Recent advances in understanding the role of supplementary cementitious materials in concrete

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Abstract

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.

Introduction

Supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, silica fume, calcined clays and natural 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, respectively [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 characterization and tests for pozzolanicity, which are reviewed next. Correspondingly, there have been significant advances in the pre-treatment of SCMs for improved reactivity or additives to improve mixture performance, 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.

Section snippets

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 CO2 emissions from concrete production. Often these high volume clinker replacements result in losses in performance at early ages, driving research into balancing

Material characterization

There is an increasing variety of SCMs being investigated, with much of the recently published literature on SCMs focusing on trial testing of new potential SCMs from waste-streams and natural sources. Some of the challenges with the introduction of so many new materials are finding methods to appropriately characterize them and recognizing the limitations of some existing methods that were developed for other materials [13]. Some of the characteristics of interest when evaluating SCMs are

Pozzolanicity testing

One of the most important characteristics of SCMs is their pozzolanicity, or their ability to consume calcium hydroxide (portlandite, CH) and form calcium silicate hydrate (C–S–H). There are several methods in use to measure pozzolanicity, and advances in these methods are described here.

One common way to measure pozzolanicity is to measure the consumption of portlandite in SCM-containing cement pastes over time using thermal analysis methods such as thermal gravimetric analysis (TGA) and/or

Thermal activation

Clays, such as kaolinite, must be calcined before use as SCMs to increase reactivity through amorphization. The optimization of the calcination process is a subject of recent research, particularly in the context of optimizing temperature and time for calcining non-kaolinite clays, such as montmorillonite or illite, or blends of clay minerals. Fabbri et al. [40] noted that while the thermal treatment of kaolinite to produce metakaolin increases pozzolanicity, thermal treatment also decreases

Effects of SCMs on cement hydration

A review on SCMs by Lothenbach et al. [27] summarizes the role of SCMs on cement hydration well, describing, in particular, impacts on cement hydration kinetics, phase assemblage in hydrated systems, and composition of C–S–H. For example, SCMs have been observed to have so-called “filler effects” on cement hydration kinetics, which can be separated into two roles depending on the timing of the effect. SCMs (and inert fillers) with very small particle sizes can enhance hydration kinetics during

Effects of SCMs on workability

Pozzolanicity is only one of the factors when selecting SCMs for use in concrete mixtures; many potential SCMs have detrimental effects on concrete workability, which may limit their application. In some cases, the water demand of a concrete mixture increases when a pozzolan is used because of small particle size, such as is the case with silica fume and nano-scale additives. In other cases, high internal porosity increases water demand through water absorption by the pozzolan, such as is the

Effects of SCMs on strength

SCMs are generally understood to increase concrete long-term strength through the pozzolanic reaction and decrease early-age strength due to dilution of cement. These trends are consistent for most SCMs, including fly ash, metakaolin, and agricultural residue ashes, as reviewed here.

Guneyisi et al. [82] examined high performance concretes containing metakaolin (MK; 5–15%) that exhibited higher (39–44%) 28-day strength than the control concretes (62–86 MPa). They concluded that the increase in

Initial surface absorption, water absorption, porosity, and sorptivity

The addition of 5–15% metakaolin improves the pore microstructure of concrete [84], resulting in a reduction in initial surface absorption [85] and sorptivity [82], [85]. This is attributed to the pozzolanic reaction whereby the free calcium hydroxide produced by cement hydration was consumed, resulting in dense C–S–H production. Regarding porosity, Antoni et al. [29], Nicolas et al. [83] and Ramezanianpour and Hooton [88] reported reduction in porosity with up to 10% MK content, but increased

Use in ultra high performance concrete

Ultra high performance concrete utilizes particle packing optimization, filler effects, and the pozzolanic reaction to dramatically increase concrete strength. While traditionally SCMs and fillers such as silica fume and silica flour are used, researchers are examining the possibility of using other SCMs. For example, 28-day compressive strengths of 150 and 180 MPa were achieved with RHA mean particle sizes of 8 and 3.6 μm, respectively, in UHPC mixtures compared to a 28-day strength of 160 MPa in

Conclusions

This paper reviews recently published literature on the effects of SCMs on concrete properties. Several new insights have been gained through recent research, which could have a significant impact on advancing the field. For example, a new way to quantitatively analyze X-ray diffraction patterns for materials containing amorphous phases, called “PONKCS,” could lead to improved ability to quantify degree of reaction of SCMs in hydrated systems [17]. Advances in treatment methods for agricultural

Acknowledgments

Maria Juenger would like to thank the U.S. National Science Foundation (CMMI 1030972) and the Texas Department of Transportation (0-6717) for sponsoring her research on supplementary cementitious materials. She would also like to acknowledge RILEM committee TC-SCM, led by Nele De Belie, for bringing together researchers for discussions of SCMs and advancing knowledge and practice.

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