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Review

The Microstructure and Modification of the Interfacial Transition Zone in Lightweight Aggregate Concrete: A Review

by
Jian Zhou
1,
Yiding Dong
2,
Tong Qiu
1,
Jiaojiao Lv
1,
Peng Guo
1 and
Xi Liu
2,*
1
Northwest Engineering Co., Ltd., Xi’an 710065, China
2
School of Architectural Engineering, Chang’an University, Xi’an 710064, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(15), 2784; https://doi.org/10.3390/buildings15152784
Submission received: 1 July 2025 / Revised: 17 July 2025 / Accepted: 31 July 2025 / Published: 6 August 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

The interfacial transition zone (ITZ) significantly influences the mechanical properties and durability of lightweight aggregate concrete (LWAC), yet existing research on the ITZ in LWAC remains fragmented due to varied characterization techniques, inconsistent definitions of ITZ thickness and porosity, and the absence of standardized performance metrics. This review focuses primarily on structural LWAC produced with artificial and natural lightweight aggregates, with intended applications in high-performance civil engineering structures. This review systematically analyzes the microstructure, composition, and physical properties of the ITZ, including porosity, microhardness, and hydration product distribution. Quantitative data from recent studies are highlighted—for instance, incorporating 3% nano-silica increased ITZ bond strength by 134.12% at 3 days and 108.54% at 28 days, while using 10% metakaolin enhanced 28-day compressive strength by 24.6% and reduced chloride diffusion by 81.9%. The review categorizes current ITZ enhancement strategies such as mineral admixtures, nanomaterials, surface coatings, and aggregate pretreatment methods, evaluating their mechanisms, effectiveness, and limitations. By identifying key trends and research gaps—particularly the lack of predictive models and standardized characterization methods—this review aims to synthesize key findings and identify knowledge gaps to support future material design in LWAC.

1. Introduction

Lightweight aggregate concrete (LWAC) has attracted significant attention due to its advantages over normal-weight concrete (NWC), including reduced density, superior strength-to-weight ratio, enhanced durability, improved thermal and acoustic insulation, and increased resistance to frost and fire [1]. These properties make LWAC particularly suitable for use in high-rise buildings, long-span bridges, and offshore structures. However, the characteristics of lightweight aggregates (LWAs) differ markedly from those of natural aggregates (NAs) due to their high porosity and water absorption capacity [2]. This high absorption alters the hydration behavior in the surrounding paste, increasing the degree of hydration and the amount of free water, leading to a more porous microstructure near the aggregate surface [3,4]. Although LWAs are generally considered chemically inert, interacting primarily with the cement matrix through physical mechanisms [5], their potential chemical influence on the interfacial transition zone (ITZ) has received limited attention.
The ITZ, located between the aggregate particles and the cement paste, is known to exhibit distinct properties due to phenomena such as the wall effect, bleeding, and the preferential orientation of hydration products (aligned growth of hydration crystals). Compared to paste, the ITZ typically has higher porosity, lower tensile strength, and reduced bonding capacity. These microstructural features, such as thickness, porosity, and composition, directly influence the macroscopic performance of concrete [6,7].
In ordinary concrete, the ITZ is often the weakest link [8], susceptible to crack initiation under mechanical loading [9,10] or environmental attack [11,12]. The water absorption rate of NAs typically ranges from 0.5% to 2% [13], while LWAs may absorb between 20% and 30%. Such a difference leads to unique microstructural developments in LWAC, particularly in the ITZ, where pre-saturation and water desorption mechanisms significantly affect hydration kinetics, as illustrated in Figure 1.
The ITZ in LWAC presents a complex and heterogeneous structure. Large plate-like calcium hydroxide (Ca(OH)2) crystals often form perpendicular to the aggregate surface, contributing to higher local porosity due to the accumulation of water films. In NWC, the ITZ can contain two to three times more pores and fewer cementitious particles than the bulk matrix [14]. While the LWA itself may be mechanically weaker than the NA, some studies suggest that the ITZ in LWAC is comparatively denser and stronger than that in NWC. However, whether it should still be regarded as the weakest phase remains a subject of debate [15]. The mechanical properties and durability of cement-based composites are strongly influenced by the structure and quality of the ITZ. For example, Konigsberger et al. [16] reported that failure under uniaxial compression frequently initiates at the aggregate–paste interface [17].
Numerous strategies have been proposed to modify and improve the ITZ, including the incorporation of supplementary cementitious materials (SCMs), nanomaterials, surface coatings, and aggregate pretreatments [18]. For example, the use of nano-silica has been shown to enhance ITZ bond strength by over 100% in early curing stages [19], while pre-wetted LWAs can serve as internal water reservoirs, promoting continued hydration and densification of the ITZ [20]. Despite these promising findings, the current body of knowledge remains fragmented, with inconsistent methodologies, limited comparative analysis, and insufficient quantitative evaluation of treatment effectiveness. The economic feasibility of various ITZ modification strategies is an important consideration for their practical implementation. Therefore, understanding the balance between cost and performance is critical for adopting ITZ enhancement methods in engineering practice.
Therefore, a systematic review of the ITZ in LWAC is urgently needed. Previous reviews have primarily focused on conventional concrete or have only touched upon ITZ effects in passing. To date, no comprehensive synthesis has addressed the combined impact of material properties, modification methods, and resulting performance on ITZ characteristics in lightweight systems.
The structure of this review is as follows: Section 2 introduces the chemical composition and microstructural development of the ITZ in LWAC. Section 3 presents key physical properties of the ITZ, including thickness, porosity, and microhardness. Section 4 evaluates how ITZ characteristics influence the mechanical and durability performance of LWAC. Section 5 discusses major ITZ improvement techniques, including mineral and nano admixtures, aggregate coatings, and pretreatment methods. Finally, Section 6 summarizes the findings and outlines future research directions.
This review aims to fill that gap by (i) analyzing the microstructure and physical properties of the ITZ in LWAC, (ii) summarizing and evaluating current ITZ modification strategies based on experimental data, and (iii) identifying unresolved challenges and future research opportunities. By integrating findings from over 100 studies, this review provides a clear reference framework to guide future material design and performance prediction of LWAC.

2. Composition of ITZ Between LWA and Cement Paste

The ITZ is a critical microstructural region in concrete, primarily composed of hydration products and unhydrated cement particles. Within the ITZ, hydration products form as a result of ion diffusion from the cement paste into the liquid film present on the aggregate surface [20]. Similar to the bulk cement matrix, the hydration products in the ITZ include calcium silicate hydrate (C-S-H) gel, calcium hydroxide (CH), and ettringite. Among these, C-S-H is the primary contributor to structural cohesion, whereas CH tends to accumulate at the interface. The large crystal size and preferential orientation of CH within the ITZ can be detrimental to mechanical performance, while C-S-H promotes pore filling and interfacial densification [21].
Although the general composition of the ITZ in LWAC is similar to that in NWC, the relative proportions of hydration products differ significantly. Huang et al. [22] reported that LWAC ITZ contains 12–15% less CH (by volume) than NWC ITZ, with Ca/Si ratios reduced by 18–25% due to enhanced pozzolanic consumption. Zhang et al. [23] observed 30% higher C-S-H density in LWAC ITZ (2.6–2.9 g/cm3) compared to NWC (2.2–2.4 g/cm3), attributed to LWA’s internal curing effect.
The chemical composition of the ITZ plays a critical role in determining its mechanical integrity and, consequently, the overall strength of concrete. A higher volume of calcium silicate hydrate (C-S-H), especially in fibrous or foil-like morphologies, contributes to enhanced adhesion between the aggregate and the surrounding paste, improving load transfer and fracture resistance. Conversely, an excess accumulation of calcium hydroxide (CH) or ettringite can introduce weak zones due to their large crystal sizes and tendency to form preferentially aligned structures. These brittle phases reduce interfacial toughness and are often associated with microcrack initiation. In lightweight aggregate concrete, the composition of the ITZ is further influenced by internal curing and pozzolanic activity, which can reduce CH content and promote the formation of secondary C-S-H, leading to a denser and stronger interface.
Microscopic analyses by Vargas et al. [24] revealed that the ITZ in concrete with natural aggregates contains higher porosity and features a more continuous distribution of hydration products, including CH and ettringite, but fewer C-S-H phases. In contrast, the ITZ in LWAC tends to show enhanced densification, particularly when the cement hydration products penetrate surface pores of the lightweight aggregates, forming mechanical interlocks that strengthen the bond between the aggregate and paste. This phenomenon, often referred to as mechanical interlacing, has been corroborated by Duxson et al. [25,26].
Moreover, the degree of hydration in the ITZ of LWAC is typically higher than that in NWC, resulting in a lower Ca/Si ratio and a noticeable reduction in CH accumulation. The porous surfaces of LWAs facilitate cement particle deposition and reduce the wall effect, promoting the formation of dense C-S-H networks. Additionally, the chemical composition, crystallinity, and surface morphology of LWAs can induce pozzolanic reactions, further enhancing ITZ densification, especially in pumice-based and artificial aggregates such as Aliven.
Figure 2 illustrates the comparative microstructure of the ITZ in concrete containing normal aggregates and lightweight aggregates. In LWAC, the interface demonstrates a greater amount of C-S-H and fewer large CH crystals, contributing to a more compact microstructure. The formation of the ITZ is influenced by multiple interrelated phenomena, among which the “wall effect” and “bleeding” are most prominent. The wall effect refers to the spatial disturbance in particle packing near the aggregate surface, where large aggregate particles prevent the close accumulation of fine cement grains, resulting in a locally higher water-to-binder (w/b) ratio and increased porosity. In contrast, bleeding describes the upward migration of water during setting, which can accumulate at the interface and further weaken the ITZ by diluting hydration products and facilitating CH crystal growth. In LWAC, the high absorption capacity of lightweight aggregates modifies both effects: It can mitigate bleeding by retaining water, while simultaneously reducing the severity of the wall effect due to the interpenetration of cement paste into the aggregate pores. As a result, the interactions between these phenomena and the LWA surface lead to a more complex ITZ development process compared to normal-weight concrete.
Furthermore, the morphology of the ITZ is strongly influenced by the water-to-binder (w/b) ratio. As shown schematically in Figure 3, different w/b ratios lead to varying degrees of internal curing and moisture gradients. At higher w/b ratios, the internal curing provided by saturated LWAs results in a denser ITZ compared to that in NWC. Conversely, at lower w/b ratios, rapid water release from LWAs can locally increase the effective w/b ratio, leading to a looser and more porous ITZ structure.
Figure 4 conceptually illustrates how LWA surface characteristics, including porosity and chemical composition, influence the microstructure of the ITZ. These features promote better bonding through both mechanical interlocking and pozzolanic activity, highlighting the significant role that aggregate properties play in determining the quality of the interfacial transition zone. Although LWAs are generally considered chemically inert, interacting primarily with the cement matrix through physical mechanisms [5], their chemical composition can induce ion exchange, alkaline activation, or dissolution–precipitation reactions at the ITZ. For instance, pumice-based LWAs release soluble silica, promoting supplementary C-S-H formation [22], while Aliven aggregates (artificial LWAs) exhibit Ca2+ leaching that modifies CH crystallization kinetics [24]. These processes, alongside pozzolanic activity, densify the ITZ by reducing pore size and enhancing interfacial cohesion.

3. Physical Properties and Microstructure of ITZ in LWAC

The physical characteristics of the ITZ, including its thickness, porosity, and microhardness, are crucial indicators of concrete performance. In LWAC, these properties differ significantly from those in NWC due to the porous nature and high water absorption capacity of lightweight aggregates. This section discusses the fundamental physical features of the ITZ in LWAC, including thickness, pore structure, and microhardness.

3.1. Thickness of ITZ

Due to the inherent heterogeneity of concrete microstructure, the thickness of the ITZ is not sharply defined. Leemann et al. suggested defining ITZ thickness as the distance from the aggregate surface at which porosity exceeds 15% of the bulk matrix value [27]. Other researchers use inflection points in spatial gradients of porosity, microhardness, CH content, or the Ca/Si ratio to demarcate the ITZ from the surrounding matrix.
The ITZ thicknesses in LWAC of approximately 50, 40, and 30 µm at different hydration ages, indicating that the ITZ becomes denser over time. The water absorption rate of the aggregate strongly influences this thickness. For example, when the 24 h water absorption of LWAs is only 1.93%, similar to that of natural aggregates, the resulting ITZ porosity may be relatively high, deviating from typical LWA-based ITZ characteristics.
Scrivener et al. [28] reported an ITZ thickness of 15–20 µm based on average porosity and CH content distributions, while Xu et al. [29] estimated 55 µm using the Ca/Si molar ratio profile. Other methods for evaluating ITZ thickness include measurements of solid volume fraction, the concentration of unhydrated cement, crystal orientation, elastic modulus, and specific surface area of solids.
The thickness of the ITZ is predominantly affected by factors such as the water-to-binder (w/b) ratio, aggregate type, and particle size. For instance, Hussin et al. found that concrete containing coarse granite aggregates exhibited a thinner and denser ITZ than concrete with coarse limestone aggregates [30]. Cwirzen et al. demonstrated that a lower w/b ratio leads to a more compact paste, thereby narrowing the ITZ to less than 5 µm [31].
Kong et al. reported that increasing the pre-wetting level of LWAs expanded the ITZ thickness from 30 to 60 µm, while Kong found that a lower pre-wetting degree produced a thicker but harder ITZ, indicating that hydration efficiency and internal curing behavior vary with moisture conditions [32]. Overall, the ITZ thickness in LWAC generally falls within 40–100 µm, but under specific conditions, it may extend up to 180 µm. A summary of ITZ characteristics in LWAC and NWC is provided in Table 1.
It is important to note that ITZ thickness measurements are highly sensitive to experimental methods and sample preparation techniques. Therefore, detailed reporting of test conditions is essential for ensuring consistency and comparability across different studies.
While most studies agree that the ITZ thickness in LWAC typically ranges between 40 and 100 μm, discrepancies exist due to differences in aggregate types, pre-wetting conditions, and measurement techniques. For example, Scrivener et al. [28] reported ITZ thicknesses as low as 15–20 μm based on CH distribution, whereas others like Xu et al. [29] estimated up to 55 μm using Ca/Si gradients. This variation highlights the lack of standardization in ITZ demarcation. Moreover, findings on the effect of pre-wetting are inconsistent—some suggest it densifies the ITZ, while others indicate excessive pre-wetting may enlarge the zone without enhancing strength. Future studies should apply multiscale imaging and modeling to unify ITZ thickness definitions and quantify how curing parameters affect the transition zone. Figure 5 illustrates three commonly used techniques for defining ITZ thickness. Subfigure (a) shows the Ca/Si molar ratio profile method, which offers insight into hydration degree gradients. Subfigure (b) shows the microhardness test results across the ITZ region, which helps identify mechanical transitions. Subfigure (c) presents porosity distribution, which correlates with both microstructure and permeability. These methods often yield varying ITZ thicknesses, reflecting the absence of a standardized demarcation approach.

