Experimental study on the effect of basalt fiber reinforced expanded polystyrene foams on the compressive strength of lightweight concrete Open Access

With the enhancement of building and construction requirements, not only does a house require a certain load-bearing function but it also requires the entire structure to be of lightweight materials.^1–4 Lightweight concrete perfectly meets the building requirements due to its low density, low thermal conductivity, good seismic performance, and other characteristics, so it is widely used in wall materials, such as lightweight wall panels and hollow blocks.^4–6 Lightweight concrete is mainly prepared by foaming and filling methods.^7–9 The production method of foam concrete is to add a foaming agent to the concrete. After the foaming agent is fully foamed, it is evenly mixed with the cement slurry to form foam concrete. Its main advantages are fast construction speed and low density. It is a lightweight, thermal insulation, fire resistance, sound insulation, and frost resistant concrete material. Its disadvantage is low compressive strength.^10,11 Lightweight concrete is prepared by using the filling method in engineering, and depending on the role, the commonly used lightweight aggregates are expanded polystyrene spheres (EPSs), hollow glass microspheres (HGMS), rubber,^12–14 biobased materials,^15–18 expanded perlite,¹⁹ and expanded clay.²⁰ Although these materials fulfill certain applications in some ways, the composites have poor mechanical strength, durability, density, and other properties.^21,22 In order to overcome the shortcomings of traditional filled lightweight concrete, millimeter and micrometer sized lightweight functional fillers (LWFs) such as EPS, HGMS, and expandable thermoplastic microspheres are widely used. Coal gangue ceramic lightweight aggregate concrete is made by converting coal gangue into low-density ceramic aggregate to make low-density lightweight aggregate concrete.^23–25 

Expanded polystyrene spheres (EPSs) are man-made lightweight aggregates (density of less than 30 kg/m³) with smooth regular spherical surfaces and a non-absorbent, hydrophobic, closed cell.^26,27 Due to its low density, it has been researched to be mixed into cement to make EPS concrete, which is low-cost and can be prepared quickly at the project site. The disadvantages of EPS concrete are that it is not resistant to high temperatures and EPS releases toxic gases when it burns, and in the preparation process, due to its low density and good fluidity, EPS floats and concentrates in the upper part of the concrete, and the filler inside the concrete is not homogeneous to delamination and causes brittle damage.^28–30 In recent years, hollow glass beads (HGMS) were added to composites due to their advantages of low price, light weight, and easy processing.³¹ Due to its high compressive strength and heat resistance (typically a softening temperature of about 650 °C)³² and smooth surface, HGMS promotes uniform dispersion of fibers in concrete. HGMS can replace the role of played by traditional fillers in many demanding lightweight concrete materials.³³ On the other hand, HGMS is not tightly bonded to the cement paste, and the cement material with HGMS added may be brittle when damaged.³⁴ 

A study found that adding fibers leads to higher strength lightweight concrete.³⁵ The role of fibers is to act as a binder to reinforce the matrix connection and as a raw material to make lightweight aggregates for filling concrete. Liu et al.³⁶ conducted research and analysis on concrete to explore whether the addition of carbon fiber and steel fiber can improve the crack resistance of concrete. Others have studied glass fibers,³⁷ steel fibers,^38,39 polypropylene fibers,^40,41 and miscanthus fibers^42,43 and found that the properties of the fibers themselves contribute to increasing the bond between the concrete matrix. Basalt fiber has the advantages of good tensile strength, corrosion resistance, friction resistance, and good elastic strength,⁴⁴ which have great application in the fields of construction and materials. In this experiment, basalt fiber powder was wrapped on the outer layer of epoxy composite balls by the “rolling ball method”^45–48 to prepare reinforced epoxy composite balls basalt fiber reinforced epoxy macrospheres (BF-R-EMS), which were used as lightweight aggregate filled concrete to prepare concrete specimens. The lightweight concrete prepared by this method^49,50 was not only of low density but also of high strength. The experiment explored the effects of the properties of epoxy composite balls and basalt fibers on the performance of lightweight concrete and analyzed the internal bonding from a microscopic point of view.

