Preparation and hydration of industrial solid waste–cement blends: A review

1.   Introduction

As one of the most widely used building materials, according to the United States Geological Survey data, cement production in 2020 was approximately 4.1 billion tons [1], and the amount of CO₂ generated was approximately 2.35 billion tons [2]. Moreover, CO₂ emissions from the cement industry have been increasing annually (Fig. 1). Overall, CO₂ emissions from the cement industry account for 5%–8% of the global total [2–3]. According to the Paris Agreement, the cement industry is facing unprecedented challenges.

Fig. 1.  Global cement production and CO₂ emissions from the cement industry [3–4].

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Globally, billions of tons of industrial solid wastes (ISWs) are generated annually [5]. In China, solid wastes, such as fly ash (FA), blast furnace slag (BFS), steel slag (SS), red mud (RM), and coal gangue (CG), are piled up in large quantities. Table 1 lists the sources, production, and disposal status of common ISWs.

Table  1.  Production and disposal of common ISWs

ISW	Source	Classification	Production	Disposal status
FA	Collected by the dust-removal systems of coal-fired power plants [6]	High-calcium FA and low-calcium FA [7]	The global annual production of approximately 750 million tons in 2020 [8–9].	The global FA utilization is only 25% of the total production [10].
BFS	Obtained from molten ironmaking slag after it underwent water quenching or air-cooled treatment [11]	Water-quenched BFS and air-cooled BFS	The global output in 2020 exceeded 500 million tons [12–13].	BFS can replace cement to a high level (70% is common) [14].
SS	Produced during basic oxygen furnace (BOF) or electric arc furnace (EAF) steelmaking [15]	BOF slag and EAF slag	The global production of BOF slag was 280–370 million tons in 2020 [12,16].	Most SS is treated as waste in landfills or stockpiles [17].
RM	Produced during the alumina-refining process of bauxite [18–19]	Bayer RM and sintered RM	The cumulative global stock of RM is approximately 600 million tons [20].	A significant amount of RM is stockpiled [21].
CG	Produced during coal mining and washing [22–23]	—	CG constitutes 15%–20% of the total global coal production [22].	China’s CG reserves have reached 5 billion tons [24].

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The safe, economical, and large-scale treatment of these solid wastes is key to enabling the sustainable development of related industries. Fortuitously, these solid wastes not only contain large amounts of SiO₂, Al₂O₃, CaO, and other valuable components [25–26], but also display high pozzolanic activity. As such, these materials have significant potential as supplementary cementitious materials [27]. Compared with cement, utilizing ISW–cement blends can synergistically reduce solid wastes and reduce cement consumption. For every ton reduction in ordinary Portland cement (OPC) production, CO₂ emissions will be reduced by 1.0 t, and 4.0 GJ energy will be saved [28]. It is worth noting that the chemistry of supplementary cementitious materials is generally characterized by lower calcium content than Portland cement (PC) (Fig. 2), which influences hydrates and strengths [25].

Fig. 2.  Relative content of silicon and aluminum components in ISWs [16,25,29–32].

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The application of ISW–cement blends provides a powerful means for the cement industry to cope with the global carbon emission reduction challenge. As a result, ISW–cement blends have received considerable research attention. This review examines the latest developments in this field, including the activation of ISWs, formation of ISW–cement blends, and associated hydration mechanisms. In addition, future directions for the development of ISW–cement blends are considered.

2.   Preparation of ISW–cement blends

2.1   Activation of ISWs

The factors determining the reactivity of ISWs include their fineness, chemical composition, degree of crystallization, and polymerization state of silica–oxygen tetrahedrons in the glassy structure [7,17,33]. ISW activation mainly includes mechanical and thermal activations.

2.1.1   Mechanical activation

In the mechanical activation process of ISWs, the action of mechanical forces is mainly divided into three categories (Fig. 3). The first mechanism is the macro-physical action, through which the particle size distribution of a material is refined and improved, thereby increasing the specific surface area and surface energy of ISWs [34–35]. The second mechanism is the micro-physical action, which can result in microscopic defects in ISW particles, increase lattice distortion energy, and reduce crystallinity [35–37]. Finally, the third mechanism is the chemical action, which can reduce the activation energy of ISWs and promote the formation of new crystal nuclei and the breaking of chemical bonds [34,38–39].

