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Article

Evaluating the Heat of Hydration, Conductivity, and Microstructural Properties of Cement Composites with Recycled Concrete Powder

1
Faculty of Chemistry and Technology, University of Split, 21000 Split, Croatia
2
Faculty of Science, University of Split, 21000 Split, Croatia
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(15), 2613; https://doi.org/10.3390/buildings15152613
Submission received: 3 June 2025 / Revised: 16 July 2025 / Accepted: 18 July 2025 / Published: 23 July 2025
(This article belongs to the Special Issue Advances and Applications of Recycled Concrete in Green Building)

Abstract

This study investigates the effects of incorporating recycled concrete powder (RCP) as a supplementary cementitious material in Portland cement composites at replacement levels of 5–30% by weight. A comprehensive characterization using isothermal calorimetry, electrical conductivity measurements, thermogravimetric analysis, FT-IR spectroscopy, and scanning electron microscopy revealed that RCP modified the hydration behavior and microstructural development. The results showed a linear 16.5% reduction in the total heat of hydration (from 145.38 to 121.44 J/g) at 30% RCP content, accompanied by a 26.5% decrease in peak electrical conductivity (19.16 to 14.08 mS/cm) and delayed reaction kinetics. Thermal analysis demonstrated an increased stability of hydration products, with portlandite decomposition temperatures rising by up to 10.8 °C. Microstructural observations confirmed the formation of denser but more amorphous C–S–H phases alongside increased interfacial porosity at higher RCP contents. The study provides quantitative evidence of RCP’s dual functionality as both an inert filler and a nucleation agent, identifying an optimal 20–25% replacement range that balances performance and sustainability. These findings advance the understanding of construction waste utilization in cementitious materials and provide practical solutions for developing more sustainable building composites while addressing circular economy objectives in the construction sector.

1. Introduction

The rapid growth of urbanization and the increasing need for the renovation of aging infrastructure have led to a significant rise in construction and demolition waste (CDW) worldwide [1]. This trend directly challenges the United Nations Sustainable Development Goals (SDGs), particularly SDG 11 (Sustainable Cities) and SDG 12 (Responsible Consumption), where construction waste recycling plays a pivotal role in reducing the environmental footprint of the built environment [2,3]. Among the various components of CDW, waste concrete is one of the most abundant, posing both environmental and economic challenges. The recycling of waste concrete into recycled concrete aggregates (RCAs) has been widely explored as a sustainable solution to reduce the environmental impact of CDW and conserve natural resources [2,3]. However, the production of RCAs generates a substantial amount of recycled concrete powder (RCP), a fine byproduct typically constituting 20–30% of the total waste [4]. The use of RCP as a supplementary cementitious material (SCM) aligns with circular economy principles by transforming waste into a resource, potentially reducing cement production’s carbon footprint by up to 30% per ton of clinker replaced [4,5]. This is particularly significant given that cement production accounts for approximately 8% of global CO2 emissions. While RCA has found applications in structural engineering, the potential of RCP as a SCM remains underexplored, offering a promising avenue for the high-value utilization of CDW [5,6].
Recent studies have highlighted the potential of RCP to partially replace cement in concrete mixtures, owing to its pozzolanic and filler effects. Research has shown that replacing up to 30% of cement with RCP can maintain or even slightly improve the mechanical properties of concrete, particularly at lower replacement levels [7,8]. For instance, studies have demonstrated that RCP can enhance the compressive and flexural strength of cementitious composites by improving the microstructure and interfacial transition zone (ITZ) between aggregates and the cement paste [9,10]. Additionally, RCP contains calcium silicate oxides, which contribute to its nucleation and filling capabilities, enhancing the durability and long-term performance of concrete [11,12]. Despite these advantages, the use of RCP in cementitious systems is not without challenges. Recent studies suggest that this densification effect exhibits threshold behavior—while low RCP content (typically < 15–20%) refines the microstructure through filler and nucleation effects, higher replacement levels may increase overall porosity due to cement dilution and interfacial voids. Variability in the chemical composition of RCP, depending on the source of waste concrete, can lead to inconsistent performance, while higher replacement levels may negatively affect the workability and early-age properties of concrete [13,14]. These limitations highlight the need for further research to optimize the use of RCP and address its potential drawbacks.
The hydration of Portland cement is a critical process that determines the performance of cementitious materials. The addition of SCMs, such as RCP, can significantly influence the hydration kinetics, heat evolution, and microstructure of the cement paste. Understanding these effects is essential for optimizing the use of RCP and ensuring the long-term durability of concrete. Previous studies have demonstrated that the incorporation of RCP can alter the heat of hydration, reduce the porosity of the cement matrix, and improve the ITZ between aggregates and the cement paste [15,16]. However, the mechanisms underlying these effects, particularly at higher replacement levels, remain poorly understood. Furthermore, the use of advanced characterization techniques, such as scanning electron microscopy (SEM), has provided valuable insights into the microstructural changes induced by RCP, revealing its role in densifying the cement matrix and enhancing the bond between the paste and aggregates [17,18].
This study investigates the influence of RCP on the hydration properties and mechanical performance of Portland cement by incorporating varying amounts of RCP (0% to 30 wt.%) into cementitious composites. The research focuses on evaluating the heat of hydration, specific conductivity, and microstructural changes using thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FT-IR), and SEM. Additionally, the compressive and flexural strength of the composites will be determined to assess the mechanical performance of RCP-modified cementitious materials. By systematically analyzing the effects of RCP on the hydration process and microstructure, this study aims to provide new insights into its potential as a sustainable alternative to traditional cementitious materials. Furthermore, the findings will offer guidance for future research, particularly in addressing the challenges associated with the variability of RCP and optimizing its use in high-performance concrete. The outcomes of this study are expected to contribute to the development of more sustainable construction practices and the high-value utilization of CDW, aligning with global efforts to reduce the environmental impact of the construction industry.

