A New Type of Mineral Admixture and Its Impact on the Carbonation Resistance of EPS Concrete

Abstract

In this study, the effect of microbead dosages (0%, 5%, 10%, 15%, and 20%) on the carbonation resistance of expanded polystyrene (EPS) concrete was investigated. Five groups of EPS concrete specimens were produced and underwent rapid carbonation testing. The carbonation depth and strength after carbonation of the specimens were measured at different carbonation ages (7 days, 14 days, and 28 days) and analyzed to determine the effect of microbead dosages and compressive strength on carbonation resistance. Results indicated that the carbonation depth increased with the progression of carbonation time. The introduction of microbeads was found to significantly improve the carbonation resistance of EPS concrete, leading to a reduction in carbonation depth of over 50% after 28 days and an increase in strength after carbonation by 18–56%. A relative compressive strength model for EPS concrete after carbonation was developed, which could accurately characterize the growth of compressive strength. Based on the analysis of EPS concrete carbonation depth data, a prediction model for the carbonation depth of EPS concrete with microbead dosage was established through fitting, providing improved accuracy in predicting carbonation resistance. The microstructure of EPS concrete was also examined using scanning electron microscopy to uncover the underlying mechanisms of microbead enhancement on carbonation resistance. These findings have potential implications for future research and engineering applications in the carbonation resistance of EPS concrete.

1. Introduction

Expanded polystyrene (EPS) is an environmentally friendly building material characterized by its thermal insulation and acoustic insulation properties [1,2,3,4,5,6,7,8]. In recent years, it has seen increasing use in insulation wall panels, marine engineering, protective engineering, and road engineering [9,10,11,12]. With the growing interest in low-carbon building materials, the scope of EPS concrete’s applications has expanded, and it has increasingly been utilized as a structural material [13,14,15,16].

As a structural material, EPS concrete not only bears loads, but also protects steel reinforcement [17,18]. The durability of EPS concrete is crucial for the safety of the structures it supports [19,20]. Carbonation, which results from the reaction between CO₂ in the atmosphere and the alkaline materials in concrete, is a key factor affecting the durability of concrete. The decrease in alkalinity caused by carbonation leads to de-passivation and corrosion of steel reinforcements, thereby reducing the service life of concrete structures in typical atmospheric environments [21,22].

EPS concrete, due to its light weight, exhibits porosity in comparison to traditional concrete. The carbonation resistance of EPS concrete has been found to be relatively inadequate [2]. As such, it is crucial to carry out a comprehensive study on the carbonation resistance of EPS concrete in order to improve the service life and overall durability performance of EPS concrete structures.

As EPS concrete is mostly used as a functional material, there is limited research on the carbonation resistance of EPS concrete currently, and the underlying mechanism of carbonation in EPS concrete remains unclear. Improving the durability of EPS concrete would have significant implications for enhancing the quality of EPS concrete structures and expanding the range of applications for EPS concrete. This study aims to incorporate a novel type of mineral admixture named microbeads into EPS concrete to improve its carbonation resistance.

Microbeads are a type of globular granular volcanic ash mineral powder material with ultra-fine and continuous particle size distribution sorted by a special process. Their unique physical and chemical properties increase the compactness of mortar or concrete, reducing harmful pores and microcracks, thus improving the corrosion resistance and impermeability of mortar and concrete. When incorporated into silicate cement mortar or concrete, microbeads exhibit the ability to resist the erosion of mortar or concrete caused by various concentrations of salts, including SO₄⁻², Cl⁻, HCO₃⁻, and Mg²⁺, as well as surface water, seawater, and sewage. Moreover, microbeads can enhance the impermeability and freeze–thaw resistance of mortar and concrete.

Currently, microbeads are primarily utilized in industrial and civil building foundation projects, including harbor terminals, water conservancies, bridges, nuclear power plants, and tunnels that are subject to erosion. The globular ultra-fine particles of microbeads have excellent filling properties and “ball effect”, resulting in the significantly reduced water consumption of concrete or mortar. They have a perfect superposition effect with chemical water reducing agents. The ultra-fine characteristics of microbeads can improve the compactness of concrete, improve the interface between mortar and coarse aggregate in concrete, reduce harmful pores, and improve the strength of concrete.

In this study, we investigate the carbonation resistance of EPS concrete with varying microbead dosages, and enhance its carbonation resistance. The findings of this research could serve as a valuable reference for practical engineering applications.

2. Experimental Overview

2.1. Materials

2.1.1. Microbeads

The microbeads utilized in this study were produced by Tianjin Zhucheng New Material Technology Co., Ltd., with an apparent density of 2520 kg/m³ and a bulk density of 760 kg/m³. The microbeads were tested according to “Mineral admixtures for high strenth and high performance concrete” (GB/T18736-2017) [23]. It is found that the microbeads have a specific surface area of over 1300 m²/kg and the average particle size is 3.1 um. The microbeads have an activity coefficient of 105–110% at 28 days and 115–120% at 56 days, and an erosion resistance coefficient index of 0.96. The chemical composition of microbeads is illustrated in

Table 1. Chemical composition of cementitious material.

Chemical Composition (%)	SiO2	CaO	MgO	Al2O3	Fe2O3	Na2O	K2O	SO3	TiO2
Microbeads	56.52	4.85	1.33	26.54	5.36	1.42	3.28	0.65	0.02
Cements	22.93	58.42	1.81	6.93	3.43	0.37	1.13	3.75	0.48
Fly ash	34.48	4.62	0.60	42.35	9.88	0.48	1.42	2.15	2.51

, and the scanning electron micrographs are presented in Figure 1.

2.1.2. Other Materials

In this study, P.O 42.5 cement produced by Conch Cement Co., Ltd., (Wuhu, China) was utilized; the chemical composition is illustrated in Table 1. The sand used was washed sea sand, with a fineness modulus of 2.7 and an apparent density of 2630 kg/m³. The water reducing agent employed was polycarboxylic acid superplasticizer with a solid content of 20%. The EPS particles had a size of 2–3 mm and a bulk density of 17.6 kg/m³, as demonstrated in Figure 2. The fly ash used was sourced from Taizhou Tianda Environmental Protection Building Materials Co., Ltd., (Taizhou, China) and belonged to Class II fly ash [24]; the chemical composition is illustrated in Table 1.

2.2. Specimen Preparation

In order to study the effect of microbead content on the carbonization resistance of EPS concrete, this study designed 5 sets of EPS concrete; the composition of each test group is presented in

Table 2. EPS concrete mix proportion.

Number	Cement/kg	Fly Ash/kg	EPS Beads/L	Water/kg	Water Reducing Agent/kg	Microbead/ kg	Cementitious Material Substitution Rate
A1	370	370	580	252	3.5	0	0%
A2	370	333	580	252	3.5	37	5%
A3	370	296	580	252	3.5	74	10%
A4	370	259	580	252	3.5	111	15%
A5	370	222	580	252	3.5	148	20%

. In the 5 sets of EPS concrete, group A1 is a blank control group; its cementitious materials only consist of cement and fly ash, without microbeads. The other four groups of concrete have the same amounts of cementitious materials as group A1 but use microbeads to replace part of the fly ash. In group A2, the amount of microbeads is 5% of the cementitious material of group A1 and is used to replace part of the fly ash. The amount of microbeads used for group A3, group A4, and group A5 is 10%, 15%, and 20% of the cementitious material of group A1, respectively.

Eight sets of 100 mm × 100 mm × 100 mm EPS light concrete cubes were prepared for each concrete type, with three specimens in each group. Three groups were used to evaluate the carbonation depth at 7 days, 14 days, and 28 days, respectively. Another three groups were used to determine the strength at the same carbonation ages. One group was used to assess the strength after curing for 28 days, and the remaining group was reserved for electron microscopy testing. After molding, the specimens were placed in a natural curing room for one day before the removal of the molds. The specimens were then transferred to a standard curing room. The dry density of EPS concrete in all five groups was found to be 1100 kg/m³.

2.3. Test Methods

2.3.1. Carbonization Test

In accordance with the “Standard for test methods of long-term performance and durability of ordinary concrete” (GB/T50082-2009) [25], the specimens were taken out of the standard curing room after 26 days of curing and subjected to a 2-day baking process in a constant temperature and humidity oven. The specimens were left with two opposite sides without wax coating, while the remaining sides were coated with paraffin wax. A parallel line was drawn on each side at 10 mm intervals to determine the location of each measurement point. The specimens were then placed in a carbonation chamber; the CO₂ concentration in the carbonization chamber was maintained at (20 ± 3)%, the humidity was maintained at (70 ± 5%), and the temperature was maintained at (20 ± 2) °C. A minimum distance of 50 mm was ensured between each specimen, as depicted in Figure 3. After undergoing carbonation for the specified age, the specimens were split and treated with a 1% phenolphthalein solution. The carbonation depth was then measured using electronic vernier calipers.

2.3.2. Strength Test

The strength testing is based on the “Standard for test methods of concrete physical and mechanical properties”(GB/T50081-2009) [26]. This study tested the strength of EPS concrete after 28 days of curing and at different carbonization ages.

2.3.3. Microstructure Analysis

Scanning electron microscopy (SEM) was employed for microstructure analysis. After 28 days of curing, a small piece was taken from the center of each crushed specimen and sprayed with gold. These samples were then placed in a scanning electron microscope for observation. The microscopic morphology of the cement matrix and the interfacial transition zone (ITZ) was captured and analyzed.

3. Results and Discussion

3.1. Effect of Microbead Dosage on the Carbonization Depth

The carbonation phenomenon of EPS concrete in the five groups for 7 days, 14 days, and 28 days is depicted in Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8, while the carbonation depth is presented in

Table 3. Carbonization depth of each specimen.

Number	Carbonation Depth of EPS Concrete at Different Carbonation Ages/mm
	7 d	14 d	28 d
A1	13.22	15.34	22.42
A2	10.35	14.46	17.61
A3	8.53	10.36	13.42
A4	6.31	9.41	13.23
A5	6.12	7.37	10.58

and Figure 9. The carbonation test was conducted for 7 days, during which the carbonation depth of the specimens increased rapidly. With the progression of carbonation age, the carbonation depth of the specimens grew gradually. Upon reaching 28 days, the maximum carbonation depth of the specimens was found to be 22.42 mm, which was significantly greater than that of ordinary concrete [20]. The relationship between the carbonation depth and carbonation age of EPS concrete with different microbead dosages is depicted in Figure 9.

As shown in Figure 9, the carbonation depth of five groups of EPS concrete specimens increased with the increase in carbonation age. The growth rate was faster during the early stages of carbonation, but the carbonation rate exhibited a slowing trend after 7 days. The reduced carbonation rate after 7 days is attributed to the coverage of CH by CaCO₃ formed in the carbonized zone and the blocking of microscopic pores in the cement mortar. Consequently, the transport rate of ions (Ca²⁺, OH⁻, and CO₃²⁻) in the solution is restricted, leading to a decrease in the amount of dissolved CH. After a short and rapid reaction, the carbonation rate of CH was significantly reduced, resulting in a significant decrease in the carbonation rate of each specimen during the later stages of carbonation [27,28,29]. The specimens without microbead doping maintained the maximum carbonation depth throughout the aging process. Compared to Group A1, the carbonation depth of the specimens in Groups A2–A5 decreased by 22.73%, 40.91%, 40.91%, and 54.55%, respectively, with the increasing microbead dosage.

3.2. Effect of Carbonization on the Compressive Strength

The 28-day curing strength of the five groups of specimens is depicted in Figure 10. It can be observed that the concrete strength increased with the increasing microbead dosage. This is due to the small particle size of the microbeads, which block harmful pores in the concrete when added, resulting in a denser concrete and increased strength [15,30].

As shown in Figure 11, the compressive strengths of specimens with different microbead dosages increased with the progression of carbonation age. The strengths increased by 18% to 56% after carbonation compared to those without carbonation. The increase in strength after carbonation is twofold: One is mainly due to the fully hydrated concrete in an environment with a CO₂ volume fraction of (20 ± 3)%, a relative humidity of (70 ± 5)%, and a temperature of (20 ± 2) °C. Adequate moisture, temperature, and humidity allow for more complete hydration, and the hydration products fill the internal pores of the concrete, making the matrix denser and improving its compressive strength. Additionally, the chemical reaction between the C-S-H gel and Ca(OH)₂ and CO₂ within the specimens produces a substantial amount of CaCO₃, filling the defective areas between the ITZ. This improves the pore structure and optimizes the structure of the ITZ, thereby enhancing the compressive strength of the concrete specimens [31,32,33].

Furthermore, the relative compressive strength can be utilized to assess the strength growth of EPS concrete at different carbonation ages. By obtaining the relative compressive strength data of EPS concrete after carbonation, as shown in

Table 4. Compressive strength and relative compressive strength of specimens after carbonization.

Number	Compressive Strength/MPa				Relative Compressive Strength ${\mathit{F}}_{\mathit{c}\mathit{u},\mathit{c}}^{}$/MPa
	0 d	7 d	14 d	28 d	0 d	7 d	14 d	28 d
A1	7.09	9.4	9.4	9.2	1.00	1.33	1.33	1.30
A2	7.18	9.3	9.7	11.2	1.00	1.30	1.35	1.56
A3	9.96	11.4	11.8	13.6	1.00	1.14	1.18	1.37
A4	9.85	12.9	12.0	12.0	1.00	1.31	1.22	1.22
A5	10.87	12.9	11.6	12.9	1.00	1.19	1.07	1.19

, the relationship curves of relative compressive strength of five groups of EPS concrete with carbonation age were obtained, as shown in Figure 12. To obtain more accurate equations for the relative compressive strength curves of five groups of EPS concrete after carbonation, a binary function was used to fit the data in this study. The fitting curve is illustrated in Figure 13. The fitting curves are in good agreement with the experimental curves except for the fourth group. The main reason for the poor fitting coefficient of the fourth group is the low strength after carbonization due to the manufacturing defects of the test specimens. The model function relationship is presented in Equation (1):

$$\frac{f_{\mathrm{cu},c}^i}{f_{cu,c}^0}{=\mathit{at}}^2+\mathit{bt}+\mathit{c}$$

(1)

where $f_{cu,c}^0$ represents the compressive strength of EPS concrete before carbonation (MPa); $f_{\mathrm{cu},c}^i$ represents the compressive strength of EPS concrete after carbonation in the i-th test; i is the number of tests under carbonization, taken as 0, 1, 2, 3, …; t is the carbonation time (day), taken as 0, 7, 14, and 28; and a, b, and c are coefficients of test parameters to be calibrated under carbonization.

Assuming $F_{cu,c}$ as the relative compressive strength of EPS concrete, we can obtain:

$$F_{cu,c}=\frac{f_{\mathrm{cu},c}^i}{f_{cu,c}^0}$$

(2)

Substituting Equation (2) into Equation (1) and finding the first and second order derivatives with respect to the carbonation time t, the following equations are obtained, respectively.

$$\frac{d{F}_{\mathit{cu},c}}{dt}=2\mathit{at}+\mathit{b}$$

(3)

$$\frac{d{F}_{\mathit{cu},c}}{d{t}^2}=2\mathit{a}$$

(4)

When the carbonation time t = 0, from Equation (3) we obtain:

$$\frac{d{F}_{\mathit{cu},c}}{dt}\left|\mathit{t}=0\right.=\mathit{b}$$

(5)

The above equations show that parameter b represents the initial velocity of the change in relative compressive strength of EPS concrete, parameter a represents the acceleration of the change in relative compressive strength of EPS concrete, and parameter c represents the initial value of the relative compressive strength of EPS concrete.

