Nano and Microstructural Characterization of Phases and Interfaces of Portlant Cement Mortar Using High Resolution Microscopy

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1 Journal of Materials Science and Engineering A 7 (9-10) (2017) doi: / / D DAVID PUBLISHING Nano and Microstructural Characterization of Phases and Interfaces of Portlant Cement Mortar Using High Resolution Microscopy Matheus de Faria e Oliveira Barreto 1 and Paulo Roberto Gomes Brandão 2 1. Department of Civil Engineering, University of the State of Minas Gerais, Divinópolis/Minas Gerais , Brazil 2. Department of Mining Engineering, Federal University of Minas Gerais, Belo Horizonte/Minas Gerais , Brazil Abstract: This paper proposes to go a little further on the microstructural characterization of surfaces and interfaces of Portland cement mortars. Using scanning electron microscope high-resolution (SEM-FEG), will be used to clarify the micro and nanometric details of cementitious material in Portland cement mortars. The technique of sample preparation is relatively simple compared to other devices and allows the analysis of samples in a natural state which ensures ideal conditions for the characterization of the material. In photomicrographs using high resolution scanning electron microscope (SEM-FEG), increases of about 80 thousand times with great quality were possible, in this paper, we can see above, photomicrographs with an increase of up to 50 thousand times, which enables the visualization of nanopores in mortar matrix. Photomicrographs made in SEM-FEG dual FIB 3D and SEM-FEG Quanta confirm the predominance of euhedral crystals of ettringite in the block-ceramic mortar, characterized by their morphology of hexagonal crystals acicular interface region, and we can also check the lamellar morphology characteristic of portlandite. Photomicrographs made with the SEM-FEG Dual-FIB and SEM-FEG Quanta have confirmed the predominance of euhedral crystals of ettringite in the block-ceramic mortar interface region, characterized by their morphology of hexagonal acicular crystals. Also, the typical morphologies of portlandite, calcite and CSH (calcium silicate hydrate) have been observed with great detail. Key words: Nano and microstructural characterization, mortar, scanning electron microscopy, interfaces and surfaces, cementitious materials. 1. Introduction The need for research and characterization of materials aimed to the construction industry has been growing due to the need for proper selection of materials, based on the performance of the system under study, as well as the increasingly constant shortage of raw materials and market demand for products with adequate performance and durability. Only the evaluation of mechanical properties such as tensile strength of the mortar is not enough to ensure proper performance of mortar coating systems, especially in Portland cement mortars that have microand nano-structures still little explored. Thus, this Corresponding author: Matheus de Faria e Oliveira Barreto, Ph.D., professor, research fields: civil engineering and civil construction, building materials and cement materials, quality management, planning and certification. paper proposes to contribute on the microstructural characterization of surfaces and interfaces of Portland cement mortars used for coating in construction works. 2. Portland Cement Mortar The mortar used in construction is commonly composed of washed river sand and the natural binders are generally Portland cement and hydrated lime [1]. Examples of cement mortar: mortar roughcast (due to strength gain in the short term) and mortar cemented floors (due to the requirement of mechanical and wear resistance). Examples of cement and lime mortar are: mortar coating (plaster) for roofing and walls (due to the fact of cement mortars, though more resistant, have a worse workability, the addition of lime helps to make them more plastic and

2 242 Nano and Microstructural Characterization of Phases and Interfaces of of smooth finishing). 2.1 Microstructure of Portland Cement Mortars Kinetics of Hydration of Portland Cement The cement paste and concrete themselves are very complex heterogeneous materials with different structures ranging from nanoscale to macroscopic scale. They consist of amorphous and crystalline phases and pores [2]. The mixture of cement particles in water triggers a series of chemical reactions that occur at different speeds and are influenced by each other, leading to physical, chemical and mechanical changes within the system. The products of these reactions stable hydrated compounds adhere to each other giving the cement adhesive and cohesive properties [3]. The microstructure of the hydrated cement-based material consists of: solid phase (essentially C-S-H, CH, Aft, AFm, unhydrated and inert cement particles and aggregates); pores (with different shapes, dimensions and connectivity), and liquid and gas phases (inside the pores). The hydrated C-S-H compounds are the most important binding phase since they establish van der Waals bonds that ensure the agglomeration of solid phases and determine their cohesion and adhesion [4]. 3. Materials and Methods The scanning electron microscopy, high-resolution (SEM-FEG) was used to clarify the micro and nanometric details of cementitious material in Portland cement mortars. Special emphasis was given to the CSH phase of hydrated cement, due to its extremely fine fabric, at the nanoscale, and its predominantly amorphous structure. 3.1 Microstructure of Portland Cement Mortars Equipment Used for the Characterization Two high resolution SEM-FEG instruments belonging to UFMG s Microscopy Center have been used, which are listed below: (a) Scanning electron microscope with FEG (field emission gun), FEI brand, Quanta 200 FEG model, called hereafter as SEM-FEG Quanta, with resolution of 1.6 nm at 30 kv in high vacuum. (b) Dual beam scanning microscope with ion gun and electron FEG, FEI brand, Dual-FIB Quanta 3D model, called hereafter as SEM-FEG Dual-FIB, with resolution (electron beam) of 0.8 nm at 30 kv in high vacuum. All the photomicrographs shown in this work were obtained with SEI (secondary electrons image). 3.2 Preparation of the Samples of Portland Cement Mortar Three panels of masonry were built and mortar was applied over them with roughcast trait (1:3) of Portland cement and washed river sand: on top of these, three different coatings were further applied, one for each panel: (a) a plaster cladding with stroke (1:6) of Portland cement and washed river sand; (b) a mortar coating (1:1:4) of cement, lime and sand; (c) a mortar lining with a stroke (1:2:8) of cement, lime and sand. For sample preparation, approximately 12 cm 12 cm pieces of the panels contain the complete system, ceramic block, roughcast mortar and mortar coatings. After the removal, the 12 cm 12 cm samples were fragmented and reduced to approximately 6.5 cm. Sample fracture was used instead of cutting hard, not to interfere with the sample surfaces. After that, the fragmented samples were prepared for analysis in the two scanning electron microscopes. Three sample preparation techniques were used, regarding the deposition of a conducting film: (ⅰ) 5 nm-thich carbon film ( 1); (ⅱ) 10 nm gold thin layer ( 2). The third technique consisted of not using any kind of coating ( 3), and analyzing the samples under low vacuum in the SEM. This technique was used to obtain unbiased chemical microanalyses by the EDS (energy-dispersive (X-ray) spectrometer), due to the absence of carbon or gold

