Recent advances in understanding the role of supplementary cementitious materials in concrete Maria C.G. Juenger a, ⁎, Rafat Siddique b a The University of Texas at Austin, 301 E. Dean Keeton St., Austin, TX 78712, USA b Thapar University, P.O. Box 32, 147004 Patiala, India abstract article info Article history: Received 29 December 2014 Revised 26 March 2015 Accepted 30 March 2015 Available online 13 April 2015 Keywords: Fly ash (D) Granulated blast-furnace slag (D) Metakaolin (D) Pozzolan (D) Silica fume (D) Supplementary cementitious materials (SCMs) are commonly used in concrete mixtures as a replacement of a portion of clinker in cement or as a replacement of a portion of cement in concrete. This practice is favorable to the industry, generally resulting in concrete with lower cost, lower environmental impact, higher long-term strength, and improved long-term durability. SCMs have been used in Portland cement concrete for decades and many of their effects are well-understood. Most recent research on SCMs has focused on a few areas: exploring new materials, increasing replacement amounts, developing better test methods, treating or modifying materials, and using additives (e.g. limestone or nanosilica) to improve performance. The advances in knowledge provided by research in these areas are reviewed in this paper, emphasizing the impact of the research on the field. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, silica fume, calcined clays and nat- ural pozzolans, are commonly blended with clinker to make portland cement or used as a replacement for a portion of portland cement in concrete. The practice of using SCMs is increasing, with the world average percent clinker in cement having decreased from 85% in 2003 to 77% in 2010, and it is projected to further decrease to 71% in the future [1]. In the U.S., SCMs are usually added to concrete rather than blended with clinker, and currently more than 60% of ready-mixed concrete uses SCMs [2]. While fly ash and ground-granulated blast furnace slag represent the majority of SCMs used, there is a shift to embrace other materials, which is driven by many factors, including supply-and-demand concerns. In 2011, 3.6 billion tons of cement were produced worldwide [3], and this is projected to rise to 5.8 billion tons by 2050 [4]. A way to meet this rising demand is to continue increasing the use of SCMs in concrete. It is understood that only part of this demand can be met through the use of fly ash and slag, since the annual global productions of these materials are approximately 1 billion tons and 360 million tons, respec- tively [5,6]. Therefore, the focus of much of the recent research on SCMs has been on the exploration of alternative SCMs and their performance in concrete. While itemizing newly discovered alternative SCMs is not the goal of this review paper, research on these materials is discussed when findings are applicable to a wider range of SCMs. This paper summarizes the advances achieved in the past four years in our understanding of SCM use in concrete. One of the primary reasons for SCM use is to reduce the environmental impact of concrete, and recent publications on this topic are reviewed first. Identifying appropriate new materials, maximizing their use, and improving their performance can best be achieved through appropriate material charac- terization and tests for pozzolanicity, which are reviewed next. Corre- spondingly, there have been significant advances in the pre-treatment of SCMs for improved reactivity or additives to improve mixture perfor- mance, particularly at the nanoscale. The interactions of SCMs with Portland cement is addressed in terms of the impact on early hydration, fresh state properties, mechanical properties, and long-term durability. Lastly, the role of SCMs in ultra-high performance concrete, is reviewed, focusing on the impact of these materials on long-term properties. 2. The role of SCMs in sustainable concrete production While the use of SCMs in concrete in relatively small amounts (5–20% replacement of clinker) is often driven by economics and improvements in the long-term mechanical properties and durability of concrete, the impetus to replace an increasing percentage of clinker with SCMs often comes from pressure on the industry to reduce CO 2 emissions from concrete production. Often these high volume clinker replacements result in losses in performance at early ages, driving research into balancing sustainability and performance and finding means of performance prediction. Cement and Concrete Research 78 (2015) 71–80 ⁎ Corresponding author. Tel.: +1 512 232 3593. E-mail addresses: mjuenger@mail.utexas.edu (M.C.G. Juenger), rsiddique@thapar.edu (R. Siddique). http://dx.doi.org/10.1016/j.cemconres.2015.03.018 0008-8846/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp