MECHANICAL INHERITANCE AND MINERALOGICAL CHARACTERISTICS OF RECYCLED CONCRETE AGGREGATES DERIVED FROM DEMOLISHED CONCRETE

ABSTRACT

The reuse of recycled concrete aggregates (RCA) is an effective strategy for reducing natural resource consumption and construction waste. However, the presence of adhered cement mortar significantly influences the mechanical and mineralogical properties of RCA. This study investigates the mechanical inheritance and phase composition of RCA obtained from demolished concrete. The parent concrete was first characterized by compressive strength testing, after which the material was crushed and separated into 20-10 mm and 10-5 mm fractions. The crushing resistance of the aggregates was evaluated before and after chemical treatment for removing adhered mortar. The results showed that acid treatment reduced mass loss and improved the crushing strength grade of RCA. XRD analysis revealed quartz as the dominant phase, while calcite, ettringite, and amorphous C-S-H were associated with the residual cement paste. The findings demonstrate that removing adhered mortar improves the mechanical quality of recycled aggregates.

АННОТАЦИЯ

Повторное использование переработанных бетонных заполнителей (RCA) является эффективным способом снижения потребления природных ресурсов и уменьшения объёмов строительных отходов. Однако наличие прилипшего цементного раствора существенно влияет на механические и минералогические свойства переработанных заполнителей. В данной работе исследуются механическое наследование и фазовый состав переработанных бетонных заполнителей, полученных из демонтированного бетона. Исходный бетон был предварительно охарактеризован методом испытания на сжатие, после чего материал подвергался дроблению и разделению на фракции 20–10 мм и 10–5 мм. Сопротивление дроблению заполнителей определялось до и после химической обработки, направленной на удаление прилипшего цементного раствора. Результаты показали, что кислотная обработка снижает потерю массы и повышает марку дробимости переработанных заполнителей. Рентгенофазовый анализ (XRD) показал, что кварц является доминирующей фазой, тогда как кальцит, эттрингит и аморфная фаза C–S–H связаны с остаточным цементным камнем. Полученные результаты свидетельствуют о том, что удаление прилипшего цементного раствора способствует улучшению механических свойств переработанных заполнителей.

Keywords: recycled concrete aggregates (RCA), adhered cement mortar, crushing resistance, mineralogical composition, X-ray diffraction (XRD), demolition concrete, sustainable construction materials.

Ключевые слова: переработанные бетонные заполнители (RCA), прилипший цементный камень, сопротивление дроблению заполнителей, минералогический состав, рентгеновская дифракция (XRD), бетон, полученный при демонтаже конструкций, устойчивые строительные материалы.

1. Introduction

The construction industry is one of the main drivers of global economic development; however, it is also among the most resource-intensive sectors in terms of natural resource consumption, waste generation, and environmental impact. Due to population growth, rapid urbanization, and the increasing demand for infrastructure, the production of concrete has been growing significantly in recent decades. Currently, concrete is the most widely produced man-made material in the world. Since aggregates constitute the primary component of concrete, their extensive use in construction significantly increases pressure on natural resource reserves [1]-[2].

The depletion of natural aggregates has become one of the most significant global challenges for the construction industry. Studies indicate that approximately 40-50 billion tons of aggregates are extracted worldwide each year, of which 70-80% are directly used for concrete production [3]. Since aggregates constitute approximately 70-75% of the total volume of concrete, any increase in concrete production inevitably leads to a significant rise in the demand for natural aggregates [4]. In many developed regions, high-quality sand and gravel deposits are becoming increasingly limited, and their extraction is complicated by environmental regulations, land resource scarcity, and growing social opposition. Moreover, excessive extraction of river and marine sand has been widely reported in the scientific literature to cause coastal erosion, disruption of hydrological balance, and degradation of natural ecosystems [5]. Therefore, the search for alternative materials capable of partially replacing natural aggregates has become an important global research priority.

