Optimization of nano-silica’s addition in cement mortars and assessment of the failure process using acoustic emission monitoring N.-M. Barkoula ⇑ , C. Ioannou, D.G. Aggelis, T.E. Matikas Department of Materials Science and Engineering, University of Ioannina, Ioannina, Greece highlights  Nano-silica’s addition results in enhanced mechanical properties and AE activity.  ‘‘Wet” is more effective process and offers stable properties at limited mixing time.  ‘‘Dry” mixing procedure can be used only in conjunction with superplasticizers.  A more tortuous path of the crack is found after the addition of nano- silica. graphical abstract article info Article history: Received 16 November 2015 Received in revised form 7 June 2016 Accepted 17 August 2016 Keywords: Nano-silica Mortar Superplasticizer Sonication Acoustic emission abstract The objective of this work was to optimize nano-silica’s addition in cement mortars using different mix- ing procedures. Nano-silica was either dispersed in water using sonication or directly added into cement and mixed using a rotary mixer. The fresh and hardened properties of nano-silica modified mortars were defined while acoustic emission (AE) activity was monitored and correlated with the failure processes. The addition of nano-silica resulted in 25–35% increase of the mechanical properties accompanied with simultaneous influence on the failure process as indicated by AE. Sonication was beneficial leading in stable mechanical properties at limited mixing duration. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction It is well established that cement-based materials offer high compressive strength in constructions however their whispered use is limited by their poor post-cracking durability. Therefore extensive work is being performed to improve the toughness and bending strength of these materials using an additional phase, such as fibres and fillers [1–5]. Nano-particles are now in the forefront of material research as filler materials to improve the mechanical properties and add functionalities to the cementitious mixes [6–8]. Among different nano-particles, nano-SiO 2 (nano-silica) has drawn considerable attention due its pore-filling effect, ability to accelerate the cement hydration and pozzolanic effect [9–12]. These mechanisms contribute to increased compressive and flexu- ral strength, decreased porosity and improved durability of the nano-silica modified cement composites [11,13–16]. However, due to their very high specific surface area and surface energy nano-silica particles tend to agglomerate and aggregate, even in the well-dispersed colloidal suspensions [17], which limits their efficiency to act as fillers at the nano-scale. Therefore very recent studies focus on the assessment of the effect of agglomeration on microstructure and properties of fresh and hardened cement based materials [18–20]. The conclusion from these studies is that the http://dx.doi.org/10.1016/j.conbuildmat.2016.08.055 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: nbarkoul@cc.uoi.gr (N.-M. Barkoula). Construction and Building Materials 125 (2016) 546–552 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat