*2.2. Fly Ash (FA)*

Fly ash (FA) is a by-product generated from coal-fired power plants. It is a fine, volatile powder emitted from chimneys alongside flue gases [10]. There are three distinct types of FA, including Class N, Class F and Class C, produced by burning black coal or brown coal, respectively. Class C and Class F are used in production of building materials, like lightweight aggregate, concrete, bricks, etc. (see Tables 2–4) [26,70]. There have been numerous studies, in which fly ash to be used as SCM in concrete have been analysed and classified according to types and characteristics. The American Society for Testing and Materials (ASTM) produced the initial report on standards for applying FA more than 40 years ago, ASTM C 618 [26].


**Table 2.** Type of fly ash as per American Society for Testing and Materials [26,70].

Coal-fired power plants produce significant solid wastes that contained FA. About 700 million tons of FA every year throughout the world can be used in cement and/or concrete production, thanks to its pozzolanic activity. Replacing with FA in cement or concrete results in gross energy requirement (GER), carbon dioxide emissions (Global Warming Potential: GWP) and natural resource consumption [71,72]. Despite these advantages, the development of optimal composition of concrete with a high volume fraction of fly ash (HVFA) remains a challenge, due to the broad variation in chemical composition (lime, sulphates, alkalis and organics), fineness and mineralogy of fly-ash [71].



**Table 4.** Type of fly ash based on boiler operations.


In recent years, the integration of fly ash as a partial substitute of cement in concrete is a common procedure. The amount of FA to substitute the cement for regular applications is restricted to 15%–35% (depending on the type of FA) by mass of the total amount of cementitious material. However, even at these levels, the use of FA has overcome some critical issues focused on sustainable construction. FA's pozzolanic activity is related to the presence of amorphous SiO2 and Al2O3 in its composition [73]. An active FA reacts with Ca(OH)2 during the cement hydration process and forms supplementary C–S–H and calcium aluminate hydrate (C–A–H) [73,74]. These hydration products allow formation of a denser matrix, resulting in better strength and superior durability (see Figure 5) [73,74].

**Figure 5.** Reaction of fly ash (FA) in cement [70].

Ashish Kumer Saha [75] studied the use of Class F fly ash as a partial substitute of binder in concrete. A set of five samples was cast—a control sample without FA and 4 samples consist of 10, 20, 30 and 40% of FA as a substitute for cement. The water-to-cement ratio and the volume of superplasticizer were maintained constant for all 5 samples. The compressive strength (see Figure 6) of the FA mixtures showed lower early compressive strength than the ones of the control samples. The positive influence of FA additions could be observed at the longer hydration times, i.e., longer than 28 days of hydration. The small sized FA particles with high surface area and high percentage of amorphous silica phase provided the pozzolanic activity, leading to enhanced strength for longer periods of hydration. In addition, the spherical shape of FA particles improves the fresh state properties by increasing the workability of the concrete mix.

**Figure 6.** Compressive strength development [75].

Bingqian Yan et al. [76] studied the changes of mechanical properties of cement mortar with an admixture of FA and lime. As brine water is used in the preparation of filling slurry of Sanshandao Gold Mine, the chloride ions in the slurry have a great negative effect on the strength of the backfill. Figure 7 shows that the uniaxial compressive strength of the cement mortar sample with an amount of FA of 5% is 0.2 MPa better than the one without FA. When the amount continued to increase, the uniaxial compressive strength of cement mortar declined with increasing FA content. Analysis revealed that the activity of FA under the excitation of lime and brine water was restricted and large amounts of FA use presented a negative effect on the strength development and on cementing properties of binder [76].

**Figure 7.** The compressive strength of the specimen and the quantity FA content [76].

FA is in the form of spherical particles composed of many phases—amorphous and also crystalline compounds, mostly silicon, calcium, aluminium and iron oxides. The versatility of fly ash for production of different types of cements is attributed to its physical features, chemical properties and phase composition [77–79].

