Second-Generation Recycled Concrete Aggregates: Comprehensive Characterization of Physical, Mechanical, and Microstructural Properties

Abstract

The rapid expansion of concrete production has intensified the depletion of natural aggregate (NA) resources, necessitating sustainable alternatives in the construction industry. Recycling construction and demolition (C&D) waste offers a solution to enhance environmental sustainability and resource efficiency. Most existing studies have mainly focused on first-generation RCAs (RCA1), with little work on second-generation RCAs (RCA2), especially fine fractions. This study examined the properties of recycled concrete aggregates (RCAs) across first and second recycling cycles, focusing on their upcycling potential. Therefore, commercially sourced NAs and RCA1 were compared with lab-produced RCA2, both coarse and fine, derived from further recycling of first-generation recycled aggregate concrete (RAC1). Comprehensive tests assessed morphology and physical, mechanical, and microstructural properties to provide a clear insight into how RCA2 differs from RCA1. Average sphericity for coarse RCA1 was 0.81, an 8% decrease from NA’s 0.88, while RCA2 had an average sphericity of 0.76, a 14% decrease. The results revealed a progressive decline in aggregate quality with each cycle. RCA1 exhibited water absorption of 9.53% (fine) and 5.55% (coarse), while RCA2 showed higher absorption at 13.16% (fine) and 6.88% (coarse). RCA1’s crushing value was 25.9%, a 41% rise over NA’s 18.09%, while RCA2’s reached 29.2%, a 61% increase. Coarse RCA2 contained 51.03% attached old mortar, 50% more than the 33.95% in RCA1. Fine RCA2 showed significant performance reductions, limiting these aggregates to non-structural downcycling applications. Microstructure analyses confirmed RCA2’s porous structure, attributed to increased adhered old mortar, including multiple weak interfaces, and numerous microcracks compared to RCA1, necessitating careful consideration when using coarse RCA2 for upcycling in sustainable construction.

1. Introduction

The rapid growth of urbanization, coupled with a sharp increase in construction activities such as major infrastructure projects and housing developments, has led to the substantial generation of C&D waste [1]. Waste concrete constitutes the largest portion of C&D waste. Its reuse in the form of recycled aggregate (RA) is among the most effective strategies for minimizing construction waste and conserving NA resources. When such RA is specifically obtained from crushed concrete, it is referred to as recycled concrete aggregate (RCA), and the concrete produced with it is termed recycled aggregate concrete (RAC). This approach ultimately supports the circular economy and enhances the sustainability of the construction sector [2]. RA has several limitations, including high porosity [3], increased water absorption [4], a high crushing index [5], the presence of microcracks in the interfacial transition zones (ITZs) [6], contamination [7], and variability in quality [8]. Due to the high porosity of residual mortar, the water absorption of RCA is typically five to ten times higher [9,10], and it has a lower particle density [11]. Additionally, microcracks in the ITZs weaken RCA [12] and may facilitate the penetration of harmful reactive substances, such as sulfate ions (SO4²⁻) [13]. As a result, the performance of RAC is inferior to that of natural aggregate concrete (NAC) [14]. New structures are made with RAs, which will finally reach the end of their service life, and consequently be demolished, and new aggregates will be formed [15,16]. It is anticipated that future C&D waste will partially consist of RAC. As a result, the characteristics of such RA will differ. Therefore, it is crucial to examine multi-RA properties and the concrete incorporating it [17].

The multi-recycling process intensifies changes in the characteristics of RA and, as a result, affects the properties of the products made from it. It is generally accepted that the properties of RA deteriorate with each recycling cycle, as the amount of adhered mortar increases [16,18,19]. Consequently, RAs have a lower density than NAs, which continues to decrease with each recycling cycle. Lei et al. [20] found that after two recycling cycles, the apparent density of coarse RCA2 is 3.5% lower than NA, while Huda and Alam [21] reported a 5.8% reduction in its apparent specific gravity. This is because, with each recycling loop, the amount of adhered mortar increases [21,22], and mortar typically has a lower density than NA. The amount of adhered mortar in RA increases until the original NA becomes nearly negligible, which occurs starting from the fourth recycling cycle [23]. Zhu et al. [24] reported that the attached mortar content of coarse RCA increased from 37.7% after the first recycling cycle to 54.8% after the second cycle. Also, the water absorption of RA increases with each recycling cycle due to a larger amount of adhered mortar. Additionally, the porosity of RA is higher, which facilitates fluid penetration [18,21]. For the coarse fractions, Kim et al. [25] reported that NA has a water absorption of 0.73%, whereas RCA1 and RCA2 range from 2.8% to 6.92%. For the fine fractions, Zhu et al. [26] stated that water absorption increased sharply, reaching 13% in the second-generation aggregate. Visintin et al. [27] showed that with increasing recycling cycles, the crushing index rises from 13.53% for NA to 22.67% and 23.70% for RCA1 and RCA2, respectively, indicating that the recycled aggregates are more susceptible to abrasion. During the first recycling cycle, the decrease in density and the increase in porosity and absorption are significantly greater compared to subsequent recycling cycles [23]. Abreu et al. [16] noted that the properties of RCA tend to stabilize with increasing recycling cycles.

Given that the quality of RA significantly affects the performance of RAC, a thorough understanding of RCA characteristics becomes increasingly important with additional recycling cycles. To date, studies on RCA2 have mainly been limited to basic properties such as density and water absorption [28] and have predominantly focused on coarse fractions [21,22,24,29]. Fine RCA2, however, remains a more problematic area due to its higher porosity and mortar content, yet has received little systematic attention. Furthermore, most previous works do not integrate morphological, mechanical, and microstructural analyses, which are essential for assessing upcycling potential. Therefore, this study aims to determine how RCA2 differs from RCA1 and NA for both fine and coarse aggregates, and whether these differences significantly impact their suitability for structural concrete.