3.2. Pore of ITZ

Porosity is a critical parameter that directly affects ITZ performance. Research on ITZ pore structure typically focuses on total porosity and pore size distribution. Wu categorized pores into four classes: harmless (<20 nm), less harmful (20–50 nm), harmful (50–200 nm), and more harmful (>200 nm) [33]. The preferential growth of CH crystals has been cited as a primary cause of elevated ITZ porosity [34,35].
Porosity in the ITZ can be measured using three main techniques: i. BSE-SEM Image Analysis: Quantitative porosity is calculated by converting micrographs into grayscale images and distinguishing pores by contrast (Figure 6) [36]. ii. Mercury Intrusion Porosimetry (MIP): Used to infer ITZ porosity indirectly by comparing the pore size distribution of cement pastes and composite samples [37]. iii. Qualitative SEM Observation: Denser microstructures observed via SEM are interpreted as indicative of lower porosity [38].
In LWAC, the ITZ generally exhibits a more uniform and compact pore distribution than in NWC, particularly under appropriate curing regimes and aggregate conditions. Zhang confirmed that porosity decreases with longer curing periods (28 to 90 days) due to continued hydration and the pore-filling effect of C-S-H [36]. Saturated LWAs can also act as internal reservoirs, promoting local hydration and reducing porosity in the ITZ [39].
While the effects of w/b ratio and curing age on ITZ porosity are well-established, the influence of aggregate type and moisture condition remains complex and context-dependent. Current studies largely offer qualitative analyses; systematic quantitative investigations of ITZ pore size distributions are still limited.
Most reviewed studies confirm that ITZ porosity in LWAC decreases with extended curing age and the use of saturated LWAs due to internal curing and enhanced hydration. However, the extent of porosity reduction is highly dependent on LWA surface properties and the applied measurement method. For instance, BSE-SEM analysis offers localized visual evidence, while MIP provides bulk data that may mask ITZ-specific features. Moreover, there is limited consensus on how LWA surface chemistry influences CH crystal growth and pore refinement. Current research lacks systematic quantification of harmful versus harmless pore types in the ITZ, especially in relation to different aggregate coatings. Future investigations should combine 3D pore structure modeling with experimental quantification to better understand ITZ permeability evolution. The combined use of lightweight aggregates (LWAs) and supplementary cementitious materials (SCMs) such as fly ash, silica fume, or metakaolin can produce synergistic effects that further reduce the porosity of the ITZ. LWAs provide internal curing by gradually releasing absorbed water, maintaining a higher local relative humidity and extending the hydration period. This sustained hydration environment enhances the pozzolanic reaction of SCMs, resulting in additional formation of calcium silicate hydrate (C-S-H) that fills pores and reduces connectivity. Moreover, the porous surface of LWAs facilitates the deposition of SCM reaction products at the interface, creating a denser microstructure. These combined effects lead to improved ITZ integrity and reduced transport pathways for aggressive ions, thereby enhancing the long-term durability of LWAC.

3.3. Microhardness of ITZ

Microhardness testing is an effective means of evaluating the mechanical integrity of the ITZ. It is typically measured using microhardness testers, Leeb hardness devices, or nanoindentation instruments. Microhardness reflects the degree of hydration, the compactness of the microstructure, and the amount of unhydrated cement or CH present in the ITZ.
In LWAC, the ITZ often displays higher microhardness than the adjacent bulk cement paste, particularly near the aggregate surface. This contrasts with NWC, where the ITZ is usually the softest region due to its high porosity and loose microstructure [40,41]. Hussin et al. found that ITZ hardness varies with aggregate type; siliceous aggregates tend to form denser and harder interfaces than calcareous aggregates [30]. Similarly, Zhang observed that LWAs with higher water absorption yielded ITZs with greater microhardness, likely due to enhanced internal curing and C-S-H polymerization [42].
Overall, the microhardness of the ITZ is governed by aggregate characteristics (type, particle size, and water absorption), w/b ratio, and age. Higher microhardness indicates a denser and mechanically stronger ITZ, contributing positively to concrete strength and durability.
Unlike NWC, where the ITZ is typically softer than the bulk paste, LWAC often shows an increase in microhardness near LWA surfaces due to internal curing and better particle interlocking. Studies consistently indicate that high water absorption aggregates improve hydration and microstructural compactness, yet the long-term evolution of microhardness and its relationship with shrinkage or cracking is rarely examined. Furthermore, comparative studies across different LWA types (e.g., natural vs. artificial, pretreated vs. untreated) are limited. A key gap remains in linking nanoindentation data with macro-scale performance metrics. Future work should focus on standardizing hardness profiles across ITZ depth and integrating them with fracture mechanics models for durability prediction.

4. Effect of ITZ on the Properties of LWAC

In cement-based composites, the mechanical performance and durability of concrete are governed not only by the properties of the cement paste and aggregates but also critically by the ITZ between them. Despite occupying a small volumetric fraction, the ITZ often serves as the weakest link in the composite structure due to its elevated porosity, inferior mechanical strength, and increased susceptibility to microcracking and chemical attack. This section highlights the influence of ITZ characteristics on the mechanical and durability performance of LWAC.

