Autor(a/res): Oscar A. Mendoza Reales, Pedro Duda, Emílio C.C.M. Silva, Maria D.M. Paiva, Romildo Dias Toledo Filho

Resumo: This work presents a comparative study of the effects of nanosilica particles on the fresh state properties of a Portland cement paste designed for 3D printing. Cement pastes were prepared with different solid substitutions of cement by pozzolanic particles and their yield stress was measured after different resting times. Rheological results were used to obtain the rate of thixotropic buildup of each paste, which was used to compute the parameters of an ideal 3D printing process (layer height, time to extrude one layer, and horizontal velocity). Results obtained for nanosilica were compared with results from other three commonly used pozzolanic nano and micro particles (microsilica, metakaolin and nanoclay). All pozzolanic particles studied were capable of increasing both the initial yield stress and the rate of thixotropic buildup of cement paste through different mechanisms; nevertheless, these increases did not translate directly into a more efficient 3D printing process due to the limitations imposed by the need to avoid cold joints between layers. It was concluded that even though nanosilica particles are more efficient thickeners for cement paste than the other particles studied, their effect is constrained by the maximum and minimum printing velocities. Additional chemical accelerators should be considered to reach the full potential of nanosilica as a structural buildup agent.

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Autor(a/res):  Giuseppe Ferrara, Marco Pepe, Enzo Martinelli, Romildo Dias Tolêdo Filho 

Resumo: The increasing attention toward environmental aspects has led, also in the sector of construction materials, to the need for developing more eco-friendly solutions. Among several options, the employment of low energy raw materials appears as an efficient solution intended to enhance the sustainability of building structures. One of the applications moving in this direction is the use of plant fibers as a reinforcement in cement-based composites, hence named as natural textile reinforced mortar (NTRM) composites. Although representing a promising technique, there are still several open issues concerning the variability of plant fibers properties, the durability, and the mechanical compatibility with the mortar. This study aims at investigating the influence of an impregnation process on the thread’s morphology and on the mechanical response. Therefore, the geometry of dry and impregnated flax threads is identified by using scanning electron microscope (SEM) images analysis, and their mechanical response in tension is assessed. In addition, the fibers-to-mortar bond behavior is investigated by means of pull-out tests. The proposed results show that the impregnation procedure employed, although not improving the fibers-to matrix bond, leads to a standardisation of the threads morphology and reduces the thread’s deformability in tension, and paves the way for further investigations on a larger scale.

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Autor(a/res): Saulo Rocha Ferreira, Lisiane Nunes Hugen, Renata Daniel dos Santos

Resumo: Hornification is phenomenon that can be promoted by wetting and drying treatment, which modify the microstructure of the lignocellulosic fibers, by promoting the stiffening of the polymeric structure of the fibercells. This process increases fiber mechanical behaviour and reduces water absorption and dimensional variation. The objective of this research is to investigate the influence of several variables on hornification process such as wetting time, fiber/water ratio and the number of wetting and drying cycles on the treatment and its effect on fibers chemical and mechanical properties. Sisal fibers were used. A pre-treatment of 1 h in water (T = 80 °C) and drying at 40°C was applied. Treatments with 1 and 5 cycles of wet and drying were also studied. In both cases two wetting time was applied (1 h and 3 h at 22 °C) and two fiber/water ratio (1:10 and 1:40). The raw and treated fibers were evaluated by thermogravimetric analysis, infrared spectroscopy, wettability analysis, and tensile test. The results showed that lower fiber/water ratios provide better hornification in sisal fibers. Immersion times of 3 hours was efficient to improve tensile strength, increasing stiffness in all studied cases.

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Autor(a/res): Josep Claramunt, Heura Ventura, Romildo D. Toledo Filho e Mònica Ardanuy.

Resumo: This paper analyses the influence of the addition of low content (0.1 to 0.8 wt%) of nanofibrillated cellulose (NFC) or cellulose nanocrystals (CNC) on the flexural performance, durability and microstructure of calcium aluminate cement (CAC) cured at 20 °C or 60 °C. The relationship between the mechanical properties and the microstructure of the cement was evaluated after curing and further after accelerated aging by flexural testing and X-ray diffraction and by backscattered electron imaging, respectively. The addition of 0.1–0.2 wt% of either nanocellulose led to an increase in the modulus of rupture (MOR) and modulus of elasticity (MOE) values. Moreover, CNC was effective to counteract the negative effects of increased porosity of CAC cements before and after aging, significantly improving their mechanical performance and durability. This effect was not observed in Portland cement systems, in which the addition of this low nanocellulose content did not lead to significant changes.

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