Cement, water, hollow glass beads, and basalt fibers were used for the lightweight concrete matrix in the test: (1) The cement was 42.5 silicate cement with a density of 3.1 g/cm³. (2) Ordinary tap water was used. (3) Hollow glass beads with a density of 0.15 g/cm³ and a compressive strength of 1.72 MPa were purchased from MN 3M, United States. (4) The basalt fibers used were three short-cut fibers of diameters of 3, 9, and 12 mm from Zibo Taixin Composite Co. Ltd.

The BF-R-EMS used in the test consisted of epoxy resin, an amine curing agent, EPS, and basalt fiber powder: (1) The epoxy resin was manufactured by Shanghai Ansheng Science and Technology Co., China. (2) The amine curing agent was manufactured by Shanghai Ansheng Technology Co. Ltd., with a density of 1.17 g/cm³. (3) The EPS beads used were produced by China Co. Ltd., with a bead diameter of 8–11 mm and a bead density of 10 kg/m³.

The curing agent and epoxy resin are mixed at a ratio of 1:3, and this is used as a base to prepare a fresh epoxy curing agent with good fluidity. The EPS and epoxy curing agent were thoroughly mixed through a mixer before covering the surface with a layer of basalt fiber powder. Finally, the basalt fiber powder reinforced epoxy composite balls were cured by step heating. One layer of BF-R-EMS was prepared after complete curing. Two and three layers of basalt fiber powder reinforced epoxy composite spheres could be prepared by the same process. As can be seen in Fig. 1, there are three main steps to prepare BF-R-EMS: covering the epoxy curing agent, wrapping the basalt fiber powder, and heating and fixing for drying.

As can be seen in Fig. 2, the steps of the specimens made of BF-R-EMS and basalt fiber are mainly mixing, filling, and fixed forming. The first step is mixing and blending, where HGMS and cement are mixed thoroughly and basalt fibers are added to them; then water is added and mixed well to form the matrix binder. The second step is to add different ratios of BF-R-EMS to the matrix to produce the initial sample. Finally, the lightweight concrete was slowly filled into the mold and compacted to avoid mixing of pores as much as possible. After completing the above-mentioned steps, cover the concrete with a heavy weight and pressurize and wait for curing. The curing and maintenance time was 28 days. The specific formulation for the experiment is demonstrated in Table I.

50 BF-R-EMS types were randomly selected using vernier calipers to measure the two-dimensional size, and then a high-precision electronic balance was used to measure the mass to calculate the density of BF-R-EMS. Lightweight concrete specimens were measured using ordinary electronic scales. The dimensions of the concrete mold are 70.7 × 70.7 × 7.07 mm. When measuring the compressive strength of concrete, the CMT5350 testing machine produced by Shenzhen Sanshi Zongheng Technology Co. Ltd. was used. The spherical wall of the BF-R-EMS as well as the bonding area of the BF-R-EMS and the substrate was analyzed by microanalysis using SEM.

In this experiment, basalt fiber powder was used to coat 10–11 mm and 8–9 mm EPS. Figures 3(a) and 3(b) show the comparison of BF-R-EMS with different inner diameters and the same number of layers and with the same inner diameter and different numbers of layers. Basalt fibers have good corrosion resistance and high mechanical strength, which can be combined with epoxy resin to form resin basalt fiber composites. Figure 3(c) shows the density scatter distribution measured after both 8–9 mm EPS and 10–11 mm EPS were wrapped with two layers of basalt fiber powder. Figure 3(d) shows the scattered density distribution measured after wrapping 8–9 mm and 10–11 mm EPS with two layers of basalt fiber powder. Studying and analyzing the graph can clarify that the average density of BF-R-EMS is 0.167 g/cm³ (8–9 mm-one layer), 0.351 g/cm³ (8–9 mm-two layers), 0.493 g/cm³ (8–9 mm-three layers), and 0.251 g/cm³ (10–11 mm-two layers). The more the BF-R-EMS layers, the smaller the inner diameter, and the higher the density.