4.1. Mechanical Properties

The ITZ in concrete is widely acknowledged as a mechanically vulnerable region, often containing microcracks, high concentrations of oriented calcium hydroxide (CH) crystals, and fewer hydration products such as C-S-H and ettringite. In concrete incorporating natural aggregates, the ITZ typically exhibits greater porosity and a less dense microstructure than the bulk matrix, significantly compromising strength and fracture resistance [43,44].
Although lightweight aggregates are generally weaker than normal aggregates, studies indicate that the ITZ in LWAC can be denser and stronger than that in NWC. This is partly attributed to the internal curing effect of LWAs and improved mechanical interlocking at the aggregate–paste interface. However, there remains debate as to whether the ITZ in LWAC should still be considered the weakest phase in the system.
For example, Kong et al. [32] reported that the ITZ surrounding LWAs exhibits greater thickness and hardness than that around NAs, suggesting enhanced interfacial performance. Conversely, Zhang and Gjørv [23] found that high-strength LWAs with dense outer shells may induce a “wall effect” similar to that observed in NWC, resulting in high porosity and weak bonding at the interface. Lo et al. [45] further emphasized that the ITZ microstructure in LWAC is strongly dependent on LWA surface texture and water absorption characteristics.
Various studies have demonstrated that the addition of pozzolanic materials such as silica fume can refine the ITZ microstructure and improve bond strength, thereby enhancing compressive strength [46]. Furthermore, Zhang et al. [36] showed that pre-wetting and surface modification of LWAs can significantly affect the ITZ properties in lightweight ultra-high-performance concrete (LUHPC), improving both mechanical strength and interface adhesion. The pozzolanic activity and internal curing capability of LWAs also contribute to ITZ densification, reducing microcracking and improving the stress transfer capacity at the interface.
In summary, the quality of the ITZ is a determining factor in the strength and toughness of LWAC. Among various strategies, the incorporation of nano-silica (NS) and metakaolin (MK) has demonstrated the most consistent improvements in mechanical properties. For instance, 3% NS increased early-age compressive strength by over 20%, while 10–15% MK replacement improved bond strength and chloride resistance significantly. These materials enhance ITZ performance through the pozzolanic reaction, filler effect, and promotion of dense C-S-H gel formation.
Surface modification methods, such as styrene-butadiene rubber (SBR) coating and grout pre-coating, have also been shown to reduce ITZ microcracks and improve adhesion, especially in self-compacting LWAC systems. However, coating uniformity and compatibility with the matrix remain technical challenges.
While current findings support the efficacy of these techniques, their performance varies depending on aggregate type, mixture design, and curing regime. Moreover, long-term behavior under mechanical fatigue and environmental exposure is still underexplored. Future studies should focus on optimizing combined strategies (e.g., NS + SBR) and establishing predictive models linking ITZ microstructure with macro-scale strength evolution.

4.2. Durability

A weak or porous ITZ not only undermines mechanical strength but also accelerates the ingress of harmful substances, thereby compromising concrete durability. The ITZ acts as a preferential pathway for moisture, ions, and aggressive chemicals, increasing the risk of corrosion, freeze–thaw damage, and sulfate attack.
In NWC, the ITZ between NA and the hardened cement paste is often porous and discontinuous due to the wall effect and bleeding during casting [47,48,49,50]. The typical ITZ width in NWC ranges from 50 to 100 µm [51]. In contrast, the ITZ in LWAC displays more variability due to the heterogeneous and porous nature of LWAs, which allow for better mechanical interlocking and potential chemical interactions, such as pozzolanic reactions, between the cementitious matrix and aggregate surface [52].
Unlike the smooth and inert surface of NA, the rough and absorptive surface of LWAs facilitates both physical and chemical bonding, forming a tighter and denser ITZ. Mehta [48] noted that the ITZ morphology is influenced by various factors, including aggregate type, pore structure, surface roughness, and bleeding behavior. In LWAC, the reduced bleeding and enhanced internal curing contribute to the refinement of the ITZ, thereby improving durability.
However, the enhancement in ITZ structure is ultimately limited by the mechanical strength of the aggregate itself. Since failure in LWAC often initiates within the aggregate rather than at the ITZ, the potential benefit of ITZ improvement on overall compressive strength may be constrained [53].
Despite these limitations, a well-structured ITZ can significantly reduce permeability, delay the onset of deleterious reactions, and improve long-term service life. As such, optimizing the ITZ remains a key strategy for improving the durability performance of LWAC, particularly in aggressive environmental conditions.

5. Effect of Treatment of LWAs on ITZ

The mechanical properties and durability of LWAC are significantly influenced by the microstructure of the ITZ. The ITZ is often characterized by elevated porosity, microcracks, and the accumulation and orientation of CH crystals, all of which contribute to its mechanical weakness. To address these limitations, various treatment strategies for lightweight aggregates have been proposed. These strategies aim to enhance the density and homogeneity of the ITZ, improve bond strength, and reduce permeability. This section reviews the primary treatment methods, including the use of mineral admixtures, nanomaterials, aggregate surface coatings, and aggregate pretreatment techniques, as shown in Table 2.

5.1. Effect of Mineral Admixtures on ITZ Microstructure and Mechanical Performance

The incorporation of active mineral admixtures such as fly ash, silica fume, and metakaolin has proven effective in refining the ITZ microstructure. These materials enhance the ITZ through pozzolanic reactions with CH, leading to the formation of additional C-S-H gel and improved matrix densification. Moreover, fine particles from mineral admixtures can fill microvoids within the ITZ and serve as nucleation sites for hydration products, particularly near the aggregate surface [68,69,70].
Silica fume (SF) is notably effective due to its high reactivity and ultrafine particle size. It reduces the ITZ pore size, improves interfacial bond strength, and decreases permeability, as shown in Figure 7. Zhang et al. [23] reported that SF significantly reduced ITZ pore diameter, thereby enhancing the durability of LWAC. However, excessive SF content can lead to increased shrinkage and potential cracking.
Metakaolin (MK) and ground granulated blast furnace slag (GGBFS) have also shown beneficial effects. Cheng et al. [54] reported that replacing 10% of cement with metakaolin increased the 28-day compressive strength by 24.6% and reduced the chloride diffusion coefficient by 81.9%, which can be attributed to the pozzolanic reaction and pore structure densification in the ITZ. Optimal replacement levels (typically around 10–15%) improve compressive strength and chloride resistance [54,56]. Similarly, fly ash (FA) can improve workability and interfacial packing due to its spherical particle shape. However, high dosages of FA may compromise early-age strength development [57].
In summary, mineral admixtures enhance the ITZ by mitigating the wall effect, reducing CH crystallization, and increasing the degree of hydration. Nonetheless, their effectiveness depends on the type, dosage, and fineness of the admixture, and trade-offs in early strength and shrinkage must be considered.

5.2. Effect of Nanophase Materials on ITZ Microstructure and Mechanical Performance

Nanomaterials offer promising ITZ modification due to their high surface area, filling effect, and potential chemical reactivity. Nano-silica (NS) is the most widely studied, known to refine microstructure, accelerate hydration, and provide nucleation sites for C-S-H formation, as shown in Figure 8. Du et al. [59] found that incorporating 3% nano-silica increased ITZ bond strength by 134.12% at 3 days and 108.54% at 28 days due to accelerated hydration and pore refinement effects. The incorporation of NS into LWAC reduces ITZ porosity, improves interfacial adhesion, and enhances compressive strength [71,72].
However, excessive NS can lead to particle agglomeration, which reduces dispersion efficiency and impedes hydration by encapsulating unreacted cement particles [73,74]. Additionally, NS improves resistance to chloride ion penetration by blocking diffusion pathways and creating a more tortuous microstructure [75].
Other nanomaterials, such as carbon nanotubes and carbon nanofibers, have also shown potential for ITZ strengthening, particularly by improving the ductility and toughness of LWAC. While promising, these materials are more expensive and require advanced dispersion techniques to realize their benefits at the interfacial level.

5.3. Aggregate Coating Subsection

Surface coating of lightweight aggregates aims to enhance the aggregate–paste bond by modifying the aggregate surface prior to mixing. Coating materials, such as silica fume, styrene-butadiene rubber (SBR), or cement grout, can mitigate the micro-bleeding effect, reduce water loss from paste to aggregate, and improve hydration uniformity near the interface.
SBR coatings improve the interfacial chemical interaction and promote the formation of stable ettringite crystals [75,76]. Wang [77] found that SBR-modified LWAC exhibited better bleeding resistance and interface stability, particularly in self-consolidating systems. Grout coatings create a denser, crack-resistant ITZ by forming a continuous shell around the aggregate [78,79].
The thickness and uniformity of the coating layer are critical. Overly thick coatings may cause segregation and reduce the plastic viscosity of the mixture [80]. As such, optimal application techniques and material combinations must be determined for different LWA types.

5.4. Pretreatment of Aggregate

Pretreatment methods, including pre-wetting, heat treatment, and carbonation, are widely used to improve ITZ quality by altering the moisture and surface characteristics of LWAs.
Pre-wetting enables LWAs to serve as internal water reservoirs, supplying moisture for continued hydration in the ITZ. This promotes the formation of dense hydration products and reduces shrinkage and porosity [65,66]. Liu et al. [81] showed that pre-wetted LWAs improved the flexural strength of ultra-high-strength concrete at later ages.
Heat treatment of bio-based LWAs decomposes organic matter, enhances dimensional stability, and improves interfacial bonding, thereby reducing microcracking and shrinkage [67]. This is particularly important for aggregates derived from agricultural waste, which may contain sugars and cellulose that negatively affect cement hydration [82,83].
Carbonation modifies the chemical composition and surface morphology of bio-based LWAs. The pyrolysis of hydroxyl groups reduces water absorption and improves hydrophobicity, leading to stronger interfacial bonds and increased mechanical performance [84,85,86].
In conclusion, aggregate pretreatment significantly influences ITZ development by controlling moisture exchange and modifying surface chemistry. However, treatment conditions must be carefully optimized to avoid negative side effects such as reduced early strength or long-term durability degradation.