2. Results and Discussion

2.1. Gradation and Shape Analysis

2.1.1. Particle Size Distribution

The sieve analysis and Camsizer 3D results highlight notable variations in particle size distribution between NAs and RCAs, as depicted in Figure 1, Table 1, and Table 2. For the fine fraction (0–4 mm), NA showed approximately 90% of particles passing the 4 mm sieve, indicating a well-graded distribution typical of NAs. In contrast, RCA1 and RCA2 exhibited progressively finer distributions, with 99% and 100% of particles passing the 4 mm sieve, respectively, suggesting increasing material breakdown with each recycling cycle. For the coarse fraction (4–16 mm), all aggregates showed similar distributions, with NA exhibiting 98% passing the 16 mm sieve, RCA1 showing 99% passing, and RCA2 reaching 100%. The Camsizer 3D analysis confirmed these trends, showing a progressive decrease in particle sizes from NA to RCA2. For coarse aggregates, NA showed a median size of 9.98 mm, decreasing to 9.60 mm for RCA1 and 7.61 mm for RCA2. Interestingly, for fine aggregates (FAs), NA contained more fine particles (median 0.954 mm) than both RCA1 (1.438 mm) and RCA2 (1.697 mm). These findings align with Florea et al. [30], who demonstrated that repeated crushing results in a finer overall particle size distribution, and Lu et al. [31] similarly concluded that the removal of adhered mortar leads to a finer particle gradation.

2.1.2. Morphology Analysis

As shown in Table 1, the morphological characteristics of coarse NA and RCAs were analyzed using the Camsizer 3D. Roundness, measured by Q3 (SPHT = 0.9) [%], which is the cumulative volume percentage of particles with sphericity (SPHT) ≥ 0.9, where a perfect sphere is 1.0, is reported at 53.66% for NA, indicating moderate roundness. For RCA1, the value increases significantly to 93.15%, while RCA2 reaches an even higher 98.62%, likely due to the effects of abrasion during the recycling process. Symmetry, measured by Q3 (Symm = 0.9) [%], which represents the cumulative volume percentage of particles with symmetry (Symm) ≥ 0.9, is 27.45% for NA. This increases markedly to 63.33% for RCA1 and further to 79.94% for RCA2. The aspect ratio, Q3 (b/l = 0.9) [%], which is the cumulative volume percentage of particles with breadth to length ratio (b/l) ≥ 0.9, is nearly identical across all samples, with values of 100% for NA, 99.83% for RCA1, and 100% for RCA2.

The average shape parameters provide detailed insight into understated differences in particle morphology. The mean value SPHT3, which represents average sphericity, is 0.88 for NA, decreasing to 0.81 for RCA1 and 0.76 for RCA2, which is a reduction of 0.07 and 0.12, compared to NA, respectively. The contrast between Q3 (SPHT = 0.9) [%] and Mean SPHT3 reflects differences in the distribution of sphericity. Recycling processes (from NA to RCA1 and then to RCA2) often involve crushing and grinding, which can preferentially create more spherical particles (increasing Q3 (SPHT = 0.9) [%]). However, these processes may also produce some irregularly shaped particles with lower sphericity, reducing the overall average (Mean SPHT3). RCA2 undergoes more processing than RCA1, which could explain why it has the highest percentage of highly spherical particles (98.62%) but also the lowest average sphericity (0.76), due to a wider range of particle shapes. This is consistent with the findings of Pacheco and Brito [32], who reported that RAs tend to be flakier and more elongated, primarily due to the nature of their constituents, which tend to fragment into elongated shapes, and the effects of the crushing process typically with a jaw crusher, this will also result in more elongated particles.

The mean value Symm3, which represents average symmetry, is 0.92 for NA, dropping to 0.89 for RCA1 and 0.87 for RCA2. Once again, the contrast between Q3 (Symm = 0.9) [%] and mean Symm3 reflects differences in symmetry distribution. RCA1 and RCA2 have a higher proportion of highly symmetrical particles due to recycling processes, but their averages are lowered by particles with reduced symmetry. The mean value b/l3, which represents the average breadth to length ratio, is 0.69 for NA, decreasing to 0.67 for RCA1 and 0.65 for RCA2, which is a reduction of 0.02 each, supporting increased elongation. The contrast between Mean b/l3 and Q3(b/l = 0.9) arises because Mean b/l3 is a volume-weighted average sensitive to the full particle population (including numerous small elongated fragments produced by crushing), whereas Q3(b/l = 0.9) is a cumulative-volume percentile dominated by the largest-volume particles; thus, the bulk volume can remain near-equiaxed (high Q3) while the mean decreases due to an increased fraction of elongated small particles.

Distribution width, measured by SPAN3, is 0.85 for NA, increases to 0.90 for RCA1, and decreases to 0.88 for RCA2, showing just minor variability. The non-uniformity factor, U3, is 1.87 for NA and RCA1, but drops to 1.68 for RCA2, indicating greater uniformity in RCA2. Overall, analysis of coarse aggregates demonstrates that the recycling process has a significant impact on particle shape and size distribution. Increased recycling from NA to RCA1 to RCA2 results in smaller average particle sizes, a higher proportion of highly spherical and symmetrical particles, but also a decrease in the average sphericity and symmetry due to the presence of irregular fragments. According to Wang et al. [33], mortar residues and internal voids contribute to the angular and irregular shapes of RAs particles, unlike the smoother, more rounded form of NAs.

Table 1. Influence of recycling cycles on the coarse aggregate shape and size properties.

Shape Parameter a	NA	RCA1	RCA2
x [mm] at Q3 = 10.0%	6.04	5.61	5.01
x [mm] at Q3 = 50.0%	9.98	9.60	7.61
x [mm] at Q3 = 90.0%	14.49	14.28	11.73
Q3 (SPHT = 0.9) [%]	53.66	93.15	98.62
Q3 (Symm = 0.9) [%]	27.45	63.33	79.94
Q3 (b/l = 0.9) [%]	100.00	99.83	100.00
Mean value SPHT3	0.88	0.81	0.76
Mean value Symm3	0.92	0.89	0.87
Mean value b/l3	0.69	0.67	0.65
SPAN3	0.85	0.90	0.88
U3	1.87	1.87	1.68

^a Shape parameters: x [mm] at Q3 = 10.0%—10% of particles are ≤ this size, x [mm] at Q3 = 50.0%—median size; 50% are ≤ this value, x [mm] at Q3 = 90.0%—90% of particles are ≤ this size, Q3 (SPHT = 0.9) [%]—% of particles with sphericity ≥ 0.9, Q3 (Symm = 0.9) [%]—% of particles with symmetry ≥ 0.9, Q3 (b/l = 0.9) [%]—% of particles with width/length ratio ≥ 0.9, mean SPHT3—average sphericity, mean Symm3—average symmetry, mean b/l3—average width/length ratio, SPAN3—width of size distribution, U3—non-uniformity factor (distribution symmetry).

Table 1. Influence of recycling cycles on the coarse aggregate shape and size properties.

Shape Parameter a	NA	RCA1	RCA2
x [mm] at Q3 = 10.0%	6.04	5.61	5.01
x [mm] at Q3 = 50.0%	9.98	9.60	7.61
x [mm] at Q3 = 90.0%	14.49	14.28	11.73
Q3 (SPHT = 0.9) [%]	53.66	93.15	98.62
Q3 (Symm = 0.9) [%]	27.45	63.33	79.94
Q3 (b/l = 0.9) [%]	100.00	99.83	100.00
Mean value SPHT3	0.88	0.81	0.76
Mean value Symm3	0.92	0.89	0.87
Mean value b/l3	0.69	0.67	0.65
SPAN3	0.85	0.90	0.88
U3	1.87	1.87	1.68

^a Shape parameters: x [mm] at Q3 = 10.0%—10% of particles are ≤ this size, x [mm] at Q3 = 50.0%—median size; 50% are ≤ this value, x [mm] at Q3 = 90.0%—90% of particles are ≤ this size, Q3 (SPHT = 0.9) [%]—% of particles with sphericity ≥ 0.9, Q3 (Symm = 0.9) [%]—% of particles with symmetry ≥ 0.9, Q3 (b/l = 0.9) [%]—% of particles with width/length ratio ≥ 0.9, mean SPHT3—average sphericity, mean Symm3—average symmetry, mean b/l3—average width/length ratio, SPAN3—width of size distribution, U3—non-uniformity factor (distribution symmetry).

The analysis of FAs, as presented in Table 2, reveals an interesting contrast to the coarse fraction results. RCAs exhibit a narrower distribution, reflected in lower SPAN3 values with RCA2 at 1.691, RCA1 at 1.934, and NA at 2.314. However, RCA1 has a higher U3 value at 4.570 compared to RCA2 at 3.804 and NA at 4.045, indicating pronounced asymmetry and irregular particle size distribution.

It is noteworthy that a slight discrepancy exists between the sieve analysis and image analysis results. While sieve analysis shows approximately 90% of NA passing 4 mm, Camsizer 3D detects 100% passing. This discrepancy arises from differences in measurement principles. Sieve analysis determines particle size based on the smallest dimension, which may cause some irregularly shaped particles to be retained. In contrast, Camsizer 3D analyzes multiple dimensions dynamically, providing a more accurate assessment of particle passing [34]. Additionally, mechanical sieving may lead to temporary particle lodging or agglomeration, whereas image analysis ensures free dispersion.

Table 2. Influence of recycling cycles on the FA shape and size properties.

Shape Parameter	NA	RCA1	RCA2
x [mm] at Q3 = 10.0%	0.303	0.392	0.537
x [mm] at Q3 = 50.0%	0.954	1.438	1.697
x [mm] at Q3 = 90.0%	2.511	3.172	3.407
a Q3 [%] at x = 1.000 mm	51.62	34.75	27.69
a Q3 [%] at x = 2.000 mm	81.33	66.46	58.87
a Q3 [%] at x = 4.000 mm	100.00	100.00	100.00
SPAN3	2.314	1.934	1.691
U3	4.045	4.570	3.804

^a Shape parameters: Q3 [%] at x = 1.000 mm, 2.000 mm, 4.000 mm—cumulative volume percentage of particles with sizes ≤ 1.000 mm, ≤2.000 mm, and ≤4.000 mm, respectively.

Table 2. Influence of recycling cycles on the FA shape and size properties.

Shape Parameter	NA	RCA1	RCA2
x [mm] at Q3 = 10.0%	0.303	0.392	0.537
x [mm] at Q3 = 50.0%	0.954	1.438	1.697
x [mm] at Q3 = 90.0%	2.511	3.172	3.407
a Q3 [%] at x = 1.000 mm	51.62	34.75	27.69
a Q3 [%] at x = 2.000 mm	81.33	66.46	58.87
a Q3 [%] at x = 4.000 mm	100.00	100.00	100.00
SPAN3	2.314	1.934	1.691
U3	4.045	4.570	3.804

^a Shape parameters: Q3 [%] at x = 1.000 mm, 2.000 mm, 4.000 mm—cumulative volume percentage of particles with sizes ≤ 1.000 mm, ≤2.000 mm, and ≤4.000 mm, respectively.

2.2. Density and Water Absorption

The apparent particle density, oven-dried particle density, and saturated surface-dried particle density were evaluated to characterize the physical properties of NA, RCA1, and RCA2 for both fine (0–4 mm) and coarse (4–16 mm) fractions, as shown in Figure 2 and Figure 3, respectively. For the fine fraction, the apparent particle density is reported at 2.57 g/cm³ for NA, decreasing to 2.43 g/cm³ for RCA1 (a reduction of 5.4%) and further to 2.35 g/cm³ for RCA2 (a reduction of 8.5% from NA). The oven-dried particle density for the fine fraction is 2.39 g/cm³ for NA, decreasing to 1.97 g/cm³ for RCA1 (a reduction of 17.5%) and further to 1.79 g/cm³ for RCA2 (a reduction of 25.1% from NA). The saturated and surface-dried particle density for the fine fraction is 2.46 g/cm³ for NA, decreasing to 2.16 g/cm³ for RCA1 (a reduction of 12.2%) and further to 2.03 g/cm³ for RCA2 (a reduction of 17.48% from NA). Jung et al. [35] investigated repeatedly recycled fine aggregates (RFAs) from crushed precast concrete, reporting specific gravities of 2.50, 2.24, and 2.20 for the first, second, and third generations, respectively, compared to 2.59 for natural river sand. Similarly, Zhu et al. [26] observed a decline in FA quality with increased recycling repetitions. Natural FA exhibited an apparent density of 2616 kg/m³, while the first and second generations decreased to 2382 kg/m³ and 2329 kg/m³, respectively. Kun et al. [36] linked the lower density to (1) residual cement paste with low density and high porosity, and (2) increased adhered cement mortar with increasing recycling, reducing the density of the next generations of RFAs.