In this study, we chose to add BF-R-EMS to the HGMS concrete matrix with spatial stacking volumes of 20%, 40%, 60%, 80%, and 90%. As shown in Fig. 4, it is clear from the analyzed graph that for the homogeneous HGMS concrete matrix, the maximum compressive strength is 17.26 MPa (0%) and the density is 1.679 g/cm³. The concrete without adding BF-R-EMS is internally dense without defects and generally does not produce stress concentration, so its density and compressive strength are maximum. After adding BF-R-EMS and gradually increasing the spatial stacking volume, its function of reducing density becomes more obvious. The density of the specimens filled with different volumes decreased by 6.8% (20%), 15.3% (40%), 20.6% (60%), 25.8% (80%), and 31.5% (90%) compared to the density of unfilled BF-R-EMS. Since the filled epoxy composite spheres are less dense than the cement matrix, the addition of BF-R-EMS reduces the density of the specimens but also adds hollow pore defects inside the concrete. When a large amount of BF-R-EMS is added, the number of epoxy composite balls in the matrix increases, the distance between the spheres is shortened, and the cement matrix between the spheres is gradually reduced, which is very likely to cause damage of the BF-R-EMS shell, resulting in stress concentration and concrete decomposition.

Figure 5(a) shows the effect of two layers of BF-R-EMS prepared using 8–9 mm EPS and 10–11 mm EPS on the density of concrete. Further analysis in conjunction with Figs. 5(a) and 5(b) shows that when the EPS size was 10–11 mm, the density decreased by 2.1% compared to the concrete using 8–9 mm EPS, and when the size of the expanded polystyrene within the concrete-filled BF-R-EMS was 10–11 mm, the compressive strength decreased by 7.1% compared to the concrete using 8–9 mm expanded polystyrene. The smaller size of the lightweight aggregate increased the density of the specimens and increased the strength. This is mainly due to the fact that the density of the filled BF-R-EMS decreases as the inner diameter increases. When the aggregates in lightweight aggregate concrete are smaller, the effective stacking volume is higher for smaller sized spheres. The internal pore defects are also smaller, and the probability of mutual contact is higher, which, together with the homogeneity principle of the material, makes it easy for force conduction to occur.

Figure 6 demonstrates the effect of lightweight aggregate with different numbers of reinforcement layers on the concrete properties. After adding BF-R-EMS with different reinforcement layers, the concrete densities are 1.225 g/cm³ (three layers), 1.150 g/cm³ (two layers), 1.093 g/cm³ (one layer), and 0.953 g/cm³ (0 layer). The compressive strength of concrete is 9.49 MPa (three layers), 8.00 MPa (two layers), 4.54 MPa (one layer), and 1.59 MPa (0 layer). The damage of concrete materials spreads out from the internal defects of the materials, and the BF-R-EMS has a hollow structure inside, and when subjected to pressure, if there are no pores in the matrix, then the BF-R-EMS will be damaged by rupture first. The strength of BF-R-EMS is related to the number of reinforcing layers in its outer layer. The more reinforcing layers it has, the higher its density, which ultimately manifests itself in compressive strength.

For concrete, not only the BF-R-EMS properties can affect the performance of lightweight concrete but also the proportion of HGMS in the matrix can affect the density of concrete. Three volume fractions of HGMS, 20%, 40%, and 60%, were added to the cement, and Fig. 7 demonstrates that the density of 20% volume fraction lightweight concrete was 1.211 g/cm³ with a strength of 8.19 Mpa, the density of 40% volume fraction lightweight concrete was 1.150 g/cm³ with a strength of 8.00 Mpa, and the density of 60% volume fraction lightweight concrete was 0.985 g/cm³ with a strength of 5.46 MPa. Using the concrete block filled with 20% HGMS proportion as a reference object, the density of the 40% HGMS proportion concrete decreased by 5% and the strength decreased by 2.3%, and the density of the 60% HGMS proportion concrete decreased by 18.7% and the strength decreased by 33.3%. The concrete block filled with 40% HGMS proportion has a lower strength decrease while the density decrease is more. According to the theory of particle random stacking parameter, Φ_max refers to the maximum amount of HGMS content that can be filled in the cement paste, within which the concrete can maintain adhesion and compatibility; Φ_max is about 40%–60%. A large amount of filled HGMS will lead to localized densification, and when subjected to pressure, the HGMS will crush each other and lead to rupture.