Fig. 3.  Mechanisms of the mechanical activation of ISWs.

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Particle size distribution and particle morphology are the main factors affecting ISW reactivity [40–41]. As shown in Fig. 4, mechanical activation mainly affects aluminosilicate glass and breaks the specific surface area of quartz [35]. In the dissolution stage, the reaction of un-milled FA is low, and the reaction activity of FA increases with the increase in the mechanical activation time.

Fig. 4.  Schematic diagram of the effects of mechanical activation on FA characteristics and reactivity: Q = quartz, M = mullite, G = glass [35]. Reprinted from Adv. Powder Technol., 28, S. Kumar, G. Mucsi, F. Kristaly, and P. Pekker, Mechanical activation of fly ash and its influence on micro and nano-structural behaviour of resulting geopolymers, 805-813, Copyright 2017, with permission from Elsevier.

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Table 2 lists common ISW mechanical activation processes. After the mechanical activation of ISWs, the D₅₀ (average size) typically drops below 10 µm, and the specific surface area increases above 350 m²/kg [34,42–43]. Compared with dry milling, wet milling has higher advantages in terms of energy intensity and production efficiency [34,38,44]. However, balls and liners are highly susceptible to corrosion during the wet milling process [44].

Table  2.  Common ISW mechanical activation processes

ISW	Mechanical activation process	Results	Ref.
FA	Ball mill, 48 r/min	D50 (53.94 µm) reduced to 9.16 µm. Specific surface area (410.6 m2/kg) increased to 803.0 m2/kg.	[40]
	Vertical mill	D50 (53.94 µm) reduced to 6.27 µm. Specific surface area (410.6 m2/kg) increased to 897.6 m2/kg.	[40]
	Ball mill	D50 (9.67 µm) reduced to 3.51 µm.	[45]
	Ultrafine grinder	D50 (30.50 µm) reduced to 4.04 µm. Specific surface area (360 m2/kg) increased to 660 m2/kg.	[46]
BFS	Vertical stirred mill, 400 r/min	D50 = 2.95 µm. The activity index reached 126.5%.	[34]
	Vertical stirred mill, 400 r/min	D50 (18.12 µm) reduced to 3.87 µm.	[42]
	High energy milling, 400 r/min	Specific surface area (290 m²/kg) increased to 1800 m2/kg. After 3 h, the BFS tended to agglomerate.	[43]
SS	Planetary ball mill, 250 r/min.	Specific surface area (234 m2/kg) increased to 359 m2/kg.	[17]
	Jaw crushers	80wt% of the slag crumbled to −45 µm.	[47]
CG	Vertical stirred mill, 450 r/min	D50 (16.3 µm) reduced to 4.97 µm. Specific surface area (1275 m2/kg) increased to 2168 m2/kg.	[44]
	Jaw crusher, planetary mill	Volume mean diameter and bulk density decreased (grinding time < 10 h), increased (grinding time = 10–14 h), and decreased (grinding time > 14 h).	[48]
FA + RM	Ball mill, 500 r/min	80% of particle sizes were less than 80 µm.	[49]
RM + CG	High-energy planetary ball mill	Blaine’s specific surface area = 22 m2/g.	[50]

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As the mechanical activation time is prolonged, although ISW reactivity will increase, the energy consumption, cost, and environmental pollution will all sharply increase. The use of grinding aids in the mechanical activation process can effectively improve the ISW activation efficiency. For instance, Prziwara et al. [51] reported that at a concentration of 0.1wt% of the grinding aid, the reduction in BFS agglomeration followed the trend of n-heptanoic acid > triethanolamine > 1-hexanol > diethylene glycol. The addition of 0.1wt% n-heptanoic acid resulted in a high fluidity index and small agglomerates of the heat-treated FA [51]. Zhao et al. [52] reported that the specific surface area of ultrafine FA with a 0.05wt% grinding aid prepared by mixing triethanolamine and ethylene glycol was 104 m²/kg higher than that of the control sample, and the content of particles of less than 16 µm was significantly increased. The 3-day strengths of blends containing 20wt%, 30wt%, and 40wt% ultrafine FA increased by 12.6%, 6.6%, and 6.6%, respectively, and the 28-day strengths increased by 6.5%, 9.3%, and 10.5%, respectively [52]. Krishnaraj and Ravichandran [53] reported that Blaine’s fineness value for chloride-based ground FA after ball milling for 120 min was five times that of a raw FA sample. At 28 d, the compressive strengths of cement containing 15wt%, 30wt%, and 45wt% of ground FA were 13%, 6%, and 2% higher than the control samples, respectively.

