An Experimental Study on Strengthening Recycled Aggregates Considering Dry Mixing before Slurry Coating

Abstract

The strengthening of recycled aggregates is a critical issue, as the low strength of recycled aggregates is the main reason that limits their widespread use. The slurry coating method can strengthen the recycled aggregates by repairing the aggregate surface, but it is hard to improve the internal strength due to the existence of pores and cracks. In this study, a new methodology considering dry mixing with fines to fill and bond the internal pores and cracks before slurry coating is proposed. Twelve strengthened samples considering different combinations of dry-mixing fines and coating solutions were prepared, and the basic physical and mechanical properties were compared, including the water-absorption rate, crushing value, and apparent density with unstrengthened aggregates. The results indicate that the proposed methodology can change the water-absorption rate significantly and improve the crushing resistance and apparent density of the recycled aggregates. A high correlation between the apparent density and the crushing value was also observed. Furthermore, the strengthening mechanism of dry mixing was also investigated by scanning electron microscopy. The micromorphology of the strengthened aggregates indicates that internal pores and cracks can be filled by dry mixing fines and then bonded together after hydration.

1. Introduction

Construction wastes can be generated during the engineering process or come from demolished buildings due to urban development or unsafe factors [1,2,3]. Moreover, a large number of construction wastes result from natural hazards [4], including earthquakes, landslides, debris flow, etc. It is estimated that the annual production of construction wastes in China is more than 2 billion tons, which is about eight times the domestic waste and accounts for about 40% of the total municipal solid waste [5]. Due to the limited land resources and potential instability problems for storage of those solid wastes [6,7] as well as the shortage of aggregates for construction needs [1,8], it is of great benefit to recycle those construction wastes [9]. However, because of the existence of adhesive mortar at the aggregate surface and numerous pores and cracks in the aggregates, recycled aggregates usually have a high water-absorption rate, a low crushing resistance, and a low apparent density, which severely limit their application [10,11,12]. It is thus valuable to investigate the strengthening of recycled aggregates.

At present, the main strengthening methods of recycled aggregates include the mechanical grinding and shaping method, removing adhesive mortar with water or acid solution, carbonization curing treatment, strengthening with an organic solvent or inorganic slurry, mixing with mineral fines, etc. [1,10,13]. In the mechanical grinding and shaping method, horizontal or vertical eccentric grinding equipment is utilized to remove impurities and old mortar at the aggregate interface and change the shape of the edges and corners of the aggregates [14,15]. It is a simple and time-efficient way to improve the performance of recycled aggregates, but much energy may be consumed, and there may be more microcracks generated in the aggregates. Clean water or acid solutions such as dilute hydrochloric acid or a dilute sulfuric acid solution can be used to remove the impurities or old mortar at the aggregate surface, but the approach may waste resources to some extent and be costly [16,17]. Some scholars tried to use the reactions between the carbon dioxide and calcium hydroxide or loose silicic acid gel to carbonize the recycled aggregates [18,19]. The generated calcium carbonate was shown to densify the recycled aggregates and reduce the water-absorption rate. Moreover, it is also regarded as an environmentally friendly method due to the consumption of carbon dioxide. The recycled aggregates can also be soaked in organic solutions such as polyvinyl alcohol (PVA) and polysiloxane to decrease the water-absorption rate [20,21]. However, the strength improvement by this method is limited. Moreover, inorganic slurry, including cement slurry, fly ash slurry, sodium silicate solution, and volcanic ash slurry, can repair the surface of aggregates and increase their strength by coating [22], whereas it is difficult to improve the internal cracks or pores of the aggregates due to the fast hydration speed. Mixing the recycled aggregates with mineral fines may help in this aspect. An improved performance of the recycled concrete aggregates was observed in [23]; for example, the tensile strength increased when mixing with silicon fumes.

In this study, a new methodology is proposed to strengthen the recycled aggregates by dry mixing with fines before slurry coating with solutions. This methodology can effectively fill the internal microcracks and pores as well as coat the aggregate surface to achieve the internal and external enhancement of recycled aggregates. Twelves combinations of samples considering different fines and solutions were prepared, and the water-absorption rate, crushing value, and apparent density of those strengthened samples were measured to show the effect of strengthening. Moreover, the microstructure of the strengthened aggregates was observed by scanning electron microscopy to analyze the strengthening mechanism of the proposed methodology.