For the coarse fraction, the apparent particle density is 2.88 g/cm³ for NA, decreasing to 2.62 g/cm³ for RCA1 (a reduction of 9.0%) and further to 2.61 g/cm³ for RCA2 (a reduction of 9.3% from NA). The oven-dried particle density for the coarse fraction is 2.78 g/cm³ for NA, decreasing to 2.29 g/cm³ for RCA1 (a reduction of 17.6%) and further to 2.21 g/cm³ for RCA2 (a reduction of 20.5% from NA). The saturated and surface-dried particle density for the coarse fraction is 2.82 g/cm³ for NA, decreasing to 2.41 g/cm³ for RCA1 (a reduction of 14.5%) and further to 2.36 g/cm³ for RCA2 (a reduction of 16.3% from NA). In the coarse fraction, from NA to RCA1, there is a significant reduction, while the reduction from RCA1 to RCA2 is more moderate. Kim and Jang [15] observed similar trends, noting that specific gravities of 2.68 for NCA, which declined to 2.41 and 2.27 for the first and second generations of recycled coarse aggregate, respectively. Yao et al. [37] reported that the apparent density of RCA1 and RCA2 was approximately 6.33% and 11.35% lower than that of NCA, respectively, with oven-dried and saturated surface-dry densities showing similar trends, primarily due to increasing adhered mortar.

These trends indicate that RCA1 and RCA2 have lower particle densities than NA across all types of densities, with reductions ranging from 5.4% to 25.1%, attributed to the porous adhered mortar and internal voids. The greater density reduction in RFAs reflects increased porosity in smaller particles, as adhered mortar content rises with decreasing particle size due to the higher surface area [38].

Water absorption indicates the RA’s capacity to absorb water, reflecting their porosity and internal structure [39]. For the fine fraction, NA is reported at 3.29%, RCA1 increases to 9.53% (an increase of 189.6%), and RCA2 further increases to 13.16% (an additional increase of 38.0% from RCA1, and 300.0% from NA), as shown in Figure 4. This sharp increase reflects higher porosity in RCAs, with RCA2 absorbing the most due to more adhered mortar [26,36]. Zhu et al. [26] reported 24 h water absorption values of 1.2%, 12.1%, and 13.0% for natural fine aggregate (NFA), first-generation recycled fine aggregate (RFA1), and second-generation recycled fine aggregate (RFA2), respectively. Similarly, Kun et al. [36] observed 30 min absorption values of 0.2%, 12.1%, 12.7%, and 14.0% for NFA, RFA1, RFA2, and RFA3. These results show that RFA1 consistently exhibits higher water absorption across both studies compared to the present work, whereas the absorption values for RFA2 closely align with those observed in this study. According to Jung et al. [35], the water absorption of RFA1 and RFA2 was comparatively lower, measured at 3.26% and 8.34%, respectively.

For the coarse fraction, NA is reported at 1.32%, RCA1 increases to 5.55% (an increase of 320.4%), and RCA2 further increases to 6.88% (an additional increase of 23.9% from RCA1, and or 421.2% from NA). Huda and Alam [21] reported absorption capacities of 5.2%, 7.1%, and 9.4% for RCA1, RCA2, and RCA3, respectively, while the natural coarse aggregate exhibited a significantly lower value of 1.2%. Abreu et al. [16] conducted a study on coarse aggregates subjected to multiple recycling cycles, reporting a progressive increase in water absorption, with values of 5.6%, 8.0%, and 9.6% for RCA1, RCA2, and RCA3, respectively, compared to 1.1% for NCA.

2.3. Crushing Value

The crushing value index, a key indicator of mechanical strength, was evaluated for NA, RCA1, and RCA2 within the coarse fraction (10–14 mm), as presented in Figure 5. This parameter measures the resistance of aggregates to crushing under a compressive load, expressed as a percentage, with lower values indicating greater resistance and suitability for load-bearing concrete applications.

For NA, the crushing value is reported at 18.09%, reflecting its robust resistance to fragmentation. For RCA1, the crushing value increased to 25.9% (an increase of 43.18% from NA), indicating a significant reduction in crushing resistance, likely due to the presence of porous adhered mortar [27] and micro-cracks [20] introduced during the initial recycling process, which weaken the aggregate structure. For RCA2, the crushing value further rises to 29.2% (an additional increase of 12.74% from RCA1, and 61.42% from NA), suggesting even lower resistance to crushing, potentially due to additional processing, or increased porosity and defects from the second recycling stage. Similarly, Zhu et al. [40] stated that the crushing index of RCA increased progressively with the number of recycling cycles, showing a linear positive correlation with the attached mortar content.

These progressive increases in crushing value from NA to RCA1 and RCA2 highlight a deterioration in mechanical strength, with RCA2 exhibiting the least resistance to crushing. The 61.42% increase in RCA2 compared to NA indicates a substantial reduction in load-bearing capacity, which could compromise the structural integrity of concrete incorporating these RAs. Visintin et al. [27] reported aggregate crushing values (%) of 13.53, 22.67, 23.70, and 26.32 for NA, RCA1, RCA2, and RCA3, respectively. Lei et al. [20] reported crushing index values, with NA at 11.63%, and RCA1, RCA2, and RCA3 at 14.79%, 16.10%, and 18.07%, respectively. They concluded that the higher crushing values for RCA1, RCA2, and RCA3 are likely linked to the porous nature of adhered mortar and the cumulative effects of recycling, such as micro-cracking and fragmentation, which reduce aggregate cohesion and strength.

2.4. Residual Mortar Content

The adhered mortar content, determined using the acid treatment method described in the test methods, was evaluated for RCA1 and RCA2 across both fine (0–4 mm) and coarse (4–16 mm) fractions, as illustrated in Figure 6. It has been demonstrated that the amount of residual mortar adhered to RCAs is one of the main factors negatively affecting their properties and performance [41]. Therefore, accurately measuring the adhered mortar content is essential for evaluating the quality of RCA.

For RCA1, the adhered mortar content is reported at 44.95% in the fine fraction and 33.99% in the coarse fraction, indicating a decrease of 24.38% from fine to coarse, reflecting reduced mortar retention in larger particles, likely due to greater surface area and higher porosity in fine fractions [42]. For RCA2, the adhered mortar content is reported at 53.95% in the fine fraction and 51.03% in the coarse fraction, showing a decrease of 5.72% from fine to coarse, suggesting a smaller reduction in mortar content for larger particles, in the second recycling stage.