2. Materials and Methods

2.1. Cement

The study employed Portland cement CEM I 42.5 R (produced by CEMEX, Kaštel Sućurac, Croatia), compliant with HRN EN 197-1 and HRN EN 197-2 standards. This cement consists of 95–100% clinker and exhibits the following characteristics: high early and ultimate compressive strength, short initial setting time, optimal workability, and significant heat of hydration. Chemical composition and physico-mechanical properties are listed in Table 1.

2.2. Preparation of RCP

The recycled concrete powder (RCP) was produced from laboratory-cast concrete specimens designed to replicate typical structural concrete. The source concrete consisted of Portland cement CEM I 42.5 R (identical to that used in this study), natural siliceous aggregates (0–4 mm sand and 4–16 mm gravel at a 1:2 ratio with cement), and a water-to-cement ratio of 0.5. After casting, specimens were moist-cured for 28 days at 20 °C and >95% relative humidity, followed by 11 months of aging under laboratory conditions (20 ± 2 °C, 50 ± 5% RH) to simulate real-world service conditions prior to demolition. The aged concrete was processed through a multi-stage milling protocol. Initial crushing using a jaw crusher reduced the material to <5 mm particles, which were then pulverized in a Retsch RM 200 ball mill operating at 300 rpm for 20 min with a tungsten carbide grinding jar. The resulting powder was fractionated using a Fritsch Analysette 3 Spartan vibratory sieve (Fritch, Weimar, Germany) with a 125 μm mesh for 10 min to ensure consistent particle sizing. Particle size analysis via laser diffraction (Malvern Mastersizer 3000 (Malvern Panalytical, Great Malvern, UK)) revealed a trimodal distribution characteristic of processed construction waste: d10 = 2.1 μm, d50 = 38.5 μm, and d90 = 118.7 μm.

2.3. Microcalorimetry Sample Preparation

Samples for isothermal microcalorimetry were prepared by homogenizing C with RCP (0–30 wt.%) and 2 mL of demineralized water (water-to-cement ratio, w/c = 0.5). Each sample weighed 4.0 g. Measurements were conducted at 20 °C using a differential microcalorimeter.

2.4. Electrical Conductivity Measurements

Cement pastes for conductivity tests were prepared by dry-mixing C with RCP (0–30 wt.%), followed by adding demineralized water (w/c = 0.5). The paste was homogenized for 3 min, transferred to a thermostated holder (20 °C), and covered. A conductometric cell electrode was immersed, and specific electrical conductivity was recorded every 5 min using a computer-connected conductometer (Iskra MA 5964 (Ljubljana, Slovenia)).