2. Materials and Methods

2.1. Materials

Combined recycled aggregates consisting of bricks and concrete taken from a crushing plant in Zhengzhou (China) were used in the study. The particle size of the combined recycled aggregates ranges between 4.75 and 15 mm. Fines including micro silicon (MSi), ultra-fine fly ash (UFFA), and nano-sized calcium carbonate (NCC) were used for dry mixing with recycled aggregates. Detailed compositions of the fine particles are shown in

Table 1. Detailed compositions of the fine particles used for dry mixing (%).

Fines for Dry Mixing	SiO2	Al2O3	CaO	Fe2O3	K2O	CaCO3	Others
MSi	96.6	0.35	1.28	1.35	-	-	0.42
UFFA	45.52	37.33	3.13	7.25	3.46	0.68	2.63
NCC	-	-	-	-	-	100	-

.

Three solutions, namely water glass (WG), the polyvinyl alcohol (PVA) solution, and cement slurry (CS), were applied for slurry coating. WG is a factory-produced sodium silicate solution with a concentration of 20% and a modulus of 2.6. The concentration of the PVA solution is 10%, which is prepared in the laboratory using PVA powder manufactured in the factory and water at a temperature of 80 °C. CS is made of ordinary Portland cement (P.O 42.5) with a water–cement ratio of 0.5.

2.2. Procedure to Strengthen Recycled Aggregates

The recycled aggregates were strengthened in two stages, with the first for dry mixing with fines and the second for slurry coating with a solution. Considering different combinations of the materials used for dry mixing and slurry coating, 12 cases are shown in

Table 2. Cases for the strengthening of recycled aggregates.

	Solution	0 *	WG	PVA Solution	CS
Fines
0		Reference	WG	PVA	CS
MSi		-	MSi + WG	MSi + PVA	MSi + CS
UFFA		-	UFFA + WG	UFFA + PVA	UFFA + CS
NCC		-	NCC + WG	NCC + PVA	NCC + CS

* 0 means the dry mixing or slurry coating was not applied.

. Furthermore, a reference case for the original recycled aggregates was also considered. A detailed methodology is illustrated as follows.

(1)

Preparation of recycled aggregates: The aggregates are soaked in water for 24 h and then placed on a plastic mesh to allow the free water at the surface to drain. In this way, the fines used for dry mixing can be better attached to the pores and cracks in the recycled aggregates. Later, the recycled aggregates are evenly divided into four groups.

(2)

Dry mixing with fines: Three groups of the soaked aggregates are mixed with MSi (group A), UFFA (group B), and NCC (group C) in the blender, respectively, as shown in Figure 1a, until the surface of the aggregates is fully and evenly coated with fines. The remaining group (group D) of prepared aggregates is not mixed with any fines for comparison.

(3)

Curing after dry mixing: After spraying water over the four groups of dry-mixed recycled aggregates, they are spread on the ground for 3 days of curing, as shown in Figure 1b.

(4)

Removing excess fines in the aggregates: Figure 1c shows the procedure when excess fines are removed from the mixture with a sieve and collected for future use.

(5)

Preparation of solution for slurry coating: Three types of solution are prepared for coating the recycled aggregates per the concentration or water–cement ratio, illustrated in Section 2.1 (shown in Figure 1d).

(6)

Coating the recycled aggregates with the prepared solution (shown in Figure 1e): Each group of the dry-mixed recycled aggregates is first divided evenly into three parts, and each part is then coated with WG, the PVA solution, and CS, respectively. The recycled aggregates are retrieved after soaking in WG and the PVA solution for 24 h. However, the aggregates are taken immediately out of CS once they are fully stirred.

(7)

Curing after slurry coating: As shown in Figure 1f, the slurry-coated recycled aggregates are spread on the ground for 7 days at room temperature to allow the slurry to cure.

