Short Review on the Application of Recycled Powder in Cement-Based Materials: Preparation, Performance and Activity Excitation

Abstract

Recycled powder is a kind of powder particle with a particle size of less than 75 μm produced in the process of preparing recycled aggregate from construction waste with concrete and brick as the main components. It has the potential to replace part of cement as an auxiliary cementitious material. This has important engineering application value for promoting the full-component and high-quality utilization of construction waste, which meets the needs of national sustainable development. Based on the physical and chemical characteristics of recycled powder, the preparation process and basic properties of the recycled powder were systematically analyzed. Based on the low activity of recycled powder, different methods of its activity excitation were described in detail. In addition, some existing problems in the current research were also prospected.

1. Introduction

With the continuous development of industrialization and urbanization, the construction and demolition of infrastructure has accelerated, thus generating a large amount of construction waste. Construction waste removal increased from 21 billion to 40 billion tons between 2007 and 2014 in China, and 60% of construction waste is waste concrete [1]. At the same time, concrete has become the world’s largest and most widely used civil engineering material, with China producing about 20 billion tons of concrete every year [2]. The production of concrete requires the consumption of large amounts of cement, which emits large amounts of CO₂ during the production of the cement, exacerbating the global greenhouse effect. Therefore, how to solve the associated construction waste and carbon emissions has become a hot topic recently. Unlike domestic waste, most construction waste can be reused as renewable resources. Generally, construction waste is mainly composed of concrete and bricks, as shown in Figure 1. Therefore, in order to reduce the consumption of natural resources and mitigate the greenhouse effect, the use of other cementitious materials to replace part of the cement has become an effective method [3]. Waste concrete will inevitably produce recycled powder with a particle size of less than 75 μm in the process of crushing and screening [4]. It accounts for about 10–20% of the raw material mass [5]. Its main components are unhydrated partial cement, inert materials, hardened cement stone, and sand and stone aggregate debris. Much attention has been paid to recycled powder and the application of cement-based materials in the research because of their good micro-aggregate filling effect and volcanic ash effect [6,7].

However, due to the large fluctuations in composition and low activity, recycled powder directly affects the hydration and hardening process, mechanical properties, and durability of the recycled powder cement-based materials. Therefore, studying the basic properties and activity excitation of recycled powder can not only promote the high-quality utilization of recycled powder, but also supplement mineral admixtures, which meets the needs of national sustainable development. In this paper, based on the physical and chemical characteristics of recycled powder, the preparation process and basic properties of the recycled powder were systematically analyzed. Based on the low activity of recycled powder, different methods of its activity excitation were described in detail. In addition, some existing problems in the current research were prospected to provide theoretical support for future research.

2. Physical and Chemical Properties and Preparation of Recycled Powder

2.1. Physical and Chemical Properties

Recycled powder (RP) is a kind of particle with a particle size of less than 75 μm produced in the process of preparing recycled aggregate from construction waste with concrete and brick as the main components [8,9]. The surface of recycled powder particles has partial joint surfaces and many fine particles, as shown in Figure 2a, which is a non-reactive powder [10]. The main chemical composition of the recycled powder was analyzed by X-ray diffraction, as shown in Figure 2b. It can be seen that the main component of the recycled powder is SiO₂, and the content is high. The main reason is that some crushed stone chips are inevitably mixed in the preparation process of the recycled powder. In addition, recycled powder also contains Ca(OH)₂, unhydrated cement particles, tobermorite, and other components. Therefore, recycled powder has the ability to act as a cement hydration crystal nucleus and continue to hydrate to form gel products, showing good pozzolanic activity. It is feasible as a cementitious material in theory [11]. In addition, due to the complex and diverse components of building materials in different regions of the world, such as concrete materials, brick–concrete materials, and clay brick materials, the properties of the prepared recycled powder are quite different. Therefore, research on recycled powder from different sources is helpful for its high-quality utilization.

2.2. Preparation Method

There are two main preparation methods for recycled powder. In the first way, the waste powder (WP) generated during RA production is treated by mechanical crushing and grinding; the second is carried out in two stages; in the first stage, the crusher is used to crush waste concrete and clay bricks into particles with a diameter of less than 5 mm. In the second stage, the products in the first stage are sent to the ball mill to produce RP with a smaller diameter, which generally does not exceed 45 μm. Figure 3 describes the method of preparing RP through the deep processing of construction and demolition waste. The total processing time including crushing, screening, and grinding is about 3–5 min [8].

Because of the limitation of separation technology and cost of construction waste, recycled aggregate (RA) generally comes from waste solid concrete and clay brick-containing mortar. Therefore, the recycled powder (RP) collected from the preparation of RA is usually a mixture of recycled concrete powder (RCP) and recycled brick powder (RBP) [12].