#### *2.3. Other Pozzolanic Ashes*

Rice Husk Ash (RHA). An agricultural by-product that is suitable for cement replacement in rice-growing regions is Rice Husk Ash (RHA) [38]. The cementitious characteristics of RHA unappreciated before the 1970s [26]. RHA is the combustion residue from rice husks, which are the stiff outer layer that accumulates during de-husking of paddy rice. Every tonne of paddy rice can yield around 200 kg of husk, which produces about 40 kg of ash after combustion. It is known that rice plants consume H4SiO4 from underground water that exists in saturated zones beneath the earth surface. H4SiO4 at this point is polymerized and leads to development of amorphous silica in the husks [80]. During combustion of the organic compounds, CO2 is produced along with the silica remaining in the ash leftovers. The researchers have demonstrated that the principal chemical composition of rice husk ash consists of biomass-driven silicon dioxide (SiO2). Table 5 summarizes its typical chemical composition and some of its properties. As a result that the nature of silica in rice husk ash is sensitive to processing conditions (see Table 5), the ash obtained through open-field burning or uncontrolled combustion in furnaces generally includes a high percentage of crystalline silica minerals, like tridymite or cristobalite, with inferior reactivity. The highest amount of amorphous silica is obtained when RHA is burnt at temperatures ranging from 500 to 700 ◦C [81]. The superior reactivity of RHA is due to its large amount of amorphous silica, which has high surface area due to the porous architecture of the host material. RHA can be used as a substitute in Portland cement (acceptable up to 15%), thanks to its pozzolanic activity. Fine RHA can increase the compressive strength of cement paste and can lead to preparation of mortars with low porosity [26,39].


**Table 5.** Typical chemical and physical properties of Rice Husk Ash (RHA) [82].

\* Minor Constituents Not Given.

As a cement substitute, the application of RHA in concrete production has advantages and disadvantages [39]. Improved compressive strength of concrete is one of the essential advantages of using RHA as substitute. Recent studies have highlighted important benefits of replacing cement with RHA in small percentages [26]. In the context of durability, the use of RHA as a substitute in concrete production can lead to notable improved water permeability resistance, Cl− penetration and sulphate deterioration [83].

Weiting Xu et al. [83] compared the pozzolanic impact of SF and ground RHA as SCMs on the properties of composite cement pastes and concretes. The authors evaluated mechanical property, workability, durability and microstructure. In the composite cement pastes, the binder (OPC) was substituted with SF and finely ground RHA (named FRHA) at 5%, 10%, 15%, 20%, 25% and 30%, respectively by mass. They reported that the optimal substitution percentage of SF and RHA were 10% by weight of cement in pastes and concretes. Compressive strength for this composition was evaluated (see Figure 8) [83].

\* Note: CRHA: RHA with 5 min grinding; and FRHA: RHA with 30 min grinding.

**Figure 8.** Compressive strength of studied pastes [83].

It can be observed that the sample with coarse rice husk ash (CRHA) had the lowest compressive strength at all curing ages, which could be because of the bigger particle size and small surface area of coarse RHA particles. Tests results showed that addition of FRHA to the paste led to an increased compressive strength compared to the control concrete, as a result of the growing specific surface area and pozzolanic activity of RHA. In addition, the morphological dissimilarity may also be implicated by the differences in compressive strength between the concrete specimens (see Figures 9 and 10) [83].

**Figure 9.** SEM images of concrete containing SF [83]. (**a**) Global Image—100 μm (**b**) Detailed image 5 μm.

**Figure 10.** SEM images of concrete containing SF [83]. (**a**) Global Image—100 μm (**b**) Detailed image 5 μm. In view of the empirical results [83], it appeared that FRHA exhibits similar pozzolanic and rheological activity to SF and can lead to notable improvement in the properties of a cementitious system.

Sugarcane bagasse ash (SBA) is a by-product of producing juice from sugar cane by crushing the stalks of the plants (see Table 6). The addition of SBA in concrete production can decrease the hydration temperature up to 33%, when 30% of OPC is substituted by SBA [42]. Furthermore, water permeability considerably decreases when compared to control concrete samples. With the aim of superior compressive strength, OPC was substituted in the range from 15% to 30%. It was also claimed that SBA aids the reduction of ASR expansion in concrete, by binding alkalis [26]. Table 6 lists the properties of SBA.



\* Minor constituents not given.

Use of SBA as limited cement substitute reduces hydration heat, compared to that of the control reference. The decrease of semi-adiabatic temperature rise (◦C) is proportional to the percentage of SBA replaced (see Figure 11). Therefore, SBA may be used to control the temperature in mass concrete pouring [82].

\* a number at the end indicates the percentage of sugarcane bagasse ash

**Figure 11.** Semi-adiabatic temperature rise (◦C) in concrete containing SBA as substitute [82]. SBA incorporation improved concrete durability. A composite concrete with additions of different amounts of SBA was studied in the context of chloride attack (see Figure 12), as well as gas and water permeability [41,82].

**Figure 12.** Effect of SBA on the chloride conductivity index of concrete [82].

Wastes of different sources have been investigated for their possibility in re-use, to reduce their environmental impact, in landfill volume and decomposition by-products [84]. Sewage sludge ash (SSA) is an urban waste that may be used as fertilizer, as well as a cement substitute [26,85–87]. SSA was not only considered as SCM in blended cements but also in a large scale of building materials like pave-stones, tiles, bricks, light aggregates production. The impact of SSA in mortar was a decrease in the compressive strength, when SSA was applied as partial cement substitute. Therefore, use of SSA as an SCM was shown to be limited, in the construction domain. The cement community does not include SSA in the group of pozzolanic materials [88].

