Layered double hydroxides (LDHs) modified cement-based materials: A systematic review

2.1 Chemical structure of LDHs

Some hydration products with layered structures generated in the hydration process of cement, such as the aluminate ferrite mono (AFm) phase, are considered hydrotalcite-like phase, namely LDHs [18]. The stability of AFm phase plays a vital role in controlling the performance of concrete [19]. Chloride ions can interact with cement hydration products of cement to form chloraluminate phase such as Friedel’s salt (3CaO·Al₂O₃·CaCl₂·10H₂O) and Kuzel’s salt (3CaO·Al₂O₃·0.5CaCl₂·0.5CaSO₄·11H₂O) [20]. Furthermore, the addition of admixtures with high Mg content, such as coal gangue [21], calcined dolomite [22,23], blast furnace slag [24,25,26,27,28], alkali activated paste [29], and MgO [30], can promote the formation of LDHs in the process of cement hydration. And the Mg element [28] contained in cement clinker can also produce MgAl–LDHs. The presence of more LDHs gives the cement a more excellent anionic curing ability [31]. Therefore, it has become a promising research trend to enhance the ability of cement-based materials to cure anions (Cl⁻, CO 3 2 − , and SO 4 2 − ) by introducing LDHs.

In common, the general formula of LDHs can be expressed as [ M 1 − x 2 + M x 3 + ( OH ) 2 ] X + [ A x / n n − ⋅ m H 2 O ] X − [32,33]. The M²⁺ and M³⁺ represent the divalent metal cation (Mg²⁺, Zn²⁺, Mn²⁺, Ca²⁺, Fe²⁺, Ni²⁺, etc.) and trivalent metal cation (Al³⁺, Ga³⁺, Fe³⁺, Cr³⁺, etc.) of the main laminate, respectively. A ⁿ⁻ is a charge compensating anion located between layers of LDHs, including NO 3 − , Cl⁻, OH⁻, CO 3 2 − , SO 4 2 − , etc.; m is the number of water molecules between layers. The typical structure of LDHs is shown in Figure 2a. A combination of tetravalent metal ions (Ti⁴⁺, Zr⁴⁺, Ce⁴⁺, Sn⁴⁺, Si⁴⁺) [34,35,36,37,38] and monovalent metal ions (Li⁺) in LDHs was found in addition to divalent or trivalent metal ions [39,40,41]. The coexistence of M⁺ and M³⁺ is only limited to Li⁺ and Al³⁺. LiA1₂(OH)₆ ⁺ A⁻ is also an example of LDHs whose properties have been extensively reported.

Figure 2

Schematic diagram of a typical structure of LDHs: (a) basic structure, (b) ion exchange of LDHs, and (c) calcination and structure reconstruction of LDHs.

2.2 Ion exchangeability and memory effect of LDHs

The effect of LDHs in cement-based materials depends on their unique chemical structure, which mainly includes the exchangeability of interlayer anions and memory effect.

3.1 Binding effect of LDHs on Cl⁻ in cement-based materials

Resistance to chloride penetration is one of the crucial parameters for evaluating the durability of concrete. Chloride ions invade concrete through pores and then react with hydration products of cement and crystallize, thus changing the composition and microstructure of concrete and deteriorating its performance. Several works have shown that [60] chloride ions in concrete pore solution can combine with hydrated calcium silicate (C–S–H) and AFm [60,61,62]. Note that AFm also belongs to LDHs and is an effective chloride adsorbent, which means that LDHs also have the potential to be used as chloride adsorbents for concrete.

Several researchers have investigated the effect of LDHs on the resistance of cement-based materials to chloride penetration by simulating concrete pore (SCP) solutions. Wu’s work [58] found that adding An-LDH to the SCP solution would lead to the replacement of the inhibitor ions in the LDHs layer by chloride ions and thus a decrease in the concentration of free chloride ions in the SCP solution. The corrosion inhibition efficiency of An-LDH for carbon steel in the SCP solution is NO₂-LDH > NO₃-LDH > C₆H₅COO-LDH > CrO₄-LDH. Research also showed that the two classic hydration products (Mg–Al LDHs, AFm) in alkali-activated slag cement pore solutions have different chloride ion adsorption mechanisms [63]. Surface adsorption is the dominant chloride ion binding mechanism for Mg–Al LDHs, accounting for about 90% of the total chloride ion binding, while the ion exchange accounts for about 10%. In contrast, the surface adsorption of chloride ions by AFM is not significant. Tang et al. [64,65] investigated the effect of CLDHs on the corrosion behavior of reinforcement in a neutralized SCP solution containing chloride ions and found that CLDHs can adsorb some chloride ions and release hydroxide. The release of hydroxide raises the pH of the solution, which can further inhibit the reinforcement corrosion. In simulated carbonated concrete pore solutions (with chloride ions), the corrosion inhibition mechanism of LDHs with nitrate intercalation lies mainly in the exchange of NO 2 − with CO 3 2 − and OH⁻ allowing the release of NO 2 − that can inhibit the corrosion of reinforcing steel [66].

In addition, the improvement of chloride permeation resistance for concrete by LDHs is related to its dosage and particle size. The appropriate amount of LDHs can refine the pore structure and reduce the pore connectivity, thus decreasing the chloride permeability. On the contrary, the overabundance of LDHs will increase the permeability and porosity of concrete and accelerate the chloride penetration [67]. Current studies have found that the appropriate content of LDHs is within 2% [51,68,69,70]. Moreover, the smaller the particle size of LDHs, the more significant its enhancement of the chloride penetration resistance for concrete, which can attribute to the filling effect of LDHs on the pores [51].

Regarding the comparison between LDHs and some auxiliary cementitious materials for chloride binding ability, researchers concluded that CLDHs > cement > slag > fly ash > silica fume [67,70,71,72]. CLDHs can adsorb chloride ions and moisture through structural memory effect and produce OH⁻ to increase alkalinity during the reconstruction of the laminar structure, which can protect reinforcement very well. In addition, slag has the characteristics of high activity and fast pozzolanic reaction, and the generated C–S(A)–H gel is more favorable for the physical adsorption and chemical binding of chloride, so its adsorption efficiency is higher than that of fly ash and silica fume.

Noted that the binding ability of LDHs to chloride ions was related to the type of laminate metal and the ratio of laminate metals. Research showed that the chloride binding capacity of Zn–Al–LDHs increased with decreasing Zn²⁺/Al³⁺, and the best binding capacity occurred when Zn²⁺/Al³⁺ was 2. The reason is that the smaller value of Zn²⁺/Al³⁺, the more charge of Al³⁺, and the higher charge density on the stack lead to a better anion adsorption capacity.

Besides, research also showed that the higher the concentration of chloride ions in the external environment, the higher the chloride ion binding rate of LDHs. Therefore, the bound behavior of LDHs to chloride ions can describe by Freundlich or Langmuir adsorption isotherms [73,74]. In the two isotherms, the absorption increases with the concentration of free chloride ions and finally reaches the saturated adsorption state. Note that the ion exchange capacity of LDHs is affected by many factors, such as the type and particle size of LDHs, the concentration of chloride ions in the external environment, etc. But the most critical factor that determines ion exchange is the selectivity order of LDHs (mentioned in Part 2).

3.2 Binding effect of LDHs on SO 4 2 − in cement-based materials

It is generally believed that sulfate reacts with the hydration products of cement in concrete to form expansive substances, which makes the hardened concrete crack and causes damage. The damage caused by ettringite is the most common type of sulfate attack. Ettringite can combine with large amounts of crystalline water to produce needle-like/rod-like ettringite crystals. This process creates internal stresses and induces concrete cracking. Cracks exacerbate the diffusion process of harmful substances (corrosive substances, air, moisture, etc.) into the concrete, leading to reinforcement corrosion, structural deterioration, and loss of load-bearing capacity [75,76,77,78,79].

In recent years, a great many studies have shown that LDHs can be used as anionic adsorbents, and the calcined products have the characteristics of uniform structure, large specific surface area, and memory effect. Some scholars have investigated the adsorption effect of LDHs on the aqueous solution, groundwater, high concentration sulfate wastewater, and soil [80,81,82,83]. These researches further support the adsorption mechanism of LDHs and CLDHs on sulfate ions as ion exchange and memory effects, respectively (as shown in Figure 3). LDHs can remove up to 90% of sulfate ions under appropriate conditions [84], which means that LDHs are very promising in the high concentration sulfate treatment.

Figure 3

The schematic illustration of sulfate attack induced crack and the modification of LDHs in concrete: (a) the schematic illustration of crack induced by sulfate attack; (b) modification mechanism of LDH on sulfate.

