PSI - Issue 47

Andrea Zanichelli et al. / Procedia Structural Integrity 47 (2023) 37–42 Zanichelli et al./ Structural Integrity Procedia 00 (2023) 000–000

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resistance and durability of the cement paste and to provide a cement paste with improved thermal performance (Jinchang and Yeming (2018)). The mechanism, by which a very low percentage of graphene or its derivatives in cement-based composites is effective to enhance the mechanical/fracture properties, is not fully understood yet (Wang et al. (2019)). For example, Lin et al. (2016) investigated the effect of GO on the hydration of cement by means of XRD and FTIR analyses. It was found that GO played a catalytic role in the cement hydration, since the oxygen functional groups provided adsorption sites for water and cement, and the water molecules on GO constituted both a reservoir and transport channels, used to further hydrate the cement. Lv et al. (2014) found that the addition of GO to cement paste could promote the formation of rod-like crystals as well as their assembly into (a) flower-like planar clusters at low percentages of GO or (b) polyhedral and lamellar crystals at high percentages of GO. Some research works reported that GO had no effect on the cement hydration process. For example, Pan et al. (2015) ascribed the improvement of the mechanical properties of an ordinary Portland cement paste, when GO was added, to the following aspects: (1) improvement of the mechanical interlocking provided by wrinkled morphology, (2) interaction with cracks, (3) promotion of the hydration process and the formation of strong interfacial forces due to the chemical reaction among carboxylic groups and hydration products. In the present paper, the flexural and compressive strengths and the fracture toughness, as well as the microstructure, of a mortar reinforced with 0.03% of GO are experimentally investigated in order to evaluate the GO effect on both the mechanical/fracture performance and the hydration products morphology. Plain mortar is also considered as a reference. 2. Materials and mixture The raw materials employed for the preparation of the nanocomposite specimens are: graphite, cement, aggregates and superplasticizer. A high pure graphite powder, characterized by an average size of 66 m µ , is oxidized to obtain GO by using the Brodie’s method (Gaboardi et al. (2020)). More precisely, 5 g of graphite powder are mixed with 40 g of NaClO 3 (1:8 wt ratio) by using a three-necked round-bottomed flask, surrounded by an ice bath. Then, 50 ml of HNO 3 are added, by keeping the mixture under continuous stirring. Finally, the suspension is: heated at 60 C ° and cooled; diluted in type-1 purified water and filtered; suspended in a 3 M solution of HCl 37% , filtered, and washed; dried at 60 C ° . Successively, in order to prepare an aqueous solution, 0.03% of GO is added to 42% of deionized water and 0.3% of superplasticizer, being such percentages expressed with respect to the weight of cement. The cement is a limestone Portland cement (42.5 R CEM II/A-LL type), complying with the UNI 9156 (2015) and the UNI EN 197-1 (2011) recommendations. The aggregates consist of a commercial silica sand with grain size distribution from 0.08 to 2.00 mm . A polycarboxylate-type superplasticizer with 40% solid content is used. The dry mixture proportions adopted to prepare both plain mortar (PM) and mortar reinforced with 0.03% of GO (GOM) specimens are: cement : water : sand (by weight) = 1:0.42:3 . The content of superplasticizer is equal to 0.3% by weight of cement. 3. Testing methods The fresh slurries, described in Section 2, are placed in moulds, compacted, and cured in laboratory for 24h. After demoulding, the specimens are submerged in water at a temperature equal to about 20 C ° for 28 days. 3.1. Flexural and compression testing Both PM and GOM specimens are tested under three-point bending loading, according to the ASTM C348 (2014). The specimens, whose sizes are equal to 40 40 160 mm × × (span equal to 120 mm ), are tested under load control with a rate of 45 N s . After such a testing, the two halves of each specimen are tested under compression (ASTM C348 (2014)).