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Cementitious composites are mainly used in the construction industry due to their good characteristics such as low cost, acceptable compressive strength, and easy access. However, there are many weaknesses in these materials, including low tensile strength, brittle behavior, and unacceptable durability (service life), which need to be improved to achieve more sustainable constructions. Nowadays, the using of nanotechnology have been growing and nanomaterials have been widely used in compound with a multitude of conventional materials. The outstanding chemical and physical properties of nanomaterials enable them to play a key role in various applications, such as modifying the material structure, ameliorating the properties of the material, and manufacturing modern multifunctional products. Recent advances in nanotechnology have led to produce nano-sized particles that can improve the durability performance of construction materials. Nanoparticles such as nano-silica, nano-Fe₂O₃, nano-clay, carbon nanotube (CNT), nano-Al₂O₃, nano-TiO₂, and graphene oxide have been used to enhance the properties of cementitious composites. The performance of halloysite nanotube on the characteristics of cementitious composites has been studied less than other nanomaterials. Although the positive effects of nanomaterials such as halloysite nanotube (HNT) on the properties of cementitious composites have been proven, the very important issue of the correct and proper dispersion of nanomaterials in the cementitious environment has not been studied acceptably. The high surface energy and interparticle forces, including van der Waals, hydrogen bonding, and electrostatic interactions, make the nanomaterials highly susceptible to agglomeration. The aggregates of nanomaterials not only decrease their benefit but also act as potential weak spots in cementitious composites that can cause stress concentration, therewith diminishing the mechanical properties of cementitious composites. In this regard, the current paper investigated the effective factors on the agglomeration of halloysite nanotube (HNT) in the cementitious alkaline environment. Finally, this paper presented an approach for solving the problem of HNT agglomeration. Results showed that Ca²⁺, K⁺, and Na⁺ ions as alkaline agents of cement environment are the main factors to provide a state for HNT agglomeration. Among them, Ca²⁺ has more effect in agglomeration of halloysite nanotube due to the bridging effect between halloysite particles. From the results, the dispersion of HNT made better with increasing the alkalinity of cement environment until pH=11, while after this pH, the agglomeration of HNT started and the intense of agglomeration raised with the increase of pH, where it reached a maximum value at pH=13.5. Common approaches to nanoparticle dispersion are through physical methods (e.g., ultrasonic, high shear mixing, ball milling, etc.) and chemical methods (e.g., chemical modification of nanoparticle surfaces, use of dispersants such as surfactants, etc.). For the cementitious systems, a combination of ultrasonic and surfactant is mostly suggested. In this research, the effect of various surfactants on overcoming the agglomeration of halloysite nanotube in the cementitious environment was studied. The results indicated that the Polycarboxylate-based surfactant has better performance on improving the dispersion of HNT compared to that of other surfactants. Furthermore, incorporation of 3 wt% HNT enhanced the compressive, flexural and sorptivity of plain mortar up to 26, 22, and approximately 28%, respectively. The outcomes of the current paper display the fact that it is necessary to have special attention on the subject of the proper dispersion of nanomaterials in the cementitious environment for achieving the maximum efficiency of nanomaterials.

Keywords: Halloysite nanotube, agglomeration, dispersion, surfactant, alkaline environment.

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