2.1.2   Thermal activation

Thermal activation is often used to activate solid wastes containing clay minerals. Thermal activation can increase the dehydroxylation degree of clay minerals, thereby increasing their reactivity [54]. The optimal activation temperature of clay minerals depends on many factors, including the mineralogical and chemical compositions of additives, removal rate of impurities (carbon and carbonate), amorphous content, and dehydroxylation degree [54–55].

The thermal activation temperature of CG is usually 400–1000°C [56–58]. The low active calcium content of CG mainly contributes to its low strengths [59]. Considering that non-ferrous metallurgical slag is low in calcium and high in iron and is dominated by crystalline minerals, it is also often mixed with calcium in the molten state and then quenched with water to increase the content and activity in the glass phase. The addition of Ca²⁺ destroys some bridging oxygen bonds in the glassy network, reduces the polymerization degree of the glassy network, and facilitates the dissolution of active silicon, aluminum, and iron ions (Fig. 5) [60].

Fig. 5.  Schematic diagram of the iron silicate glass structure: BO = bridging oxygen, NBO = non-bridging oxygen [60]. Reprinted from J. Clean. Prod., 232, Y. Feng, Q.X. Yang, Q.S. Chen, J. Kero, A. Andersson, H. Ahmed, F. Engström, and C. Samuelsson, Characterization and evaluation of the pozzolanic activity of granulated copper slag modified with CaO, 1112-1120, Copyright 2019, with permission from Elsevier.

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During thermal activation, an alkali source, such as sodium hydroxide, is usually added. For instance, RM was used as an aluminosilicate source after being activated by NaOH at 800°C [61–62]. Ke et al. [62] identified the C₃A, C₂S, and CAS₂ phases from RM treated with NaOH. RM reacted with NaOH at 800°C, and reaction products, such as calcium silicate and sodium aluminum silicate, were hydrated when in contact with water [63].

2.2   Curing of ISW–cement blends

The main factors influencing the curing process are the curing temperature, relative humidity (RH), and curing age. The curing of ISW–cement blends can be divided into room-temperature curing (20–30°C) and high-temperature curing (40–80°C) [64]. As the hydration temperature increases, the glassy network structure is subjected to thermal stress, making Si–O and Al–O bonds likely to break, which is beneficial to the depolymerization of the glassy network. Within an appropriate range, high temperatures are conducive to the formation of a dense microstructure and fine pore distribution [65]. When an upper-temperature limit was exceeded, compressive strengths decreased with further temperature increases [66]. This behavior is attributed to rapid gel formation, which prevents the transition to a dense structure. Moreover, microcracks can form as a result of high-temperature drying and shrinking processes [65,67]. In addition to temperature, RH needs to be controlled during curing, as shown in Table 3. Cementitious materials often cause weathering, microcracking, and subsequent decreases in compressive strengths due to dehydration, so curing is typically performed under saturated conditions (RH = 100%). At present, most of the curing of cementitious materials uses a single-stage curing process, i.e., maintaining a certain curing temperature and humidity to a prescribed age. The hydration activity of the blends cannot be fully activated easily in the early stage while ensuring stable mechanical properties in the later stage.