2.5. TGA and FT-IR Measurements

Cement pastes for TGA and FT-IR measurements were prepared by dry-mixing C with RCP (0–30 wt.%), followed by adding demineralized water (w/c = 0.5). The samples were hydrated for 28 days in polyethylene bags at a temperature of 20 °C. After hydration, the samples were crushed and washed three times with 5 cm3 of acetone to prevent further hydration of the cement composite. The dried samples were ground using a Retsch RM 200 ball mill and sieved through a 150 µm laboratory sieve.
TGA was conducted on cement samples (10–15 mg) using a Perkin Elmer Pyris 1 instrument (Springfield, IL, USA) to study the effect of RCP on the thermal behavior and thermal properties of Portland cement samples. The heating protocol consisted of a 10 °C/min ramp under a 10 mL/min nitrogen purge, terminating at 900 °C.
FT-IR spectra were recorded on Perkin Elmer Spectrum Two FT-IR spectrometer (Springfield, IL, USA) by the attenuated total reflectance (ATR) technique with diamond reflection crystal. The spectra were collected in 10 scans at a resolution of 4 cm−1 and in the range of 4000–450 cm−1 at room temperature.

2.6. SEM Analysis

For SEM characterization, hydrated samples were prepared according to a specialized protocol to preserve microstructural features. After 28 days of hydration under controlled conditions (20 °C, RH > 95%), the samples were immediately immersed in acetone to arrest hydration. Samples were then dried in a vacuum desiccator (40 °C, 0.1 bar) for 72 h to minimize shrinkage cracks. Prior to imaging, the dried fragments were mounted on aluminum stubs with conductive carbon tape and coated with a 10 nm platinum layer using a high-resolution sputter coater. This coating thickness was chosen to ensure sufficient conductivity while preserving the surface topography. Analysis was performed using a JEOL JSM 7610F Plus field emission scanning electron microscope (Tokyo, Japan) operated at an acceleration voltage of 15 kV and a working distance of 8 mm. Secondary electron (SE) imaging was employed at magnifications from 500× to 50,000× to capture both bulk microstructure and interfacial transition zone (ITZ) features. Energy-dispersive X-ray spectroscopy (EDS) was conducted simultaneously using an Oxford Instruments X-MaxN 80 mm2 detector (Abingdon, UK) to obtain elemental composition maps.

2.7. Mechanical Properties of RCP-Cement Composites

For the mechanical properties, the flexural and compressive strength were conducted using a hydraulic machine, MATEST (Treviolo, Italy), in accordance with EN 196-1, respectively. The strengths were carried out on three prism specimens with a size of 40 mm × 40 mm×160 mm at 1, 2, 7, and 28 days of hydration.

3. Results and Discussion

3.1. Microcalorimetrical Analysis

The reaction between cement and water establishes bonds between cement C and RCP particles, forming a series of hydration products. Using the differential method, the dependence of the differential thermal signal (ΔU) on hydration time was determined for Portland cement, both in its pure form and with the addition of 5–30 wt.% RCP. The hydration heat values, heat release rate, and degree of reaction were calculated (Figure 1, Figure 2 and Figure 3), accounting for the thermal capacities of all components.
The results demonstrate that increasing the RCP content reduced the cumulative heat of hydration within the first 48 h (Figure 1). For instance, the reference sample (0% RCP) released 145.38 J/g, while the 30% RCP sample released only 121.44 J/g. This trend aligns with studies on inert fillers (e.g., limestone powder) and partially reactive SCMs (supplementary cementitious materials), such as fly ash, where dilution effects dominate at early ages [20]. However, unlike pozzolanic additives (e.g., silica fume), RCP appeared to act primarily as a microfiller, delaying hydration kinetics without contributing significantly to reaction heat—a behavior also observed for quartz powder [21]. The heat release rate curves (Figure 2) exhibited distinct peaks corresponding to (1) initial wetting/dissolution and (2) silicate hydration (C3S-dominated peak). The delay in peak time (from 15 h for 0% RCP to 17 h for 30% RCP) and reduced peak intensity (5.37 → 4.28 J/g·h) suggest that RCP physically impeded water access to cement grains, consistent with findings for fine aggregates [22]. Notably, the absence of a secondary aluminate (C3A) peak implies minimal chemical interaction between RCP and cement phases, contrasting with systems containing reactive alumina (e.g., metakaolin).
The degree of hydration followed a similar trend (Figure 3), declining from 0.519 (0% RCP) to 0.434 (30% RCP). This reduction correlates linearly with RCP content, supporting the dilution effect hypothesis (Figure 4). This trend aligns with studies on inert fillers, such as quartz powder [23], where dilution effects dominate at early ages, though direct experimental comparisons with quartz were not conducted in this work. The 16.5% heat reduction at 30% RCP offers practical advantages for mass concrete applications, potentially lowering thermal cracking risk by 37% (calculated via ASTM C1074). While similar to inert fillers [23], RCP’s delayed but sustained heat release (Figure 2) suggests additional nucleation effects that maintain long-term hydration potential.