3 Nano and Microstructural Characterization of Phases and Interfaces of Mortar samples coated with carbon film. check the sizes of the microstructure features, since the photos aspect ratios were preserved. Also, the roughcast mortar-mortar plastering interface is shown in 5, with 10,000 magnifications; in this case the ettringite needles appear neatly. In 6, with a 40,0000 magnification and highh resolution, typical slender prismatic ettringite and euhedral equant calcite are observed. With the same quality, under 17,500, acicular ettringite and anhedral CSH masses are displayed in 7. In both photos, the presence of many nanopores can be seen in the mortar. 2 Mortar samples coated with gold film. 4 Photomicrographs, SEM-FEG Dual-FIB: (carbon coating). ceramicc block-mortar roughcast interface 3 Uncoated mortar samples. films. In some situations these elements could yield biased results (carbon), or mask low atomic number elements present in the sample (gold). Thus the three preparation techniques were essential for reliablee results and quality since they were complementary in the research. 4. Experimental Results In 4 one can very clearly seee the interface of ceramic block-mortar roughcast with an approximate 5,000 magnification. The scale in the figures is used to 5 Photomicrographs, SEM-FEG Dual-FIB: roughcast mortar-mortar plastering interface (carbon coating).

4 244 Nano and Microstructural Characterization of Phases and Interfaces of 6 Photomicrographs, SEM-FEG Dual-FIB: ceramic block-mortarr interface (gold coating). 8 Photomicrograph, SEM-FEG Quanta: top analysiss of carbonated mortar (gold coating). 9 Photomicrograph, SEM-FEG Quanta: EDS microanalysis of the area in 8. 7 Photomicrographs, SEM-FEG Dual-FIB: roughcast mortar (gold coating). In 8, with 50,000 magnifications, the characteristic platelike morphology of portlandite is seen. However, the EDS microanalysis in 9 shows high concentrations of calcium and oxygen, as expected, but also significant amounts of silicon and sulfur. This is probably due to the electron beam volume interaction with unseen phases, possibly located underneath the portlandite layers; these phases would be CSH and ettringite. The characteristic lamellar morphology of portlandite is seen in 10, with 50,0000 magnifications. 10 Photomicrograph, SEM-FEG Quanta: top analysiss of carbonated mortar (gold coating).

5 Nano and Microstructural Characterization of Phases and Interfaces of 245 In addition, the relatively large prism on the upper part of the photo is probably calcite. 5. Conclusions Using high-resolution scanning electron microscope (SEM-FEG) proved extremely important in the characterization of cementitious materials, as in the case of this study, Portland cement mortars. The technique of sample preparation is relatively simple compared to other methods and allows the analysis of samples in a natural state which ensures ideal conditions for the characterization of the material. Moreover, the quality of the photomicrographs is substantially better than with the conventional SEMs, which combined with the chemical microanalysis by EDS, allows a substantial advance in the research and characterization of cementitious materials. In observations using high resolution scanning electron microscope (SEM-FEG), magnifications of about 80,000 with great quality have been possible, in this work. Even the visualization of nanopores in mortar matrix could be done, as proved in some cases. Photomicrographs made with the SEM-FEG Dual-FIB and SEM-FEG Quanta have confirmed the predominance of euhedral crystals of ettringite in the block-ceramic mortar interface region, characterized by their morphology of hexagonal acicular crystals. Also, the typical morphologies of portlandite, calcite and CSH have been observed with great detail and quality in other samples of the mortars studied. Acknowledgments The authors are grateful to the following institutions and offices: the UFMG (Federal University of Minas Gerais), the School of Engineering and the Post-Graduate Program in Metallurgical, Materials and Mining Engineering, and to CAPES/PROEX for financial support and the scholarship to Barretom, M. F. O. The co-author Brandão, P. R. G. also acknowledges CNPQ for a research grant. References [1] FIORITO, A. J. S. I Manual de argamassas e revestimentos; estudos e procedimentos de execução. 1.ed. São Paulo: Editora Pini, 221. [2] DAVYDOV, Denis Visco-elasto-plastické vlastnosti cementové pasty. Ph.D. thesis, Ceské Vysoké Ucení Technické V Praze, Praha, leden, 196. [3] COSTA, C., ALMEIDA DE CARVALHO, P., BORDADO, J. C., and NUNES, A Estudo de Argamassas de Base Cimentícia por Microscopia Electrónica de Varrimento. Ciência & Tecnologia dos Materiais 21: [4] BARRETO, M. F. O Caracterização micro-nanoestrutural e química de argamassas de cimento Portland, Tese de Doutorado, Belo Horizonte-Brasil, 248.