Sustainable Performance and Interfacial Characteristics of Fully Recycled Concrete with Combined Recycled Concrete, Brick, and Ceramic Aggregates

Abstract

The rapid growth of construction and demolition (C&D) waste calls for sustainable recycling in concrete production. However, most studies address single-source recycled aggregates, leaving the behavior of mixed recycled systems insufficiently understood. To address this gap, this study investigates concrete in which natural aggregates are fully replaced by recycled concrete aggregate (RCA), and part of the RCA is further substituted with waste ceramic tile (WCT) or recycled crushed brick (RCB) at controlled proportions. Aggregate type and blending ratio were systematically examined, and Vickers microhardness was used to characterize the interfacial transition zone (ITZ). This study quantitatively establishes a micro–macro correlation between ITZ microhardness profiles and mechanical performance in fully recycled concretes with hybrid aggregate systems. Results show that high replacement levels (100% WCT and 100% RCB) reduced density by 5.6% and 7.0%, respectively, and increased water absorption, reaching 17.0% for 100% RCB. Optimal ultrasonic pulse velocity (UPV) occurred at intermediate blending ratios—50% for WCT and 75% for RCB. Mechanically, a 25% WCT substitution enhanced splitting tensile strength by 50%, whereas a 100% RCB substitution reduced compressive strength by 10.6%. Microhardness profiling revealed ITZ widths of about 70 μm for RCA and WCT, and 80 μm for RCB, consistent with corresponding macro-scale trends. These quantitative findings demonstrate that the rational blending of recycled aggregates can fine-tune the microstructure and performance of fully recycled concrete, providing insight into the high-value recycling of C&D waste.

1. Introduction

The construction industry generates a substantial amount of construction and demolition (C&D) waste annually, accounting for nearly 40% of the total solid waste produced worldwide [1]. However, the recycling rate of C&D waste remains below 10% in most developing countries, far behind the 80–95% achieved in developed regions [2]. Each year, the global construction industry generates over 10 billion tons of construction and demolition waste, of which China accounts for more than 2 billion tons—nearly one-quarter of the total [3]. Concrete, as the world’s most consumed man-made material, plays a dominant role in this massive resource flow, underscoring the urgent need for sustainable recycling strategies [4]. Consequently, recycling C&D waste into recycled coarse aggregates (RCA) to replace natural aggregates in concrete has become a vital strategy for alleviating resource scarcity and promoting sustainable construction practices.

To address these challenges, numerous scholars have explored the feasibility of using recycled aggregates derived from C&D waste to produce recycled concrete. RCA, produced from crushed old concrete, is the most widely studied recycled aggregate. However, it often contains adhered mortar, high porosity (3–10%), and pre-existing microcracks, which lead to deterioration in strength and durability relative to natural aggregate concrete [5,6]. Despite these defects, recycled aggregate can still achieve 85–95% of the compressive strength of conventional concrete when properly designed [7,8]. Other types of C&D waste, such as recycled crushed brick (RCB) and waste ceramic tile (WCT), have also attracted attention due to their abundance in demolition debris. WCT, being dense and vitrified, exhibits low water absorption (0.55–1.4%) and high apparent density (2278–2390 kg/m³), often improving stiffness and reducing permeability [9,10,11,12]. In contrast, the porous texture of RCB results in high water absorption (10.2–18.9%) and lower density (2146–2412 kg/m³) [13,14], typically causing a decline in strength beyond 30% replacement [6,13,15]. Moderate WCT incorporation (15–25%) can enhance tensile strength by up to 11% [16]. These findings highlight that the physical properties of different recycled aggregates significantly influence the mechanical behavior of RAC.

At the microstructural level, the ITZ between aggregates and the surrounding mortar is generally considered the weakest link in recycled concrete. SEM observations have shown that, compared with natural aggregate concrete, RAC exhibits looser and more porous ITZs with fewer hydration products [10,17,18]. The RCA–paste interface often contains multiple weak interfaces between old and new mortar, while the RCB–paste ITZ is irregular and highly porous, and the WCT–paste ITZ, though denser, tends to have reduced hydration bonding [17,19]. However, most of these investigations are based on single-type recycled aggregates. In practice, C&D waste from demolition sites is a heterogeneous mixture that contains concrete fragments, bricks, and ceramics, making precise separation extremely difficult. As a result, the RCA used in real construction recycling is typically a blend of multiple materials, rather than a single aggregate type. Despite this reality, research on mixed or hybrid recycled aggregate concrete remains limited. Moreover, previous ITZ studies have mainly relied on qualitative SEM observations of morphology, while quantitative evaluations of ITZ width and microhardness are still scarce [20]. Therefore, understanding how the blended composition of recycled aggregates affects ITZ compactness and width—and how these micro-scale characteristics correlate to macroscopic performance—remains a key scientific and practical challenge.