Table  3.  Effect of curing on the mechanical properties of ISW–cement blends

Curing schedule		Blends	Results	Ref.
Temperature	RH and age
20°C	Water (120 d)	BFS + PC	After 3 h of mechanical activation, the 120-d compressive strengths of the sample increased by 8.8%.	[43]
	> 95% (7 d)	FA + PC	Compared with the unactivated samples, the 1-d and 7-d compressive strengths of (K,Na)2SiO3-activated samples were increased by 45% and decreased by 21%, respectively.	[68]
	≥95% (1 d), water (1–60 d)	BFS + PC	Compared with the unactivated samples, the 1-d, 3-d, 7-d, 28-d, and 60-d compressive strengths of Na2SO4-activated samples increased by 15.8%, 19.5%, 39.0%, 25.3%, and 4.7%, respectively.	[69]
23°C	100% (7 d)	FA + PC	The early compressive strengths of Na2SO4-activated samples were approximately 40% higher than that of unactivated samples.	[70]
25°C	Covered with a polyethylene film (1 d), >95% (1–28 d)	BFS + PC	Compared with the unactivated samples, the 1-d compressive strengths of NaOH-activated and Na2SiO3/NaOH-activated samples increased by 4.5 and 10.8 times, respectively.	[71]
27°C	90% (1 d), water (1–28 d)	BFS + PC	The 7-d and 28-d BFS activity indexes after mechanical activation and Na2SiO3 activation increased by 65.94% and 72.19%, respectively.	[72]
40°C	Covered with wet hessian (91 d)	BFS + PC	The 3-d, 7-d, 28-d, 56-d, and 91-d compressive strengths of samples after mechanical and Na2SO4 activation increased by 138.1%, 72.7%, 49.2%, 37.5%, and 27.0%, respectively.	[73]
20–60°C	>90% (9 h)	FA + OPC	After curing for 9 h, the compressive strengths of samples with 1.0wt% and 4.0wt% nano-SiO2 increased by 22% and 106%, respectively, compared with that without nano-SiO2.	[74]

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3.   Hydration mechanisms

3.1   Hydration of ISW–cement blends

After the high-temperature calcination and rapid cooling of cement clinker, many defects can form in the crystal structure, which enhances the material reactivity. Clinker minerals include silicon-phase (C₃S and C₂S), aluminum-phase (C₃A), and iron–aluminum-phase solid solutions (C₄AF). The cement hydration reaction is exothermic and based on the hydration heat release curve as measured by isothermal calorimetry experiments, the hydration of cement can be divided into the (I) pre-induction period, (II) induction period, (III) acceleration period, (IV) deceleration period, and (V) stable period [75]. The thermodynamic simulations showed that, over time, the formation of C–S–H and Ca(OH)₂ increases until a certain saturation level is reached (Fig. 6).

Fig. 6.  Evolution of the main solid phases during the PC hydration [76].

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During the first few hours of the hydration of paste containing cement and ISWs, ISW particles became inert. Berodier and Scrivener [77] replaced 40wt% cement with BFS or FA, and the resulting changes in hydration kinetics during the first 24 h were similar to those shown by the quartz–cement curve. This result showed that BFS and FA particles were inert during this period, and their physical presence can promote the hydration of cement, termed the filler effect [78].

The filler effect of solid wastes usually occurs on the first day of hydration when the microstructure and performance rapidly develop [25,77]. The filler effect is attributed to three mechanisms: the dilution effect (forming an extra space for hydrates or increasing the water–cement ratio), heterogeneous nucleation effect (solid waste provides additional surfaces and hydrates catalyze the nucleation process by lowering the energy barrier), and accelerated dissolution effect [25,77,79].

When the ISW–cement blend system was mixed with water, the C₃S, C₂S, C₃A, C₄FA, and gypsum components in the cement simultaneously dissolved with the soluble minerals on the ISW surface. Several cations, such as Si⁴⁺ and Ca²⁺, and anions, such as OH⁻ and ${\text{SO}}_{\text{4}}^{{2 - }}$, were released [80–81] (Fig. 7(a)). Cement first underwent hydration reactions to form a large number of products, such as Ca(OH)₂ and C–S–H gel. Ca(OH)₂ formed a hydration film on the surfaces of the ISW particles via interactions at the phase boundaries [80]. Simultaneously, OH⁻ reacted with the surfaces of the ISW particles, causing the aluminosilicate network structure to break down and different complexes [81]. Thus, hydration products were further formed [82] (Fig. 7(b)). Many hydration products combined to form structural networks (Fig. 7(c)) and filled the pores among the particles. When the ISW surface was completely covered by the products, the ISW hydration switched to a diffusion-controlled reaction [81].

Fig. 7.  Schematic diagram of the FA–CG–cement microstructure formation: (a) dissolution period, (b) hydration period, and (c) flocculated-structure formation and growth period [81]. AFt = ettringite.