The model parameters obtained by fitting the curves are tabulated in

Table 5. Model parameters of relative compressive strength.

Number	a	b	c	R2
A1	−0.0011	0.0414	1.0262	0.8918
A2	−0.0006	0.0345	1.0235	0.958
A3	−0.00007	0.00146	1.0112	0.9781
A4	−0.0009	0.0307	1.0376	0.6688
A5	−0.0005	0.0211	1.0188	0.8301

. As shown in Table 5, a value of b greater than 0 indicates that the relative compressive strength of EPS concrete varies with b at the beginning of the carbonation test, and the relative compressive strength varies with an acceleration of 2a during the subsequent carbonation process. Given that the value of parameter b is considerably greater than that of a, the change rate of EPS concrete’s relative compressive strength, represented by 2at + b, is positive, implying that the relative compressive strength of concrete increases with the extension of the carbonated specimen t. This result is consistent with the observed test results.

By comparing the model parameters of five groups of EPS concrete in Table 5, it can be observed that the initial velocity of relative compressive strength of Groups A1 and A2 is higher than that of the last three groups. This is because there is no or less addition of microbeads, resulting in faster early carbonation and faster growth of compressive strength. This is also consistent with the described pattern of relative compressive strength change after carbonization.

3.3. Predicting the Carbonation Depth of EPS Concrete

It is generally concurred that there is a linear relationship between the depth of concrete carbonation and the square root of the carbonation time. Based on the available experiments and relevant theories for ordinary concrete [34,35,36,37,38,39], a prediction model for the carbonation depth of EPS concrete has been established.

$$Y=K\sqrt{t}$$

(6)

where Y is the carbonization depth, mm; t is the carbonization time, d; K is the carbonization coefficient.

The test results were fitted based on Equation (6) with the requirement that the fitting curve passes through zero (when t = 0, Y = 0). This requirement reflects the fact that before the carbonation test began, the specimen was not carbonized, and its carbonation depth was 0. The fitting curve is illustrated in Figure 14, while the carbonation coefficient is presented in

Table 6. Parameters of carbonation depth fitting curve of EPS concrete.

	Group	A1	A2	A3	A4	A5
Fitting Parameters
K		4.30002	3.55636	2.69253	2.48342	2.02069
R2		0.98448	0.98025	0.97301	0.99906	0.98838

.

Table 7. Comparison of the measured and calculated carbonation depth.

Number	Carbonation 7 d			Carbonation 14 d			Carbonation 28 d
	Measure Value/mm	Calculated Value/mm	Error/%	Measure Value/mm	Calculated Value/mm	Error/%	Measure Value/mm	Calculated Value/mm	Error/%
A1	13.22	11.38	13.92	15.34	16.09	−4.89	22.42	22.75	−1.47
A2	10.35	9.41	9.08	14.46	13.31	7.95	17.61	18.82	−6.87
A3	8.53	7.12	16.53	10.36	10.07	2.80	13.42	14.25	−6.18
A4	6.31	7.12	−12.84	9.41	10.07	−7.01	13.23	14.25	−7.71
A5	6.12	5.35	12.58	7.37	7.56	−2.58	10.58	10.69	−1.04

Note: The positive error value indicates that the measured value is greater than the calculated value, while the negative error value indicates that the measured value is less than the calculated value.

compares the measured and calculated carbonation depth.

