Cement and Concrete Research 144 (2021) 106417 Available online 7 March 2021 0008-8846/© 2021 Elsevier Ltd. All rights reserved. Effects of anionic and nonionic surfactants on the dispersion and stability of nanoSiO 2 in aqueous and cement pore solutions Yogiraj Sargam a , Kejin Wang a, * , Ayuna Tsyrenova b , Fei Liu b , Shan Jiang b, * a Iowa State University, Department of Civil, Construction, and Environmental Engineering, Ames, IA 50011, United States of America b Iowa State University, Department of Material Science and Engineering, Ames, IA 50011, United States of America A R T I C L E INFO Keywords: Nanosilica Cement paste Surfactants Dispersion Stability ABSTRACT It has been well recognized that the benefts and effectiveness of nanoparticles in cement-based materials could not be maximized if these are not well dispersed. To address this issue, in this study, different anionic (SDS and PCE) and nonionic surfactants (Tweens and Tritons) were used to disperse nanosilica (NS) in aqueous solution and cement pore solution. The results show that the dispersibility of NS in cement pore solution was improved, and the compressive strength of the cement-NS pastes increased linearly with critical micelle concentration (CMC) of nonionic surfactants. Among all surfactants studied, Triton X-405 led the paste to the highest increase in strength (33% at 1-day and 41% at 3-days) since it had the highest CMC. TEM and EDS analysis evidenced that this strength increase might be attributed to the nucleation of outer product CSH gel and its densifcation with calcite nanocrystals, attributed to Triton X-405 addition. 1. Introduction Nano-engineering has garnered increasing attention in cement and concrete technology in the last decades. Cement-based composites like concrete can be nano-engineered by the incorporation of various nanosized materials, such as nanoparticles (NP) (zero dimension, 0D), nanofbers (1D), nanoplates/sheets (2D, like graphene), etc. [1]. The nanomaterials can be either blended and embedded into a cement ma- trix or hybridized onto the surfaces of cement particles, thereby modi- fying the particle interactions, altering the microstructures, and improving the properties of cement-based materials [1,2]. The benefts of the use of NPs in cement-based materials have been revealed by many researchers [3–9]. They include - (1) facilitating cement hydration by providing nucleation sites for the hydration products to form and grow, (2) improving particle packing of cement matrix and reducing the porosity of mortar/concrete, (3) immobilizing free water in the cementitious system, (4) reducing the porosity of interfacial transition zone (ITZ) between cement paste and aggregate, and (5) consequently enhancing the mechanical properties and durability of concretes. Be- sides, some NPs also participate in cement hydration. For example, nanosilica (NS) can undergo pozzolanic reaction in a cement matrix, which consumes calcium hydroxide and forms more calcium silicate hydrate (CSH) gel, thus further improving the mechanical properties and durability of concrete [10]. There are several challenges for the use of NPs in cement-based materials, among which uniform dispersion and the cost-to-beneft ratio are key barriers [1]. Despite being produced and supplied in the size of particles below 100 nm, the commercial NP tends to agglomerate and form clusters in the orders of μm in aqueous solutions [11]. The high surface energy and interparticle forces, including van der Waals, hydrogen bonding, and electrostatic interactions, make them highly susceptible to agglomeration [11–13]. When NPs are added to the cementitious system, other factors such as capillary force and inter- locking due to particle shape also contribute to their agglomeration [14]. Large, random aggregates not only diminish the beneft of using NPs but also act as potential weak sites for stress concentrations, thereby reducing the mechanical properties of concrete [13]. In this regard, great efforts have been made, and various approaches have been pro- posed for the dispersion of NPs [4,14–16]. Common approaches to nanoparticle dispersion are through physical methods (e.g., high shear mixing, sonication, milling, etc.) and chemical methods (e.g., use of dispersants, chemical modifcation of nanoparticle surfaces, etc.). For the cementitious systems, a combination of sonicat- ion and surfactant is often suggested. Researchers [12,17–22] have indicated that various surfactants exhibit different effcacy in dispersing nanomaterials. For example, Sindu and Sasmal [20] analyzed fve * Corresponding authors. E-mail addresses: kejinw@iastate.edu (K. Wang), sjiang1@iastate.edu (S. Jiang). Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres https://doi.org/10.1016/j.cemconres.2021.106417 Received 13 August 2020; Received in revised form 15 February 2021; Accepted 23 February 2021