Use of binary and ternary blends in high strength concrete

Abstract

Combinations of cement additions may provide more benefits for concrete than a single one. In this study, 80 high strength concretes containing several types and amounts of additions were produced. In the first stage, silica fume contents in binary blends that give the highest strengths were determined for different binder contents. In the second stage, a third binder (Class F or Class C fly ash or ground granulated blast furnace slag) was introduced to the concretes already containing Portland cement and silica fume in the amounts found in the first stage. Results indicated that ternary blends almost always made it possible to obtain higher strengths than Portland cement + silica fume binary mixtures provided that the replacement level by the additions was chosen properly. Moreover, the performance of slag in the ternary blends was better than Class F fly ash but worse than Class C fly ash.

Introduction

Mixture proportioning of high strength concrete (HSC) is more critical than that of normal strength concrete (NSC) since, usually, specially selected pozzolanic and chemical admixtures are employed, and a low water-to-binder ratio (w/b) is considered essential. Optimum mixture proportions are obtained after a greater number of trial batches than needed for NSC [1]. There is no single method for the proportioning of HSC, and this leads to different approaches to the issue [1], [2], [3].

The advantages of using cement additions in concrete are, mainly, the improved concrete properties in fresh and hardened states, and economical and ecological benefits. The achievement of these advantages becomes more important for HSC proportioning since HSC requires high amounts of cementitious materials. However, the selection of additions needs more attention due to their different properties: initial reactions of fly ash (FA), ground granulated blast furnace slag (S) and natural pozzolans are slower than that of Portland cement (PC), which causes a slower rate of strength development and a longer curing period. Slags develop strength more rapidly than do most fly ashes [4]. Silica fume (SF), due to its high surface area, increases water demand and superplasticizer requirement for a certain consistency [5], [6]. In addition, the use of SF should be limited due to the high costs of SF itself and the superplasticizer required. High-calcium fly ash (FA/C) is much more reactive than low-calcium fly ash (FA/F) and, therefore, exhibits some similarity to the behavior of S [7], [8]. In some cases, combined usage of the additions may reduce the efficiency of the cementitious system. In other words, the compatibility of the additions is another point to be considered in concrete proportioning. For example, the reaction of FA/F requires a high alkalinity of the pore water and this alkalinity is reduced when SF or S is present in the mix. Consequently, the reactivity of FA in such mixes is reduced [8], [9].

Fortunately, combinations of additions may provide more benefits for concrete when compared to a single one [10], [11]. Some examples of the researches on the use of ternary blends are given below.

Since the additions having low surface area and reactivity do not contribute to the strength quickly, they are not able to improve early strengths of concrete. The solution given by Mehta and Gjørv is to use a mixture of normal and highly reactive additions, such as FA and SF, respectively [10]. A number of reports demonstrated that the combined usage of SF and FA resulted in improvements in compressive strength [6], [10], [11], pore size distribution [10], chloride permeability [11], [12], alkali-silica reactions [12], [13] and sulfate resistance [12].

The results of a study by Shannag [14] suggest that certain natural pozzolan-SF combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan or SF alone.

The incorporation of a combination of finely ground FA and S with PC was reported to produce higher compressive strengths at all ages than their binary blends [15]. Similarly, according to Li and Zhao, blending FA and S presents an excellent behaviour in both short- and long-term compressive strengths and in resistance to H₂SO₄ attack; and improves the microstructure and hydration rate [16]. Jianyong and Pei concluded that blending S and SF synergizes the advantages of these two admixtures so that the compressive strength, split tensile strength and rupture strength are improved while the fresh concrete mixture keeps a good workability [17]. The ternary blends of PC, SF and FA or S were also studied and it was found that these blends can increase the ASR resistance [13] and chloride resistance [18] of concrete.

This study investigates the compressive strengths of the high strength concretes containing (a) PC only, (b) PC + SF binary binders, and (c) PC + SF + FA/F or FA/C or S ternary binders. The binder content in the concrete mixtures was also varied. The main objective of this research is to determine the effects of using a third binder (FA/F or FA/C or S) accompanying PC and SF to produce high strength concretes. By using different amounts of the most widely known additions (silica fume, fly ash and slag) as binary and ternary binders in 80 high strength concrete mixtures, a large database was formed in this study.