6. Conclusions

This review systematically analyzed the microstructural characteristics, physical properties, and modification strategies of the interfacial transition zone (ITZ) in lightweight aggregate concrete (LWAC), providing a comprehensive evaluation of its role in mechanical performance and durability. Based on the reviewed literature and comparative analysis, the following key conclusions can be drawn:
(1)
The ITZ in LWAC exhibits unique microstructural features compared to that in normal-weight concrete, including higher degrees of hydration, lower Ca/Si ratios, and improved interfacial compactness due to internal curing and mechanical interlocking effects of porous lightweight aggregates. However, its porosity, microcrack sensitivity, and local heterogeneity remain critical factors limiting performance in many cases.
(2)
Quantitative evaluation confirms the effectiveness of several ITZ modification strategies. For example, 3% nano-silica addition increased ITZ bond strength by over 130% at early ages, and 10% metakaolin replacement enhanced 28-day compressive strength by 24.6% while reducing the chloride diffusion coefficient by 81.9%. Such findings highlight the significant potential of nanomaterials and mineral admixtures in enhancing interfacial performance.
(3)
Among current approaches, nanomaterials (nano-silica, carbon nanofibers) offer the most substantial ITZ densification due to their high reactivity and surface area, though issues like dispersion and cost persist. Surface coatings (SBR latex and grout layers) improve bonding and crack resistance but may affect mixture workability. Pretreatment methods, such as LWA pre-wetting or carbonation, enable internal curing and microstructural refinement, but their influence varies with aggregate type and environmental conditions.
(4)
Despite significant progress, the field still lacks standardization in characterization techniques, data comparability, and predictive modeling. Few studies provide long-term durability data or link microscale ITZ properties to macro-scale behavior using multiscale simulation or machine learning tools. In addition, environmental assessment of ITZ enhancement methods is largely absent.
(5)
Future research should focus on the following: i. establishing unified protocols for ITZ characterization (porosity, hardness, and CH content); ii. developing performance-based models linking ITZ structure to global concrete behavior; iii. exploring hybrid strategies that combine nanomaterials and mineral modifiers with surface treatments; and iv. evaluating the environmental and economic sustainability of ITZ treatments in LWAC systems.
In summary, targeted ITZ modification is a powerful tool to improve the mechanical and durability performance of LWAC. A deeper mechanistic understanding, combined with data-driven design and sustainability considerations, will be essential for the next generation of high-performance lightweight concretes.

Author Contributions

Conceptualization, J.Z. and X.L.; methodology, P.G. and X.L.; software, J.L.; validation, T.Q. and Y.D.; formal analysis, T.Q.; investigation, J.Z.; resources, Y.D. and J.L.; writing—original draft preparation, Y.D.; writing—review and editing, X.L.; visualization, Y.D.; supervision, J.Z.; funding acquisition, T.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China, China (52478297), the Key Research and Development Program of Shaanxi Province, China (2024SF-YBXM-609, 2024GX-YBXM-177), and Scientific Research Plan Projects of Shaanxi Education Department, China (23JE004).