3. Properties of Recycled Powder

Recycled powder in relation to the mechanical properties of cement-based materials has a close relationship with the amount of its admixture, with the appropriate amount of recycled powder instead of cement having a certain promotion effect on its mechanical properties. The research [13] shows that the flexural strength of mortar tends to increase firstly, and then decrease with the increase in recycled powder content, and the best content is 10%. However, other studies [4] have shown that the compressive strength of cement-based materials decreases with the increase in recycled powder content after the incorporation of recycled powder without active treatment. The 28d compressive strength of concrete mixed with 15–45% recycled powder decreases by 25~73% [10], which is similar to the conclusion of Pera et al. [14]. The reason is that recycled powder contains hydrated and partially unhydrated cement and inert materials with low activity and a high ratio surface. The increase in water demand makes the hydration reaction difficult to occur [15].

The durability of cement-based materials is also crucial, it directly determines the service life of buildings. Therefore, the durability of recycled powder has attracted great attention. The carbonation test [16] shows that the carbonation resistance of concrete can be effectively improved by adding 10% recycled powder, which is 7.24% lower than the carbonation depth of the reference group, and the carbonation resistance of concrete can be further improved by grinding refinement treatment. The freeze–thaw test of recycled powder mortar shows that the freeze–thaw performance of recycled powder increases with the amount of activator content [17]. Compared with other mineral admixtures, the dry shrinkage of recycled powder self-compacting concrete is better than slag concrete and limestone concrete, but the composite incorporation with slag is conducive to improving the performance of concrete [18].

Currently, recycled powder is mainly added to concrete as a mineral admixture rather than as a mixture to prepare cement [19]. Due to the low activity of recycled powder, the strength of mortar or concrete prepared by adding recycled powder will be significantly affected, so the application of recycled powder is greatly limited. Therefore, it is essential to study the activation of recycled powder to further enhance its activity and utilization efficiency [20].

4. Active Excitation of Recycled Powder

Although there are few reports on the utilization of mixed recycled powder, some studies on the performance of hybrid recycled powder have been carried out. Waste concrete has high potential activity as one of the main sources of recycled powder. Due to the residual C₂S, C₃S, silica, and free calcium oxide in recycled powder, the compressive strength of a 100% recycled powder sample can reach more than 80% of that of ordinary Portland cement sample [5,21]. In addition, mixed recycled powder occupies a large proportion of construction waste [22]. Therefore, it is of great significance to recycle recycled powder and stimulate potential activity. At present, the common excitation methods are: thermal, physical, chemical, and biological activation [23,24,25,26,27]. In addition, some studies used carbon dioxide carbonization and nano-reinforcement to enhance the compressive strength and optimize the microstructure of the cement materials [28,29,30,31,32].

4.1. Fineness

Mechanical activation is to release the activity of the wrapped material by increasing the particle size of the material, and thereby improving the specific surface area of the material. However, the grinding particles should not be too fine; otherwise, they will lead to material agglomeration and reduce the strength of the materials.

Improving the fineness of recycled powder is an effective method to enhance the activity of recycled powder. The specific surface area of recycled powder has a positive impact on the hydration reaction and the pozzolanic reaction [33,34]. Recycled powder particles tend to spheroidize by mechanical grinding. The specific surface area of the recycled powder increases and the surface binding energy decreases with the increase in grinding time, which enhances the volcanic ash activity. At the same time, grinding can also affect its microstructure; for example, due to mechanical force, the tetrahedral structure of silicon dioxide (α-SiO₂) is distorted and transformed amorphously, and the amorphous nature of silicon dioxide can increase the activity of recycled powder; however, too long a grinding time will cause the agglomeration of recycled powder, which reduces the refining efficiency [35]. In addition, some studies have shown that when the grinding time is more than 20 min, the improvement efficiency of the activity is not high; however, after 30 min of grinding time, the fine particles are finer and the particle size distribution is relatively reasonable, which can better fill the internal pores of the recycled mortar and achieve the highest compressive strength and flexural strength [20]. Figure 4 shows the XRD patterns of recycled powder under different grinding times. It can be seen that the main mineral components of the recycled powder are CaCO₃ and SiO₂. With the increase in grinding time, the particle size of recycled powder gradually decreases, and the encapsulated mineral components can be detected. Figure 5 shows the SEM of the recycled powder with different grinding times. By comparison, it was found that the morphology of the recycled powder particles without mechanical grinding was very irregular. With the increase in grinding time, the particle size in the micro powder decreased significantly and the particle shape became more regular.

Mehdizadeh et al. [30] found that recycled powder with a particle size of less than 75 μm can promote the formation of calcium alumite through XRD analysis. However, when recycled powder with a particle size greater than 75 μm was used, no similar phenomenon occurred. The reason for the latter is that recycled powder particles with a large particle size cannot improve the pore structure of concrete and have a negative impact on the microstructure. In addition, as the particle size of the recycled powder decreases, the content of amorphous oxides in the gelling material increases, thus improving the activity of the recycled powder [31].