Palm oil fuel ash (POFA) is an important cash-crop in tropical countries, especially in Malaysia and Indonesia [82]. For every 100 t of fresh fruit bunches handled, there will be about 20 t of nut shells, 7 t of fibres and 25 t of empty bunches released from the mills. POFA can be used in concrete either as aggregates, SCM or as filler material [89,90]. Comparable to RHA and SBA, the amorphous SiO2 (around 76%) content of POFA offers relatively high pozzolanic activity, when used as binder in concrete production. Even though a few performance parameters of concrete (especially setting time and strength) are negatively influenced by POFA, several studies claimed that palm oil fuel ash may be appropriate in different applications [91,92]. It may be an important resource in developing countries, although more studies are certainly needed, to support the use of POFA in structural applications [82,93,94].

Mine tailings. The amount of mine tailings has grown excessively with an ever increasing demand for metal and mineral resources [95]. Mining wastes are produced during mineral extraction by the mining industry and is at present one of the largest waste flows worldwide [25,95,96]. Mine tailings are finely ground in the time of mineral processing and separation of minerals of interest [96]. A significant part of milling processes and separation procedure uses water as the transport medium. Therefore, tailings obtained by mining enterprises usually consist of small particle slurries with high water content, flow ability and poor mechanical stability [95]. Mine tailings are deposited in dammed ponds along with industrial wastewater or as thickened pastes in piles close to the extraction sites [96]. At present, their principal use is as backfill in mined-out underground areas or as deposits in tailing ponds. As such, they pose potential long-term risks as environmental pollution. However, use of tailings is not only relevant to environmental conservation, but can also benefit the mining industry. These solid wastes contain compounds with potential pozzolanic properties and can decrease the amount of cement used to produce concrete, reducing simultaneously the ecological impact of the cement and mining industries. An additional benefit of mine tailings is that they are already finely ground. Most of the other SCMs require mechanical grinding, as a pre-treatment for use, to improve their reactivity. However, mine tailings already come with favourable physical and chemical properties as particle dimensions, crystal structures and even surface properties [96].

Therefore, depositories of tailings have lately gained global attention [95]. However, despite the potential of mine tailings as substitutes in the production of cementitious materials, there is a striking lack of studies regarding this topic in the cement and concrete literature.

Marble dust. Marble is a finely crystallized metamorphic rock originating from the low-intensity metamorphism of calcareous and dolomitic rocks. Calcium carbonate (CaCO3) can form up to 99% of the total amount of this carbonated rock. Additional phases may also include SiO2, MgO, Fe2O3, Al2O3 and Na2O and, in minor ratio, MnO, K2O, P2O5, F, Cu, S, Pb and Zn [97]. For a long time, marble has been a significant building material. For instance, annual production of marble in Turkey accounts for 7 Mt and represents 40% of the global storage; Turkey has an annual total production of block marble of ca. 1,500,000 m3, generating approximately 375,000 m3 of marble powder [98]. Throughout the shaping, sawing and polishing operations, around 20%–25% of processed marble is converted into powder or lumps. As a result, dumps of marble dust have become an important environmental issue worldwide [99]. Marble powder (MP) has successfully been demonstrated as a viable SCM in self-compacting concrete (SCC). The research proved that marble powder used as mineral substitute of cement can enhance some properties of fresh concrete and/or hardened concrete [100]. In the cement-related literature, there are just a few research studies related to the application of marble powder in concrete or mortar production. Thus, more detailed studies are needed in order to define the properties of concretes or mortars with marble powder. The use of marble powder in ternary cementitious blends demands further caution to remove or reduce its adverse effects on the fresh properties of self-compacting concrete and/or mortar [98].

Construction and demolition debris (CDD) constitute one of the massive flows of solid waste generated from municipal and commercial activities of modern society [101]. Usually, CDD are in the shape of brick bats, mortars, aggregates, concrete, glass, ceramic tiles, metals and even plastics. They must be mechanically sorted according to size and quality level. They are then crushed down to desired size [102]. It is essential to study the life cycle of construction materials to develop a global understanding of sustainable building construction and the feasible use of CDD as SCMs for OPC substitution. The life cycle of some construction materials, such as concrete, has been analysed to evaluate their environmental consequences. While substantial effort has been applied to LCA-based

sustainability assessment of construction materials and buildings, the specialty literature needs more detailed studies regarding the LCA of recycled construction materials, ones that take into account both process and supply chain-related outcomes as a whole [103]. The sheer mass and heterogeneity of CDD materials and absence of data classification across non-standardized tracking systems have led to new challenges [101]. In addition, the lack of knowledge about the possible savings and implications correlated with recycling of this kind of construction materials from the life cycle outlook still limits their use [103].