To address these limitations, this study fully replaced natural coarse aggregates with RCA. Furthermore, it substituted portions of RCA with RCB and WCT at controlled proportions ranging from 0% to 100%. The physical and mechanical properties of the resulting concretes were systematically examined through measurements of density, water absorption, ultrasonic pulse velocity, and strength. In addition, the Vickers microhardness method was employed to quantitatively characterize the width and compactness of the ITZ, encompassing three types of interfaces: between new mortar and old aggregate, between new mortar and adhered old mortar, and between old aggregate and old mortar within RCA. Beyond the single-aggregate systems commonly reported in previous research, this work further compares the ITZ characteristics of hybrid concretes incorporating both RCB and WCT, thereby elucidating the combined effects of multiple recycled aggregates on ITZ development. Unlike previous studies that mainly examined a single ITZ type, this work quantitatively compared three distinct ITZs formed between different recycled aggregates and new mortar. This comparative approach provides deeper insight into the interfacial heterogeneity of fully recycled concrete, an aspect that has been relatively less explored in the existing literature. Although many previous studies have characterized the ITZ of RAC using SEM to observe microstructural features such as cracks, voids, and hydration products, these methods remain largely qualitative. SEM observations can only provide morphological insights, without quantifying the local mechanical heterogeneity of the ITZ. In contrast, microhardness testing enables a quantitative assessment of ITZ compactness and mechanical uniformity, offering a more direct correlation with macroscopic performance. Therefore, in this study, a quantitative ITZ microhardness method was adopted to complement SEM observations, providing deeper insight into the microstructural uniformity and interface integrity of recycled aggregate concrete incorporating recycled aggregates. This study further establishes a direct correlation between the macroscopic mechanical properties of concrete and the width of the ITZ, providing quantitative insight into the interface behavior of mixed recycled aggregate concrete and a technical basis for its application in sustainable construction.

2. Materials and Methods

2.1. Materials

Ordinary Portland cement (OPC) of grade P.O 42.5, conforming to Chinese standard GB 175-2007 [21] and equivalent to ASTM C150 Type I [22], was supplied by Chongqing Qingpeng Cement Co., Ltd. (Chongqing, China). Its chemical composition and key performance indicators are summarized in

Table 1. Chemical composition and performance indicators of OPC.

SO3	MgO	Cl-	Slag	Gypsum	Ignition Loss	Specific Surface Area	Density
2.85%	2.52%	0.04%	12%	6%	3.06%	369 m2/kg	3.17 g/m3

. Tap water was used throughout the experiments (Chongqing, China). Natural river sand (NRS), employed as fine aggregate, was classified as Grade III natural sand according to GB/T 14684-2011 [21], with a fineness modulus of 1.70, a moisture content of 6.83%, and an oven-dried bulk density of 2704 kg/m³ (Chongqing, China).

The RCA was sourced from demolition waste and laboratory-reconstructed debris (Chongqing, China). The RCA was derived from demolished concrete blocks (compressive strength ≈ 35 MPa, age 20–25 years) collected from local construction sites. The original concrete consisted of natural granite coarse aggregate and ordinary Portland cement (P.O. 42.5). As illustrated in Figure 1a–c, RCA, WCT, and RCB were processed by mechanical crushing. During mechanical crushing, excessive needle-like and flaky particles were minimized by adjusting the crusher opening, and visible impurities such as wood chips and glass were manually removed. These processes helped improve particle shape regularity and reduce weak interfaces, thereby enhancing the consistency of aggregate performance and interfacial bonding quality in the recycled concrete. All coarse aggregates were graded within a continuous particle size range of 5–20 mm, with specific gradation details provided in

Table 2. Coarse aggregate particle grading.

RCA/WCT/RCB	4.75 mm	10 mm	16 mm	20 mm
Cumulative passing percentage (%)	100%	75%	——	5%
Fractional passing residue (%)	25%	70%	——	5%

and Figure 2. Their fundamental physical properties, tested in accordance with GB/T 14685-2011 [21], are presented in

Table 3. Basic physical properties of recycled aggregates.

Type of Coarse Aggregates	Needle Flake Particle Content Ratio(%)	Ruggedness Coefficient(%)	Crushing Indicator (%)	Apparent Density (kg/m3)	Packing Density (kg/m3)	Water Absorption (%)
RCA	3	7.9	16	2650	1276	5.7
WCT	10	5.2	7	2407	1267	1.2
RCB	7	10.1	29	2431	980	17

.