When comparing the fine fractions, RCA1 exhibits an adhered mortar content of 44.95%, which is 20.02% lower than that of RCA2 (53.95%), indicating a higher accumulation of residual mortar in RCA2. For the coarse fractions, RCA1 has an adhered mortar content of 33.99%, which is 50.16% lower than that of RCA2 (51.03%), highlighting a substantial increase in residual mortar content in RCA2.

The literature indicates that most studies focus on measuring the adhered mortar content of coarse aggregates, likely due to the greater complexity and difficulty associated with assessing mortar content in RFAs. Zhu et al. [24] reported a similar trend in the adhered mortar content of coarse aggregates, with values of 37.7%, 54.8%, and 61.9% for RCA1, RCA2, and RCA3, respectively. Another study reported adhered mortar contents of 40.3%, 45.8%, and 55.4% for RCA1, RCA2, and RCA3, respectively [43]. Thomas et al. [23] concluded that after the third recycling cycle, adhered mortar accounts for over 80% of the total aggregate volume. Their trend analysis predicts that from the fourth cycle onward, the original aggregate becomes nearly negligible, with the RCA consisting almost entirely of residual mortar. This elevated mortar content could increase water absorption and decrease concrete strength, necessitating careful mix design strategies such as pre-treatment to remove excess mortar or blending with NAs to mitigate adverse effects and ensure suitability for structural applications. Juan and Gutiérrez [38] asserted in their study that only RCAs with an adhered mortar content below 44% are suitable for use in structural concrete.

The optical microscope images highlighted clear structural variations between NA, RCA1, and RCA2. As can be seen in Figure 7, NA samples display a consistent, uniform mineral composition, devoid of adhered mortar, pores, or cracks, reflecting their superior structural quality. RCA1 particles, on the other hand, exhibit patches of adhered mortar alongside scattered pores, consistent with their measured mortar content of 44.95% in the fine fraction and 33.99% in the coarse fraction, where finer particles retain more mortar due to their greater surface area. RCA2 shows a marked increase in adhered mortar, especially in the fine fraction, accompanied by noticeable pores, cracks, and a fragmented appearance, aligning with its higher mortar content of 53.95% (fine) and 51.03% (coarse). This indicated progressive degradation from repeated recycling. The transition from NA’s uniformity to RCA2’s disordered structure illustrates the increasing presence of porous, mortar-rich phases in RCAs. The amount of adhered mortar in RCAs is influenced by factors such as particle size, the strength of the original concrete, and the specific crushing process employed [44,45]. Ultimately, the properties of RCAs are largely governed by both the quantity and quality of the adhered mortar [45].

2.5. Microstructural Analysis

2.5.1. Mineralogical Characterization

Figure 8 presents the X-ray Diffraction (XRD) spectra of the NA, RCA1, and RCA2 for both fine (0–4 mm) and coarse (4–15 mm) fractions. The mineralogical analysis indicates the presence of several key crystalline phases across all aggregate types. Quartz (SiO₂), calcite (CaCO₃), and dolomite (CaMg(CO₃)₂) were consistently identified in all samples, indicating that these minerals are intrinsic to the original aggregate sources and persist through recycling cycles. Their presence in NA confirms their origin in the natural resources, typically quartzitic or carbonate-based. In RCA1 and RCA2, their continued detection reflects the mineralogical stability of these phases during the concrete’s life cycle and subsequent recycling. Specifically, calcite in RCAs may originate from the original rock as well as from further processes such as carbonation of cement paste, where calcium hydroxide (portlandite) reacts with atmospheric CO₂ to form calcium carbonate [46]. The XRD spectra show that Quartz is the dominant phase across all samples, with its peak at approximately 26.59° 2θ being the most intense, consistent with its prevalence in both NA and RCAs. This is consistent with previous XRD studies on RCAs, which frequently report quartz and calcite as the dominant mineral phases identified in the diffraction spectra [47,48]. Albite (Na_0.84Ca_0.16Al_1.16Si_2.84O₈) and muscovite (KAl₂(Si₃Al)O₁₀(OH)₂) were also detected across all aggregate types, belonging to the family of feldspar and mica minerals group. Limbachiya et al. [49] reported comparable findings in their study of commercially produced coarse RCAs and NAs within the 4–16 mm size fraction. XRD analysis revealed the presence of calcite, portlandite, and minor peaks attributed to muscovite or illite in the RCA. These mineral phases were found to correspond closely to the composition of the original concrete, indicating that the mineralogical characteristics of RCAs are largely governed by the properties of their parent material.

Notably, portlandite/calcium hydroxide (Ca(OH)₂) was identified exclusively in RCA2. As expected, this phase is absent in the NA, which contains no cementitious components. Its absence in RCA1 suggests that any portlandite originally present has undergone complete carbonation, likely transforming into calcite over time. In contrast, the reappearance of portlandite in RCA2 indicates the presence of residual, uncarbonated cement paste. This can be attributed to the relatively young age of RCA2’s source concrete. RAC1, from which RCA2 was produced, was approximately six months old at the time of recycling. On the other hand, RCA1 was commercially sourced and likely derived from much older, long-demolished concrete, allowing for more advanced carbonation [50]. Wang et al. [33] also studied RA treatment methods and noted that the newly formed attached mortar contains a large amount of calcium hydroxide. In addition, Evangelista et al. [51] demonstrated that the mineralogical composition of RCAs is not significantly influenced by particle size, a finding that is also confirmed by the results of the present study.

2.5.2. Scanning Electron Microscopy (SEM)

The microstructural evaluation of the fine (0–4 mm) and coarse (4–16 mm) fractions of NA, RCA1, and RCA2 was conducted using SEM, as presented in Figure 9. These images offer evidence of the morphological transformations induced by further recycling cycles, revealing the evolution of porosity, microcracks, and structural defects across generations.

