Performance of sustainable concretes containing very high volume Class-F fly ash and ground granulated blast furnace slag

Abstract

In recent years, with the aim of lowering the environmental impact of concrete, the partial replacement of cement in concrete with fly ash (FA) and ground granulated blast furnace slag (GGBS) has received significant attention. This paper presents the first experimental study on the properties of concrete containing ternary binders with high volume Class-F FA and GGBS partially replacing cement up to 90%. A total of 15 batches of concrete were manufactured with binary and ternary binders based on FA, GGBS, and cement. Experimental tests were performed to establish the density, workability, compressive strength, elastic modulus, flexural strength, splitting tensile strength, and water absorption of different mixes. The results indicate that the compressive strength of concrete containing FA decreases significantly with an increase in the FA content from 50% to 90%. The concretes containing GGBS at up to 90% cement replacement exhibit a similar compressive strength to that of conventional concrete after 28 days. It is found that ternary mixes with a 70% replacement ratio and relative FA to GGBS ratio (Ψ) of 1:2, and those with a 50% replacement ratio and Ψ of 1:1 develop a similar 28- and 90-day compressive strength to that of conventional concrete. A further increase in the replacement ratio and Ψ results in a decrease in the compressive strength. The results also show that GGBS mixes develop a slightly higher 28-day elastic modulus than that of conventional concrete mix. Although the elastic modulus of FA and the majority of the ternary mixes is lower than that of conventional concrete, ternary mixes with 70% (Ψ of 1:2) and 50% (Ψ of 1:1) replacement ratios develop a similar elastic modulus to that of conventional concrete. It is observed that an increase in the FA and GGBS content, respectively, results in a significant increase and moderate decrease in the water absorption of concrete. All ternary mixes with up to 90% cement replacement exhibit lower water absorption than conventional concrete mix. These highly promising findings suggest that the technology used in this study can provide an attractive avenue for high volume use of FA and GGBS in concrete with the possibility of significantly reducing its environmental impact.

Introduction

Concrete is the most widely used construction material. High volume use of concrete leads to the consumption of large amounts of energy on the production, transportation, and use of raw materials (Torgal et al., 2012). The production of the ordinary Portland cement (OPC) results in the release of a significant amount of greenhouse gases (essentially CO₂) into the atmosphere. It was reported that approximately one ton of CO₂ is generated and approximately 2.5 tons of materials are consumed to produce one ton of OPC (Mehta and Meryman, 2009). It is estimated that the total amount of CO₂ produced by OPC production accounts for approximately 5–7% of global anthropogenic CO₂ emissions (Huntzinger and Eatmon, 2009, Turner and Collins, 2013). Therefore, it is recommended to seek an alternative material to replace cement in concrete to reduce the environmental impact of concrete.

One alternative for reducing the environmental impact of concrete is to use industrial by-products as mineral admixtures to replace the OPC in concrete (Crossin, 2015). Over the past three decades, fly ash (FA) and ground granulated blast furnace slag (GGBS) have been used as alternative cementitious materials to improve the green credentials of the construction industry. It is now recognized that the use of FA and GGBS in the concrete mixes is a highly promising technology to reduce the environmental impact of both the industrial by-products and concrete. It was reported that the worldwide generation of FA and GGBS is approximately 450 and 530 million tons, respectively (Zhao et al., 2015), with only about 25% of FA (Ahmaruzzaman, 2010) and 65% of GGBS (Tsakiridis et al., 2008) are currently being used. Disposal of large volumes of FA and GGBS has become increasingly costly, while also causing environmental concerns (Topcu, 1997).

In the past two decades, a large number of studies have been conducted to understand the performance of concrete containing FA and GGBS. Mo et al. (2017) reported that the use of FA and GGBS reduces costs in the concrete production and waste disposal, especially for high volume replacement. Bouzoubaa et al. (2001) and Mehta (2004) showed that the use of Class-F FA in the concrete modifies the properties of both fresh and hardened concrete, such as workability, strength, abrasion, heat evolution, shrinkage, and durability. Song and Saraswathy (2006) and Atis and Bilim (2007) showed that the inclusion of GGBS in concrete can reduce the porosity and increase the corrosion resistance of concrete. Siddique, 2004, Crouch et al., 2007, Kayali and Ahmed (2013), and Shafigh et al. (2016) showed that the compressive strength of concrete containing FA decreases with an increase in the FA replacement ratio. Chidiac and Panesar, 2008, Bilim et al., 2009, and Mo et al. (2016) found that the 28-day compressive strength of GGBS concrete increases with an increase in the GGBS replacement ratio up to 40–60% replacement level, beyond which the strength starts to decrease. Li and Zhao (2003) and Berndt (2009) reported that combined use of 25% of Class-F FA and 15–25% of GGBS in concrete resulted in a slightly lower or similar 28-day compressive strength to conventional concrete. Berndt (2009) and Zhao et al. (2015) showed that the elastic modulus of Class-F FA concrete decreases with an increase in the FA replacement ratio. They also reported that the elastic modulus of GGBS concrete increases with an increase in the GGBS content up to 50%, beyond which it starts to decrease. The tensile strength of concrete containing FA or GGBS has been shown to have a strong correlation with its compressive strength. Berndt, 2009, Mehta, 2004, and Yijin et al. (2004) showed that the 28-day tensile strength of concretes decreases with an increase in the FA content and increases with an increase in the GGBS content up to 50% replacement level. Berndt (2009) reported that beyond this replacement level the tensile strength of GGBS concrete starts to decrease with a similar trend to its compressive strength.