2.2. The Casting of Concrete Specimens and Mechanical Testing Methods

2.2.1. Concrete Mix Ratios and Samples

The physical properties of recycled aggregates vary considerably depending on their source and composition [23]. In this study, natural coarse aggregate was fully replaced by recycled aggregate, with RCB and WCT used to partially substitute RCA at varying replacement ratios. A total of nine concrete groups were designed, incorporating RCB or WCT at replacement levels of 0%, 25%, 50%, 75%, and 100%. The ratios were determined based on preliminary tests and literature reports to balance mechanical performance, water absorption, and workability, while also reflecting the typical composition of mixed C&D waste. Considering the high water absorption of RCB (17%), all recycled aggregates were pre-conditioned to a saturated surface dry (SSD) state before mixing. The concrete mix proportion was set as W:C:S:G = 0.5:1:0.91:1.62, and specimens were cast as 100 mm × 100 mm × 100 mm cubes. The detailed mix designs for each group are provided in

Table 4. Experimental mix ratios of recycled aggregate concrete (kg/m³).

Mix ID	RCA	RCB	Cement	Water	RCA	MA	NRS
RCA-100	100	0	690	345	1115.6	0	628
RCB-25	75	25	690	345	836.7	253.3	628
RCB-50	50	50	690	345	557.9	506.7	628
RCB-75	25	75	690	345	278.8	760.0	628
RCB-100	0	100	690	345	0	1013.3	628
WCT-25	75	25	690	345	836.7	253.3	628
WCT-50	50	50	690	345	557.9	506.7	628
WCT-75	25	75	690	345	278.8	760.0	628
WCT-100	0	100	690	345	0	1013.3	628

Note: Mixed aggregate (MA) refers to mixtures containing RCB or WCT at specific replacement ratios. The Mix ID is composed of the specific aggregate type and its substitution rate. For example, RCA-100 indicates 100% usage of RCA as the coarse aggregate, while RCB-25 signifies that RCB was used as a replacement for a portion of the RCA at a rate of 25%.

.

2.2.2. Mechanical Property Tests

The properties of the hardened concrete, including density, ultrasonic pulse velocity (UPV), water absorption, cube compressive strength, and splitting tensile strength, were evaluated. All tests were performed in triplicate.

Density was determined at the specified curing age by measuring the specimen dimensions with a vernier caliper and its mass using an electronic balance. The density ρ (kg/m³) was calculated as follows:

$$\rho ={\displaystyle \frac{\mathit{M}}{\mathit{A}}}\times {10}^6$$

(1)

where M is the mass (g), and A is the volume (mm³) of the specimen.

UPV was measured in accordance with ASTM C597-16 [24] using a non-metallic ultrasonic tester (ZBL-U520). The UPV V (m/s) was computed as follows:

$$\mathit{V}={\displaystyle \frac{\mathit{L}}{\mathit{T}}}$$

(2)

where L is the transmission path length (mm), and T is the wave transit time (μs).

Water absorption was tested at 28 days. After curing, the saturated surface-dry mass of each specimen was recorded. Samples were then immersed in water at 20 ± 2 °C, supported 25 mm above the tank base, and weighed periodically until the mass change between consecutive 24 h intervals was less than 0.2%. Subsequently, specimens were oven-dried at 105 ± 5 °C until mass stability (change < 0.2% over 24 h) was achieved, and the dry mass (m_d) was recorded. Water absorption W_a (%) was calculated as follows:

$${\mathit{W}}_{\mathit{a}}={\displaystyle \frac{{\mathit{m}}_{\mathit{s}}-{\mathit{m}}_{\mathit{d}}}{{\mathit{m}}_{\mathit{d}}}}\times 100\%$$

(3)

where $W_a$—the water absorption of recycled concrete after curing 28 days, measured in percentage (%); m_s—the surface dry mass of the water-saturated specimen of recycled concrete, measured in grams (g); m_d—the mass of the drying specimen of recycled concrete, measured in grams (g).

The drying temperature of 105 ± 5 °C was selected following standard GB/T 14685-2021 [21]. It is acknowledged that this temperature may cause minor microcracking, but it was uniformly applied to all samples to ensure consistency in comparative analysis.

Mechanical properties, including cube compressive strength and splitting tensile strength, were evaluated in accordance with GB/T 50081-2019 [25]. Compressive strength tests were performed using a WAW-1000D electro-hydraulic servo universal testing machine, which had a calibrated accuracy of ±1% and a loading rate of 0.5 MPa/s. While splitting tensile tests were performed on a WAW-300B machine, both at a loading rate of 0.5–0.8 MPa/s.