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The hydration of ISW–cement blends is mainly affected by the inherent properties of ISWs, such as their crystal structure and physical and chemical properties [83]. The crystal structure of ISWs is mainly composed of silicon–oxygen and aluminum–oxygen tetrahedrons. Each silicon–oxygen tetrahedron is connected to bridging oxygen (Si–O–Si). Aluminum ions also occupy the middle positions of some tetrahedra. Substituting AlO₄ for SiO₄ produces negative charges that are balanced by exchangeable cations [84–85], increasing the depolymerization of the Si–O–Si network. The early stage dissolution mechanism of aluminosilicate glass proposed by Duxson and Provis [86] is shown in Fig. 8. Due to their large ion radius, alkaline earth metals (Ca²⁺ and Mg²⁺) in the aluminosilicate glass structure result in a great degree of distortion. Some weaker Al–O–Al bonds are formed, and the number of non-bridging oxygen atoms is increased, which reduces the polymerization degree [87]. Moreover, the degree of “damage” to the network structure increases after it is replaced in the dissolution process. This finding explains the activity of Ca²⁺- and Mg²⁺-rich BFS, which is better than that of high-calcium and low-calcium FA.

Fig. 8.  Dissolution mechanism of an aluminosilicate glass during the early stages of reaction: (a) exchange of H⁺ for Ca²⁺ and Na⁺, (b) hydrolysis of Al–O–Si bonds, (c) breakdown of the depolymerized glass network, and (d) release of Si and Al. P. Duxson and J.L. Provis, J. Am. Ceram. Soc., 91, 3864-3869 (2008) [86]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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3.2   Alkali activation of ISW–cement blends

ISW–cement blend hydration is usually conducted under the action of alkali activators, which results in a highly alkaline medium [7]. The high alkalinity (pH > 13) of these solutions is conducive to the rapid dissolution of ISWs. Table 4 lists the chemical activation processes of common ISW–cement blends. Commonly used activators are NaOH, Na₂SiO₃, Na₂CO₃, and Na₂SO₄ [68–71,88].

Table  4.  Common ISW–cement blend chemical activation processes

Blends	Activator	Results	Ref.
FA + OPC	Na2CO3/potassium citrate	Na2CO3 did not or only slightly enhance the pozzolanic reaction of FA. Potassium citrate hindered the reaction between FA and OPC.	[68]
	Sodium oxalate/(K,Na)2SiO3	Sodium oxalate and (K,Na)2SiO3 accelerated the solidification of the FA–OPC mixture, increased the early compressive strengths of the FA-OPC mixture, and reduced the later strengths.	[68]
	Na2SO4	During 3–7 d, the compressive strengths of activated samples increased by approximately 40% compared to the samples without an activator.	[70]
	Nano-SiO2	After curing for 9 h, compared with no nano-SiO2 added, 1.0wt% and 4.0wt% nano-SiO2 increased the compressive strengths of the samples by 22% and 106%, respectively.	[74]
	NaOH	The prevalence of N–A–S–H gel interfered with the normal hydration of the calcium silicate phase in the clinker.	[88]
	Moderately alkaline compounds	The initial rapid hydration of calcium silicate in the clinker produced sufficient Ca and alkalinity to convert part of the soluble alkaline salt into NaOH and then activated the glass phase in the FA.	[88]
BFS + OPC	Na2SO4	Compared with the unactivated samples, the compressive strengths of Na2SO4-activated samples at 1, 3, 7, 28, and 60 d increased by 15.8%, 19.5%, 39.0%, 25.3%, and 4.7%, respectively.	[69]
	Na2CO3	The compressive strengths of samples at 60 d after Na2CO3 activation were more than 1.5 times that at 28 d.	[69]
	Na2SiO3	The compressive strengths of Na2SiO3-activated samples at 1 and 60 d were 2.18 and 27.08 MPa, respectively.	[69]
	NaOH	The compressive strengths of samples activated by NaOH before 28 d were higher than that of Na2SiO3 and Na2CO3 activations, and the compressive strengths after 60 d were the lowest.	[69]
	NaOH + Na2SiO3	Compared with the unactivated samples, at 1 d, the compressive strengths of samples after activation by NaOH increased 4.5 times, and the compressive strengths of samples after activation by Na2SiO3 and NaOH increased 10.8 times.	[71]
SS + OPC	Na2SiO3/Na2SO4	Compared with the unactivated samples, the compressive strengths of Na2SiO3- and Na2SO4-activated cement-slag slurry at 28 d increased by 12.63% and 2.59%, respectively.	[17]
	NaOH/Na2CO3	Compared with the unactivated samples, the NaOH- and Na2CO3-activated cement-slag binder paste showed lower 28-d compressive strengths.	[17]