2.3. Tests of the Aggregate Properties

After strengthening, the water-absorption rate W, crushing value Q_c, and apparent density ρ₀ of the recycled aggregates were measured according to the national standard for pebble and crushed stone for construction (GB/T 14685-2022) [24].

The water-absorption rate W was measured for the recycled aggregates in all 13 cases. Then, 2 kg of the aggregates were placed in a clean container and soaked for 24 h at room temperature. The water level should be 5 mm higher than the top surface of the sample. After removing the free water on the surface of the soaked aggregates, the weight of the aggregates m (g) was measured. Then, the aggregates were fully dried in an oven and weighed with a dry weight m_d (g). The water-absorption rate W can be estimated by

$$\mathit{W}=\frac{\mathit{m}-{\mathit{m}}_{\mathit{d}}}{{\mathit{m}}_{\mathit{d}}}\times 100\%$$

(1)

For each kind of strengthened recycled aggregate shown in Table 2, the water-absorption rate W was measured twice to obtain an average value. To illustrate the improvement in water-absorption of strengthened recycled aggregates compared with the unstrengthened ones, the decreasing ratio R_w is calculated by

$${\mathit{R}}_{\mathit{w}}=\frac{\mathit{W}-{\mathit{W}}_{\mathit{R}}}{{\mathit{W}}_{\mathit{R}}}\times 100\%$$

(2)

where W_R is the water-absorption rate of the unstrengthened recycled aggregates (reference case). Moreover, to investigate the effect of dry mixing on water absorption, the decreasing ratio R_wm is calculated by

$${\mathit{R}}_{\mathrm{wm}}=\frac{\mathit{W}-{\mathit{W}}_{\mathit{Nm}}}{{\mathit{W}}_{\mathit{Nm}}}\times 100\%$$

(3)

where W_Nm is the water-absorption rate of the case without considering dry mixing before slurry coating.

The crushing value Q_c reflects the crushing resistance of aggregates, and reducing Q_c is the main aim of aggregate strengthening. For each case shown in Table 2, three of the same tests were conducted for each kind of strengthened aggregate to obtain an average value of the crushing value Q_c. The decreasing ratio R_c of Q_c compared with the unstrengthened case and the decreasing ratio R_cm compared with the case without dry mixing can be calculated, respectively, by

$${\mathit{R}}_{\mathit{c}}=\frac{{\mathit{Q}}_{\mathit{c}}-{\mathit{Q}}_{\mathit{cR}}}{{\mathit{Q}}_{\mathit{cR}}}\times 100\%$$

(4)

$${\mathit{R}}_{\mathit{cm}}=\frac{{\mathit{Q}}_{\mathit{c}}-{\mathit{Q}}_{\mathit{cNm}}}{{\mathit{Q}}_{\mathit{cNm}}}\times 100\%$$

(5)

where Q_cR and Q_cNm is the crushing value of the reference case and the case without considering dry mixing before slurry coating, respectively.

The apparent density ρ₀ reflects the density of aggregate as well as the content of cracks and pores. Therefore, the increase in the apparent density of recycled aggregates can reflect the filling degree of cracks and pores by strengthening materials. In the tests, two samples were taken from every case for measurement of the apparent density, and the average value was calculated for ρ₀.

2.4. Scanning Electron Microscopy (SEM)

To analyze the strengthening mechanism of the proposed methodology, the strengthened recycled aggregates (case MSi + CS) were observed under scanning electron microscopy (TESCAN MIRA LMS, TESCAN, Kohoutovice, Czech Republic). The sample was directly glued to the conductive adhesive, and the Oxford Quorum SC7620 sputtering coating machine (TESCAN, Kohoutovice, Czech Republic) was used to spray gold for 45 s. SEM photos were taken for the recycled aggregates, recycled aggregates after mixing with MSi, and recycled aggregates coated with CS, respectively.