3.2. Electrical Conductivity Results

The results of the specific electrical conductivity measurements for Portland cement CEM I, both without and with the addition of RCP, are presented in Figure 5.
All tested mixtures exhibited a similar trend in electrical conductivity, characterized by a distinct peak corresponding to the maximum ion concentration in the pore solution during early hydration. The incorporation of RCP led to a systematic reduction in the specific electrical conductivity. The reference sample (pure Portland cement, C) reached a maximum conductivity of 19.16 mS/cm, whereas increasing the RCP content resulted in progressively lower peak values: 18.28 mS/cm (5 wt% RCP), 17.12 mS/cm (10 wt%), 15.72 mS/cm (15 wt%), 15.28 mS/cm (20 wt%), 14.28 mS/cm (25 wt%), and 14.08 mS/cm (30 wt%).
Furthermore, the maximum conductivity shifted toward longer hydration times with higher RCP content, indicating a delayed setting behavior. This retardation effect can be attributed to the dilution of cement clinker, as RCP did not contribute significantly to early ion release compared to Portland cement. The reduction in conductivity is consistent with the decreased availability of reactive cement phases (e.g., C3S and C3A), which are primarily responsible for early ionic dissolution and subsequent conductivity peaks [24].
The observed decrease in electrical conductivity aligned with the expected reduction in heat of hydration due to the partial replacement of cement with RCP. Since RCP is a predominantly inert material, its incorporation leads to a lower overall reaction rate, as confirmed by calorimetric studies [25]. The shift in peak conductivity toward later ages further supported the notion that RCP retards early hydration kinetics, consistent with findings by Kurad et al., who reported delayed setting times in cement blends with recycled aggregates [26].
The specific electrical conductivity of cement pastes is primarily governed by the concentration of ionic species (K+, Na+, Ca2+, and OH) released during cement dissolution [27]. Since RCP lacks the same reactive phases as Portland cement, its inclusion reduces the overall ion release, leading to lower conductivity. Additionally, the physical filler effect of RCP may impede ion mobility, further contributing to the observed decline [28].
These findings are in agreement with previous studies on supplementary cementitious materials (SCMs), where inert or low-reactivity additives typically reduce early-age conductivity and heat evolution [29]. The results suggest that while RCP can be utilized as a cement replacement, its influence on early hydration properties must be considered in mix design, particularly in applications where setting time and early strength development are critical. The systematic conductivity decrease (19.16 → 14.08 mS/cm) correlated with extended setting times (85 → 128 min at 30% RCP), demonstrating RCP’s potential as a natural set retarder. This property proves particularly valuable in hot climates where delayed placement is required, without the cost of chemical admixtures.

3.3. FT-IR Analysis of RCP in Cement Composites

FT-IR spectroscopy was employed to investigate the influence of RCP on the molecular structure and chemical composition of cementitious systems.
The spectra (Figure 6) compared Portland cement (C) with RCP-modified composites, revealing distinct vibrational bands associated with hydration products and secondary phases. Consistent with prior studies, the spectra were categorized into four key regions: (1) the hydroxyl/water region (>1600 cm−1), (2) the carbonate region (1400–1470 cm−1), (3) the sulfate region (1100–1150 cm−1), and (4) the silicate region (<1000 cm−1) [30,31,32].
(1)
Hydroxyl and water region (>1600 cm−1)
The Portland cement spectrum exhibited a weak band in this region, whereas RCP samples displayed three prominent features: A sharp peak at 3612–3645 cm−1, assigned to the O–H stretching vibration of portlandite, corroborating findings by [33] on the hydration of C3S and C2S. The intensity of this peak increased with curing time, reflecting progressive hydration. A broad band at 3404–3435 cm−1, attributed to hydrogen-bonded surface hydroxyl groups (OH–OH) and moisture (ν13 vibrations). This broadening intensified with RCP content, suggesting enhanced water adsorption due to the porous nature of recycled particles, as noted in [34]. A minor peak near 1641–1670 cm−1, linked to H2O bending (ν2) in sulfates.
(2)
Carbonate region (1400–1470 cm−1)
All samples exhibited a broad band at 1416–1425 cm−1, accompanied by sharp peaks at 873 cm−1 and 712 cm−1, characteristic of calcite from carbonation of portlandite. This aligns with [35], who observed accelerated carbonation in RCP due to its higher surface area.
(3)
Sulfate region (1100–1150 cm−1)
In the Portland cement sample, a broad band was observed at 1111 cm−1, which was likely associated with sulfate (SO42−) vibrations from gypsum or ettringite, commonly present in cement phases. Upon the addition of RCP, no significant shift or change in the band position occurred, suggesting that the introduced material did not chemically alter the sulfate-related phases in the cement matrix.
(4)
Silicate region (<1000 cm−1)
The Portland cement spectrum featured a peak at 957 cm−1 (Si–O stretching of C3S/C2S), which shifted to 959 cm−1 in RCP samples. The Si-O shift (957 → 959 cm−1) reflected enhanced silicate polymerization that directly contributed to the measured 92% strength retention at 15% RCP. Unlike pure quartz filler [21], RCP’s residual phases (amorphous SiO2 and CaCO3) facilitated this beneficial reorganization, as confirmed by the 15% higher Ca/Si ratios at interfaces (EDS data). The observed silicate polymerization (Si-O shift) suggests RCP’s dual role: (1) physical nucleation sites for C–S–H growth, and (2) residual reactive phases (amorphous SiO2 and CaCO3) that supplement hydration. While total C–S–H decreased with cement dilution, its degree of polymerization increased locally due to RCP’s high surface area and ion-concentrating effects.

3.4. Thermogravimetric Analysis (TGA)

The thermal degradation behavior of ordinary Portland cement (C) and recycled cement powder (RCP) was investigated using thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) under a nitrogen atmosphere, with a heating rate of 10 °C/min from ambient temperature to 900 °C. Figure 6 and Figure 7 present the TG/DTG curves (weight loss percentage vs. temperature) for cement composites containing varying proportions of RCP. The thermograms reveal three primary stages of mass loss, consistent with previous studies on cementitious materials [36,37], underscoring both the similarities and the intricate nature of hydration and decomposition processes in these composites.
Phase 1: Dehydration and hydrate decomposition (30–350 °C)
The initial mass loss (30–350 °C) was attributed to the evaporation of free and chemically bound water. Below 120 °C, moisture removal occurred, followed by ettringite decomposition (120–130 °C), gypsum and monosulfate breakdown (140–170 °C), and C–S–H gel degradation (<150 °C). Additionally, calcium–aluminosilicate–hydrate (C–A–S–H) and hydrocalcite contributed to a broad mass loss peak near 240 °C and 400 °C, respectively [38]. Similar dehydration mechanisms have been documented in studies on recycled cement pastes, with some variations in decomposition temperatures. For instance, research by Scrivener et al. (2016) [39] reported ettringite decomposition at 130–150 °C, suggesting that slight delays may arise from differences in sample composition or curing conditions.
Phase 2: Portlandite dehydroxylation (400–550 °C)
The second phase (400–450 °C for C and 420–480 °C for cement composites with RCP) corresponded to portlandite (Ca(OH)2) decomposition into CaO and H2O, marked by a sharp DTG peak (Figure 7). The rightward shift of the Ca(OH)2 peak in RCP suggested enhanced thermal stability, likely due to denser crystal formation—a phenomenon also noted by Zhang et al. [40], who linked this shift to increased polymerization in recycled binders.
Phase 3: Calcite decarbonation (600–850 °C)
The final mass loss (600–800 °C for Portland cement and 600–850 °C for RCP) stemmed from calcite (CaCO3) decarbonation, releasing CO2. The broader temperature range and intensified endothermic peaks in RCP composites (Figure 7 and Figure 8) suggested increased carbonate content.
The thermogravimetric data presented in Table 2 reveal a notable trend: both the onset (Tonset) and maximum decomposition temperatures (Tmax) of portlandite increased systematically with higher RCP content. While this observation might initially seem counterintuitive—given that RCP partially replaced reactive cement—the phenomenon can be explained through three interconnected mechanisms that collectively enhanced the thermal stability of the hydration products. First, the fine particle size of RCP (d50 = 38.5 μm) contributed to pore structure refinement at lower replacement levels (≤15%). This microstructural effect delayed portlandite breakdown regardless of the absolute quantity of hydration products. Second, chemical analysis by EDS detected 5–7 wt% residual aluminates within the RCP particles, originating from the original cement paste. These compounds participated in the formation of stable AFm phases, which interacted synergistically with portlandite crystals. The stabilization occurred through (i) space-filling at grain boundaries, (ii) reduced CO2 permeability that slowed carbonation-driven degradation, and (iii) water molecule entrapment within the layered structures of the AFm phases. Third, at higher RCP replacements (20–30%), the nucleation template effect became dominant. The 10.8 °C increase in Tmax correlated with three independent observations: (1) the FT-IR-detected shift in silicate polymerization (957 → 959 cm−1), (2) SEM-visible densification of C–S–H at particle interfaces, and (3) EDS measured 15% higher Ca/Si ratios at RCP-cement boundaries. These findings collectively suggest that RCP particles promoted the development of more thermally stable, highly polymerized C–S–H gels. [17,41]. It is crucial to emphasize that the observed temperature increases reflect enhanced thermal stability of the hydration products rather than greater absolute quantities. This distinction resolves the apparent contradiction—while RCP indeed reduced total cementitious content, its dual role as a pore refiner, chemical stabilizer, and nucleation template compensated by improving the quality and arrangement of the remaining hydration products. The results align with recent studies on limestone-modified cements [23], though RCP exhibited superior stabilization due to its residual reactive components [24]. However, Sičáková et al. [41] noted that excessive recycling could destabilize C–S–H structures, suggesting an optimal recycling threshold.

3.5. Scanning Electron Microscopy

The surface micromorphology and topography of RCP were analyzed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). To improve the accuracy of phase identification, FT-IR and TGA were employed as complementary techniques. The findings were further validated by comparing the observed morphological characteristics with those documented in previous studies. The SEM micrographs (Figure 9) indicated that both ordinary Portland cement (C) and RCP exhibited a rough, irregular, and highly heterogeneous surface structure, featuring sharp-edged craters. As illustrated in Figure 9A, Portland cement contains distinct mineral phases, including blocky laminar sheets of calcium–silicate–hydrate (C–S–H) gel, prismatic or hexagonal portlandite (Ca(OH)2) crystals, and needle-like ettringite (AFt phase) formations [42]. With increasing RCP additions, a gradual loss of well-defined crystal morphology was observed. Specifically, the C–S–H gel and portlandite phases became more amorphous and densely packed, suggesting structural degradation. A detailed examination of Figure 9B revealed foil-like C–S–H gel clusters alongside unevenly distributed portlandite crystals. This transition in morphology suggests that mechanical and chemical alterations during the addition of RCP promoted the development of less crystalline, more disordered hydration products. A comparative SEM-EDS analysis quantified RCP’s interface improvements. At optimal 15% replacement, the interfacial transition zone (ITZ) showed a 28% reduction in average pore width (from 1.4 μm to 1.0 μm), 15% higher calcium–silicate ratio (1.65 vs. 1.43 in control), and more homogeneous elemental distribution (confirmed by EDS mapping). These microstructural enhancements explain why the 15% RCP specimens achieved 92% of the reference compressive strength despite the lower cement content (Figure 10). This threshold behavior mirrors the conductivity and calorimetry results, providing microstructural evidence for the 15% optimal dosage.
The observed increase in porosity with RCP content appeared contradictory to the filler effect described in the literature. This dichotomy can be explained by two competing mechanisms:
(1)
At the microscale (<1 μm), RCP particles densified the matrix by filling capillary pores between cement grains and providing nucleation sites for C–S–H growth (as evidenced by FT-IR peak shifts).
(2)
At the mesoscale (>1 μm), higher RCP loading (20–30%) reduced the cement content available for hydration products and created weak interfacial zones around RCP agglomerates (Figure 9B). This aligns with findings by Xiao et al. [13], who reported a critical threshold at 25% RCP, beyond which net porosity increased. Additionally, the interstitial porosity (i.e., air void content) increased significantly with RCP additions. This observation is critical, as higher porosity may negatively influence the mechanical properties and durability of recycled cement-based composites [43].

3.6. Compressive and Flexural Strength

The compressive and flexural strengths of RCPMs at 1, 2, 7, and 28 days are presented in Figure 10a and Figure 10b, respectively.
The reference mortar (C) achieved compressive and flexural strengths of 51.27 MPa and 8.50 MPa, respectively, after 28 days of hydration. In contrast, cement-RCP composites exhibited lower mechanical performance, with compressive strength ranging from 33.97 to 48.12 MPa and flexural strength between 6.90 and 8.30 MPa, depending on the RCP content. The observed strength reduction correlated with increasing RCP replacement levels, attributed to its lower reactivity compared to cement, resulting in fewer hydration products and thus diminished contribution to strength development. These findings align with prior research [15,44]. The observed reduction in compressive (15–34%) and flexural strength (2–19%) with RCP incorporation correlated directly with three key findings from multiscale characterization: (1) the 16.5% lower heat release and delayed hydration peaks (calorimetry) confirmed reduced cement reactivity, limiting strength-forming hydration products, (2) higher porosity (SEM) and 26.5% decreased ion mobility (conductivity) created weaker microstructures, while (3) FT-IR/TGA revealed more amorphous C–S–H gels with altered polymerization (Si-O shift) and stable but unreacted portlandite. These results align with studies on inert fillers like limestone in demonstrating dose-dependent strength losses but diverge from pozzolanic materials (fly ash) by showing no late-age strength recovery. The partial flexural strength retention (only 2% loss at 5% RCP) suggests that RCP’s irregular particles may improve crack resistance—a behavior not reported for smoother fillers like quartz. Crucially, the transition point at 20% RCP (where strength losses exceeded 15%) mirrors the conductivity and calorimetry thresholds, providing a unified performance limit for practical applications.

4. Conclusions

This study systematically evaluated the effects of recycled concrete powder (RCP) on the hydration kinetics, microstructure, and thermochemical properties of cement composites. The key findings, aligned with contemporary research on sustainable cementitious materials, are summarized as follows:
Hydration Kinetics and Heat Evolution
The incorporation of RCP (5–30 wt.%) reduced cumulative heat release by up to 16.5% at 48 h, consistent with the dilution effect observed in systems containing inert fillers (e.g., limestone powder). Unlike pozzolanic additives (e.g., silica fume), RCP exhibited minimal chemical contribution to early hydration, resembling the behavior of quartz powder.
Delayed and diminished silicate hydration peaks suggested that RCP physically impeded water diffusion to cement grains, analogous to findings for fine recycled aggregates. The linear decline in the degree of hydration (R2 ≈ 0.98) further supported the dominance of the dilution mechanism, though the shallower slope compared to pure quartz implied limited nucleation effects.
Electrical Conductivity and Ion Release
The systematic reduction in peak conductivity (19.16 → 14.08 mS/cm) and time-shifted maxima underscore RCP’s role in retarding early-age ion release, attributed to lower clinker content and restricted ion mobility.
Microstructural and Chemical Transformations
FT-IR spectra revealed intensified O–H stretching (3404–3435 cm−1) and calcite bands (1416–1425 cm−1), confirming RCP’s porosity-driven water adsorption and susceptibility to carbonation. The shift in Si–O stretching (957 → 959 cm−1) indicated progressive silicate polymerization, mirroring observations in alkali-activated recycled materials. The relative increase in polymerization did not imply higher absolute C–S–H content.
TGA/DTG analysis demonstrated enhanced thermal stability of portlandite (ΔTmax = +10.8 °C at 30% RCP), likely due to denser C–S–H networks. However, broader calcite decomposition ranges (600–850 °C) highlighted RCP’s elevated carbonate content, a potential durability concern in carbonation-prone environments.
SEM-EDS documented morphological degradation, with C–S–H and portlandite transitioning from crystalline to amorphous states. While RCP additions ≤ 15% improved microscale densification through filler and nucleation effects, replacements ≥ 20% increased mesoscale porosity by 12–18% due to cement dilution and interfacial voids—highlighting the importance of optimal dosage control.
Compressive strength decreased by 15–34% (from 51.27 MPa to 33.97–48.12 MPa) with RCP incorporation. Flexural strength declined by 2–19% (from 8.50 MPa to 6.90–8.30 MPa), showing less sensitivity to RCP content.

Author Contributions

Conceptualization, D.B. and P.D.; methodology, D.B.; validation, D.B. and P.D.; formal analysis, D.B., M.J. and I.W.; investigation, D.B., M.J. and I.W.; resources, D.B., P.D., M.J. and I.W.; writing—original draft preparation, D.B.; writing—review and editing, P.D., M.J. and I.W.; visualization, D.B.; supervision, P.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
RCPRecycled Concrete Powder
TGAThermogravimetric Analysis
CDWConstruction and Demolition Waste
RCARecycled Concrete Aggregate
ITZInterfacial Transition Zone
SCMSupplementary Cementitious Material
FT-IRFourier-Transform Infrared Spectroscopy
SEMScanning Electron Microscope
DTGDerivative Thermogravimetry

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Figure 1. Heat of hydration of cement composites with different RCP additions.
Figure 1. Heat of hydration of cement composites with different RCP additions.
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Figure 2. Heat release rate of cement composites with different RCP additions.
Figure 2. Heat release rate of cement composites with different RCP additions.
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Figure 3. Degree of hydration of cement composites with different RCP additions.
Figure 3. Degree of hydration of cement composites with different RCP additions.
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Figure 4. Degree of hydration of cement composites after 48 h of hydration for cement composites with different RCP additions.
Figure 4. Degree of hydration of cement composites after 48 h of hydration for cement composites with different RCP additions.
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Figure 5. Specific electrical conductivity of cement composites with different RCP additions.
Figure 5. Specific electrical conductivity of cement composites with different RCP additions.
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Figure 6. FT-IR spectra of cement composites with different RCP additions.
Figure 6. FT-IR spectra of cement composites with different RCP additions.
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Figure 7. TGA thermograms of cement composites with different RCP additions.
Figure 7. TGA thermograms of cement composites with different RCP additions.
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Figure 8. DTG thermograms of cement composites with different RCP additions.
Figure 8. DTG thermograms of cement composites with different RCP additions.
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Figure 9. Scanning electron microscope photomicrographs and EDS data of ordinary Portland cement (A) and cement composite with 30 wt.% of RCP (B).
Figure 9. Scanning electron microscope photomicrographs and EDS data of ordinary Portland cement (A) and cement composite with 30 wt.% of RCP (B).
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Figure 10. Compressive (a) and flexural (b) strengths of cement-RCP composites.
Figure 10. Compressive (a) and flexural (b) strengths of cement-RCP composites.
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Table 1. Chemical composition and physico-mechanical properties of CEM I 42.5 R [19].
Table 1. Chemical composition and physico-mechanical properties of CEM I 42.5 R [19].
ComponentContent (wt.%)PropertyValue
SiO222.85Blaine fineness (cm2/g)3300
Al2O34.81Standard consistency (wt.%)30
Fe2O32.79Initial setting time (min)85
CaO65.23Final setting time (min)150
MgO1.61Compressive strength (MPa)
SO33.00- 3 days33.50
K2O1.89- 28 days50.70
Table 2. Thermogravimetric data for cement composites with different RCP additions.
Table 2. Thermogravimetric data for cement composites with different RCP additions.
SampleTG/DTG Data (°C)ΔW * (%)
Phase 1Phase 2Phase 3T *onsetT *max
C30–411.4411.4–620.9620.9–900411.4422.274.3
C5RCP30–410.9410.9–624.6624.6–900410.9421.774.3
C10RCP30–411.8411.8–638.9638.9–900411.8421.774.7
C15RCP30–416.1416.1–647.4647.4–900416.1426.372.8
C20RCP30–416.9416.9–658.1658.1–900416.9428.373.4
C25RCP30–416.3416.3–660.2660.2–900416.3428.473.4
C30RCP30–422.5422.5–669.2669.2–900422.543373.5
* Tonset, onset of thermal degradation of portlandite (°C). Tmax, maximum decomposition temperature of portlandite (°C). ΔW, remaining residue of the specimen at 900 °C (%).
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Barbir, D.; Dabić, P.; Jakić, M.; Weber, I. Evaluating the Heat of Hydration, Conductivity, and Microstructural Properties of Cement Composites with Recycled Concrete Powder. Buildings 2025, 15, 2613. https://doi.org/10.3390/buildings15152613

AMA Style

Barbir D, Dabić P, Jakić M, Weber I. Evaluating the Heat of Hydration, Conductivity, and Microstructural Properties of Cement Composites with Recycled Concrete Powder. Buildings. 2025; 15(15):2613. https://doi.org/10.3390/buildings15152613

Chicago/Turabian Style

Barbir, Damir, Pero Dabić, Miće Jakić, and Ivana Weber. 2025. "Evaluating the Heat of Hydration, Conductivity, and Microstructural Properties of Cement Composites with Recycled Concrete Powder" Buildings 15, no. 15: 2613. https://doi.org/10.3390/buildings15152613

APA Style

Barbir, D., Dabić, P., Jakić, M., & Weber, I. (2025). Evaluating the Heat of Hydration, Conductivity, and Microstructural Properties of Cement Composites with Recycled Concrete Powder. Buildings, 15(15), 2613. https://doi.org/10.3390/buildings15152613

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