Increasing the fiber content in the matrix can form more fiber crossings in the matrix. Adding too many fibers to the matrix reduces the bond of the concrete, affects the compatibility of the concrete, and is not conducive to the dense formation of the specimens, so the water–cement ratio was set at 2:5 in the test group with the addition of fibers. The basalt fiber with a length of 9 mm was added to concrete at ratios of 0.5%, 1%, 1.5%, and 2% of cement mass, and the influence of different fiber contents on concrete was compared. As can be seen from Fig. 8, the density and strength of the test with basalt fibers are directly proportional to the added fibers and content, and the density of the specimens from small to large was 1.109 g/cm³, 1.110 g/cm³, 1.115 g/cm³, and 1.118 g/cm³; the specimens’ compressive strength from small to large was 6.37, 6.48, 6.58, and 7.07 MPa. It can be assumed that while increasing the quality of the fibers in the matrix, the strength and density of the concrete will gradually increase. The addition of basalt fibers in the cement matrix will form a lattice structure inside, increasing the bonding force inside the matrix, and when the concrete is damaged, the basalt fibers will prevent it from rupturing and quickly disperse and conduct the external force. The greater the number of basalt fibers added, the greater the damage resistance and strength of the concrete.

Fibers have the ability to bond with concrete, and different lengths of fibers bond with concrete to different degrees. By analyzing the study shown in Fig. 9, we can clearly see the increase in density and compressive strength of the specimens after adding three lengths of basalt fibers of 3, 9, and 12 mm to the concrete. The density of the specimens with added basalt fibers is 1.113 g/cm³ (3 mm), 1.118 g/cm³ (9 mm), and 1.120 g/cm³ (12 mm); and the compressive strength is 7.03 MPa (3 mm), 7.07 MPa (9 mm), and 8.63 MPa (12 mm). The compressive strength exhibited by the lightweight concrete was higher when the basalt fibers were longer (12 mm). This is due to the three-dimensional distribution of the long fibers in the HGMS–cement matrix, which can link the specimens together more tightly as a whole, creating a network structure that forces the tension-expanding forces to be easily dispersed so that tensile stresses due to drying or stresses in the matrix are attenuated. Longer basalt fibers are more tightly bound to the HGMS–cement matrix and have a greater ability to resist concrete damage when the lightweight concrete is subjected to loads.

Figure 10 demonstrates a model of basalt fiber reinforcement in lightweight concrete. The BF-R-EMS prepared using regular spherical EPS also has a regular spherical shell. Figure 10(a) demonstrates that, after the specimen is subjected to external force, the presence of pore defects in the matrix leads to stress concentration at the defects when the force is transmitted downward, and when the stress at the defects exceeds the maximum value that can be carried by the matrix concrete, the concrete matrix at the defects first ruptures, then transmits outward, and ultimately expands outward, resulting in the destruction of the overall material. In addition, although the uniform wall thickness of BF-R-EMS is conducive to fit with the matrix, the basalt fiber powder in the surface layer and the cement belong to different materials, and there are voids at the interface between the two. When damaged by external forces, since the strength of BF-R-EMS is not as good as that of the concrete matrix, the damage first starts from the weakest contact point between the BF-R-EMS. When a certain part of the BF-R-EMS is damaged, the spherical structure of the BF-R-EMS is broken and cannot conduct the force downward, forming pore defects, which ultimately lead to material fragmentation. As shown in Fig. 10(b), by adding fibers, the fibers are uniformly and three-dimensionally distributed in the matrix, and when subjected to external pressure, the network of fibers surrounding the pore defects rapidly transfers and disperses the force downward due to the bonding effect of basalt fibers with the matrix and the tensile effect of the fibers, thus reducing the stress concentration at the defects. Through the comparison between the specimens without added fibers and the specimens with added fibers after destruction shown in Fig. 10(c), it is found that due to the bonding effect of fibers, the matrix of the lightweight concrete with added fibers is still intact on the whole and does not break up when it is destroyed.

