the phases present, mainly ettringite, portlandite, calcite and CSH. All these phases were analyzed and characterized on their morphology, chemical compositions and microstructural fabric, on the surfaces of mortar samples and also at ceramic block/mortar and roughcast roughcast mortar/grout coating interfaces. Furthermore, atomic force microscopy (AFM) was used in an unprecedent way for micro-and nanostructural investigation of mortars. All this experimental investigation indicated that the tensile bond strength is greater in mortars containing higher proportions of Portland cement in relation to the amount of fine aggregates. Mortars that had lime in their composition have lower tensile bond strength values after 28 days, compared with mortars without lime in the mix. For the characterization by X- ray diffraction, the sample preparation method of enriching the cementitious material, performed via comminution, by optimizing the fragmentation of the cementitious materials and preserving the aggregate was efficient, because it allowed the detection of minor phases. The X-ray results along with the X-ray fluorescence spectrometry fulfilled the overall chemical and mineralogical characterization of the mortars. Three techniques for sample preparation were used for the mortar samples for SEM-FEG and EDS microanalysis: two conventional methods consisting in the deposition of a gold or carbon layer onto the samples to make them conductive; the third technique was not using any type of coating, and analyze the samples in high and low vacuum SEMs. The coatless technique was used in order to obtain reliable EDS microanalysis, since the gold films tend to shield chemical elements present in the mortar of Portland cement mortar, such as sulfur and carbon itself. Carbon films could generate ambiguous results, when carbon occurs originally in the sample. Thus the three preparation techniques were essential for reliable and quality results. The uncoated technique was used in an unprecedented manner in the characterization of mortars by high-resolution SEMs. Completing the micro and nanostructural investigation, atomic force microscopy was an original method for mortar study, obtaining topographical and 3-dimentional micrographs of features such as groups of ettringite crystals surfacing within pores and skeletal (partially hollow) calcite crystals. Especially, CSH morphologies were observed such as colloform/botryoidal grains and compositions of coalesced spherical nodules, what are typical of amorphous phases.