3. Results and Discussion

3.1. Water-Absorption Rate

Figure 2 shows the water-absorption rate W for strengthened recycled aggregates in the 13 considered cases, and aggregates using the same coating material are presented in the same color. The variation of the decreasing ratio R_w and R_wm for the water-absorption rate of strengthened recycled aggregates with different strengthening methods is shown in Figure 3. By comparing the cases without considering dry mixing with fines (reference case and cases WG, PVA, and CS) in Figure 2 and Figure 3, it indicates that the water-absorption rate W of recycled aggregates strengthened by different slurry coating materials is significantly different. The water-absorption rates for cases PVA, WG, and CS are 8.94%, 11.9%, and 17.41%, respectively. Compared with the unstrengthened reference case, the water-absorption rate was reduced by 37.48%, 16.78%, and −21.7%, respectively. The results suggest that the PVA solution has the most pronounced effect in reducing the water-absorption rate W of recycled aggregates, followed by WG. However, the water-absorption rate W is increased when cement slurry is used for strengthening. The results correspond well with those in the literature, where the PVA solution and WG can reduce the water-absorption rate [20,25], while CS can increase the water-absorption rate [13]. It can be attributed to the different strengthening mechanisms for the three kinds of coating solutions. When using the PVA solution prepared at a water temperature of 80 °C for aggregate strengthening, a layer of hydrophobic membrane forms at the surface of aggregates, which can limit the water entering into the aggregates. Unlike the physical protection of the PVA solution, sodium silicate in the WG can react chemically with calcium hydroxide at the surface of or inside the aggregates to generate silica gel to prevent water infiltration or block the internal seepage channel. As CS has a greater water-absorption capacity after hardening, the water-absorption rate of recycled aggregates strengthened by CS significantly increases.

Figure 2 and Figure 3 also indicate considering dry mixing before slurry coating to strengthen recycled aggregates can effectively reduce the water-absorption rate W of the aggregates, and dry-mixing materials have an impact on the water-absorption rate. Taking the recycled aggregates wrapped by WG for example, the water-absorption rates are 11.9%, 9.54%, 9.85%, and 10.37% for cases WG, MSi + WG, UFFA + WG, and NCC + WG, respectively. The decreasing ratio R_wm is 19.83%, 17.23%, and 12.86% for cases MSi + WG, UFFA + WG, and NCC + WG, respectively. It suggests that dry mixing with MSi works best in reducing the water-absorption rate, followed by UFFA and NCC, as shown in Figure 3b. The same trends are observed for cases coated by PVA and CS. It can be attributed to the particle size and composition, the hydration speed of the dry-mixing fines, as well as the reactions with slurry-coating materials.

3.2. Crushing Value

Figure 4 shows the crushing value Q_c of recycled aggregates for the 13 cases considered in this study, while Figure 5a,b shows the decreasing ratio R_c and R_cm of Q_c, respectively. It is shown in Figure 4 and Figure 5a that the crushing value Q_c decreases in all the 12 kinds of strengthened recycled aggregates, among which the decreasing ratio R_c of Q_c for cases wrapped with CS is larger than those with WG or PVA solution. The decreasing trend of the crushing value of strengthened recycled aggregates agrees well with those presented in previous literature [1,10]. It is due to the high strength of CS after curing. For example, when dry mixing is not considered, the decreasing ratios R_c of Q_c for cases CS, WG, and PVA are 20.45%, 13.6%, and 5.19%, respectively.

It is indicated in Figure 4 and Figure 5b that dry mixing before slurry coating can further improve the crushing resistance of the recycled aggregates. For instance, compared with the case of CS without considering dry mixing before slurry coating, the decreasing ratio R_cm of Q_c for cases MSi + CS, UFFA + CS, and NCC + CS is 13.76%, 8.71%, and 7.53%, respectively. It is observed that NCC is the least significant in the decrease of Q_c among the three dry-mixing fines. For recycled aggregates coated with CS and the PVA solution, dry mixing with MSi works best in reducing Q_c, while for those coated with WG, dry mixing with UFFA leads to a higher decreasing ratio R_cm.

