*Review* **End-of-Life Materials Used as Supplementary Cementitious Materials in the Concrete Industry**

**Adrian Ionut Nicoara 1,2, Alexandra Elena Stoica 1,2, Mirijam Vrabec 3, Nastja Šmuc Rogan 3, Saso Sturm 4, Cleva Ow-Yang 5,6, Mehmet Ali Gulgun 5,6, Zeynep Basaran Bundur 7, Ion Ciuca <sup>8</sup> and Bogdan Stefan Vasile 1,2,\***


Received: 22 March 2020; Accepted: 19 April 2020; Published: 22 April 2020

**Abstract:** A sustainable solution for the global construction industry can be partial substitution of Ordinary Portland Cement (OPC) by use of supplementary cementitious materials (SCMs) sourced from industrial end-of-life (EOL) products that contain calcareous, siliceous and aluminous materials. Candidate EOL materials include fly ash (FA), silica fume (SF), natural pozzolanic materials like sugarcane bagasse ash (SBA), palm oil fuel ash (POFA), rice husk ash (RHA), mine tailings, marble dust, construction and demolition debris (CDD). Studies have revealed these materials to be cementitious and/or pozzolanic in nature. Their use as SCMs would decrease the amount of cement used in the production of concrete, decreasing carbon emissions associated with cement production. In addition to cement substitution, EOL products as SCMs have also served as coarse and also fine aggregates in the production of eco-friendly concretes.

**Keywords:** construction debris; cement; recycling; circular economy; eco-friendly concretes; fly ash (FA); silica fume (SF); palm oil fuel ash (POFA); rice husk ash (RHA); sewage sludge ash (SSA) and sugarcane bagasse ash (SBA); mine tailings; marble dust; construction and demolition debris (CDD)

### **1. Introduction**

In the face of rapidly expanding urbanization, environmental sustainability represents a serious challenge for the construction industry, whose consumption of concrete requires a significant quantity of natural reserves worldwide and necessitates the development of alternative materials and sources. The fabrication of concrete consumes around 27 billion tonnes of feedstock, representing 4 tonnes of

concrete for each person every year. By 2050, concrete production will be four times higher than in 1990. Aggregates and binder (i.e., cement) represent around 60%–80% and 10%–15% of the total weight of concrete, respectively [1]. Along with processing of substantial amount of aggregates and around 2.8 billion tonnes of cement products per year, concrete generates approximately 5%–7% of the global total carbon dioxide emissions. By 2025, around 3.5 billion tonnes of carbon dioxide is foreseen to be released to the atmosphere during cement production. One solution for more sustainable production can be harvesting locally available end-of-life (EOL) and/or recyclable materials [1,2].

Global quarry practices to obtain coarse aggregates have substantially modified the ecological equilibrium. Figure 1 shows the amount, in tonnes of aggregates produced per capita in 39 countries. For the sustainable future of our planet, it is essential to find substitutes for virgin materials to harvest for producing binder and aggregates necessitated by the construction industry. Meanwhile, a lot of EOL materials are disposed of in open fields as landfill. One example of this kind of waste is construction and demolition debris with enormous potential for recycling as a profitable recycled concrete aggregate (RA) [2,3].

**Figure 1.** Aggregates production, in tonnes per capita, in 39 countries [3].

RA can be found in almost all developed and developing countries as a result of the demolition of older buildings and structures. Moreover, in war-afflicted regions in other parts of the world, a large number of buildings have been destroyed by bomb attacks. Such buildings have become impractical and are of no-value, but offer significant potential as material sources for reconstruction projects. Currently, a significant volume of RA is being disposed of as zero-value debris. Therefore, RA can play an important role in sustainable development. RA has recently become an important domain in construction as substitutes for natural aggregate raw materials. Several studies [2,4–9] claimed that the incorporation of RA in concrete production would alter the hardened properties. Several studies demonstrated that 100% substitution of aggregates with RA in concrete is unacceptable, due to a significant decrease in the hardened strength varying from 15 to 25% [2].

In addition, concrete has been produced with traditional supplementary cementitious materials (SCMs) that possess a high pozzolanic activity [10], such as fly ash (FA) [11–14], silica fume (SF) [15–17] and ground granulated blast slag (GGBS) [2,18–21] that yielded notable improvement in strength and durability. Numerous industrial solid by-products containing calcareous siliceous, and aluminous materials (fly ash, ultrafine fly ash, silica fume, etc.), along with some natural pozzolanic materials [22] (volcanic tuffs, diatomaceous earth, sugarcane bagasse ash, palm oil fuel ash, rice husk ash, mine tailings, etc.) can be used as SCMs, because they possess cementitious and/or pozzolanic properties [23]. The abundance of these classes of materials and their broad diversity in chemical and physical composition compel the development of a common strategy for their application in concrete production industry (see Figure 2) [24,25].

**Figure 2.** Most common industrial by-products used as substitutes [26].

Although traditional SCMs have been attractive due to their superior long-term durability [27], sustainable SCMs can also be developed to decrease the quantity of cement required for concrete production with lower ecological impact. One example is the incorporation of industrial EOL by-products into conventional cement. Concrete containing up to 30% of cement substituted by SCMs has been regarded as environmentally-friendly concrete [2,10,28]. In recent years, the use of SCMs and/or natural pozzolans has increased in the concrete industry, due to their superior long-term performance. Consequently, there is a strong interest in activating large amounts EOL materials to replace traditional SCMs as different sustainable resources that are otherwise deemed as of-zero-value [2,10].