Life Cycle Assessment (LCA)-based studies have shown that the use of recycled concrete aggregates can reduce energy consumption by 15-30% and CO₂ emissions by 10-25% compared with the production of natural aggregates [1], [6], [7]. In particular, the local recycling of construction and demolition waste can further enhance environmental efficiency by reducing transportation distances. However, the production of recycled concrete aggregates also requires energy and imposes a certain environmental burden. Therefore, the advantages of recycled aggregates can only be fully realized through scientifically based technological approaches and proper engineering evaluation.

Recycled concrete aggregates (RCA) typically exhibit relatively high water absorption (usually 4-12%) and lower density (2,1-2,5 g/cm³) compared with natural aggregates [8], [9], [10]. The presence of adhered cement paste within RCA increases the overall porosity of the material. Moreover, the amount of cement paste contained in recycled fine aggregates (RFA) is generally higher than that in recycled coarse aggregates [11]-[12], For this reason, the reuse (valorization) of RFA remains one of the most challenging issues in recycled aggregate technology. The water absorption of recycled fine aggregates is particularly high, reaching up to 10,9% [13], and even 13,1% for fractions smaller than 1,25 mm [14]. As a consequence, concrete produced with such aggregates often requires a significantly higher water demand to maintain adequate workability [15], [16]. A detailed investigation of the microstructural characteristics of recycled aggregates derived from construction and demolition waste is essential for understanding their behavior in concrete. In a study conducted by Angulo et al., the microstructure of C&D aggregates was evaluated using a combination of XRF, XRD, TGA-DTG, and selective HCl dissolution methods, allowing the identification of cement paste residues, carbonation products, and clay minerals within the aggregates [17]. Their results showed that the residual hardened cement paste present in recycled aggregates possesses high porosity, which significantly reduces the microstructural density of recycled aggregate concrete (RAC) [17], [18]. According to Mehta and Monteiro, the microstructure of concrete and its pore network are key factors governing the durability of cement-based materials under aggressive environmental conditions [4]. In particular, studies have shown that very fine fractions (<0.15 mm) contain a significantly higher proportion of cement paste and clay minerals, which explains their high water absorption and lower mechanical stability [17], [19].

Despite extensive research on recycled concrete aggregates, the relationship between mechanical properties inherited from the parent concrete and the mineralogical composition of the adhered mortar layer remains insufficiently understood. In particular, the influence of residual cement paste on the crushing resistance and microstructural characteristics of RCA requires further investigation. Therefore, the main objective of this study is to evaluate the mechanical inheritance and phase composition of recycled concrete aggregates obtained from old concrete. Special attention is given to the influence of adhered cement paste on the crushing resistance of RCA and the changes in mineralogical composition before and after chemical removal of the mortar layer using acid treatment.

2. Materials and Methods

2.1 Source and characterization of parent concrete

The parent concrete used in this study was obtained from demolished concrete elements collected from existing structures. In order to evaluate the mechanical condition of the old concrete, the material was cut into 10 ⅹ 10 ⅹ 10 cm cubic specimens. The cubes were tested for compressive strength in accordance with the relevant standard procedures [20]. The obtained strength values were used to assess the quality of the residual cement matrix and natural aggregates present in the parent concrete.

2.2 Production of recycled concrete aggregates

Recycled concrete aggregates (RCA) were produced by mechanically crushing the parent concrete under laboratory conditions. After crushing, the material was sieved and separated into three particle size fractions: 20-10 mm, 10-5 mm, and 5-0 mm. These fractions were selected for further analysis due to their common use in recycled aggregate concrete production.

2.3 Determination of the crushing strength grade of RCA

The crushing strength grade of RCA fractions (20-10 mm and 10-5 mm) was determined according to the applicable standard testing procedures [21]. During the test, aggregate samples were subjected to controlled compressive loading, and the degree of particle fragmentation was evaluated. The results were used to classify the RCA according to their crushing strength grade and to compare the mechanical performance of different aggregate fractions.