2.3. Test Method for Microhardness of the ITZ

Microhardness testing has been widely recognized as a reliable method for characterizing the mechanical properties of ITZs [26,27,28]. In general, the ITZ exhibits higher porosity and lower compactness than the bulk cement paste, making it the weakest link within concrete. Consequently, when pursuing overall improvements in concrete performance, priority should be given to enhancing the ITZ rather than solely optimizing the cement matrix [29].

2.3.1. Interface Transition Zone Sample Processing

For this study, Vickers microhardness testing was employed to characterize three distinct types of ITZs: (1) ITZ₁—between the old aggregate (original natural aggregate) and the adhered old mortar on the surface of recycled concrete aggregates (RCAs); (2) ITZ₂—between the adhered old mortar and the newly formed mortar; and (3) ITZ₃—between the old aggregate and the new mortar, as illustrated in Figure 3a.

RCA mortar specimens with a water-to-cement ratio of 0.5 were cast into 40 mm cubes and cured for 28 days. The specimens were then sectioned to expose the ITZ, embedded in epoxy resin, and sequentially polished to obtain a smooth surface suitable for indentation. Before testing, the surfaces were flushed with anhydrous ethanol and oven-dried to ensure complete dehydration. Polishing was particularly critical to achieving a uniform surface finish, which directly influences the accuracy and reproducibility of the microhardness measurements.

Indentation tests were measured using an HVS-1000 digital Vickers microhardness tester equipped with a regular tetrahedral diamond pyramid indenter. A load of 0.25 N (25 gf) and a dwell time of 15 s were applied, which were selected considering the relatively low hardness of the recycled aggregates and cement paste. Representative epoxy-mounted specimens of RCA, WCT, and RCB concretes are shown in Figure 3b–d, where (1) denotes the epoxy resin, (2) the new mortar, and (3) the recycled coarse aggregate. The corresponding Vickers indentation patterns for the three types of ITZ are presented in Figure 3e–g.

• Concrete cubes (40 mm) cured for 28 days were sectioned into microhardness specimens measuring 15–30 mm. Each specimen was embedded in a 30 mm silicone mold using epoxy resin under vacuum conditions to ensure complete impregnation.
• After resin curing, surface preparation was carried out using a metallographic grinding machine. The specimens were sequentially ground with 800-grit (22.1 μm) and 1200-grit (14.5 μm) sandpapers, followed by polishing with oil-based diamond lapping films of 9 μm, 3 μm, and 1 μm particle sizes, and finally with a polishing cloth. The grinding and polishing durations were 5 min, 30 min, and 30 min, respectively. After each polishing step, ultrasonic cleaning in ethanol for 5 min was performed to remove residual abrasives. Throughout the process, a light load of 20 N, a rotation speed of 150 rpm, and a lubricant feed rate of 0.1 mL/s were maintained.
• Upon completion, the specimens underwent a final ultrasonic cleaning for 10 min to eliminate any remaining dust or diamond residue. The surface quality and smoothness were verified under a microscope, and the prepared samples were subsequently stored in airtight containers before microhardness testing.

2.3.2. Vickers Hardness Test Dotting Method

In conventional recycled concrete, the ITZ is strongly influenced by factors such as the spatial distribution of coarse and fine aggregates and internal porosity [30]. To minimize the excessive variability in microhardness values between the aggregate and paste, a single-aggregate specimen preparation approach was adopted in this study. Furthermore, a Z-shaped indentation layout was employed to accurately characterize the ITZ microhardness [31], as illustrated in Figure 4.

In this method, indentation points were positioned along the aggregate–paste interface at horizontal intervals of 10 μm. Starting from the surface of the recycled aggregate, nine indentation points were successively placed on each side, resulting in a total of nineteen measurement points across each interface. This procedure was consistently applied to all types of recycled concrete incorporating RCA, WCT, and RCB.

2.3.3. Test Principle and Data Processing

The prepared specimens were mounted on the platform of the Vickers hardness tester, and the contact between the diamond indenter and the specimen surface was carefully adjusted. After a specified load was applied and maintained for a predetermined dwell time, the indenter was removed, and the two diagonals of each indentation were precisely measured. The indentation area was calculated using Equation (4), and the corresponding Vickers hardness (HV) was determined from Equation (5).

$$\mathrm{Indentation} \mathrm{area}:\mathit{F}={\displaystyle \frac{{\mathit{d}}^2}{2\mathit{sin}\theta }}$$

(4)

$$\mathrm{Vickers} \mathrm{hardness}:\mathit{HV}={\displaystyle \frac{\mathit{P}}{\mathit{F}}}$$

(5)

where F—indentation area in mm²; d—diagonal length in mm; P—load in g; θ—angle between the indenter of the hardness tester and the specimen surface, and θ = 68°; and HV—Vickers microhardness in kgf/mm².