#### **2. Supplementary Cementitious Materials (SCMs)**

SCMs play a pivotal role in concrete performance across many civilizations. They encompass a wide spectrum of alumino-silicious materials, including natural or processed pozzolans and industrial by-products, like GGBS, FA/UFFA and SF [29]. In spite of broad variations in properties across the various types of SCMs, they share in common the capacity to react chemically in concrete and form cementitious binders replacing those obtained by OPC hydration [30]. The key feature of SCMs is their pozzolanicity, i.e., their capability to react with calcium hydroxide (portlandite, CH) aqueous solutions to form calcium silicate hydrate (C–S–H) [31].

In the right proportion, SCMs can improve the fresh and hardened properties of concrete, especially the long-term durability. The use of SCMs in concrete composition is an ancient technique [32–34], as evidenced by the widespread use of natural pozzolans, like volcanic ash, in Greek and Roman civilizations. The testament to their efficacy is the persistence of an important number of constructions built using pozzolanic materials that are still standing today [30].

The most frequently employed SCMs in the cement industry are discussed below: fly ash (FA) and/or ultrafine fly ash (UFFA) [35], silica fume (SF) [36,37] and natural pozzolanic materials like rice husk ash (RHA) [38–40], sugarcane bagasse ash (SBA) [41–43], sewage sludge ash (SSA) [44,45], palm oil fuel ash (POFA) [46–49], mine tailings [50–53], marble dust [54–58], construction and demolition debris (CDD) [59–61].

#### *2.1. Silica Fume (SF)*

Silica fume (SF) used to be EOL products harvested from industrial processes. However, lately the demand for the silica fume for high-performance concrete has increased so strongly that there are now foundries dedicated to producing SF for ultra -high-performance concrete. SF may be in the form

micro-silica, condensed silica fume or volatilized silica [26]. It is a fine powder produced in silicon foundries, where it is ultimately condensed from the vapour phase upon cooling [10]. As a consequence, SF is composed almost entirely of very small round particles of amorphous SiO2, whose fineness contributes to a relatively high pozzolanic activity [62]. The size distribution of regular SF particles is on the order of 0.1 μm, where the majority of the particles—more than 95%—should be smaller than 1 μm. They are about 100 times smaller than cement particles, with a specific surface of SF is around 20,000 m2/kg, therefore their specific surface is 10 to 20 times larger than that of other pozzolanic materials. Considering its characteristics, SF is a very reactive pozzolanic material [63]. The use of SF can notably increase mechanical properties of concrete due to effective filling and pozzolanic properties [64].

SF's most important effect on concrete is on the short- and long-term strength and long-term durability [26]. SF is claimed to be capable of increasing the bonding between cement paste and aggregates at the interfacial transition zone (ITZ). With a small particle size, SF not only improves the packing in the ITZ but also uses the localized CH in the ITZ during pozzolanic reaction, in combination with additional C–S–H. The net effect is creating better adhesion between the cement paste and aggregate. The high surface area of SF also provides a large reaction surface for enhancing the hydration process, resulting in improved density [26]. SF's influence on the improved density, or reduced macro porosity, also stems from its fluidized bed effect. The perfectly round fine particles of SF facilitate denser packing in concrete. Improved flow properties of concrete in the fresh state reduce bleeding and segregation, leading to superior performance of fresh and hardened concrete. Additional pozzolanic activity helps fill the excess water volume during further hydration [26,65].

A study by Seong Soo Kim et al. [66] demonstrated the improvement in compressive strength due to SF incorporation. Three types of coarse aggregates (basalt, quartzite and granite) were used to produce different concrete samples. The main binder used was OPC (according to ASTM C150), while SF was used to substitute 10 wt. % of cement. Figure 3, from this study, demonstrates the increase in compressive strength increased for all coarse aggregates types (i.e., basalt, quartzite and granite), when SF was added (see Figure 3).

**Figure 3.** Compressive strength test results (the specimens made with only cement as binder were noted with the acronyms CG, CQ and CB for concrete with granite, with quartz and with basalt aggregates, respectively). Those containing silica fume were called SFG, SFQ and SFB, accordingly [66].

Tensile performance was investigated in another study, in which 3 different series of concrete samples (see Table 1) with 0 wt.%, 12 wt.% and 16 wt.% SF as a cement substitute were produced and tested [67]. The results indicated that SF added to concrete yielded superior mechanical properties under dynamic tensile loading. Moreover, the dynamic tensile strength of samples rose with the silica fume (SF) amount, as well as with strain rate. In addition, the strain rate sensitivity of concrete augmented with SF appeared to have increased in comparison to concrete without SF (see Figure 4) [67].


**Table 1.** The proportion of concrete constituents with different SF levels by weight [67].

**Figure 4.** Dynamic tensile strength under different impact velocities and silica fume levels [67].

SF additions to concrete improve several other physical and chemical properties besides the mechanical strength: lower adverse environmental impact, decreased permeability, increased corrosion protection for steel bars and improved resistance against the sulphate and chemicals attacks [68,69].