The SEM of NAs, shown in Figure 9a,b, illustrates the high structural integrity. Both the fine and coarse fractions of NA exhibit dense and homogeneous surfaces with no signs of microcracks. Following one cycle of recycling, RCA1 displays clear signs of microstructural degradation. Figure 9c,d depict the internal morphology of RCA1 fine and coarse fractions, respectively. These images reveal the presence of old adhered mortar surrounding the original aggregate particles, with a visibly porous and low compact interfacial zone. This zone, referred to as the ITZ, is a region of weakness often characterized by the presence of high porosity, unhydrated cement particles, and microcracking [52]. In Figure 9c, the interface between the original aggregate and old mortar is highlighted by increased porosity and localized defects, while Figure 9d shows the presence of voids and cracks propagating through the adhered mortar layer. Previous studies [53,54] have consistently reported that NAs typically exhibit smooth surfaces with minimal porosity, while RCAs are characterized by irregular textures, a high number of pores, and loosely attached particles.

The RCA2, represented in Figure 9e,f, exhibits more complex and deteriorated microstructures. Fine RCA2 (0 to 4 mm), shown in Figure 9e, contains extensive cracking and multiple generations of adhered mortar. The coarse fraction in Figure 9f reveals a heterogeneous matrix with distinct zones formed during repeated recycling. Three interfacial zones can be identified. ITZ1 refers to the interface between the original NA and the old adhered mortar. ITZ2 represents the interface between the original aggregate and the newly added mortar from the second recycling cycle. ITZ3 is the interface between the old mortar and the newly added mortar. The coexistence of the three ITZs within a single RCA2 particle compromises its overall structural integrity. The high porosity of the adhered mortar and the microstructural incompatibility between old and new hydration products often result in interlayer detachment and poor internal cohesion, especially evident in Figure 9f. It has been demonstrated that the microstructure of RCAs progressively deteriorates with each additional recycling cycle, primarily due to the increased content of adhered mortar [40]. Similarly, Liu et al. [43] demonstrated that cracks and other forms of damage in RCA increase significantly with each recycling cycle. In the third generation of RCA, the gradual expansion of surface micro-pores and micro-cracks leads to a rough, loose, and porous structure, resulting in longer capillary pathways.

Moreover, a distinction between fine and coarse fractions is consistently observed. Fine RCA particles tend to retain more adhered mortar and exhibit a higher degree of internal porosity due to their greater surface area [55]. This suggests that finer RCA fractions may contribute more significantly to strength reduction and require special consideration in mix design.

2.5.3. Backscattered Electron (BSE)

BSE imaging complemented SEM analysis by enhancing atomic number contrast and brightness intensity, thereby outlining compositional variations and microstructural heterogeneity in aggregates [56]. Figure 10a–f illustrate the internal morphology of both fine and coarse fractions, offering a clear visualization of aggregate phases, adhered mortar phases, ITZs, and associated microdefects.

Figure 10a and Figure 10b show the BSE images of NA for the 0 to 4 mm and 4 to 16 mm size fractions, respectively. These images reveal a dense and compact matrix with minimal heterogeneity and no signs of porosity. The high atomic number uniformity across the matrix confirms the purity and homogeneity of the NA particles.

In contrast, RCA1 presents a significantly altered microstructure. Figure 10c and Figure 10d depict the BSE images of RCA1 fine and coarse aggregates, respectively, with the corresponding BSE observations in Figure 10c (200× magnification) and Figure 10d (100× magnification) highlighting the clear distinction between the original aggregate and the adhered old mortar, identified by differences in brightness levels due to compositional contrast. ITZ1, the transition zone between the NA and the old mortar, is characterized by a porous zone with weak bonding and noticeable microvoids. Figure 10c highlights this ITZ1 region, where fragmented old mortar surrounds the aggregate. Figure 10d reveals the presence of large cracks crossing ITZ1, as well as internal pores within the adhered mortar. These microstructural deficiencies, magnified through BSE contrast, are indicative of cracks in RCA that evolve into larger pores, linking microcracking to increased porosity [57].

The BSE images of RCA2, shown in Figure 10e,f, further emphasize the microstructural complexity introduced by further recycling cycles. In Figure 10e, the RCA2 (0 to 4 mm fraction) displays a complex matrix containing multiple ITZs. Consistent with the SEM observations, the BSE images reveal three discrete interfacial zones: ITZ1 outlines the boundary between the original NA and the residual old mortar, ITZ2 marks the interface between the original aggregate and the newly introduced mortar during the second recycling cycle, and ITZ3 separates the adhered old mortar from the freshly applied mortar matrix. These ITZs are visually distinguishable by varying grayscale intensities, highlighting compositional differences and structural discontinuities. Figure 10f, representing the coarse RCA2 fraction, reveals a highly porous new mortar phase and a large void next to the original aggregate. Hanif et al. [58] used BSE image analysis to assess the porosity of old mortar in RCAs, highlighting a weak aggregate–paste interface that limits their use in high-performance concrete and supports the need for surface pre-treatment to reduce porosity. For example, Yunsheng et al. [59] analyzed BSE images and demonstrated that carbonated RCA exhibits a much denser old ITZ compared to untreated RCA, due to the chemical reactions between CO₂ and the hydration products.

Kumar and Singh [60] classify RCA into four quality levels (High, Medium, Low, and Poor) based on parameters derived from international standards and literature consensus (

Table 3. Quality classification of RCA (adapted from [60]).

Characteristics	HQ a	MQ b	LQ c	PQ d
Specific gravity	≥2.6	2.3–2.6	2.0–2.3	<2.0
Water absorption (%)	≤3	3–5	5–7	>7
Crushing value (%)	≤25	25–35	35–45	>45

^a High quality can be used for all structural elements, including pavement construction (load-bearing infrastructure). ^b Medium quality can be used for structural concrete. ^c Low quality can be used for non-structural elements. ^d Poor quality can be used for backfilling or temporary use, and subbase course for road construction.

