Artificial alkali-activated aggregates developed from wastes and by-products: A state-of-the-art review

Abstract

Natural resources depletion is gradually becoming a critical burden on the environmental and ecological balance, pushing the development of artificial aggregates forward. In order to curb the shortage issues of natural aggregates and minimize the destruction of land topography, alkali-activated aggregates (AAA) have gradually become a hot topic in recent years as a new application of alkali-activated materials (AAM) with the benefits of utilizing industrial by-products and waste materials. This article provides an overall review of the manufacturing process and engineering properties of two types of artificial aggregates, cold-bonded AAA (CB-AAA) and sintered AAA (ST-AAA). Their applications in concrete, besides the mechanical evaluation, durability performance, and leaching behavior, are summarized based on the existing research outcomes. Finally, the future perspectives and challenges of artificial aggregates development are also proposed.

Introduction

The irreversible destruction of natural topography caused by the overexploitation of natural resources has introduced restricted policies in many countries and regions (Agrawal et al., 2019) to address the concerns of natural aggregates (NA) scarcity. On the other hand, the increase of discarded industrial by-products/wastes has forced modern society to face the challenges of landfill shortage and environmental pollution (Balapour et al., 2020, Yliniemi et al., 2017). The utilization of industrial by-products or wastes in soil stabilization of peatlands or manufacturing bricks is an innovative approach to increase waste consumption and reduce the usage of cement materials (Gavali and Ralegaonkar, 2020, Gavali and Ralegaonkar, 2020, Gavali and Ralegaonkar, 2020, Gavali et al., 2021, Nguyen Hoc Thang et al., 2021, Vincevica-Gaile et al., 2021). However, considering that the aggregates occupy 60–70% volume of concrete, the approach of recycling waste materials to produce artificial aggregates is another one-stone two-bird solution for the aforementioned issues.

Part of the wastes is feasible for direct use as fine or coarse aggregates in concrete, such as agricultural wastes, plastic wastes, crumb rubber, ferrous slag, boiler ash, and waste glass (Kim and Lee, 2011, Saikia and De Brito, 2012, Brand and Roesler, 2015, Jie et al., 2020, Liu et al., 2019, Poinot et al., 2018, Cardoso et al., 2018, Dong et al., 2021), as shown in Fig. 1. However, such directly-used wastes may adversely affect the properties of concrete composites (Manaswini C, 2015). For instance, the application of agricultural wastes, plastic wastes, and crumb rubber as aggregates can cause noticeable strength loss of mortar or concrete composites (Saikia and De Brito, 2012, Jie et al., 2020, Gomaa et al., 2020). On the other hand, alkali-silica reaction is a major concern of waste glass (Choi et al., 2018). Regarding steel-furnace slag aggregates, expansion may occur when such aggregates are hydrated due to their high content of free calcium and magnesium oxides (Brand and Roesler, 2015). In addition, the hazardous elements or ions contained in some wastes may impose risk on the environment when used as construction materials without prior treatment (Nguyen Hoc Thang et al., 2018, Hoc Thang Nguyen et al., 2018). Herein not all of such wastes are listed in this article. For example, the leached heavy metals of untreated municipal solid waste incineration primarily include Zn, Cu, Mn, Pb, Cr, and Cd (Wang et al., 2021a). A previous study on mortars constituting waste glass as fine aggregates showed that the leaching of Pb and Cr was not completely prevented (Choi et al., 2018). An experimental study on porous concrete utilizing coal bottom ash as coarse aggregate indicated acceptable leaching results for several substances (Park et al., 2009). However, the concentration of these hazardous elements varies depending on the origin of coal.

Considering the above-mentioned concerns, turning unutilized wastes into artificial aggregates for use in concrete has become a worthy topic. Currently, the common manufacturing methods of artificial aggregates include cold bonding and sintering (Bijen, 1986). The cold bonding method typically utilizes cementitious materials (such as cement or lime) to bind the waste materials, such as coal fly ash (CFA), furnace bottom ash, ground granulated blast furnace slag (GGBFS), and so on, at curing temperatures below 100 °C (Aljerf, 2015). In comparison, sintered aggregates (SA) require high temperatures over 1000 °C (Wei et al., 2020, Nadesan and Dinakar, 2018) to harden the pellets by fusing raw material particles (Kim and Lee, 2011). In recent decades, alkali-activated materials (AAM) have been rapidly developed (Alrefaei and Dai, 2018, Alrefaei et al., 2019, Wang et al., 2021, Provis, 2018, Thang, 2020, Minh and Thang, 2020, Do Quang Minh and Nguyen, 2019), promoting a new type of aggregates, termed alkali-activated aggregates (AAA). Such aggregates include cold-bonded AAA (CB-AAA) and sintered AAA (ST-AAA), which will be the main target of this article. The feasibility of ST-AAA has been demonstrated by the high thermal stability of AAM at high temperature (Thang, 2021, Thang, 2021). Both CB-AAA and ST-AAA are universally described as "AAA" in this article unless otherwise specified.

Similar to AAM, CB-AAA gain strength from the alkali-activation of aluminosilicate precursors, resulting in a steady and solid cross-linked aluminosilicate structure (Xu et al., 2021a). In this article, all raw materials are termed precursors of AAA unless otherwise specified. CB-AAA and cementitious materials-based aggregates (CMA) are usually discussed together in previous research as they both use ambient temperatures for production (both belong to cold-bonded aggregates production). Notices should be paid that the CB-AAA are separated explicitly from CMA in this article because each type has completely different binder molecular structures (sodium/calcium aluminosilicate structures in CB-AAA, while calcium-silicate structures in CMA). Further, the alkali-bearing SA are classified as ST-AAA in the present article, although no clear categorizing criterion was defined for traditional SA and ST-AAA in previous literature. For ST-AAA, alkali may react with the raw materials before sintering, and the fresh aggregate may include reacted and unreacted precursor particles, as expressed in Fig. 2. Such a reaction can change the glassy phase of the sintered materials and thus may differentiate the mechanism of the ST-AAA from that of traditional SA.

Fig. 3 shows the timeline versus annual publication numbers of the above-mentioned four types of artificial aggregates (SA, CMA, CB-AAA, and ST-AAA), excluding the aggregates produced from natural resources, such as expanded clay/shale aggregates. Books and conference publications are not included in this figure, and the actual numbers of publications may possibly be more than the presented values herein. The early history of artificial aggregates was back to the 1960s with the launch of commercial sintered aggregate (SA) using CFA as a raw material, named "Lytag", while the literature of CMA probably first appeared in the 1990s (Orangun, 1967). On the other hand, the CB-AAA were primarily proposed in 2007 (Jo et al., 2007), although the AAM have been developed for several decades as possible alternative binders of ordinary Portland cement. Besides, ST-AAA were firstly manufactured in 2013, and only a few studies were available in the literature.

Accordingly, the research area of CB-AAA and ST-AAA is still relatively new, and their values and applications as aggregates have been far from being comprehensively investigated. Previous research (Tajra et al., 2019a, Nadesan and Dinakar, 2017, Ren et al., 2020) reviewed the SA and CMA, yet not much detailed information has been reviewed on CB-AAA and ST-AAA. Therefore, this article aims to provide a state-of-the-art review of the development of CB-AAA and ST-AAA, at which the structure of this review consists of a) the manufacturing process of AAA, including raw materials and manufacturing process, as summarized in Section 2, b) the engineering properties of AAAs and their applications in concrete reviewed in Section 3 and 4, respectively, c) the environmental impacts in Section 5, and d) the summary and future perspectives provided in Section 6.