After completing the indentation tests, the acquired microhardness data were statistically processed using box plot analysis, as illustrated in Figure 5a. Outliers below the lower quartile or above the upper quartile were excluded. The median value of the valid data points was taken as the representative microhardness of the corresponding region and used to define the standard microhardness of the new mortar matrix. Similar criteria have been adopted in previous studies [32], providing consistent and reproducible boundary identification. The filtered results were subsequently plotted as dot distributions for each recycled aggregate type (RCA, RCB, and WCT) to analyze microhardness variation across the ITZs.

Taking the interface between old aggregate and new mortar in RCA concrete as an example, microhardness values were recorded across the old aggregate, ITZ, and new mortar matrix. A large number of indentations were made across both sides of the interface to obtain statistically robust data. As shown in Figure 5b, box plot processing was used again to determine the standard microhardness of the mortar matrix. The ITZ width was defined as the region where the microhardness fell below the lower quartile value of the matrix standard zone. To clearly distinguish different ITZ widths, only the lower boundary of the matrix hardness zone is indicated in the final microhardness profiles.

3. Results and Discussion

3.1. Density of Recycled Aggregate Concrete

The density of recycled aggregate concrete reflects both the compactness of the microstructure and the degree of cement hydration [33]. As shown in Figure 6a, the density of WCT concrete exhibited a gradual decrease with increasing replacement ratio. At 100% WCT and 28 days of curing, the density was reduced by 5.6% compared with the RCA-100 control. This reduction arises mainly from two factors: (1) the smooth and dense surface texture of WCT, which weakened the mechanical interlock and interfacial adhesion with the new paste, and (2) the limited infiltration of hydration products into the ITZ, leading to localized porosity. Consequently, despite adequate curing, the internal compactness of WCT concrete remained slightly inferior to that of the control. (3) The lower density of WCT concrete can be attributed to both the intrinsic low density of ceramic aggregates and the formation of a weaker ITZ with higher porosity, which collectively reduces the overall compactness of the matrix.

For RCB concrete, as shown in Figure 6b, a similar decreasing trend was observed, with a 7% reduction in density at full replacement. The presence of adhered old mortar on RCB aggregates introduced microcracks and inherent voids, which impaired the overall packing density. Although the rougher surface of RCB improved mechanical bonding at moderate contents, its higher water absorption induced a locally higher effective water–cement ratio during mixing, further increasing internal porosity. Prolonged curing only slightly compensated for this effect, suggesting that the intrinsic porosity of recycled aggregates, rather than hydration extent, governs the final density. Overall, RCA concrete consistently exhibited the highest density, highlighting the adverse influence of recycled aggregate defects on bulk compactness.

3.2. Ultrasonic Pulse Velocity Test

The ultrasonic pulse velocity (UPV) characterizes the homogeneity and integrity of the internal structure of concrete. As illustrated in Figure 7a, the UPV of WCT concrete decreased progressively with increasing WCT content, by 3.4%, 0.7%, 3.4%, and 5.1% for 25%, 50%, 75%, and 100% replacement, respectively. Interestingly, the 50% WCT mix exhibited a slightly higher velocity, implying that moderate incorporation of WCT improved the particle packing and internal stress distribution. With curing age, all groups showed increasing UPV, consistent with the continuous formation of C–S–H gel that filled capillary pores and enhanced the connectivity of the solid skeleton.

At higher replacement ratios, however, the weak ITZ bonding and smooth WCT surface produced acoustic impedance discontinuities, causing greater energy attenuation and reduced pulse transmission. This suggests that the UPV decline was dominated by ITZ degradation rather than bulk paste porosity.

For RCB concretes, as shown in Figure 7b, the UPV initially increased and then decreased with the replacement ratio, peaking at 75% RCB. This reflects the dual influence of RCB’s rough surface texture and adhered old mortar. The rough surface enhanced interfacial bonding, promoting efficient stress transfer and wave propagation at moderate replacement levels. Beyond 75%, the higher proportion of porous adhered mortar and pre-existing cracks caused excessive scattering and absorption of ultrasonic energy, resulting in a 16.4% reduction at 100% replacement. Thus, the acoustic results confirm that an optimal replacement ratio (50–75%) achieves a balance between improved bonding and minimized porosity, while excessive replacement compromises structural integrity.

3.3. Mechanical Properties

3.3.1. Compressive Strength

As shown in Figure 8a, the compressive strength of WCT concrete decreased with increasing replacement ratio. At low replacement levels (≤25%), the pre-saturated aggregates provided internal curing water, accelerating hydration and improving early age strength relative to the RCA control. However, with further replacement, the smooth WCT surface and weak ITZ bonding limited mechanical interlocking. The subsequent release of water from saturated aggregates increased the local water–cement ratio, producing excess capillary porosity and deteriorating long-term strength. At 28 days, the strength decreased by 3.8% at 25% WCT and 23.8% at 100%.

For RCB concrete, as shown in Figure 8b, compressive strength initially increased with replacement ratio, reaching a maximum at 75% RCB, and then declined at 100%. The improvement at moderate levels was due to the rougher surface and adherent old mortar, which enhanced chemical bonding and provided nucleation sites for secondary hydration. However, at full replacement, the low intrinsic strength and high porosity of RCB aggregates led to premature microcrack propagation, offsetting the interfacial gains. The results demonstrate that compressive strength is controlled by a competition between ITZ densification and aggregate skeleton quality.

3.3.2. Splitting Tensile Strength

The tensile strength results, in Figure 8c, exhibited trends similar to those of compressive strength but were more sensitive to ITZ quality. For WCT concrete, tensile strength increased by 50% at 25% replacement, driven by improved internal curing and interfacial hydration. However, above 50%, tensile strength declined sharply, reaching 1.3 MPa at 100% replacement. The reduction was primarily due to the smooth surface of WCT aggregates, which hindered bonding and promoted interfacial crack initiation under tension.

For RCB concretes, tensile strength increased with replacement ratio up to 75%, where the optimal ITZ structure provided enhanced adhesion and stress transfer. Beyond this point, excessively old mortar introduced weak zones and micro voids, leading to tensile strength loss. At 100% RCB, the tensile strength dropped to 0.86 MPa, indicating poor load transfer capability due to interconnected ITZ cracks and high local porosity. The notable increase in splitting tensile strength, despite a minor reduction in compressive strength, may be attributed to the internal curing effect of porous RCB particles and the rougher surface texture, which enhances mechanical interlocking at the ITZ.

Overall, both compressive and tensile tests reveal that moderate incorporation (50–75%) of RCB or low incorporation (≤25%) of WCT produces favorable mechanical performance. The mechanical evolution of recycled concretes is therefore governed by two competing mechanisms: (1) beneficial effects of interfacial densification and secondary hydration from internal curing water; and (2) detrimental effects of increased porosity and weaker aggregate skeletons at high replacement ratios. These findings emphasize that optimizing the ITZ microstructure—rather than solely improving paste strength—is the key to achieving durable recycled aggregate concrete.

3.4. Microhardness Analysis of the ITZ

3.4.1. ITZ Microhardness in RCA Concrete

Figure 9 illustrates the microhardness distribution across the three types of ITZs in RCA concrete. The old natural aggregate exhibited the highest hardness (161.8–171.0 kgf/mm²), followed by the old mortar (71.2–80.8 kgf/mm²), whereas the ITZ regions showed a pronounced decline. The weakest zones were observed at the ITZ₁ (old aggregate–old mortar, 56.2–59.3 kgf/mm², width ≈ 50 μm) and ITZ₂ (old mortar–new mortar, 48.7–51.7 kgf/mm², width ≈ 70 μm), while ITZ₃ (old aggregate–new mortar) showed relatively higher hardness (53.6–57.5 kgf/mm², width ≈ 60 μm).

The hardness gradient indicates that the weakest bonding interface in RCA concrete is the transition between the old and new mortar (ITZ₂). This region suffers from a local increase in water–cement ratio and poor hydration continuity, where the existing hydrated layer on old mortar restricts further diffusion of ions and water. As a result, the formation of C–S–H and Ca(OH)₂ is limited, leading to a loose microstructure and low hardness.

By contrast, ITZ₃ exhibited slightly higher microhardness due to the wall effect: water accumulated along the rough old aggregate surface enhanced localized hydration, forming denser C–S–H and ettringite (Aft) products [34,35,36]. Thus, the microhardness hierarchy is as follows: Old aggregate > Old mortar > ITZ₃ > ITZ₁ > ITZ₂, indicating that the discontinuity between the old and new mortar layers primarily constrains the overall integrity of RCA concrete.

Figure 10 is the schematic diagram of ITZ₂. As shown in Figure 10, the ITZ₂ between the old mortar and the new mortar exhibits a porous and loose interfacial structure with relatively low microhardness, confirming the weak bonding characteristics observed in the microstructural analysis.

3.4.2. ITZ Microhardness in WCT Recycled Aggregate Concrete

As shown in Figure 11, the glazed surface of WCT exhibited exceptionally high hardness (≈550 kgf/mm²), approximately 10 kgf/mm² higher than that of the unglazed surface. This is attributed to the vitrified structure formed by high-temperature sintering, characterized by compactness and low porosity [37]. However, the same dense microstructure leads to brittleness and weak chemical bonding with the cement paste.

Both ITZs—between glazed and new mortar and between unglazed and new mortar—had comparable widths (~70 μm) with microhardness values of 49.1–57.3 kgf/mm². The unglazed surface, with its rougher texture and micro-porous structure, provided stronger mechanical interlocking and local anchoring effects [38]. Consequently, its ITZ exhibited slightly higher hardness and smaller local voids.

Mechanistically, the bonding strength of WCT concrete is governed by the competition between physical interlocking and chemical adhesion [39]. The glazed surface enhances compressive resistance due to its intrinsic hardness but restricts interfacial hydration. The unglazed surface, in contrast, promotes a denser ITZ through partial infiltration of hydration products. The duality of “dense–brittle vs. porous–adhesive” characterizes the WCT system’s interfacial behavior.

3.4.3. Microhardness of the ITZ of RCB Recycled Aggregate Concrete

As shown in Figure 12, the RCB aggregate displayed relatively low microhardness values (79.8–93.5 kgf/mm²), owing to its porous ceramic–clay composition. The ITZ between RCB and new mortar exhibited hardness values of 46.0–50.2 kgf/mm², with a width of approximately 80 μm, around 20 μm wider than that of RCA.

This broader, weaker ITZ originates from the high water absorption and low crushing resistance of RCB. During mixing, pre-saturated RCB releases water gradually during curing, locally increasing the water–cement ratio near the interface. This results in a heterogeneous hydration environment, producing coarser C–S–H and loosely packed CH crystals. Although the internal curing effect promotes secondary hydration, the released water is excessive and uneven, leading to microcracking and low ITZ hardness. Hence, the RCB concrete ITZ is characterized by a wide, porous, and mechanically fragile transition layer, which directly explains its inferior compressive and tensile performance.

3.4.4. Microhardness of ITZs of Three Kinds of Recycled Aggregate Concrete

The comparative microhardness profiles of RCA, WCT, and RCB concretes (Figure 13) reveal consistent gradients from the aggregate surface toward the mortar region, with soft zones typically within 10 μm of the interface. However, the magnitude and slope of this gradient vary significantly among the three systems.

• WCT concrete exhibited the highest aggregate hardness (≈ 550 kgf/mm²) and the narrowest ITZ (~70 μm), due to its sintered ceramic structure and low crushing index. The glazed surface formed a continuous barrier with uniform hardness, while the unglazed surface enhanced local bonding through rough interlock.
• RCA concrete showed moderate microhardness and a multi-layered ITZ structure, reflecting the coexistence of old and new cementitious phases. The relatively denser ITZ₃ explains the superior compressive performance of RCA concrete.
• RCB concrete had the lowest microhardness and widest ITZ (~80 μm), attributed to the high porosity and water-retentive nature of brick aggregates, which weakened interfacial compactness.

A negative correlation between ITZ width and microhardness was observed: narrower ITZ → higher microhardness → stronger mechanical performance. This trend mirrors the macroscopic compressive strength results (WCT > RCA > RCB at 28 days).

Mechanistically, ITZ development is governed by the balance between water migration, surface reactivity, and mechanical compatibility [33]. In RCA, residual cementitious products act as nucleation sites promoting secondary hydration; in WCT, sintered density suppresses interfacial hydration; in RCB, excessive water release dilutes hydration near the boundary. Consequently, the ITZ microhardness sequence (WCT > RCA > RCB) and width sequence (RCB > RCA > WCT) collectively reflect the coupled effects of aggregate porosity, chemical reactivity, and interface densification.

In essence, the microhardness results substantiate that the durability and strength of recycled concretes are dominated not by aggregate strength alone, but by ITZ densification and microstructural continuity, which determine the efficiency of stress transfer and the onset of microcracking under load.