2.4 Removal of adhered cement mortar

To investigate the influence of adhered cement paste on the mineralogical composition of RCA, a portion of each aggregate fraction was subjected to a cement mortar removal procedure. The adhered mortar layer was removed using a chemical dissolution method, allowing comparison between RCA samples with adhered mortar and those with the mortar removed. After treatment, the aggregates were thoroughly washed with water and dried before further analysis. This procedure followed the methodologies previously proposed by Tam (2005) and Silva & de Brito (2016) [22]-[23].

2.5 Mineralogical analysis (XRD)

The mineralogical composition of RCA samples was analyzed using X-ray diffraction (XRD). Diffraction measurements were performed separately for RCA samples with adhered mortar and those after mortar removal. Phase identification was carried out by comparing the obtained diffraction patterns with standard reference data. As a result, the main crystalline phases were identified, including quartz (SiO₂), calcite (CaCO₃), portlandite (Ca(OH)₂), calcium silicate hydrate (C-S-H), and ettringite (AFt). The relative presence of these phases was qualitatively evaluated based on the intensity of the diffraction peaks.

3. Results

The physical and mechanical properties of the parent concrete specimens are summarized in Table 1. The weight of the tested cubes ranged from 2451 g to 2474 g, while the moisture content varied between 0,9% and 1%. The compressive strength determined using the compression testing machine (press method) showed values of 19,83 MPa and 18,2 MPa for samples 1 and 2, respectively. The corresponding strength values obtained using the Schmidt hammer were 18 MPa and 17 MPa. The average compressive strength measured by the press method was approximately 19 MPa, whereas the average Schmidt hammer value was 17 MPa.

Determination of the crushing strength grade of RCA

The crushing resistance of recycled concrete aggregates (RCA) was evaluated for two size fractions (20-10 mm and 10-5 mm), both in untreated condition and after acid treatment. The results are summarized in Table 1.

Table 1.

Results

For the 20-10 mm fraction, the mass loss under crushing load was 12,57%, corresponding to a crushing strength grade of M800. After acid treatment, the mass loss decreased significantly to 5,5%, resulting in an improved crushing strength grade of M1000. Similarly, the 10-5 mm fraction exhibited a mass loss of 15,49% in untreated condition, corresponding to grade M600. Following acid treatment, the mass loss was reduced to 10,79%, and the crushing strength grade increased to M800.

Removal of adhered cement mortar

The results show that the adhered mortar content in the 20-10 mm fraction was approximately 15,96%, while the 10-5 mm fraction exhibited a higher value of 19,34%. The increase in adhered mortar with decreasing particle size indicates that smaller coarse particles retain a larger proportion of residual hardened cement paste.

This behavior is mainly attributed to the crushing process, during which fragmentation increases the exposure of the old mortar layer and generates additional microcracks along the interfacial transition zone (ITZ). As a result, the 10-5 mm fraction contains a higher surface-to-volume ratio of adhered mortar compared to the 20-10 mm fraction.

Mineralogical analysis (XRD)

The mineralogical composition of recycled concrete aggregates (RCA) obtained from old concrete was investigated using X-ray diffraction (XRD). Diffractograms for two aggregate size fractions (20-10 mm and 10-5 mm) before and after acid treatment (AT) are presented in Fig. 1. The analysis reveals the presence of several crystalline phases originating from both the natural aggregate and the adhered cement mortar.

Figure 1. X-ray Phase Analysis of Recycled Concrete Aggregate (RCA)

The dominant crystalline phase detected in all samples is quartz (SiO₂), which exhibits the most intense diffraction peak at approximately 2θ ≈ 26.6°. Additional quartz reflections are observed at around 20.8°, 36.5°, 50°, 60°, and 68°, confirming that siliceous natural aggregates constitute the main component of the recycled material. The persistence of these peaks in both untreated and acid-treated samples indicates that the treatment process does not affect the mineralogical structure of the natural aggregate particles.