Conflicts of Interest

Authors Jian Zhou, Tong Qiu, Jiaojiao Lv and Peng Guo were employed by the company Northwest Engineering Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Figure 1. ITZ between lightweight aggregate and cement paste.
Figure 1. ITZ between lightweight aggregate and cement paste.
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Figure 2. Microstructure of ITZ between different aggregates and cement: (a) ITZ between NA and cement; (b) ITZ between LWA and cement ((A)-100 μm, (B)-10 μm, (C)-2 μm).
Figure 2. Microstructure of ITZ between different aggregates and cement: (a) ITZ between NA and cement; (b) ITZ between LWA and cement ((A)-100 μm, (B)-10 μm, (C)-2 μm).
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Figure 3. Schematic diagram of the difference in the ITZ microstructure according to aggregate types (NWA and LWA).
Figure 3. Schematic diagram of the difference in the ITZ microstructure according to aggregate types (NWA and LWA).
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Figure 4. Illustration of the ITZ between different aggregates and cement.
Figure 4. Illustration of the ITZ between different aggregates and cement.
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Figure 5. Different methods for defining ITZ thickness: (a) Ca/Si ratio; (b) microhardness; (c) porosity.
Figure 5. Different methods for defining ITZ thickness: (a) Ca/Si ratio; (b) microhardness; (c) porosity.
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Figure 6. Example of the BSE-SEM image analysis procedure: (a) the BSE-SEM image, (b) pore segmentation, and (c) strip delineation [36].
Figure 6. Example of the BSE-SEM image analysis procedure: (a) the BSE-SEM image, (b) pore segmentation, and (c) strip delineation [36].
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Figure 7. ITZ SEM micrographs for LWAC with and without silica fume: (a) sample MD (×50), (b) sample MD (×200), (c) sample MD (×2000), (d) sample MD (×2000), (e) sample MPC (×50), (f) sample MPC (×200), (g) sample MPC (×2000), (h) sample MPC (×5000) [46].
Figure 7. ITZ SEM micrographs for LWAC with and without silica fume: (a) sample MD (×50), (b) sample MD (×200), (c) sample MD (×2000), (d) sample MD (×2000), (e) sample MPC (×50), (f) sample MPC (×200), (g) sample MPC (×2000), (h) sample MPC (×5000) [46].
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Figure 8. The modification mechanism diagrams of nano-SiO2 (binder includes cement, fly ash, and silica fume).
Figure 8. The modification mechanism diagrams of nano-SiO2 (binder includes cement, fly ash, and silica fume).
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Table 1. ITZ properties of concrete with LWAC and NWC.
Table 1. ITZ properties of concrete with LWAC and NWC.
AggregateInterfacial CharacteristicITZ WidthITZ Characteristic
Lightweight aggregateSmooth with closed voids40–100 μmPorous
CH-assembling
Crack-inducing, dense
80–180 μm
Porous with open voids- -
Normal aggregateDense,
smooth		50–100 μmPorous
CH- and AFm-assembling
Crack-inducing
Table 2. Methods to improve interface transition zone.
Table 2. Methods to improve interface transition zone.
MethodEffectAdvantageDisadvantageReference
Mineral admixtureMetakaolin, slagWhen the replacement level of metakaolin and slag is 10%, the optimal effect is achieved. When the w/b ratio is 0.3, the 28-day compressive strength increases by 24.6%, and the chloride diffusion coefficient decreases by 81.9%.(1) The porosity of the interfacial transition zone is reduced, and the bond property of the interfacial transition zone is improved.
(2) Conducive to waste disposal.		(1) Reduce the early strength of concrete.
(2) The properties of different mineral admixtures vary greatly, and there is a lack of systematic theoretical guidance.		[54]
MetakaolinThe amount of MK used was less than 3.6% of the total adhesive content, and the bond strength between LWAC and corroded rebar was improved. Using up to 15%MK modified concrete, pumice light aggregate has higher corrosion resistance, enhancing the paste matrix and interfacial transition zone.[55]
Silica fumeImproves the strength of the aggregate–cement paste interface, decreases ITZ pore size, and increases ITZ density. [55]
Fly ashThe 28-day compressive strength of oil palm shell LWAC was increased by 4% with 10% FA, while with the higher contents of 30% and 50%, the compressive strength was reduced by 14% and 32%, respectively.[56]
Ferrochromium slagThe bonding strength of the interface is increased by 27.98%, and the ITZ thickness is reduced by 20%.[35]
nanophase materialsNano-silicaWith 3% nano-SiO2 incorporation, the 3d, 7d, and 28d-strength of LWAC made with ceramsite N and ceramsite Y increased by 23.5%, 23.7%, 16.8% and 10%, 9.1%, 9.6%, respectively.The porosity of the interface transition zone was reduced, and the performance of the interface transition zone was optimized.The high specific surface area and the surface can cause it to disperse unevenly in concrete.[57,58,59,60]
Carbon nanofibersThe compressive strength of CNF increased by 18.5%, 16.5%, and 12.7% on days 1, 7, and 28, respectively.[61]
Aggregate coatingstyrene butadiene
rubber (SBR)		Replacing NS with FA significantly improves compressive strength and transport performance, and FA enhances paste and paste–aggregate interfaces through its combined effects (pozzolanic reactivity and pore refinement of concrete skeletons). [62]
Grouting coatingThe ITZ of the grout coating enhances the interfacial bond between the LWA particles and the grout matrix, which is completely different from the traditional weak porous ITZ with microcracks. [63,64]
Pretreatment
aggregate		Pre-wettingThe effects of LWAs on the ITZ quality of dry and saturated surfaces were studied. The inner curing provided by the water stored in the saturated LWA enhances the hydration of the surrounding cement paste, thereby densifying the ITZ.The internal curing provided by the water stored in the saturated LWA enhances the hydration of the surrounding cement paste, thereby densifying the ITZ and improving its quality of the ITZ.Excessive pre-wetting of aggregate will affect the water-cement ratio and thus the strength.[65,66]
PreheatingHeat treatment of biological aggregate can effectively improve the dimensional stability of biological aggregate and reduce the generation of microholes and cracks in light aggregate concrete of biological aggregate.Heat treatment of biological aggregate can effectively improve the dimensional stability of biological aggregate and reduce the generation of microholes and cracks in light aggregate concrete of biological aggregate.Reduces the long-term performance of concrete.[67]
Aggregate carbonizeApricot shell and peach shell carbonated aggregates can improve the strength of concrete by increasing the bonding capacity of the ITZ.Mineral precipitation fills the pores in the interfacial transition zoneReduces the long-term performance of concrete.[55]
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MDPI and ACS Style

Zhou, J.; Dong, Y.; Qiu, T.; Lv, J.; Guo, P.; Liu, X. The Microstructure and Modification of the Interfacial Transition Zone in Lightweight Aggregate Concrete: A Review. Buildings 2025, 15, 2784. https://doi.org/10.3390/buildings15152784

AMA Style

Zhou J, Dong Y, Qiu T, Lv J, Guo P, Liu X. The Microstructure and Modification of the Interfacial Transition Zone in Lightweight Aggregate Concrete: A Review. Buildings. 2025; 15(15):2784. https://doi.org/10.3390/buildings15152784

Chicago/Turabian Style

Zhou, Jian, Yiding Dong, Tong Qiu, Jiaojiao Lv, Peng Guo, and Xi Liu. 2025. "The Microstructure and Modification of the Interfacial Transition Zone in Lightweight Aggregate Concrete: A Review" Buildings 15, no. 15: 2784. https://doi.org/10.3390/buildings15152784

APA Style

Zhou, J., Dong, Y., Qiu, T., Lv, J., Guo, P., & Liu, X. (2025). The Microstructure and Modification of the Interfacial Transition Zone in Lightweight Aggregate Concrete: A Review. Buildings, 15(15), 2784. https://doi.org/10.3390/buildings15152784

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