4. Conclusions

This study systematically investigated the mechanical performance, density, ultrasonic pulse velocity, and interfacial microstructure of fully recycled concretes in which natural coarse aggregates were entirely replaced with recycled concrete aggregates (RCAs) and further partially substituted with waste ceramic tile (WCT) or recycled crushed brick (RCB). The results revealed distinct correlations between aggregate characteristics, ITZ microhardness, and the macroscopic mechanical behavior of recycled concretes. The detailed findings are shown as follows:

• At the macroscopic level, both apparent density and ultrasonic pulse velocity (UPV) decreased with increasing replacement rates of WCT or RCB, reflecting the influence of aggregate porosity and stiffness on the overall packing density and internal homogeneity of concrete. While the overall trend showed a decrease in density and UPV with increasing replacement, the RCB concrete exhibited a local maximum UPV at 75% replacement, indicating a non-linear relationship between replacement ratio and compactness. WCT concrete exhibited the lowest density reduction (≈5.6% at 100% substitution), while RCB concrete showed the largest (≈7%), owing to its higher water absorption and microstructural heterogeneity. These trends indicate that the internal continuity and compactness of the composite skeleton are governed not only by the intrinsic aggregate density but also by the degree of hydration achieved at the interfacial zones.
• Compressive and splitting tensile strengths followed trends consistent with the UPV results, underscoring the dominant role of the ITZ in load transfer efficiency. For WCT concrete, strength initially increased at a 25% substitution level due to the filler and nucleation effects of fine ceramic particles, which promoted ITZ densification. However, excessive WCT incorporation (≥75%) led to interfacial debonding and brittleness, attributed to the smooth, impermeable surface of the ceramic aggregates that hindered hydration product anchoring. Conversely, RCB replacement improved strength up to 75%, benefiting from its rough, porous surface, which enhanced mechanical interlocking and localized hydration. Beyond this threshold, the low intrinsic strength and weak structure of brick aggregates caused a decline in compressive and tensile strength. These competing effects suggest that mechanical behavior in recycled concretes is controlled more by ITZ microstructure evolution than by the inherent strength of aggregates themselves.
• Vickers microhardness profiling provided direct microstructural evidence for these macroscopic trends. The ITZ between old aggregate and new mortar in RCA concrete exhibited a hardness of 53–57 kgf/mm² and a width of approximately 60 µm, whereas the ITZs in WCT and RCB concretes showed wider and softer interfacial zones—around 70 µm and 80 µm, respectively. The glazed surface of WCT displayed a dense, high-hardness shell (~550 kgf/mm²) yet formed a weak mechanical bond with the surrounding matrix, while the rough RCB surface promoted more extensive C–S–H formation but increased the ITZ porosity through water release during curing. A strong inverse relationship was established between ITZ width and microhardness, confirming that narrower and denser interfacial zones correspond to superior mechanical properties and higher overall concrete compactness.
• Comparative analysis of the three ITZ types—old aggregate–old mortar, old mortar–new mortar, and old aggregate–new mortar—further revealed that the weakest bonding occurred at the old–new mortar interface (ITZ₂), primarily due to incomplete hydration and water redistribution during mixing. The old aggregate–new mortar interface (ITZ₃) benefited from a wall effect that promoted localized precipitation of calcium hydroxide and ettringite, leading to slightly higher hardness and improved bonding. These observations indicate that optimizing interfacial compatibility between recycled and new phases is critical for enhancing mechanical stability.

In summary, this study establishes a clear structure–property relationship linking aggregate type, ITZ morphology, and mechanical performance in fully recycled concretes. The microhardness method proved to be an effective quantitative tool for assessing ITZ integrity and correlating microstructural heterogeneity with macroscopic mechanical response. Among the tested systems, WCT and RCB aggregates exhibited distinct yet complementary effects—WCT contributing stiffness and density, and RCB providing enhanced adhesion and hydration continuity. These findings not only clarify the micro–macro mechanisms governing recycled concrete durability but also offer practical insights for optimizing hybrid recycled systems to achieve high-value utilization of mixed construction and demolition waste in sustainable concrete production. It is acknowledged that this study primarily focused on the mechanical and interfacial behavior of mixed recycled aggregate concrete. Future work will extend to durability aspects, such as freeze–thaw resistance, water impermeability, and carbonation resistance, to comprehensively evaluate the long-term performance of fully recycled concrete.

Besides, this study provides practical guidance for the effective utilization of mixed C&D waste in sustainable concrete production. The results reveal that incorporating blended RCA, WCT, and RCB can achieve a reasonable balance between mechanical performance and interfacial integrity when properly proportioned. These findings can serve as a reference for mix design optimization and quality control in fully recycled concrete, especially for applications in non-structural components, paving materials, and eco-friendly construction elements. By promoting the recycling of mixed C&D waste, this work contributes to resource conservation and the reduction in environmental burden in the construction industry.