Infrastructure, a series of fundamental facilities and structure systems, performs indispensable support for society, expected to achieve sustainability and economic efficiency. However, safety hazards caused by progressive deterioration with age [1] and serious disasters related to severe environmental conditions (freeze–thaw cycles [2,3], marine environment [4], high temperature [5], etc.) are the potential issues during the lifespan of infrastructure, possibly leading to a shortened service life and high maintenance/reconstruction costs. Concrete, regarded as an affordable, durable, and dramatic building material, has been widely applied in infrastructure [6,7,8,9,10]. Due to the adverse impact of intrinsic (material self-defects [11], deficient structural design and construction, etc.) and extrinsic (severe environment, accidental loading [12], etc.) factors, concrete structures usually undergo deterioration during the whole lifespan, such as concrete cracking [13], steel corrosion [14,15], spalling [16], and structure collapse [17].

Most serious durability and safety issues for concrete-based infrastructure are usually the cumulative consequence induced by the service environment [18,19], aging [20], and self-defects [21]. At different periods of its lifespan, there are various dominant factors for degeneration. In the concreting process, fresh concrete can be easily influenced by temperature, humidity, and rheological properties, possibly leading to self-defects (cracks and pores). During the service life, aggressive action related to the invasive substances (chloride, carbon dioxide, and sulfate, etc.) and environmental change is the major reason for the deterioration (e.g., corrosion, carbonation, and cracking). As concrete structures age, the deterioration will decrease the ultimate load capacity and further bring safety and serviceability risks. The application of eco-friendly materials (e.g., recycled concrete [22] and aggregate [10], seawater [23] and sea sand [24], and geopolymer [25,26]) in construction is another challenge for the structural performance. Due to durability and safety issues, concrete-based infrastructure will struggle to maintain functionality, and the most affordable solution (e.g., repair, upgrade, and partial reconstruction) needs to be filtrated.

Considering the severe consequences caused by structural deterioration, there is a strong demand for implementing identification strategies and protection for concrete-based infrastructure. An innovative, reliable, and cost-effective structural health monitoring (SHM) technique for constructing/existing infrastructure has been an essential item to diagnose and mitigate damages and further ensure its functionality, thereby elongating the service life. The strategy of efficient and accurate SHM systems with intelligent materials (e.g., optical fiber [27], piezoelectric materials [28]) installed in concrete structures has attracted lots of attention in recent decades [29,30,31,32]. Piezoelectric materials can be encapsulated as smart aggregates and embedded into the concrete structure, thereby monitoring the deterioration process. The distinction in acoustic impedance, density, and mechanics properties leads to the lower compatibility between the sensing element and host structures, resulting in signal capture disturbance. Table 1 summarizes the major parameters that lead to the incompatibility among cement, concrete and piezoelectric ceramics. One of the critical factors for signal acquisition is acoustic impedance, determined by the density and acoustic velocity [33]. The acoustic velocities of piezoelectric ceramic, cement, and plain concrete are similar, while the density of piezoelectric ceramic is much higher, resulting in the acoustic impedance mismatching problem. The piezoelectric composite is an alternative approach to address this issue. In 2002, cement-based piezoelectric composites (CPCs), innovated by Li et al. [34], are regarded as a pioneering inorganic piezoelectric composite, more adaptable to the concrete structure. Based on the connectivity, piezoelectric composite materials can be divided into 10 basic types [35]. The superiority of the 0-3/1-3/2-2 type CPCs applied in SHM has been demonstrated. The 1-3 [36]/2-2 [37] types can be regarded as the development based on 0-3 by controlling the distribution of piezoelectric materials in the cement matrix. Despite the lower piezoelectric strain constant (d₃₃), 0-3 CPCs show an excellent overall performance (e.g., higher piezoelectric voltage constant (g₃₃), acoustic impedance matching, and flexibility) as a better alternative material for sensing elements, and also show a prospect in combining intelligent manufacturing. However, the difference in fabrication parameters and various raw materials sources leads to a great variety in the final piezoelectric performance.

In the last two decades, the performance improvement of CPCs has been performed. The significant variety in the final piezoelectric performance illustrates the existing shortcoming and insufficient understanding around the composite. Our main objective in this review is to recapitulate the previous studies related to CPCs and discuss the influence of raw materials and the main problems in fabrication, thereby promoting advanced piezoelectric composite design, fabrication, and application. Moreover, the present review will summarize previous research to sort out the critical influencing factors and potential directions. Then, to clarify the influencing factors of the composite materials, theories and fundamentals, experimental and modeling analysis, raw materials, the fabrication process, and application are presented in different sections. At the end of each section, insightful viewpoints and prospective studies on the composite will be provided.

The piezoelectric performance (e.g., piezoelectric, electromechanical coupling, and dielectric properties) of CPCs has been comprehensively characterized and discussed. Initially, Li et al. [30] reported the feasibility of CPCs and characterized their performance, including the piezoelectric strain factor (d₃₃), piezoelectric voltage factor (g₃₃), electromechanical coupling coefficient (K_t and K_p), and dielectric constant (ε_r). Subsequently, Huang et al. [46] studied the polarization process of PZT/sulphoaluminate cement composites; Chaipanich et al. [47] demonstrated the properties of PZT–ordinary Portland cement composites. These studies have demonstrated the feasibility of CPCs and revealed that the piezoelectric particles’ higher content could improve the piezoelectric performance. Additionally, Chaipanich et al. [48] studied the particle size effect, showing that a larger particle of the function phase is beneficial for improving the d₃₃ and ε_r. Pan et al. [49] and Chomyen et al. [50] demonstrated improved piezoelectric performance by adding fly ash, respectively. However, the final piezoelectric performance shows a tremendous difference, mainly attributed to the difference in piezoelectric materials and polarization parameters. According to the above researches, the main influencing factors on the all-round performance can be summarized as the (a) matrix; (b) functional phase (piezoelectric materials); (c) fabrication process; (d) aging.

Due to the complex hydration product composition and heterogeneous microstructure, the matrix effect on piezoelectric performance has been investigated. Among these, enhancing d₃₃ is still the primary target. The low-efficiency stress transfer between matrix and piezoelectric particles caused by the poor connectivity and porosity is the main reason for the lower d₃₃. Therefore, a denser matrix is a penitential approach to optimize it. Chaipanich et al. [51] revealed that the d₃₃ shows a slight increase with adding silica fume. Wang et al. [52] found that adding silica-based material can improve the d₃₃ even up to 99.0 pC/N due to the ITZ optimization under the conditions of compression forming, steam curing, and aging. Subsequently, to decrease porosities, slag, fly ash, and kaolin was studied by Pan et al. [49,53], tracing the piezoelectric properties in different ages, and the highest d₃₃ can reach 111.1 pC/N. Wittinanon et al. [38] employed PVDF to modify the ITZ and reduce the porosity, showing a significant optimization in reducing leakage current. Considering the positive effect of the higher matrix conductivity during the polarization process, carbon materials (carbon addition [54], carbon black [55,56], carbon nanotube [57], and graphene nanoplatelets [58]) were also applied to optimize the piezoelectric properties. The matrix properties can also affect the g₃₃, K_t, K_p, ε_r, and dielectric loss (tan δ), and the increase of parameter values (K_t, ε_r, and g₃₃) with the help of admixture has been reported [49,55]. The output voltage (V) of the composite mixed with basalt fiber, which affects the matrix’s elastic modulus, was also characterized [43].

Meanwhile, the fabrication process optimization was also carried out, and some steps demonstrate significant improvement. Huang et al. [59] applied the forming pressure to fabricate the CPCs, which could help obtain a denser matrix and further enhance the d₃₃. Furthermore, the curing is also essential to obtain the higher piezoelectric performance because inadequate curing would cause interfacial cracks and lead to a locally poor value of d₃₃. Considering the positive effect of the high temperature on the hydration evaluation, Wang et al. [52] carried out the hot water and steam curing process, respectively. Pan et al. [60] found that the pre-heating treatment could improve the polarization efficiency due to decreased moisture. The above fabrication process development could contribute to a better microstructure and mitigate the negative effect caused by the ITZ between the cement matrix and piezoelectric ceramic particles. The significance of polarization has been studied by Huang et al. [46] and Dong et al. [61], demonstrating that the voltage, polarization time, and temperature can directly play a decisive role.

Recently, the multi-factors coupling for the design and fabrication of the composite has been considered. Among those factors, aging is the key influential factor combined with the materials and fabrication process for the performance of CPCs. Dong et al. [37] and Huang et al. [62] revealed the d₃₃ increase with time, even though the matrix phase in their studies is different. Later, Chaipanich et al. [63] found the increased trend of d₃₃ in PZT-Portland cement composites with time; Pan et al. [53] also characterized this phenomenon during their investigation into the effect of admixture. In 2016, Pan et al. [60] found that heat treatment could improve the comprehensive performance after aging. Subsequently, the effect of the water/cement ratio and time on the piezoelectric performance was also studied [64].

The application of this composite has been carried out. Lu et al. [29,30,40,65] systemically monitored the different states in concrete using embedded CPCs sensors, including hydration, crack, and corrosion. Xing et al. [66] tested the electrical response of this material under different mechanical loadings. Pan et al. [31] applied the composite for monitoring the strength growth of concrete via electromechanical impedance. Those applications reveal the feasibility and superiority of CPCs as a potential sensing element.

Regarding the environmental issues, lead-free piezoelectric material has attracted increasing attention. Rianyoi et al. [38,67,68,69] prepared the barium titanate-cement composites and characterized the influence of the particle size and polyvinylidene fluoride (PVDF). Chaipanich et al. [39,50,70,71,72,73] fabricated barium zirconate titanate-cement composites and studied their microstructure and piezoelectric performance. Hunpratub et al. [74] believe that BCTZO (Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃) particle is an alternative material as function phase and revealed the effect of particle size on dielectric and piezoelectric properties. Additionally, BNBT (0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃) [75] and BNBK (0.88Bi_0.5Na_0.5TiO₃-0.08Bi_0.5K_0.5TiO₃-0.04BaTiO₃) [76] have been used as a function phase to fabricate the composite. Although lead-free piezoelectric materials are potential functional materials, the lower piezoelectric properties and poor temperature stability still limit their application in CPCs.

Numerous studies have illustrated the feasibility to fabricate and employ this composite as a sensing element. However, the effect of the fabrication process and polarization parameters are still essential to further study due to the physical and chemical distinction between cementitious materials and piezoelectric materials. The properties of lead-based/lead-free piezoelectric ceramic and its cement-based composites in recent studies, including d₃₃, g₃₃, ε_r, K_t, and acoustic impedance, are intuitively summarized in Table 2. The highest value of d₃₃ in lead-free CPCs is 61.5 pC/N, while that of lead-bearing CPCs is 87 pC/N initially. However, the d₃₃ of the lead-bearing composite can reach over 140 pC/N after aging, close to the piezoelectric ceramic. It should be mentioned that these higher parameters almost attribute to the positive effect of aging, and the typical studies for tracing the change of d₃₃ with aging are illustrated in Figure 1. Only a few studies can obtain high piezoelectric performance at the initial stage after polarization. Santos et al. [77] reported that the curing process of CPCs has a direct relationship with its dielectric properties and electrical conductivity, attributing to the existence of unstable dipoles, which would be a suitable example for understanding the performance variation, illustrating the significant effect of the matrix properties on the final performance. Kantakam et al. [78] revealed the tremendous influence of the matrix material on the dielectric properties.