Research showed that CLDHs have higher sulfate ion adsorption efficiency than uncalcined LDHs due to a large interlayer vacancy [85,67,86,87,88]. Chen et al. [87] added Mg–Al–CO₃ LDHs and metakaolin (MK) as modifiers to concrete and found that they could adsorb SO 4 2 − and optimize the pore structure. Another work found that adding LDHs and CLDHs to Na₂SO₄ solution resulted in 1.70 and 6.0% SO 4 2 − adsorption after 12 h, respectively, demonstrating that CLDHs are more effective than LDHs. CLDHs can adsorb large sulfate ions from the cement hydration environment in a short period, thus significantly inhibiting the formation of ettringite and gypsum. Research has shown that adding Mg–Al LDHs to SCP solutions (containing SO 4 2 − ) significantly retarded the reinforcement corrosion. Owing to Mg–Al LDHs solidified SO 4 2 − and Cl⁻ and released NO 3 − and OH⁻, thus alleviated the reinforcement corrosion. As shown in Figure 4, adding Mg–Al LDHs reduced the stable corrosion potential values (E _corr) of the reinforcement in the test solution, suggesting that the addition of Mg–Al LDHs can effectively mitigate the reinforcement corrosion [85,89]. Noted that LDHs have a slightly lower adsorption capacity for SO 4 2 − than Cl⁻ when only SO 4 2 − or Cl⁻ is present in the SCP solution. However, SO 4 2 − will affect the adsorption of Cl⁻ by LDHs when both SO 4 2 − and Cl⁻ are contained in SCP solution. And the adsorption capacity of LDHs on Cl⁻ decreased significantly with the increase in SO 4 2 − concentration.

Figure 4

Evolutions of corrosion potentials of steel specimens in the solutions: (a) with the Mg–Al LDHs; (b) without the Mg–Al LDHs [85].

3.3 Binding effect of LDHs on CO 3 2 − in cement-based materials

The carbonation of concrete can destroy the passive film of reinforcement and diminish the protection of concrete to reinforcement, thus leading to reinforcement corrosion. Industrial wastes, such as blast furnace slag and fly ash, are already widely used as supplementary cementitious materials for concrete. Research has shown that using fly ash or slag powder to replace 50% or higher proportion of cement to prepare concrete reduced the alkalinity and carbonation resistance of concrete and increased the risk of steel corrosion [90]. The anions between the layers of LDHs can exchange with the anions in the solution [89], and the order of the exchange capacity is CO 3 2 − ≥ SO 4 2 − ＞OH⁻＞F⁻＞Cl⁻＞Br⁻＞ NO 3 − . Therefore, the interlayer anions of non- CO 3 2 − intercalated LDHs can easily exchange with CO 3 2 − in solution, thus binding CO 3 2 − . But the high affinity of LDHs for CO 3 2 − harms the chloride binding ability of LDHs. In addition, if LDHs already bind chloride ions, they may release the bound chloride ions and combine with carbonate when exposed to carbonate [63].

Several researchers have also investigated the binding mechanism of LDHs to CO 3 2 − in cement-based materials. Ma et al. [91] found that the adsorption mechanism of LDHs (Mg–Al–CO₃ type) on CO 3 2 − is different from that of CLDHs (calcination at 600°C). Mg–Al–CO₃ type LDHs cannot exchange ions with CO 3 2 − , but their surface can adsorb CO 3 2 − (as shown in Figure 5). In contrast, CLDHs (600°C calcination) provide a lot of sites for CO 3 2 − to enter the interlayer. Hence, the adsorption mechanism of CLDHs on CO 3 2 − lies in the reconstruction of the structure through the memory effect. In addition, research has revealed that the addition of LDHs can improve the carbonation resistance of cement-based materials, especially in the post-conservation phase [92]. In a work by Shui et al. [93], three types of Mg–Al–LDHs were added into concrete: original LDHs (O-LDHs), LDHs after calcination at 600°C (C-LDHs), and LDHs reconstructed after calcination (R-LDHs). The results showed that the carbonation depth of concrete using C-LDHs was significantly lower than that of R-LDHs and O-LDHs because C-LDHs adsorbed a lot of CO 3 2 − during the structural reconstruction process (as shown in Figure 6). And R-LDHs have more OH⁻ compared with O-LDHs, which improves their binding ability to CO 3 2 − . Therefore, the binding capacity of LDHs to CO 3 2 − in concrete is C-LDHs > R-LDHs > O-LDHs. Research [47,93,94,95,96,97] concluded that CLDHs have the best carbon binding capacity (due to their structural reconfiguration effect) and promising application potential. However, because marine concrete usually serves in an environment where CO 3 2 − and Cl⁻ coexist, the synergistic binding mechanism of LDHs to Cl⁻ and CO 3 2 − in concrete still needs further research.

Figure 5

Process of thermal decomposition, structural reconstruction, and CO 3 2 − binding process of LDHs.

Figure 6

Carbonation depth of the designed concrete with LDHs materials at different curing days [93].

4.1 Effect of LDHs on the physical properties of cement-based materials

Research has shown that a larger dose of LDHs and CLDHs makes agglomeration more likely to occur in cement pastes, which can attribute to their large specific surface area and absorbing a large amount of mixed water and some free water [100,101]. LDHs and CLDHs can absorb water from the external environment through surface adsorption and interlayer water storage functions, thus reducing the fluidity of the cement paste. Correspondingly, the water in the cement paste decreases, leading to an increase in the consistency of the cement paste and a decrease in the setting time [102]. On the other hand, the reduction of cement paste fluidity by LDHs also relates to its lamellar structure, which can significantly reduce the fluidity of cement. LDHs are less effective than CLDHs in reducing cement setting time, which can attribute to the presence of water molecules and anions in the interlayer of LDHs, thus making LDHs less absorbent than CLDHs [103]. The specific surface area [104,105] and pore volume of CLDHs increase significantly after calcination, and they can absorb lots of water through physical adsorption and memory effects. Therefore, the incorporation of CLDHs shows a significant influence on the setting time of cement. In addition, the addition of LDHs can bind some sulfate ions (from gypsum), which accelerates the hydration and shortens the setting time (as shown in Figure 7) [106]. Ma et al. [107,108,109] investigated the effect of LDHs–MK on concrete collapse. The results showed that the workability of concrete significantly decreased due to a large amount of water adsorbed by the layered structure of LDH and MK. But the “rolling bearing effect” generated by the combination of ultrafine fly ash and limestone powder can effectively compensate for the loss of concrete workability.

Figure 7

Effect of LDHs on setting time of cement paste [106].

4.2 Effect of LDHs on the mechanical properties of cement-based materials

The particle size of LDHs has significant effect on the mechanical properties of cement-based materials. Research has found that the smaller the particle size of LDHs, the higher the early compressive strength, which can be attributed to the improvement of the pore structure by the filling effect [110]. Another work also showed [111] that the smaller the particle size of LDHs, the easier it is to fill the cement pores, thus promoting the hydration reaction of tricalcium silicate (C₃S) and increasing the early strength of the cement paste (as shown in Figure 8(d)). Note that with the increase in dosage, LDHs will cover the surface of cement particles, thus hindering the hydration of cement particles and decreasing the growth rate of strength gradually. On the other hand, when incorporating LDHs with large particle size and high crystallinity, the filling effect of LDHs is weaker. When their dosage is low, the “nucleation effect” promotes the local hydration of C₃S and generates a large amount of AFm covering the surface of C₃S, thus hindering its further hydration and adverse to the early strength. When adding more LDHs, LDHs adsorbed SO 4 2 − in the pore solution and inhibited the generation of AFm, thus accelerating the hydration reaction of C₃S and promoting the growth of strength. Qu et al. [51] prepared LDHs with different particle sizes by controlling the pH value and obtained the smallest particle size of LDHs at a pH of 13. Their further study found that adding 1% LDHs resulted in a 17 and 55% increase in compressive and flexural strength of cement mortar, which can explain by the filling effect of LDHs resulting in a more dense microstructure [112,113].

Figure 8

(a) Porosity of LDHs blended cement pastes after 28 days of curing. (b) Compressive strength of cement containing LDHs at 3, 7, and 28 days. (c) Effects of LDHs on the microstructure of cement pastes [106]. (d) Schematic of the hydration process [115].

Table 1 summarizes the relevant research on the effect of incorporation of LDHs on the mechanical properties of cement-based materials. Duan et al. [114] found that incorporating 1% O-LDHs increased the compressive strength while 1% C-LDHs was detrimental, but the dosage and type of LDHs had a less significant impact on the compressive strength. Li et al. [102] pointed out that 1–2% of LDHs can greatly improve the compressive strength of C30 concrete. Because LDHs have a unique layered structure and a large specific surface area, which can absorb water through surface adsorption and reduce the water-cement ratio [9]. In addition, LDHs can fill in pores and reduce the content of harmful pores, thus improving the pore structure and increasing the compressive strength of concrete. But excessive LDHs will make the aggregate cannot cement effectively, which is detrimental to the concrete strength.