Most of the existing studies on the use of FA and GGBS in concrete were concerned with a relatively low volume replacement of cement (i.e., below 50%) with up to 30% and 40%, respectively. Although there are a number of studies that investigated the high volume Class-F (Bouzoubaa et al., 2001, Kayali and Ahmed, 2013, Oner et al., 2005) and Class-C (Kuder et al., 2012, Rivera et al., 2015) FA concrete (i.e., over 50% replacement), only one study (Kuder et al., 2012) considered a replacement ratio of over 80%. As high volume FA concrete shows excellent workability and good long-term strength gain, attributed to the pozzolanic reaction of FA with calcium hydroxide of OPC resulting in a reduction in the porosity of the concrete in the longer term (Crouch et al., 2007, Kayali and Ahmed, 2013), the investigation of FA concrete at an ultra-high volume replacement is of significant interest. Likewise, there are only a limited number of studies (i.e., Bouikni et al., 2009, Elchalakani et al., 2014, Güneyisi and Gesoğlu, 2008, Richardson, 2006, Sivasundaram and Malhotra, 1992, Tomisawa and Fujll, 1995, Wang and Lin, 2013, Yazıcı et al., 2010) on the high volume use (i.e., higher than 50%) of GGBS in concrete. The results of these studies showed that concretes containing high volume GGBS exhibit excellent mechanical properties and good long-term strength gain. Although there are a number of studies on the individual use of FA and GGBS in concrete, only two studies have investigated the properties of concrete with high volume replacement of cement by combined FA and GGBS (i.e., Kuder et al., 2012 (Class-C FA); Rashad, 2015 (Class-F FA)). Furthermore, only one study (i.e., Kuder et al., 2012) to date has evaluated the mechanical properties, namely the compressive strength and elastic modulus, of concrete containing both FA (Class-C) and GGBS at a replacement ratio as high as 90%. However, the other important properties, such as the splitting tensile strength, flexural strength, and water absorption were not studied. As it is evident from the above review of the literature, additional experimental studies are required to understand the behavior of concrete containing high volume Class-F FA and GGBS.

Water absorption is a key parameter in the investigation of the durability of concrete. Kungskulniti et al. (2011) and Mukherjee et al. (2012) showed that the water absorption of concrete significantly increases with an increase in the FA replacement ratio from 10% to 70%. This behavior was attributed to the rough surface of FA as well as the presence of entrapped air on the surface of FA particles, which increase the porosity of concrete (Kungskulniti et al., 2011). On the other hand, Sakai et al. (1992) and Atis and Bilim (2007) reported that the water absorption reduces with an increase in the amount of GGBS. This was explained by the reduction in the capillary pore volume and creation of a discontinuous capillary pore structure in the concrete with an increase in the GGBS content (Sakai et al., 1992). To date, only two studies have investigated the water absorption of the concrete containing high volume GGBS (Elchalakani et al., 2014, Güneyisi and Gesoğlu, 2008) and there has been no study on the water absorption of concrete containing the combination of FA and GGBS at high volume.

This paper presents the first experimental study to investigate the physical and mechanical properties of concretes containing high volume Class-F FA and GGBS binary and ternary binders with cement replacement ratios up to 90%. This study is also the first to investigate the splitting tensile strength, flexural strength, and water absorption of concrete containing the combination of high volume FA and GGBS (i.e., replacement ratio >50%). The paper initially provides a summary of the experimental program, including material properties, specimen properties, and testing procedures, which is followed by the results of the experimental program. A detailed discussion on the results is subsequently presented to discuss the effects of the cement replacement with FA and GGBS on the properties of concrete. The promising technology presented in this study allows the reduction of the environmental impact of both the industrial by-products and concrete and has the potential of significantly contributing toward a greener construction industry.