Another major phase identified in the diffractograms is calcite (CaCO₃), with characteristic peaks near 2θ ≈ 29.4°, 39-40°, 43°, and 47-48°. The presence of calcite is attributed to the carbonation of the hydrated cement paste attached to the aggregate surface. Over long service periods, calcium hydroxide present in the cement paste reacts with atmospheric carbon dioxide according to the reaction:

Ca(OH)₂+CO₂→CaCO₃+H₂O

As a result, significant amounts of calcite accumulate within the adhered mortar layer. The intensity of calcite peaks is particularly noticeable in the untreated samples, indicating a substantial amount of carbonated cement paste surrounding the aggregate particles.

Weak diffraction peaks corresponding to ettringite (AFt) are observed at low diffraction angles (approximately 7-10° 2θ). These peaks represent sulfate-bearing hydration products formed during the early hydration of Portland cement. The persistence of AFt in old concrete suggests that some hydration products remain stable within the microstructure of the adhered mortar even after long-term service life.

In addition, a broad diffuse region between approximately 20° and 35° 2θ indicates the presence of calcium silicate hydrate (C-S-H), the main binding phase of hydrated cement paste. Due to its poorly crystalline nature, C-S-H does not produce sharp diffraction peaks but appears as a broad hump in the XRD patterns.

Comparison of the two aggregate fractions reveals slight differences in phase distribution. The 10-5 mm fraction exhibits relatively stronger signals associated with hydration products, indicating a higher amount of adhered mortar. This observation is consistent with previous studies showing that finer RCA particles generally retain larger quantities of residual cement paste compared with coarser fractions.

After acid treatment (AT), the diffraction patterns remain generally similar; however, slight reductions in the intensity of peaks related to hydration and carbonation products can be observed. This suggests that the acid treatment partially removes or dissolves the adhered cement paste, thereby exposing the natural aggregate core. Nevertheless, the dominant quartz peaks remain unchanged, confirming that the treatment process primarily affects the mortar layer rather than the aggregate itself.

4. Conclusion

Based on the experimental investigation of recycled concrete aggregates derived from demolished concrete, the following conclusions can be drawn:

1. The parent concrete exhibited compressive strength values ranging from approximately 14 MPa to 20 MPa, indicating a moderate-strength concrete source for recycled aggregate production.
2. The crushing resistance of RCA is strongly influenced by the presence of adhered cement mortar. The untreated 20-10 mm and 10-5 mm fractions showed crushing strength grades of M800 and M600, respectively.
3. Chemical treatment using hydrochloric acid effectively reduced the amount of adhered cement paste, resulting in improved mechanical performance. After treatment, the crushing strength grades increased to M1000 for the 20-10 mm fraction and M800 for the 10-5 mm fraction.
4. The adhered mortar content increased as particle size decreased, confirming that smaller RCA fractions retain a larger proportion of residual cement paste.
5. XRD analysis revealed that quartz is the dominant crystalline phase originating from natural aggregates, while calcite, ettringite, and amorphous C-S-H phases are associated with the residual cement paste surrounding the aggregate particles.
6. The results demonstrate that the mechanical performance of recycled concrete aggregates is closely related to the microstructural characteristics inherited from the parent concrete, particularly the amount and composition of adhered mortar.

References:

1. Faleschini, F., Jiménez, C., Barra, M., Aponte, D., Vázquez, E., & Pellegrino, C. (2014). Rheology of fresh concretes with recycled aggregates. Construction and Building Materials, 73, 407–416. DOI: 10.1016/j.conbuildmat.2014.09.068
2. Dhir, R. K., de Brito, J., Silva, R. V., & Lye, C. Q. (2019). Sustainable Construction Materials: Recycled Aggregates. Woodhead Publishing. DOI: 10.1016/C2017-0-00645-1
3. UNEP. (2019). Global Resources Outlook 2019: Natural Resources for the Future We Want. United Nations Environment Programme
4. Mehta, P. K., & Monteiro, P. J. M. (2014). Concrete: Microstructure, Properties, and Materials. McGraw-Hill Education
5. Peduzzi, P. (2014). Sand, rarer than one thinks. Environmental Development, 11, 208–218. DOI: 10.1016/j.envdev.2014.04.001
6. Coelho, A., & de Brito, J. (2013). Environmental analysis of a construction and demolition waste recycling plant in Portugal. Waste Management, 33(2), 466–476. DOI: 10.1016/j.wasman.2012.10.002
7. Hossain, M. U., Poon, C. S., Lo, I. M. C., & Cheng, J. C. P. (2016). Comparative environmental evaluation of aggregate production from recycled waste. Resources, Conservation and Recycling, 109, 67–77. DOI: 10.1016/j.resconrec.2016.02.009
8. Rodrigues F, Carvalho MT, Evangelista L, De Brito J (2013) Physical-chemical and mineralogical characteriza tion of fine aggregates from construction and demolition waste recycling plants. J Clean Prod 52:438–445
9. de Juan MS, Gutie´rrez PA (2009) Study on the influence of attached mortar content on the properties of recycled con crete aggregate. Constr Build Mater 23(2):872–877
10. Delobel F, Bulteel D, Mechling JM, Lecomte A, Cyr M, Re´mond S (2016) Application of ASR tests to recycled concrete aggregates: influence of water absorption. Constr Build Mater 124:714–721
11. Zhao Z, Remond S, Damidot D, Xu W (2013) Influence of hardened cement paste content on the water absorption of f ine recycled concrete aggregates. J Sustain Cem Mater 2(3–4):186–203
12. ZhaoZ,CourardL,MichelF,RemondS(2017)Influenceof granular fraction and origin of recycled concrete aggregates on their properties. Eur J Environ Civ Eng. https://doi.org/ 10.1080/19648189.2017.1304281
13. Hansen TC (1986) Recycled aggregates and recycled aggregate concrete second state-of-the-art report develop ments 1945–1985. Mater Struct 19(3):201–246
14. Evangelista L, De Brito J (2010) Durability performance of concrete made with fine recycled concrete aggregate. Cement Concr Compos 32:9–14
15. SaniD,MoriconiG,FavaG,CorinaldesiV(2005)Leaching and mechanical behavior of concrete manufactured with recycled aggregates. Waste Manag 25:177–182 21
16. Cartuxo F, De Brito J, Evangelista L, Jimenez JR, Ledesma EF (2015) Rheological behavior of concrete made with fine recycled concrete aggregates: influence of the superplasti cizer. Constr Build Mater 89:36–47
17. S. C. Angulo, C. Ulsen, V. M. John, H. Kahn, and M. A. Cincotto, “Chemical–mineralogical characterization of C&D waste recycled aggregates from São Paulo, Brazil,” Waste Management, vol. 29, pp. 721–730, 2009
18. B. Bianchini, E. Marrocchino, R. Tassinari, and C. Vaccaro, “Recycling of construction and demolition waste materials: a chemical and mineralogical appraisal,” Waste Management, vol. 25, pp. 149–159, 2005
19. M. C. Limbachiya, E. Marrocchino, and A. Kouloris, “Chemical–mineralogical characterisation of coarse recycled concrete aggregate,” Waste Management, vol. 27, pp. 201–208, 2007
20. GOST 10180-2012
21. GOST 8267-93
22. S. Tam, C. Tam, and W. Tam, “Assessment of durability of recycled aggregate concrete produced by two-stage mixing approach,” Cem. Concr. Res., 35 (6), 1195-1203(2005).[https://doi.org/10.1016/j.cemconres.2004.09.025
23. R. V. Silva, J. de Brito, and R. Dhir, “Use of recycled aggregates arising from construction and demolition waste in new construction applications,” J. Build. Eng., 8 (2016) 75-89. [https://doi.org/10.1016/j.jobe.2016.09.008