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High-alkalinity activators, such as NaOH and Na₂SiO₃, accelerate the rupture of Si–O and Al–O bonds. Soluble silica exists in the form of silicate ions, which can enhance the polymerization rate of ionic species present in the system [89]. The hydration of cement is affected by the alkalinity and presence of soluble silica [88]. The hydration mechanism of NaOH- and sodium silicate-activated FA–cement blends was proposed by García-Lodeiro et al. [90]. They reported that the C–S–H gel development proceeds as follows: C–S–H → C–(A)–S–H → C–A–S–H and N–A–S–H gel evolved as follows: N–A–S–H → (N,C)–A–S–H → C–A–S–H. Compared with high-alkalinity activators, the activation of inorganic salts, such as Na₂CO₃ and Na₂SO₄, has less influence on the hydration reaction of the cement clinker. Cement first generates portlandite with water and then reacts with inorganic sodium carbonate or sodium sulfate salts to generate insoluble calcium salts or NaOH [88,91]. The generated NaOH stimulates the glassy phase in FA, resulting in paste freeze. The use of different alkalinity activators has a direct impact on hydration kinetics and modifies the proportion of each gel in the mixture of C–A–S–H and (N,C)–A–S–H gels [88].

4.   Environmental impact and cost comparison

Table 5 lists the energy intensity, carbon emissions, and raw material costs of blended cement based on Chinese data. Wu et al. [92] used the material sustainability indicators to quantify that the prepared ultrafine cement containing 70wt% FA and ground granulated BFS (GGBFS) had 59%, 53%, and 30% reductions in CO₂ emissions, energy consumption, and cost, respectively, compared to those of PC. Nguyen et al. [93] evaluated the CO₂ emissions and costs of cementitious materials, such as OPC, FA–cement (30wt% FA + 70wt% OPC, FAC), and BFS–cement (30wt% GGBFS + 70wt% OPC, SC), using the life cycle assessment (LCA) techniques. The results show that SC and FAC consumed 44% and 47% less energy consumption than OPC, respectively, and the SC–FAC blend reduced CO₂ emissions by up to 68% and energy consumption by up to 68%. Hossain et al. [94] evaluated the environmental performance and associated costs of normal concrete (NC) produced by OPC, FA concrete (30wt% FA, FC), and GGBFS concrete (30wt% GGBFS, SC). Compared with NC, the results showed that SC and FC reduced the cost by 11% and 15%, respectively.

Table  5.  Energy intensity, carbon emissions, and costs of blended cement components [92]. Reprinted from J. Clean. Prod., 196, M. Wu, Y.S. Zhang, Y.S. Ji, G.J. Liu, C. Liu, W. She, and W. Sun, Reducing environmental impacts and carbon emissions: Study of effects of superfine cement particles on blended cement containing high volume mineral admixtures, 358-369, Copyright 2018, with permission from Elsevier

Materials	Energy intensity / (MJ·kg−1)	CO2 emission / (kg·t−1)	Cost / (CNY·t−1)
PC	5.5	930	600
FA	0.1	8	100
GGBFS	1.6	83	300
gypsum	1.8	120	650

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Radwan et al. [95] evaluated the eco-mechanical performance of blended mortars based on the calculated warming-potential-to-strength and particulate-matter-to-strength ratios. The study showed that the FA–BFS–cement ternary hybrid system with a substitution level of 40% is a feasible environmental protection solution, and the eco-mechanical properties were improved by 37%. Kulasuriya et al. [96] compared the energy, ecological impacts, and financial costs of the alkali pozzolan cement (APC) mixture (70wt% APC + 30wt% OPC), the high volume FA (HVFA) mixture (70wt% FA + 30wt% OPC), and OPC. The results showed that the APC mixture has lower energy, carbon, and financial costs than the HVFA mixture and OPC. Table 6 lists the durability calculations for the mixtures. In terms of the combined strength and chloride durability, the HVFA mixture (water/binder ratio = w/b = 0.4) outperformed the APC mixture (w/b = 0.4) and OPC (w/b = 0.4). Espuelas et al. [97] performed the LCA analysis of cement with different BFS or FA contents and combinations of 15 alkali-activated binders. They concluded that GGBFS 6M is optimal in terms of acidification and eutrophication, close to optimal in terms of climate change and dust impacts, and is the best overall combination from an environmental perspective.