). Experimental evaluation revealed that both RCA1 and RCA2 predominantly fall into medium-to-low quality categories. For water absorption, RCA1 (5.55% coarse, 9.53% fine) and RCA2 (6.88% coarse, 13.16% fine) specimens are classified as low quality (LQ: 5–7%) or poor quality (PQ: >7%). The crushing values of RCA1 (25.9%) and RCA2 (29.2%) place both materials in the medium-quality category (25–35%). Although RCA2 shows a higher crushing and abrasion susceptibility than RCA1, it still falls within the range recommended for structural concrete. Density measurements (RCA1: 2.16–2.62 g/cm³; RCA2: 2.03–2.61 g/cm³) further confirm their medium-to-low quality classification (2.0–2.3 g/cm³), primarily due to their significant attached mortar content. Such classifications are supported by SEM and BSE observations, which show that both RCA1 and RCA2 contain old mortar with a porous structure and visible microcracks, along with a weak ITZ. Compared to RCA1, RCA2 displays more pronounced microcracking and greater porosity at the interface between the core aggregate and the surrounding mortar, aligning with its higher water absorption and lower density. These quality assessments suggest that while fine RCA1 and fine RCA2 may be suitable for non-structural or low-strength applications (consistent with DIN [61] and AASHTO [62] guidelines), coarse RCA1 and coarse RCA2 have the potential to be used in structural applications, which would require quality enhancement. The literature indicates that appropriate treatment methods (including removing or strengthening the old mortar) could potentially upgrade these materials to higher quality classes, improving their suitability for more demanding applications [63].

2.6. Correlation Analysis of Old Mortar Content with Aggregate Properties

Figure 11 illustrates the correlation between old mortar content and key physical properties of fine RCAs, specifically particle density and water absorption. The results revealed clear and consistent trends, highlighting the influence of adhered mortar on aggregate performance.

In Figure 11, three forms of particle density, including apparent, oven-dried, and saturated surface-dried, exhibit a declining trend with increasing old mortar content. The strength of the linear relationships is confirmed by high coefficients of determination (R² > 0.95), indicating a strong negative correlation. This behavior can be attributed to the intrinsic lower density of the adhered mortar phase compared to the NA core [64]. Among the three, oven-dried particle density presents the highest R² value (0.9804), suggesting it is the most sensitive to variations in old mortar content.

In addition, Figure 11 further highlights the hydrophilic nature of the adhered mortar, with water absorption increasing markedly with old mortar content. A strong linear relationship (R² = 0.9542) indicates that higher mortar content significantly enhances the porosity and capillarity of fine RCAs. Poon et al. [65] observed that the porosity of RCA, measured by mercury intrusion porosimetry, was approximately 6.5 times greater than that of NA. This finding reinforces the idea that residual mortar not only reduces density but also exacerbates water uptake, which may critically influence mix design parameters and the durability performance of concrete incorporating fine RCAs.

Figure 12 illustrates the influence of old mortar content on key properties of coarse RCAs. A consistent pattern appears, revealing that increased adhered mortar adversely affects both the physical characteristics and geometrical uniformity of the aggregates.

As seen in Figure 12a, particle density, across all three measured states, declines with increasing mortar content. Among them, oven-dried density exhibits the strongest correlation (R² = 0.9589), confirming its reliability as a sensitive indicator of internal porosity induced by residual mortar. Similarly, water absorption shows a strong linear increase (R² = 0.9894), confirming the porous nature of the adhered phase. This is consistent with the findings of Akbarnezhad et al. [44]. They examined RCA from the same concrete source and identified a strong correlation between adhered mortar, water absorption, and oven-dry density, with R² values exceeding 0.98 for each correlation.

Importantly, Figure 12b demonstrates a significant rise in the crushing value (R² = 0.9984), indicating a clear degradation in mechanical integrity as mortar content increases. This emphasizes the structural vulnerability introduced by excessively adhered mortar, which weakens the aggregate’s resistance under load. Consistently, Hanif et al. [66] stated that adhered mortar may enhance mechanical disintegration and reduce aggregate strength due to both the quantity and quality of old mortar bonded to the aggregate surface.

Finally, morphological characteristics in Figure 12b show that both sphericity and symmetry deteriorate slightly with increasing mortar content, suggesting that mortar accumulation impairs the shape regularity of coarse particles, which is a factor that may affect workability. Similarly, Martín-Morales et al. [67] noted that RCA is more angular than the original coarse aggregate, as adhered mortar at corners sharpens rounded particles.

3. Experimental Program

3.1. Materials

In this experimental program, CEM II/B-M (S-LL) 42.5 N with a specific gravity of 3.1 was used as the primary binder for mixing RAC1, along with an Austrian supplementary cementitious material (AHWZ-Flumix). This material consists of a mixture of Ground Granulated Blast-Furnace Slag (GGBS), fly ash, and limestone and has a specific gravity of 2.71. Additionally, a commercially available superplasticizer was used as an admixture, with a specific gravity of 1.05.

3.2. Preparation of RCA2

To evaluate the RCA2’s performance, NA and RCA1 were obtained from a recycling plant in Austria. The commercial RCA1 used in this study was categorized as RB-A2 according to the Austrian standard ÖNORM B 3140 [68], which defines RAs predominantly originating from demolished concrete with at least 90% concrete content. This category allows only minor amounts of other constituents (e.g., masonry). The RCA1 was then used to produce RAC1 with varying replacement levels of RA while maintaining a consistent water-cement ratio (w/c) of 0.53. The concrete mixtures incorporated both coarse and fine RCA1 at volumetric replacement levels of 30–15%, 65–40%, and 100–0%, respectively. The 28-day compressive strength of the mixes ranged approximately between 50 and 60 MPa. The mixtures with varying RCA1 replacement ratios were prepared to simulate the variability often encountered in practical construction scenarios. Cement was used at 315 kg/m³, and AHWZ was used at 35 kg/m³ in all mixtures.

After 28 days of curing followed by six months of aging to ensure adequate hydration, RCA2 was produced by jointly crushing all RAC1 mixtures; the resulting material was then combined, mixed thoroughly to ensure uniformity, and finally sieved. This process involved controlled crushing to break down the hardened concrete, followed by sieving to classify the aggregate into appropriate size fractions. To minimize impurities, the aggregate was thoroughly washed. Both coarse and fine fractions of RCA2 were collected and characterized for further analysis. This method ensured consistency in aggregate quality and allowed for a direct comparison between NA, RCA1, and RCA2 in subsequent experimental investigations. The preparation process of RCA2 is shown in Figure 13.

The aggregates were sieved to obtain fine and coarse fractions of 0–4 mm and 4–16 mm, respectively, which were used in this research study. Figure 14 illustrates the NA and different generations of RCAs.

3.3. Test Methods

In this study, various physical, mechanical, and microstructural tests were conducted to evaluate and compare the properties of NA, RCA1, and RCA2.

3.3.1. Sieve Analysis

The grading of aggregates was determined through sieve analysis in accordance with ÖNORM EN 933-1 [69], ensuring proper classification of particle size distribution.

3.3.2. Particle Morphology

To further assess the shape and morphological characteristics, the CAMSIZER 3D, based on Dynamic Image Analysis (ISO 13322-2) [70] was employed, providing precise measurements of sphericity, symmetry, and aspect ratio.

3.3.3. Density & Water Absorption

The water absorption and density of the aggregates were measured following ÖNORM EN 1097-6 [71], which is critical in evaluating porosity and overall quality, particularly for RCAs.

3.3.4. Crushing Value

To assess the mechanical properties, the aggregate crushing value test was performed according to BS 812-110 [72], determining the resistance of aggregates to compressive loads.

3.3.5. Adhered Mortar Content

The residual mortar content on RCA1 and RCA2 was quantified using a modified version of the method developed by Duan and Poon [73], as illustrated in Figure 15. This approach enabled an estimation of the adhered old mortar, which is a critical factor influencing RCA performance. To determine the adhered mortar content on the RCAs, a step-by-step procedure was followed. Initially, the aggregate samples, categorized into 0–4 mm and 4–16 mm fractions after sieving and washing, were dried in an oven at 105 °C for 24 h to remove any residual moisture. Once dried, a precisely measured portion of the sample (m1, 100 g per test) was immersed in a 10% hydrochloric acid (HCl) solution for 24 h. This acid treatment helped dissolve the cementitious components attached to the aggregate surface. Following the acid immersion, the sample was thoroughly washed with water to eliminate loosened cement particles and was subsequently dried again at 105 °C for 24 h to ensure complete removal of moisture. At this stage, a significant portion of the adhered mortar was detached from the aggregate. However, to enhance the effectiveness of the removal process, the entire procedure, including acid immersion, washing, and drying, was repeated once more to dissolve any remaining mortar residue. After completing both cycles of acid treatment, the final mass of the cleaned aggregate (m2) was measured. The adhered mortar content was then calculated using Equation (1), providing a quantitative assessment of the residual mortar percentage in the RCAs. To validate the effectiveness of the mortar removal process, the water absorption of the RCAs was measured after the adhered mortar had been removed. The results showed that the water absorption values of the cleaned aggregates were comparable to those of the NAs. For the FAs, the 24 h water absorption of the NA was 3.3%, while fine RCA1 and fine RCA2 exhibited slightly higher values of 3.7% and 3.9%, respectively. Similarly, the water absorption for natural coarse aggregate was 1.3%, compared to 1.6% for coarse RCA1 and 1.4% for coarse RCA2.

$$Attached mortar content \left(\%\right)={\displaystyle \frac{m1-m2}{m1}}\times 100$$

(1)

Figure 16a shows the CAMSIZER 3D (Microtrac Retsch, Haan/Düsseldorf, Germany), which was employed to analyze particle morphology. Figure 16b presents the KEYENCE VHX-6000 optical microscope (KEYENCE Corporation, Osaka, Japan), which was used to acquire optical images of polished sections of embedded RCA samples, enabling clear visualization of the residual mortar.

3.3.6. Microstructural Analysis

To gain deeper insights into the microstructural characteristics of NA, RCA1, and RCA2, a comprehensive set of analytical techniques was employed. XRD was utilized to identify the crystalline phases present in both the residual cement mortar and the aggregate core. Detailed microstructural imaging was conducted using both SEM and BSE imaging, which allowed for the analysis of porosity, surface morphology, and microstructural defects. This combination of physical, mechanical, and microstructural analyses ensured a comprehensive evaluation of the differences between NA, RCA1, and RCA2, contributing to a better understanding of RCA2 properties.

4. Conclusions and Recommendations

This study presents a comprehensive evaluation of RCA2, produced through an additional recycling cycle of RAC1. Unlike previous studies, which mainly assessed RCA1 or focused only on coarse RCA2, this work provides a dual-scale analysis of both coarse and fine RCA2 fractions, supported by correlated microstructural interpretation. The main findings are:

• Particle morphology and size distribution: Recycling progressively altered aggregate geometry. As cycles increased from NA to RCA1 and RCA2, average particle size decreased and irregular fragments became more frequent, especially in RCA2. Fine RCA2 exhibited coarser grading trends compared to RCA1.
• Density: Both RCA1 and RCA2 showed lower densities than NA, but the difference between RCA1 and RCA2 was small, suggesting near-stabilization after the first cycle.
• Water absorption: RCA2 exhibited the most severe degradation, with water absorption about four times higher than NA for fines and nearly five times for coarse fractions, due to increased adhered mortar and microcracking.
• Mechanical strength: RCA2 reached a crushing value of 29.2%, placing it in the medium-quality category (25–35%). While this indicates higher crushing susceptibility than RCA1, RCA2 remains within the range suitable for structural concrete.
• Adhered mortar content: RCA2 contained 53.95% adhered mortar in the fine fraction and 51.03% in the coarse fraction, confirming adhered mortar as a key parameter controlling quality.
• Mineralogical composition: XRD confirmed quartz, calcite, and dolomite as dominant phases, with portlandite present in RCA2 due to relatively young parent concrete, indicating incomplete carbonation.
• Microstructural analysis: SEM and BSE revealed highly porous mortar zones, multiple weak ITZs, and extensive microcracking, highlighting structural weaknesses induced by repeated recycling.
• Correlation analysis: Strong correlations between adhered mortar content and aggregate properties underline its predictive value for assessing RCA quality.

Overall, the results demonstrate that RCA2 is of lower quality compared to RCA1 but still falls within limits that allow cautious structural application. Fine RCA2 shows significant performance limitations, restricting its use to non-structural applications, while coarse RCA2 remains a potential candidate for structural upcycling under controlled conditions.

This study was limited to aggregate-level characterization without direct RAC2 concrete testing. Future research should therefore focus on (i) quality improvement strategies (mechanical, chemical, or surface treatments to mitigate the impact of adhered mortar), (ii) the influence of parent concrete properties (age, strength class, and service history), and (iii) comprehensive life-cycle assessments to evaluate environmental impacts. Such efforts will provide critical knowledge to support the sustainable implementation of multi-generation RCA within circular construction practices.