The results showed that the difference between the prediction model and the measured values is minimal, as seen in Table 7. The comparison between the test curve and fitting curve in Figure 14 indicated that the carbonation depth prediction model is suitable for EPS concrete. The carbonation rate of Group A1 was the highest among all groups, and it gradually decreased as the microbead dosage increased. Compared to Group A1, the carbonation coefficients of Groups A2, A3, A4, and A5 decreased by 17.44%, 37.44%, 42.26%, and 53.00%, respectively.

3.4. Microscopic Analysis

The SEM images of the five groups of EPS concrete in Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19 demonstrated the presence of an ITZ between EPS particles and the cement matrix [1,40,41]. The surface of EPS particles is non-polar, while the cement matrix is polar, leading to poor compatibility between the two [15]. Due to the influence of surface tension, the cement mortar is prone to disengage from the EPS particles and to coat the surface of the particles, resulting in the formation of a weak interfacial layer. This weak layer increases the permeability of water and CO₂, thus contributing to the reduction of concrete strength and durability. EPS particles are viewed as harmful pores in concrete and can have a significant impact on the overall quality of the material.

As can be seen from Figure 15 and Figure 16, the gap between the ITZ is obvious. Figure 17, Figure 18 and Figure 19 demonstrate that EPS particles are closely embedded into the cement matrix, and there is a small gap at the ITZ between the EPS particles and the cement matrix. This is because with the increasing microbead dosage, some microbeads enter the ITZ between the EPS particles and the cement matrix, making the ITZ denser. Additionally, the smaller particle size of the microbeads compared to cement and fly ash can effectively fill the pores in the cement matrix, resulting in a denser matrix and hindering the infiltration of CO₂.

Moreover, due to the nanoscale size of some microbead particles, their chemical activity is significantly higher than that of typical materials. The microbeads are bonded to the hydration products of cement in large quantities, and play a crystalline nucleation role for the hydration products of cement around them, which can adsorb the ions in the cement matrix and become its precipitation site. The hydration product of cement, hydrated calcium silicate gel, has an adsorption effect and can be adsorbed in the form of smaller particles on the surface of gel particles and in the mesh structure of hydrated calcium silicate gel with microbeads as the core, making it difficult for large crystals to form. It reduces the degree of crystal orientation, which is beneficial for improving the interfacial structure and increasing the interfacial bond strength [42,43].

Meanwhile, the water reducing agent aids in the full dispersion of microbeads, thereby forming a finer and denser mesh structure [44]. Microbeads can refine the grains in the cement matrix and fill the pores in the cement matrix, making it denser and less porous. Microbeads improve the microstructure of EPS concrete, increase the diffusion resistance of CO₂, and reduce the total amount of CO₂ diffusion. Therefore, microbeads can improve the carbonation performance of EPS concrete.

4. Conclusions

The following conclusions were obtained from the study of the carbonation resistance of EPS concrete with different microbead dosages of the same density:

• The change of carbonation depth of EPS concrete is similar to that of ordinary concrete. The carbonation depth increases with the increasing carbonation age, but the carbonation rate slows down.
• The introduction of microbeads has a significant effect on preventing the carbonation of EPS concrete, and the 28-day carbonation depth can be reduced by more than 50%.
• The relative compressive strength of EPS concrete after carbonation is modeled, and the strength of EPS concrete after carbonation increases by 18–56%. The model parameter values of relative compressive strength of EPS concrete with different microbead dosages are provided. The relative compressive model agrees well with the measured curves and provides a reference for the strength assessment of such EPS concrete after carbonation.
• Based on the relationship between the traditional carbonation depth and carbonation time, the carbonation depth prediction model of EPS concrete under different microbead substitution rates is established, and the experimental data of each group are compared with the calculated values of the carbonation depth prediction model. The error is small, and the degree of fitting is good.
• The effect of introducing microbeads on the carbonation resistance of EPS concrete is analyzed from the microscopic perspective, which is mainly attributed to the improvement of the interfacial transition zone and the matrix compactness of microbeads.