The cross section of the specimen shown in Fig. 11(a) demonstrates that the specimen is completely filled with BF-R-EMS, and the hollow spherical wall can be seen after removing the EPS inside the BF-R-EMS. Due to air ingress, during the preparation of the material and incomplete vibration and compaction at a later stage, pore defects due to air ingress during preparation existed within the matrix between the BF-R-EMS and the BF-R-EMS. When subjected to external force, the defects could not effectively conduct the force to form a stress concentration, and when the stress exceeded the bearing capacity of the matrix, fracture occurred at the pore defects and spread rapidly, resulting in concrete crushing. Figure 11(b) shows the BF-R-EMS shell, and the thickness of the BF-R-EMS shell is measured to be about 450 µm; in addition, the uniform spherical wall facilitates a better fit to the substrate. Figure 11(c) demonstrates the interface part of BF-R-EMS and the concrete matrix; by zooming in, we can see that there are voids at the interface of BF-R-EMS and the matrix, and this is because the epoxy composite ball shell and matrix material are different, and the voids at the interface will affect the concrete compressive strength. Adding HGMS to the matrix can fill some of the pore defects, significantly reducing the number of pores in the matrix.

In this paper, the properties of lightweight concrete specimens filled with BF-R-EMS reinforced with basalt fiber powder and an epoxy resin curing agent system were studied. For this purpose, several sets of epoxy resin composite ball specimens with different volume ratios, EPS sizes, and wall thicknesses were designed using the “rolling ball method.” Concrete specimens with different HGMS ratios at fixed volume ratios and with different lengths and contents of fibers were also designed and fabricated according to the specimens, and the micro-interfaces of the specimens were scanned by SEM. The results of this study show that the volume ratio of epoxy composite spheres in the sample can significantly affect the properties of concrete materials. With a fixed volume ratio of epoxy composite balls, smaller epoxy composite balls and larger wall thicknesses help to increase the compressive strength of the material. Addition of an appropriate amount of HGMS can reduce the number of pores in the matrix and increase the strength of concrete, but too much HGMS addition can reduce the adhesion of the concrete matrix. Basalt fibers are uniformly distributed in the concrete, and longer basalt fibers can make the concrete matrix more tightly connected and connect the concrete specimens into a whole. The mesh structure formed by the basalt fibers also helps to inhibit the expansion of defects in the matrix and to conduct and disperse the forces in time when the lightweight concrete receives damage from external forces. When two layers of BF-R-EMS with a diameter of 8–9 mm are added to the concrete and 12 mm basalt fibers with a content of 2% are added to the matrix, the maximum compressive strength was increased by 7.9% compared to the unadulterated basalt fibers, and the maximum compressive strength was increased by 443% compared to the EPS concrete.

This work was financially supported by the Science and Technology Commission of Shanghai Municipality and Shanghai Engineering Research Center of Ship Intelligent Maintenance and Energy Efficiency (Grant No. 20DZ2252300) and the Shanghai High-level Local University Innovation Team (Maritime Safety and Technical Support).

The authors have no conflicts to disclose.

Zheng Cao: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Tao Jiang: Data curation (equal); Software (lead); Writing – review & editing (equal). Ying Wang: Visualization (equal). Erke Wang: Software (equal). Lixue Xiang: Investigation (equal). Bo Tang: Investigation (equal). Xinfeng Wu: Methodology (equal); Writing – review & editing (equal). Wei Shao: Resources (equal). Wenge Li: Resources (equal). Kai Sun: Validation (equal). Danda Shi: Methodology (equal).

The data that support the findings of this study are available within the article.