A variety of supplementary cementitious materials are used in the concrete structure in the project to improve the performance of concrete and save energy [116,117,118]. Scholars have studied the mechanical properties of LDHs with different supplementary cementitious materials. Liu et al. [70] studied the effects of MgO, LDHs, and CLDHs content on alkali-activated fly ash slag (AAFS) and slag blends and found that the addition of MgO and CLDHs increased the compressive strength of AAFS mainly due to the refinement of mesopores and the decrease of porosity, while LDHs caused minor effects on the compressive strength. Chen et al. [67] prepared ultrahigh-performance concrete (UHPC) using cement, fly ash, and silica fume as raw materials with the addition of CLDHs. They found that adding 1% CLDHs increased the compressive strength of the specimens from 145 to 157 MPa after curing for 56 days. And when the dosage of CLDHs was 2%, the compressive strength decreased due to the increased harmful pores. Qiao et al. [71] investigated the impact of incorporating Mg–Al–CLDH on the mechanical properties of seawater marine sand concrete containing fly ash and slag. On the one hand, CLDHs combined part of chloride ions through structural reconstruction, which weakened the early strength effect of chloride ions; on the other hand, as an ultrafine powder, CLDHs can fill pores and improve the pore structure, thus enhancing the latter strength of seawater marine sand concrete.

In summary, the laminate metal, particle size, crystallinity, and dosage of LDHs all affect the mechanical properties of cement-based materials. The smaller the particle size of LDHs, the more significant the positive effect on strength. The mechanism is that LDHs can fill the pores and improve the pore structure of cement-based materials. The effect mechanism of crystallinity on mechanical properties is that LDHs can act as seeds. And the incorporation of LDHs can provide nucleation points for cement hydration products and promote early hydration, thus improving the strength of cement-based materials. The dosage of LDHs has a significant effect on the strength of concrete-based materials. The high dosage can cause the agglomeration of particles, which is detrimental to strength development; 2% is a safe amount of LDHs in cement-based materials.

4.3 Effect of LDHs on hydration and hardening properties of cement-based materials

The hydration mechanism of cementitious materials is the basis for its application. The hydration mechanism and the hardening process differ for different cementitious materials. The current research focuses on the effect of LDHs on the hydration of CSA cement. The precipitation-dissolution equilibrium exists in the aqueous solution of LDHs, and the dissolved ions in CSA cement paste vary with the laminate elements of LDHs. Different metal ions have various effects on the hydration process of CSA cement. For example, Li⁺ has a significant role in promoting the hydration process of CSA cement, but Zn²⁺ retards the setting of cement [120]. In the early hydration stage of cement, Zn²⁺ ions can combine with Ca²⁺ to form Zn–Ca complexes such as Ca(Zn(OH₃)₂)·2H₂O, leading to a decrease in the concentration of Ca²⁺ and OH⁻ in the pore solution, thus delaying the hydration of cement. However, there is also research showing that Li–Al, Mg–Al, Zn–Mg–Al, and Zn–Mg–Al LDHs contribute to the hydration of CSA cement. It can attribute to the fact that LDHs can also act as seeds, reducing the amount of nucleation during the formation of hydration products and thus accelerating the hydration process. Therefore, the promotion of hydration of CSA cement by LDHs may be the result of the combination of crystalline nucleation and interlayer metal ions [106,122].

Recently, Xu et al. [121] found that the addition of CaAl–LDHs accelerates the formation of hydration products (especially for C–S–H), thus increasing the early strength. Guan et al. [123] found that the early hydration heat rate and total heat release rate of CSA cement clinker increased with the increasing of LiAl–LDHs dosage. The addition of LiAl–LDHs accelerated the early hydration, shortened the setting time of CSA cement paste, and no new phase formed in this hydration system. At the same time, Li–Al–LDHs can work as a nucleation site for hydration products, reducing the energy barrier of the hydration product precipitation process, thus accelerating the hydration process and increasing the hydration products [124]. Based on previous research, Ke et al. [8] used CLDHs to control the reaction kinetics of sodium carbonate-activated slag cement. The results showed that adding 10% CLDHs could significantly accelerate the reaction kinetics. Because adding CLDHs accelerated the consumption of carbonate and increased the pH value, thus promoting the dissolution of slag and reducing the water–cement ratio. In summary, the synergistic effects of interlayer metal ions and nucleation effects of LDHs promote (or delay) cement hydration. Adding LDHs to cement-based materials cannot form new hydration products but increases the number of hydration products. The nucleation effect of LDHs reduces the precipitation energy barrier of hydration products and promotes the hydration of cement (Figure 9), leading to a more compact microstructure of hydration products.

Figure 9

Hydration heat flow and total heat released from cement pastes with LDHs [106].