4.2. Two-Component Excitation of Recycled Concrete Powder (RCP) and Recycled Brick Powder (RBP)

Since the RCP and RBP components are different, the mixing of the two components can enhance the activity of recycled powder and the performance of concrete. Li et al. [32] showed that a regenerated micronized group mixed with 70% RBP and 30% RCP had 6.0% and 6.7% higher concrete compressive strength than the regenerated micronized group with 100% RBP and 100% RCP, respectively. Zhou et al. [36] showed that when 30% recycled powder is added, the compressive strength of the RCP-RBP mixture was 6.0–8.0% higher than that of RBP. The reason for the above phenomenon may be that the mixing of RCP and RBP improved the composition of the composite and promoted the occurrence of a pozzolanic reaction, thus promoting the production of cementitious materials and improving the activity of recycled powder.

Using other SCM materials to replace cement can also improve the performance of recycled powder concrete, especially fly ash [37]. RCP and fly ash or mineral slag contribute to improving the strength of concrete in the long term [38]. Liu et al. [39] further found that the mechanical properties of RCP concrete were further improved by replacing 10% cement with lime, because the addition of lime promoted the formation of calcium hydroxide and could weaken the negative impact of low content of hydration products of recycled powder concrete.

In addition to the above methods, optimizing the mix proportion or limiting the content of recycled powder can also significantly enhance the performance of recycled powder concrete [40,41,42]. Adding nanomaterials also helps to enhance the performance of RP concrete. For example, when Yang Lin et al. [43] added 0%, 5%, and 8% nano-SiO₂, the compressive strength of RCP concrete was 31.8 MPa, 33.8 MPa, and 37.0 MPa, respectively.

4.3. Heat Treatment

Concrete will decompose at high temperatures. When the ambient temperature exceeds 500 °C, calcium carbonate will decompose into calcium oxide. In addition, C-S-H gel and calcium hydroxide in the cementation material will also decompose into a larger surface area and stronger activity of calcium oxide [44], in general. The content of calcium oxide in RCP is higher, which makes RCP more active and helps to improve the performance of concrete. Based on the experimental results of XRD, it is found that new active calcium silicate (Ca₃SiO₅ and Ca₂SiO₄) can hydrate with water at 800 °C, which further shows the SEM and XRD images of RCP before and after heat treatment, as show in Figure 6 [45,46,47].

Therefore, heat treatment is an effective way to enhance the activity of RCP, and there was found about a 16.4% increase in activity of RCP after heat treatment at 800 °C compared to untreated [48]. Yang et al. [43] found that after heat treatment at 600–800 °C, the compressive strength of concrete with 30% RCP content was 16.7–26.9% higher than that of concrete without heat treatment. In addition, the study [49] showed that increasing the heat treatment time can also enhance the activity of RCP. However, higher temperatures (above 800 °C) lead to significant decomposition of RCP, reducing RCP activity, and higher heating temperatures consume more energy. Considering the strengthening effect and reducing energy consumption, the best temperature of RCP heat treatment is 600–800 °C. In this temperature range, the performances of RCP and RCP concrete are remarkably improved. In addition, the best heat treatment time of RCP is recommended to be 30–60 min.

4.4. Carbon Dioxide Curing

Cementitious material contains a large amount of calcium hydroxide, and cementitious materials have more strength after carbonization [50,51,52]. In recent years, carbon dioxide curing technology has been widely used to improve the performance of recycled materials. The results of the study showed that the concrete made from recycled aggregates treated with carbon dioxide curing technology had significantly better compatibility, durability, and mechanical properties than the untreated recycled aggregate concrete [53]. Equations (1)–(4) are the reaction equations of carbon dioxide.

$$Ca{(OH)}_2+C{O}_2\to CaC{O}_3+H_{2}O$$

(1)

$$3CaO\cdot 2Si{O}_2\cdot 3H_{2}O+3C{O}_2\to 3CaC{O}_3\cdot 2Si{O}_2\cdot 3H_{2}O$$

(2)

$$3\left(3CaO\cdot Si{O}_2\right)+\left(3-x\right)\cdot C{O}_2+y{H}_{2}O\to xCaO\cdot Si{O}_2\cdot y{H}_{2}O+\left(3-x\right)CaC{O}_3$$

(3)

$$2\left(3CaO\cdot Si{O}_2\right)+\left(2-x\right)\cdot C{O}_2+y{H}_{2}O\to xCaO\cdot Si{O}_2\cdot y{H}_{2}O+\left(2-x\right)CaC{O}_3$$

(4)