3.3. Apparent Density

The variation in the apparent density of recycled aggregates under different strengthening methods is presented in Figure 6. It can be seen that the apparent density of the recycled aggregates is improved after strengthening. In particular, the apparent density of the recycled aggregates increases significantly when coated with CS, agreeing well with the results shown in [10]. For instance, the apparent density ρ₀ increases by 6.26% from 2302 kg/m³ for the unstrengthened condition to 2446 kg/m³ for case MSi + CS. Moreover, the increase in the apparent density ρ₀ of recycled aggregates coated with the PVA solution is less pronounced than those coated with WG or CS. Moreover, after considering dry mixing with fines before slurry coating, the apparent density ρ₀ also increases compared with the cases without considering dry mixing. The NCC is found to be less significant to the increase of the apparent density ρ₀ than MSi or UFFA.

The results indicate that the trends of the variation of the crushing value Q_c and the apparent density ρ₀ under different strengthening methods are correlated. It shows that a high apparent density ρ₀ is usually observed if the strengthened recycled aggregates have a high crushing value Q_c. It is because both ρ₀ and Q_c are related to the compactness degree of the recycled aggregates. In general, the higher the hydration degree of the reaction product between the aggregates and the strengthening materials, the denser the strengthened recycled aggregates and the higher the crushing resistance. To better capture their relationship, the apparent density ρ₀ was plotted against the crushing value Q_c for all 13 cases, as shown in Figure 7. A curve-fitting equation was also obtained as follows with a correlation coefficient R² of 0.9075.

$${\rho }_0=-2098{.5\mathit{Q}}_{\mathit{c}}+2735.4$$

(6)

Figure 7 and Equation (6) indicate that the apparent density ρ₀ decreases with the crushing value Q_c. Therefore, for the strengthening of the same recycled aggregates, apparent density ρ₀ can be predicted with the crushing value Q_c directly, which is valuable, as the laboratory tests for apparent density are rather complicated.

3.4. Mechanism for the Strengthening of Recycled Aggregates

The reasons for the high water-absorption rate, low strength, and crushing value of the recycled aggregates are not only related to the adhesive broken mortar layer and pores on the aggregate surface but also related to the internal pores or microcracks of the recycled aggregates. Therefore, the strengthening of recycled aggregates is dependent on improving broken mortar layers and filling and bonding of pores and microcracks. In this study, dry-mixing fines are used to first fill the pores and microcracks and then bonded after hydration. Then, the slurry used for coating can repair the large pores and cracks at the aggregate surface as well as improve the old mortar layer. To analyze the mechanism of the strengthening of recycled aggregates, scanning electron microscopy (SEM) was employed to observe the microstructure of the recycled aggregates.

Figure 8a,b shows the original recycled aggregates under SEM, for which a rough and uneven surface and obvious voids can be observed at the surface of the aggregates. As shown in Figure 8c, when the recycled aggregates are mixed with MSi and cured for 3 days, many silica gels, marked in the red in Figure 8c, are generated in the sample because of the reactions between the micro silicon (MSi) and calcium hydroxide in the pores or cracks. It suggests that micro silicon can fill the voids and cracks after dry mixing and can bond them after curing. Figure 8d further shows the microstructure of dry-mixed recycled aggregates after being wrapped in CS. More clusters of silica gels are generated at the interface, indicating that the internal compactness of the recycled aggregates is significantly improved after strengthening.

4. Conclusions

In this paper, macroscopic laboratory tests and microscopic morphology analysis are conducted to investigate the strengthening of recycled aggregates by dry mixing before slurry coating. The following main conclusions are obtained:

(1)

The proposed methodology to strengthen recycled aggregates by dry mixing before slurry coating can significantly change the water-absorption rate and improve the crushing value and apparent density of recycled aggregates. It is indicated that dry mixing with MSi and slurry coating with the PVA solution can significantly reduce the water-absorption rate, while dry mixing with MSi and slurry coating with CS can effectively increase the crushing resistance and apparent density of the recycled aggregates.

(2)

A high linear correlation is observed between the apparent density and the crushing value of the strengthened recycled aggregates. In general, a higher crushing value causes a lower apparent density.

(3)

Microscopic morphology of the strengthened recycled aggregates shows that dry-mixing fines can fill the voids and cracks and bond them after hydration. Slurry coating can further repair and strengthen the weak contact to improve the performance of the recycled aggregates.