Table  6.  Estimation of the mixture durability in carbonized and chloride environments [96]. Reprinted from J. Clean. Prod., 287, C. Kulasuriya, V. Vimonsatit, and W.P.S. Dias, Performance based energy, ecological and financial costs of a sustainable alternative cement, art. No. 125035, Copyright 2021, with permission from Elsevier

Mixture	w/b	Average carbonation durability / a	Average chloride durability / a
APC Mixture	0.3	17.7	>100
APC Mixture	0.4	4.4	25
HVFA Mixture	0.3	2.1	>100
HVFA Mixture	0.4	1.0	65
OPC	0.3	>100	69
OPC	0.4	>89	11

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The environmental impact and cost comparison findings show that the blended cement prepared from ISWs exhibits greater advantages in terms of carbon emission reduction, energy and cost savings, ecological environmental protection, and improvement of mechanical properties and durability.

5.   Conclusions

ISWs, such as FA, BFS, SS, RM, and CG, are often used as supplementary cementitious materials due to their pozzolanic activities. Activation techniques for ISWs include mechanical and thermal activations. The actions of mechanical activation on ISWs can be divided into macro-physical action, micro-physical action, and chemical action. Grinding aids are usually added during the mechanical activation of ISWs to improve their activation efficiency and reduce energy consumption. Furthermore, thermal activation can increase the glassy-phase content and reactivity of ISWs, where the CG activation temperature is usually set at 400–1000°C. ISWs mainly play a physical role in the early stage of hydration of ISW–cement blends, which is mainly attributed to the dilution effect, heterogeneous nucleation effect, and accelerated dissolution effect. The chemical role of ISWs becomes apparent as ISW–cement blends continue to hydrate. ISWs are activated by the Ca(OH)₂ produced by the cement and produce C–(A)–S–H gels that fill the pores between cement particles. Moreover, alkali activation affects the hydration kinetics of ISW–cement blends and modifies the proportion of gels.

Few ISWs have the same reactivity as FA and BFS, and their clinker replacement level is generally low. Fortunately, considering the similarity of their chemical properties, we can apply the mature activation technology and formation process of FA and BFS to other solid wastes. However, some areas are not shared, such as the environmental and economic impact of blended cement. FA–cement blends generally exhibit higher energy and cost savings than BFS–cement blends. FA and BFS seem to have greater advantages in terms of environmental impact and cost as supplementary cementitious materials.

The key points to allow a great large-scale and diverse utilization of ISWs are also the microstructural development of blended cement and the safe use of ISWs. The addition of ISWs to cement generally retards its hydration process and affects its strength and durability. Furthermore, some added ISWs may present a risk of heavy metal leaching.

6.   Prospects

ISW–cement blends can synergistically dispose of solid wastes and reduce cement consumption. Solid wastes, such as FA and BFS, have a very mature activation technology and forming process. More attention should be paid to the application of low-reactivity solid wastes, such as SS and RM. By combining research practices and other literature findings, further research should focus on the following areas: (1) rapidly assess the reactivity of ISWs and establish quantitative relationships between material composition structure and reactivity; (2) based on the quantitative relationship between the composition structure and reactivity, multiple activation techniques or activators are matched to promote the activation of ISW reactivity; (3) develop models to effectively predict the strengths of blended cement to improve the effective utilization of ISWs; (4) precisely calculate the material flow and energy flow in the blended cement manufacturing process to quantitatively analyze its sustainability indicators.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (Nos. 2019YFC1907101 and 2021YFC1910504), Key R&D Program of Ningxia Hui Autonomous Region (Nos. 2020BCE01001 and 2021BEG01003), National Natural Science Foundation of China (Nos. U2002212 and 51672024), Xijiang Innovation and Entrepreneurship Team (No. 2017A0109004), the Fundamental Research Funds for the Central Universities (Nos. FRF-BD-20-24A, FRF-TP-20-031A1, FRF-IC-19-017Z, FRF-GF-19-032B, and 06500141), and Integration of Green Key Process Systems MIIT.

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.