The effect of carbonized RCP on the performance of Portland cement was investigated by Lu et al. [28] It was shown that the microstructure and mechanical properties of recycled powder Portland cement were significantly improved after the carbon dioxide curing treatment. Such as the increase in compressive strength of 3.3 MPa and 14.2 MPa for concrete with 10% and 20% carbon dioxide cured RCP compared to concrete with untreated RCP. In addition, carbon dioxide curing treatment can also enhance other performance of recycled powder concrete. For example, the resistance to chloride ion permeability of recycled powder concrete is significantly improved after carbon dioxide curing treatment. Concrete prepared with 20% RCP has no loss of strength [54]. Mehdizadeh et al. [30] further found that cement compressive strength can be increased to 65.8 MPa by replacing cement with 10~15% carbonized RCP, because of the promotion of phase heterogeneous nucleation and carboaluminate formation by carbonated RCP.

4.5. Chemical Activation

Chemical alkali excitation is to add acid and alkali salt into the recycled powder to enhance the activity of recycled powder, in order to enhance its hydration hardening ability [55,56]. Some studies showed that the activity of recycle powder with a particle size of less than 0.075 mm is significantly enhanced by an alkali activator [57,58]. Li Qin et al. [59] assessed the relationship between five different activators on the activity of recycled powder by compressive strength, porosity, and SEM images of mortar. The conclusions showed that the effects of activators were CaCl₂ > CaSO₄·2H₂O > NaOH, Ca(OH)₂, and Na₂SO₄ > no excitation. From the SEM images analysis, it can be found that the mortar without an activator presents a blocky slab and has poor cementing action (Figure 7a); the mortar samples excited by NaOH, Ca(OH)₂ and Na₂SO₄ activators showed a honeycomb morphology because the ettringite was wrapped and filled with gel [60] (Figure 7b,d); and the samples excited by CaSO₄·2H₂O showed a large amount of gel and no acicular ettringite (Figure 7e), which made the microstructure more dense; due to the easy diffusion of Ca²⁺ and Cl⁻ and the continuous reaction with active Al₂O₃ and CaCl₂, the samples excited by the CaCl₂ activator generated large expansive hydrated calcium chloroaluminate to fill the microscopic pores, which generated gel, ettringite, and rod-like Frediel salt [61] (Figure 7f). The chemical excitation mechanism is mainly through increasing the concentration of OH⁻ in the recycled powder slurry to form free polyunsaturated bonds [62]. This allows network polymer of SiO₂ and Al₂O₃ to be formed on its surface with a lower degree of polymerization, which promotes the reaction of active components in the slurry phase, resulting in the increase in the C-S-H gel content [63]; or under the action of Ca²⁺, SO₄²⁻ produced by the promoted hydrolysis reacts with Al₂O₃ to form ettringite.

It is well known that the replacement of some auxiliary cementitious materials with recycled powder can not only reduce the consumption of natural resources, but also protect the ecological environment. For example, the geopolymer composite prepared by partially replacing metakaolin and fly ash with recycled powder has higher mechanical strength, which provides a new research direction for the development of the future construction industry [64].

5. Conclusions

Based on the analysis, collation, and summary of the existing literature, it is known that the academic and industrial circles at home and abroad have realized the importance of recycled powder as an alternative cementitious material. However, in the practical application process, there are still deficiencies in the durability and strength of recycled powder concrete. At the same time, due to the low activity of recycled powder, it is of great significance to study the activity of recycled powder. Here, the relevant summary is as follows.

1. Mechanical grinding can increase the specific surface area of recycled powder, and also change its microstructure, such as the silica structure, so as to stimulate the activity of recycled powder. However, the grinding time should not be too long, and the grinding particle size should not be too small, so as to prevent agglomeration and reduce the strength of concrete.

2. Heat treatment is an effective method to improve the activity of RCP, but there are certain requirements for temperature and treatment time. The optimum temperature of RCP heat treatment is 600–800 °C. In this temperature range, the performance of RCP and RCP concrete is significantly improved. At the same time, the best heat treatment time of RCP is recommended to be 30–60 min.

3. Carbonization treatment can not only change the mechanical properties but also other properties, such as resistance to chloride ion permeability.

4. Chemical excitation mostly adopts alkaline excitation and has a better activation effect than mechanical excitation.

5. In 2021, the world’s annual cement production was 4.31 billion tons. If 1% of cement is replaced by recycled micro powder, then 43.1 million tons of cement can be saved every year. Assuming that the price of cement per ton is 80 dollars, 3.448 billion dollars can be saved every year.

Autor Contributions

Y.Y., Z.K. and B.Z. wrote the paper; P.G., Q.Y., J.W., W.Z. and Y.Z. provided writing ideas, W.B., C.Y., Y.B., J.D. and Y.C. reviewed the literature. All authors have read and agreed to the published version of the manuscript.