Improving the performance of cement-based composites containing superabsorbent polymers by utilization of nano-SiO 2 particles Ali Pourjavadi a,⇑ , Seyed Mahmoud Fakoorpoor a , Alireza Khaloo b , Payam Hosseini b a Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Azadi Ave., P.O. Box 11365-9516, Tehran, Iran b Department of Civil Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran article info Article history: Received 15 March 2012 Accepted 17 May 2012 Available online 30 May 2012 Keywords: A. Concrete A. Polymer A. Nano-materials abstract The application of superabsorbent polymer (SAP) as an internal curing agent for cement based composites results in benefits such as reduced autogenous shrinkage and cracking. However, a reduction in compres- sive and flexural strength usually occurs due to the empty voids remained in the matrix after deswelling of SAP particles. Nanoparticles are good candidates for improving the mechanical performance of cementi- tious materials, due to their multiple mechanisms of action, not the least their high pozzolanic activity. In the present work, the capability of amorphous nano-SiO 2 (NS) as the most widely used nanoparticle in cementitious materials, for retrieving mechanical properties of SAP-containing pastes was evaluated, and its impact on setting time and rheological properties was measured. It was found that small dosages of NS could offset the negative effect of SAP on compressive strength but flexural strength was not fully compen- sated. Optimization of the dosages of NS and SAP could reduce the negative influences on the yield stress and viscosity whilst improving mechanical performance. Scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy were used to monitor the changes in microstructure and composition. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Concrete, with a global annual production of about 10 billion tons is second only to water in terms of consumption per capita [1]. The popularity of concrete as a construction material results from its ready availability, relatively low cost, good mechanical properties, fire resistance, versatility for different applications, and capability of implementing various architectural designs. Because of this tremendous turnover and economical signifi- cance, concrete industry has always been a good target for deploy- ment of latest achievements in science and technology. In particular, the developments in nanoscience have had a great im- pact on concrete industry. Nano-materials have been used in con- crete industry over the past decade. Extensive research has shown that incorporation of nanoparticles in cement matrix could improve durability and mechanical properties of cement based materials [2]. NS in particular has found wide usage in this field because of its rel- atively low cost of production, high reactivity and very large spe- cific surface area, which results in a high degree of pozzolanic activity [3–5]. NS further accelerates the dissolution of C 3 S and for- mation of CASAH with its activity being inversely proportional to the size, and also provides nucleation sites for CASAH [6,7]. Even small additions (0.6 wt.% binder) of NS are very efficient and compare to much larger amounts of silica fume in terms of improvement in mechanical properties of cement based materials. This is especially pronounced at early ages [8–10] and for concretes with regular strength grade [11]. Therefore, application of NS could be a successful method for improvement of otherwise low com- pressive or flexural strength of cement based materials. Porosity in cement matrix could be a ground for low mechanical performance and is usually a result of excessive air content in cement paste. A similar situation is expected when SAP is used in cement matrix for the purpose of internal curing [12–14]. SAPs are lightly crosslinked networks of flexible polymer chains and can absorb a large amount of water. Upon gradual release of the absorbed water during the hydration of cement, these polymers greatly enhance the properties of cement based materials by reducing the self-desiccation, autogenous shrinkage, and cracking. On the other hand they leave voids in the matrix after releasing their water and deswelling. These hollow spaces cause a reduction in compressive and flexural strength, thus partly cancel out the benefits of internal curing [15]. Addition of 5% of entrained water in the form of SAP even in cement pastes containing 20% of silica fume reduces the early age strength of the pastes by about 20%, while at later ages the strength reduction is about 10% [16].A similar reduction in strength is also reported for internal curing of concrete with saturated lightweight aggregates [17–19]. In view of the success of the SAP in reduction of autogenous deformation, many researchers have recently paid their attention 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.05.030 ⇑ Corresponding author. Tel.: +98 21 6616 5311; fax: +98 21 6602 9165. E-mail address: purjavad@sharif.edu (A. Pourjavadi). Materials and Design 42 (2012) 94–101 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes