# Physical, Mechanical, and Durability Behavior of Sustainable Mortars with Construction and Demolition Waste as Supplementary Cementitious Material

Sandra Cunha, Kubilay Kaptan, Erwan Hardy, José Aguiar

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Sandra Cunha, Kubilay Kaptan, Erwan Hardy, José Aguiar. Physical, Mechanical, and Durability Behavior of Sustainable Mortars with Construction and Demolition Waste as Supplementary Cementitious Material. Buildings, 2025, 15, 10.3390/buildings15152757. [https://hal.science/hal-05368347v1](https://hal.science/hal-05368347v1)

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Article

# Physical, Mechanical, and Durability Behavior of Sustainable Mortars with Construction and Demolition Waste as Supplementary Cementitious Material

Sandra Cunha <fnref n="1" />,* [DOI], Kubilay Kaptan <fnref n="1" />, Erwan Hardy <fnref n="2" /> and José Aguiar <fnref n="1" />[DOI]

## Abstract

The construction industry plays a major role in the consumption of natural resources and the generation of waste. Construction and demolition waste (CDW) is produced in substantial volumes globally and is widely available. Its accumulation poses serious challenges related to storage and disposal, highlighting the need for effective strategies to mitigate the associated environmental impacts of the sector. This investigation intends to evaluate the influence of mixed CDW on the physical, mechanical, and durability properties of mortars with CDW partially replacing Portland cement, and allow performance comparisons with mortars produced with fly ash, a commonly used supplementary binder in cement-based materials. Thus, three mortar formulations were developed (reference mortar, mortar with 25% CDW, and mortars with 25% fly ash) and several characterization tests were carried out on the CDW powder and the developed mortars. The work's principal findings revealed that through mechanical grinding processes, it was possible to obtain a CDW powder suitable for cement replacement and with good indicators of pozzolanic activity. The physical properties of the mortars revealed a decrease of about 10% in water absorption by immersion, which resulted in improved performance regarding durability, especially with regard to the lower carbonation depth (–1.1 mm), and a decrease of 51% in the chloride diffusion coefficient, even compared to mortars incorporating fly ash. However, the mechanical performance of the mortars incorporating CDW was reduced (25% in terms of flexural strength and 58% in terms of compressive strength), but their practical applicability was never compromised and their mechanical performance proved to be superior to that of mortars incorporating fly ash.

**Keywords:** construction and demolition waste; supplementary cementitious material; mortars; physical behavior; mechanical behavior; durability

## 1. Introduction

The onset of the Industrial Revolution triggered profound and lasting socioeconomic transformations, most notably the processes of urbanization and industrialization. Since its emergence in Western societies, the interdependence between urbanization and industrialization has been extensively recognized and studied on a global scale. This interconnection has significantly enhanced manufacturing capacity, hence promoting economic growth in

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Academic Editors: Yijiang Wang,
Yukun Ji and Chengyang Liu

Received: 10 July 2025

Revised: 29 July 2025

Accepted: 4 August 2025

Published: 5 August 2025

**Citation:** Cunha, S.; Kaptan, K.; Hardy, E.; Aguiar, J. Physical, Mechanical, and Durability Behavior of Sustainable Mortars with Construction and Demolition Waste as Supplementary Cementitious Material. *Buildings* **2025**, *15*, 2757.
https://doi.org/10.3390/buildings15152757

**Copyright:** © 2025 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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<footnote n="1">¹ Centre for Territory Environment and Construction (CTAC), Department of Civil Engineering, School of Engineering, University of Minho, Campus de Azurém, 4800-058 Guimaraes, Portugal; kaptankubilay@gmail.com (K.K.); aguiar@civil.uminho.pt (J.A.)</footnote>

<footnote n="2">² Département Genie Civil, Polytech Nantes, Gavy Océanis BP 152, 44603 Saint-Nazaire, CEDEX, France; erwan100710@gmail.com</footnote>

<footnote>* Correspondence: sandracunha@civil.uminho.pt</footnote>
both established and emerging economies [1]. Conversely, extensive energy consumption in industrial and domestic sectors resulted in the generation of pollutant emissions, such as carbon dioxide (CO₂) and other greenhouse gases [2]. Thus, urbanization and industry provided society with modernization and improved living standards, accompanied by significant health repercussions, environmental pollution, depletion of natural resources, increased waste production, and alterations in land use [3,4].

Because of the growth in industrialization and urbanization, the civil construction sector has emerged as one of the predominant consumers of natural resources, producing solid waste that adversely affects the environment [5]. The extensive and intricate manufacturing cycle of the construction industry consumes more raw resources (about 3000 Mt/year) than any other economic sector, demonstrating its unsustainability for global development [6]. The construction sector is expected to consume over 40% of the resources of the world [7], with one-third of these resources being transformed into cement-based products [8].

Global usage of concrete and mortar presently constitutes more than 45% of all materials obtained from the planet [9]. Cement has been widely utilized globally. Nonetheless, its manufacturing for concrete and mortar constitutes 12–15% of total industrial energy consumption and roughly 6–8% of world carbon dioxide emissions [10]. Furthermore, the cost of cement constitutes around 20% of the total concrete expense [11].

This urban metabolism towards construction imposes significant strain on the environment along four primary dimensions: (i) The extraction of raw materials utilized in the making of cement and concrete, including limestone, gravel, and sand. (ii) The elevated ambient CO₂ emissions originating from Portland cement manufacture; these are nearly evenly split between the combustion of fossil fuels required to heat kilns to approximately 1450 °C and the calcination of the primary raw material, which consists of 80% CaCO₃. (iii) The production of substantial quantities of waste through infrastructure development, renovation, and demolition. Construction and demolition waste (CDW) is currently produced at a rate of 10,000 Mt/year [12], accounting for approximately 30–40% of the global solid waste stream [12–14]. Projections indicate that the volume of solid waste generated by the construction industry will increase from 12.7 billion metric tons to 27 billion metric tons by the year 2050 [15]. (iv) The recycling management strategies for CDW differ between nations, with less than 1% in Mexico [16], under 10% in China and India [17], and exceeding 90% in Denmark [18], Germany [19], and Japan [20]. Conversely, it is projected that merely 35% of CDW is appropriately landfilled [12], while an indeterminate portion is managed through unregulated methods, posing environmental risks that may contaminate air, soil and water [21].

Recently, the construction industry has pursued strategies to minimize raw material usage, CO₂ emissions, and waste production in its operations [22]. Consequently, there is a necessity to identify substitutes for cement and natural aggregate from both economic and environmental perspectives [11]. Comprehensive studies have been conducted in recent years to identify sustainable substitutes for natural aggregate and cement. Coarse material was substituted with lightweight aggregate derived from several sources, including palm oil [23], expanded clay [24], and lava [25]. Fine aggregate was substituted with tire rubber [26], copper slag [27], and mica [28]. Conversely, partial substitution of cement was achieved using supplementary cementitious materials, including ground granulated blast furnace slag [29], cement kiln dust [30], fly ash [31,32], silica fume [32], rice husk ash [33], and bottom ash [34].

The increasing need for cement to address housing and infrastructure shortages, along with the imperative to mitigate CO₂ emissions, necessitates the exploration of innovative supplementary cementitious materials (SCMs) to reduce the environmental impact of the cement sector [35]. In addition, conventional SCMs, including natural pozzolans, fly
ash, and granulated blast furnace slag, exhibit restricted availability [36]. In contrast, the analysis conducted by Bogoviku and Waldmann [37] indicates that CDW might satisfy the need for materials required to produce new cement for a minimum of the next 80 years.

Recent research has increasingly explored the potential of CDW as an SCM, given that the manufacture of cement is linked to significant CO₂ emissions. However, very few studies refer to the use of mixed CDW; most essentially refer to the individual use of residues that constitute CDW, such as recycled concrete [38–40] and ceramics [41,42]. Recently, some researchers have concentrated their attention on the use of mixed CDW as supplementary cementitious materials. However, such research is still relatively scarce. Duan et al. [43] investigated recycled powders obtained from different CDW sources, such as waste concrete, mixed brick debris, and aerated brick residues. Their findings indicated that recycled powders derived from mixed CDW and waste bricks, particularly when finely ground, demonstrated a high strength activity index of 80% at a 30% replacement level. This index was calculated as the ratio between the compressive strength of mortars containing supplementary cementitious materials and that of reference mortars composed of 100% Portland cement. In turn, Liu et al. [44] acquired recycled hybrid powder derived from dust-collecting systems as an SCM for their study; they determined that the percentage of clay brick particles in the recycled powder significantly influenced mortar performance. Rosales et al. [45] assessed the mechanical performance of mortars incorporating 20% and 30% mixed CDW powder by volume as a partial replacement for cement. At 90 days, the compressive strength showed reductions of about 10% and 18%, respectively, in comparison to the reference mortar produced with ordinary Portland cement. Velardo et al. [46] demonstrated that substituting 5 wt.% of ordinary Portland cement with mixed CDW powder yielded a compressive strength at 180 days similar to that of the reference mortar. However, higher replacement contents of 10% to 20% led to strength reductions ranging from 5% to 21%.

Based on the existing literature, this study aims to promote the advancement of knowledge on the incorporation of mixed CDW as a substitute for cement in mortars. Therefore, the primary objective of this study was to evaluate the influence of mixed CDW on the physical, mechanical, and durability properties of mortars incorporating 25% CDW as a partial replacement for Portland cement. Additionally, the research explored the effects of substituting 25% cement with fly ash, providing insights into an alternative approach to SCMs and enabling a comparative analysis with those currently employed in construction practices.

## 2. Research Significance

Existing studies on the use of CDW as supplementary cementitious material have primarily concentrated on the individual examination of concrete and ceramic waste powders. Currently, many research efforts focus on reducing waste variability by using materials produced in the laboratory or on a small scale, ensuring consistency in waste type, origin, composition, and other key parameters.

In this study, the main innovation is associated with the use of CDW as a supplementary cementitious material, which was collected and treated to be sold as recycled aggregate by a CDW management company. Thus, the CDW used is made up of waste from different locations (real construction and demolition sites) and contains waste from different construction materials (concrete, mortar, brick, tile, and others), representing the practice of civil construction and avoiding the pre-separation of the CDW constituents.

This study makes a substantial contribution to the field by addressing innovative aspects that have been scarcely or not previously explored in the existing literature, namely, an exploration of physical and mechanical properties and an evaluation of the durability of
mortars incorporating CDW as supplementary cementitious materials in comparison with other supplementary cementitious materials commonly used in construction.

# 3. Experimental Details

## 3.1. Raw Materials

In this study, different raw materials were used to produce mortars: CEM I 42.5R Portland cement, fly ash, CDW powder, natural sand, and water.

The cement used was produced by a Portuguese company (Secil), located in Outão, Portugal, and classified according to the European standard EN 197-1 [47]. The fly ash was sourced from a coal-fired power plant situated in Pego, Abrantes, Portugal. The CDW was provided by a Portuguese CDW processing plant (RCD) located in Figueira da Foz, Portugal. In Portugal, this waste is commonly marketed and used as mixed recycled aggregate possessing a grain size distribution from 0 to 10 mm. It originates from various demolition sites, which results in a heterogeneous composition, composed of different construction materials, namely, concrete, mortars, ceramics, and other materials [48]. The characteristics of the CDW were previously determined and presented in a publication developed by our research team [48]. Therefore, its use as a supplementary cementitious material required pre-treatment, specifically mechanical grinding, to obtain the characteristics appropriate for its application. The mechanical grinding process was carried out in the laboratory, using Los Angeles equipment, according to the process described in the work previously published by our research team [49]. Figure 1 schematically represents the process of obtaining CDW powder, from the demolition site to its incorporation as supplementary cementitious material.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Illustrative diagram of the CDW powder production process, detailing the stages from demolition site collection to its use as a supplementary cementitious material.</figcaption></figure>

Table 1 shows the density and specific surface area of cement, fly ash, and CDW powder. The raw material densities were determined according to LNEC Specification E 64 [50]. It was possible to verify that the density of CDW powder is lower than the density of cement, but higher than the density of fly ash. Comparable findings have been reported by other researchers [51,52], and the results obtained in this research fall within the range of values previously documented in the literature [53].

Figure 2 shows the particle size distribution of cement, fly ash, and CDW, determined using a laser granulometer (Malvern Instruments, Mastersizer 3000, Worcestershire, UK).
<table><caption>**Table 1.** Density and specific surface area of cement (CEM I 42.5 R), fly ash (FA), and construction and demolition waste (CDW).</caption><thead><tr><td>Material</td><td>Density (kg/m³)</td><td>Specific Surface Area (m²/kg)</td></tr></thead><tbody><tr><td>CEM I 42.5R</td><td>3142</td><td>599</td></tr><tr><td>Fly ash</td><td>2364</td><td>461</td></tr><tr><td>CDW</td><td>2707</td><td>14,640</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Granulometric particle distribution of cement (CEM I 42.5 R), fly ash (FA), and construction and demolition waste (CDW).</figcaption></figure>

Figure 3 presents the particle size distributions of cement, fly ash, and CDW. Both OPC and fly ash exhibit a unimodal distribution, while CDW displays a bimodal pattern, as previously observed by Malladi et al. [54]. Cement shows a narrower peak, compared to fly ash and CDW, with most particles ranging between 2 µm and 60 µm. Fly ash demonstrates a wider distribution, with particle sizes spanning from 1 µm to 300 µm. In contrast, CDW features a distinct narrow peak between 0.01 and 0.5 µm, along with a broader distribution ranging from 1 to 300 µm.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Particle size distribution of cement (CEM I 42.5 R), fly ash (FA), and construction and demolition waste (CDW).</figcaption></figure>
As depicted in Figure 2, it was possible to determine the D10, D50, and D90 values (Figure 4). This work aimed to produce a CDW powder with a D50 value similar or slightly lower to that of cement and fly ash particles. Based on Figure 3, it was possible to verify that the CDW particles had D10 and D50 values that were about 96% and 14% lower than the cement particles, respectively. On the other hand, the D90 value had a particle size approximately 77% larger than the cement particles. Regarding fly ash, it was possible to verify that after the mechanical grinding process, the CDW particles presented D10, D50, and D90 values lower by approximately 97%, 29% and 12%, respectively. These findings are linked to the large dispersion of CDW particles, showing that the grinding process is in fact effective both on larger and smaller particles of the CDW aggregate. However, the grinding process applied, as well as the equipment used, was defined on a laboratory scale, so it is believed that the application of this process on an industrial scale will allow greater control over the granulometric distribution of the CDW powder.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** D10, D50, and D90 values of cement (CEM I 42.5R), fly ash (FA), and construction and demolition waste after mechanical grinding (CDW).</figcaption></figure>

The chemical compositions of cement, fly ash, and CDW are detailed in Table 2. It was possible to verify that the main component of cement is calcium oxide, followed by silicon oxide, aluminum oxide, iron oxide, and sulfuric oxide. Fly ash has silicon oxide as its main component, followed by aluminum oxide, iron oxide, potassium oxide, and calcium oxide. The CDW powder presents silicon oxide as a major component, followed by calcium oxide, aluminum oxide, and iron oxide. The contents of CDW components are in accordance with those indicated in Kaptan et al.'s study [53]. CDW contains a significant content of calcium oxide (CaO) and silicon oxide (SiO₂), due to the high proportion of concrete and natural aggregates in its original composition [38,53,55,56]. The CaO content in CDW is intermediate, lower than that of Portland cement but higher than that of fly ash, aligning with values reported in the literature [55–57]. In CDW, the CaO content predominantly occurs as inert calcium carbonate (CaCO₃), whereas in cement, CaO is present primarily as active clinker phases [52]. The presence of a lower amount of CaO compounds can be attributed to the lower content of cement [53,57,58]. In turn, the SiO₂ content in CDW is higher than in cement and fly ash [53,56–59], because the CDW powder was obtained by grinding and the SiO₂ is present mainly due to the non-hydrated cement and silica sand [57,58].
<table><caption>**Table 2.** Chemical composition of cement (CEM I 42.5R), fly ash (FA), and construction and demolition (CDW).</caption><thead><tr><th>Chemical Component (%)</th><th>CEM I 42.5 R</th><th>FA</th><th>CDW</th></tr></thead><tbody><tr><td>Calcium Oxide (CaO)</td><td>69.5</td><td>2.4</td><td>24.5</td></tr><tr><td>Iron Oxide (Fe<sub>2</sub>O<sub>3</sub>)</td><td>3.2</td><td>8.5</td><td>3.3</td></tr><tr><td>Silicon Oxide (SiO<sub>2</sub>)</td><td>16.8</td><td>51.0</td><td>56.7</td></tr><tr><td>Aluminum Oxide (Al<sub>2</sub>O<sub>3</sub>)</td><td>4.9</td><td>28.3</td><td>10.4</td></tr><tr><td>Magnesium Oxide (MgO)</td><td>1.6</td><td>1.5</td><td>1.1</td></tr><tr><td>Titanium Oxide (TiO<sub>2</sub>)</td><td>0.4</td><td>2.4</td><td>0.6</td></tr><tr><td>Sodium Oxide (Na<sub>2</sub>O)</td><td>0.0</td><td>1.0</td><td>0.6</td></tr><tr><td>Potassium Oxide (K<sub>2</sub>O)</td><td>1.3</td><td>3.5</td><td>2.0</td></tr><tr><td>Sulfuric oxide (SO<sub>3</sub>)</td><td>2.3</td><td>1.3</td><td>0.8</td></tr></tbody></table>

According to the ASTM C618 standard [60], pozzolanic materials must contain reactive silicate and aluminate compounds that can interact with calcium hydroxide (Ca(OH)<sub>2</sub>) present in cement. Therefore, supplementary cementitious materials must present a minimum mass fraction for the cumulative primary oxides (SiO<sub>2</sub> + Al<sub>2</sub>O<sub>3</sub> + Fe<sub>2</sub>O<sub>3</sub>) that is higher than 70%. Some researchers have employed this indicator to assess the pozzolanic activity of various waste materials [53,54]. According to Table 2 the CDW powder used in this study met the minimum mass fraction requirement for the cumulative primary oxides, exhibiting a value of 70.4%.

Sicakova and Kovac [61] indicate that the Rankin hydraulicity index, calculated as the ratio between CaO and SiO<sub>2</sub> contents, serves as a reliable indicator of pozzolanic activity, with values below 0.5 suggesting the presence of pozzolanic potential. The CDW powder analyzed in this study exhibited a value of 0.43, thereby supporting its viability as a supplementary cementitious material. Additional indicators commonly used to control the composition of the raw mix of Portland cement clinker include the silica modulus (SM) and alumina modulus (AM), which are calculated using Equations (1) and (2), as described by Malladi et al. [54] and Sicakova and Kovac [61].

$$SM = \frac{\text{SiO}_2}{\text{Al}_2\text{O}_3 + \text{Fe}_2\text{O}_3} \qquad (1)$$

$$AM = \frac{\text{Al}_2\text{O}_3}{\text{Fe}_2\text{O}_3} \qquad (2)$$

An SM value exceeding the threshold of 2.0 is indicative of enhanced pozzolanic reactivity, which can contribute positively to the mechanical strength and long-term durability of the cementitious matrix. In parallel, an AM value higher than 1.0 suggests a potential for mitigating the alkali-silica reaction, thereby improving the chemical stability of the system. As shown in Table 2, the CDW powder exhibits highly favorable characteristics, with an SM of 4.09 and an AM of 3.15, both significantly surpassing the respective benchmark values reported in the literature.

Natural sand was used as aggregate, which was supplied by a Portuguese company (Extractopuro) located in Santarém, Portugal. Figure 5 shows the granulometric distribution of the sand, where it is possible to observe D10, D50, and D90 values of 0.16 mm, 0.68 mm, and 3 mm, respectively. Its density and water absorption were determined according to the national standard NP 581 [62], presenting a density of 2569 kg/m³ and a water absorption of 1.19%.
<figure><img src="image_8.png" /><figcaption>Figure 5. Granulometric particle distribution of natural sand.</figcaption></figure>

## 3.2. Mixtures Design

Mortar formulations were developed to evaluate the incorporation of CDW as supplementary cementitious material. Thus, three mortars were formulated: (i) mortar with 100% Portland cement as a binder (M_REF), acting as the reference mortar; (ii) mortar with the incorporation of 25% fly ash (M_FA25), by volume, as a cement replacement, in order to study a known supplementary cementitious material; (iii) mortar with the incorporation of 25% CDW powder (M_CDW25), by volume, as a cement replacement. The selection of a 25% binder replacement level was based on previous work conducted by the research team [49].

Table 3 presents the mortar mix designs, which were prepared in compliance with the procedures outlined in European Standard EN 196-1 [63].

<table><caption>Table 3. Mortar formulations (kg/m³).</caption><thead><tr><th>Mortar</th><th>CEM I 42.5 R</th><th>FA</th><th>CDW</th><th>Sand</th><th>Water</th></tr></thead><tbody><tr><td>M_REF</td><td>512</td><td>0</td><td>0</td><td>1535</td><td>256</td></tr><tr><td>M_FA25</td><td>384</td><td>97</td><td>0</td><td>1535</td><td>256</td></tr><tr><td>M_CDW25</td><td>384</td><td>0</td><td>110</td><td>1535</td><td>256</td></tr></tbody></table>

## 3.3. Testing Methods

The experimental procedure was specially designed to evaluate the mortars' performance according to (i) physical behavior; (ii) mechanical behavior; (iii) durability (Figure 6).

The workability of the mortars was assessed in accordance with the European standard EN 1015-3 [64]. The test results were deemed valid when the measured spreading diameter satisfied the condition specified in Equation (3).

$$19.5 \text{ cm} \le D \le 21.5 \text{ cm}, \qquad (3)$$

where

D—mortar spreading diameter, cm.
<figure><img src="image_9.png" /><figcaption>Figure 6. Schematic representation of mortars’ experimental tests.</figcaption></figure>

At 28 days, the density of the mortar specimens was evaluated from the ratio of their dry mass to their geometrically measured volume.

The mortars’ water absorption by immersion capacity was evaluated in accordance with Portuguese Specification E 394 [65], based on Equation (4), at 28 days of curing.

$$A = \frac{m_1 - m_3}{m_1 - m_2} \times 100\%, \qquad (4)$$

where

A—mortar water absorption by immersion, %;

$m_1$—specimen saturated mass, g;

$m_2$—specimen hydrostatic mass, g;

$m_3$—specimen dry mass, g.

Capillary water absorption tests were carried out in compliance with EN 1015-18 [66]. In accordance with the procedure, specimens were initially subjected to flexural testing, generating six fragment halves with nominal dimensions of 40 × 40 × 80 mm³, which were subsequently employed for the capillarity assessment. The four lateral faces of the specimens were sealed using a specified sealing material. The water absorption coefficient was determined from the capillarity, considering the slope of the tangent line between the measurements made between the masses of the specimens measured between 10 min and 105 min of testing.

The flexural and compressive tests of the hardened mortars were carried out in accordance with the standard EN 1015-11 [67], according to Equations (5) and (6), at 28 days. Flexural strength was assessed using a Lloyd LR50K hydraulic press operating at a constant loading rate of 50 N/s, while compressive strength was evaluated with an ELE Auto Test hydraulic press at a loading rate of 350 N/s.

$$F_s = 1.5 \times \frac{F \times l}{b \times d^2}, \qquad (5)$$

where

$F_s$—mortar flexural strength, MPa;

F—mortar maximum load, N;

l—distance between supports, mm;

b and d—internal mold dimensions, mm.

$$C_s = \frac{F}{A}, \qquad (6)$$
where

C_s—mortar compressive strength, MPa;

F—maximum load, N;

A—cross-sectional area, mm².

The strength activity index of the mortars was determined in reference to the specification in the Portuguese standard NP EN 450-1 [68], based on the compressive strength ratio of the test mortar and reference mortar, according to Equation (7).

$$SAI = \frac{A}{B} \times 100\%, \qquad (7)$$

where

SAI—strength activity index, %;

A—test mortar's compressive strength, MPa;

B—reference mortar's compressive strength, MPa.

The carbonation test was performed in a controlled carbonation chamber in accordance with Standard EN 12390-12 [69]. Environmental conditions within the chamber were controlled to sustain a CO₂ concentration of 5% ± 1%, a relative humidity of 60% ± 5%, and a temperature of 23 ± 3 °C. The specimens underwent carbonation for 42 days, with measurements taken at 2, 7, 14, 21, 28, and 42 days. Afterward, the carbonation depth of each specimen was measured. The sample surfaces were initially cleaned and subsequently treated with a 1% phenolphthalein solution. Following color stabilization, carbonation depth was determined at multiple surface locations using a digital caliper. This method has been extensively employed by other researchers [70–72], allowing them to determine the pH value.

The electrical resistivity was assessed based on the standard NP EN 12390-19 [73]; measurements were carried out at 2, 7, 14, 21, 28, 59, and 90 days. The samples were saturated before all testing measurements. The electrical measurements were conducted using a Wenner Probe device manufactured by Proceq SA. Several researchers use this method to indirectly evaluate the durability of concrete and mortar [74–76]. When there is a higher amount of pore liquid, the current transmission speed increases, resulting in a lower resistivity value, in turn indicating weaker concrete durability. Conversely, higher resistivity suggests stronger durability.

The determination of the mortars' chloride diffusion coefficient by migration in a non-steady state was determined at 90 days in accordance with the LNEC specification E 463 [77]. The chloride diffusion coefficient determined with this method represents a measure of the chloride penetration resistance. The chloride diffusion coefficient by migration in a non-steady state was determined based on Equation (8):

$$D = \frac{0.0239(273 + T)L}{(U - 2)t} \left( x_d - 0.0238 \sqrt{\frac{(273 + T)Lx_d}{U - 2}} \right), \qquad (8)$$

where

D—mortar's chloride diffusion coefficient by migration in a non-steady state, ×10¹² m²/s;

U—absolute value of the applied voltage, V;

T—average value of the initial and final temperature in the anolyte solution, °C;

L—specimen thickness, mm;

x_d—average value of the penetration depth, mm;

t—test duration, h.
### 3.4. Specimen Preparation

Test specimens with two geometries were produced (Table 4): (i) prismatic specimens measuring $4 \times 4 \times 16 \text{ cm}^3$; (ii) cylindrical specimens measuring 5 cm in height and 10 cm in diameter. The specimens remained inside the mold for 24 h, having been placed in a curing chamber with a relative humidity of at least 90%. In accordance with NP EN 196-1 [63], the demolded specimens were cured in water at ambient temperature until the time of testing. A total of 18 prismatic specimens and 3 cylindrical specimens were assessed for each composition, totaling 63 test specimens for the experimental campaign.

<table><caption>**Table 4.** Specimen geometry and number of samples for experimental tests.</caption><thead><tr><th>Test</th><th>Number of Samples</th><th>Specimens Geometry</th></tr></thead><tbody><tr><td>Flexural strength</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Compressive strength</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Density</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Water absorption by immersion</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Water absorption by capillarity</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Activity index</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Carbonation depth</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr><tr><td>Chloride diffusion coefficient</td><td>3</td><td>h = 5 cm; Φ = 10 cm</td></tr><tr><td>Electrical resistivity</td><td>3</td><td>4 × 4 × 16 cm<sup>3</sup></td></tr></tbody></table>

## 4. Results and Discussion

### 4.1. Workability

According to Figure 7 it was possible to verify the spreading diameter obtained for the developed mortars which meet the requirements established in Equation (1). An increase of approximately 6.4% in the spreading diameter was observed when 25% of the cement was replaced with fly ash. The results obtained are in agreement with previous studies conducted by other authors [32,78]. Gao et al. [78] reported that the incorporation of fly ash particles into cementitious materials can exert a lubricating effect, reducing interparticle friction among cement particles and thereby enhancing the workability of the resulting composites. On the other hand, mortars incorporating 25% construction and demolition waste powder instead of cement showed a tendency to reduce the spreading diameter by approximately 4%. This behavior is associated with the larger specific surface area of construction and demolition waste powder and particles with smaller dimensions compared to cement (Table 1 and Figure 2), and even due to the high-water absorption of CDW, consuming more mixing water, as mentioned by other researchers [38,54,55].

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Spreading diameter of the mortars.</figcaption></figure>
## 4.2. Density

According to Figure 8, it was possible to verify a slight tendency for the density of the mortars to decrease with the presence of CDW and fly ash. This behavior was found to be associated with the lower density of the CDW powder and fly ash (Table 1) and the lower cement content (Table 2) [79,80].

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Density of the mortars.</figcaption></figure>

## 4.3. Water Absorption by Immersion

The water absorption assessed via immersion of the mortars was evaluated and is presented in Figure 9. The water absorption assessed via immersion is a key indicator of concrete durability, as it indirectly reflects the extent of open porosity within the material [81,82].

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Water absorption by immersion of the mortars.</figcaption></figure>

Compared to the reference mortar (100% cement), the mortar incorporating 25% CDW powder as a binder replacement (M_CDW25) resulted in a decrease in water absorption from immersion of approximately 10%. This behavior can be attributed to the filler effect of CDW powder, which promotes improved particle packing, contributing to a reduction in porosity through more efficient pore filling and the development of a denser structure, which reduces water absorption due to immersion and, consequently, decreases porosity [46,54]. Regarding the mortar incorporating 25% fly ash (M_FA25), a contrasting trend was observed, with an increase in water absorption capacity through immersion of about 7%. This behavior is already mentioned in research with fly ash from other sources [82] or other supplementary cementitious materials [81] and could be associated, in addition to the more spherical shape
of its particles (which allows for reduced compaction of the mixture), with the fact that fly ash has hygroscopic properties, absorbing humidity from the concrete and consequently causing greater water absorption.

### 4.4. Water Absorption by Capillarity

The capillary water absorption test provided temporal data (Figure 10) as well as the determination of the water absorption coefficient by capillarity during the initial hours of testing (Figure 11).

<figure><img src="image_13.png" /><figcaption>Figure 10. Water absorption by capillarity of the mortars.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Figure 11. Water absorption coefficient by capillarity of mortars.</figcaption></figure>

Figure 10 illustrates the capillary water absorption profiles over 54 h of the mortar specimens under investigation. It was possible to observe that the reference mortar exhibited the lowest water absorption, with a value of 4.4 kg/m². This trend is in accordance with the results obtained by Tokareva and Waldmann [83]. This phenomenon can be explained by the increased reactivity of cement compared to fly ash and CDW, providing a more compact
mortar microstructure. The incorporation of fly ash and CDW resulted in a decrease in the capillary water absorption capacity of approximately 18% and 30%, respectively. This phenomenon can be attributed to the reduced content of hydrated cement in these mortars [83]. Even though mortars incorporating CDW presented the lowest water absorption capacity by immersion (Figure 8), they also presented the highest water absorption capacity by capillarity. It is important to mention that both properties are indirectly related to the porosity of the mortars. However, water absorption by capillarity is largely influenced by microporosity [84], in which the hydration products have a greater influence. On other hand, some researchers state that cementitious matrix incorporating recycled powders, specially recycled CDW, generally exhibits a more porous microstructure, reduced density (Figure 7), and increased microcracking relative to reference mixtures [41,53,85–89].

The results depicted in Figure 11 indicate that the reference mortar exhibits the lowest water absorption coefficient by capillarity, while the mortar incorporating CDW shows the highest. This tendency corroborates the behavior previously documented in Figure 10. The addition of 25% fly ash and 25% CDW powder led to an increase in the capillary water absorption coefficient of approximately 13% and 58%, respectively.

### 4.5. *Flexural and Compressive Strength*

Regarding the flexural strength of the developed mortars (Figure 12), 25% replacement of cement with either fly ash or CDW powder resulted in a reduction in flexural strength of approximately 25% in both cases.

<figure><img src="image_15.png" /><figcaption>**Figure 12.** Flexural strength of the mortars.</figcaption></figure>

Figure 13 presents the compressive strength results, revealing decreases of approximately 38% and 31% for mortars incorporating 25% fly ash and 25% CDW powder, respectively.

The lower mechanical performance of the mortar with the incorporation of CDW, compared to the reference mortar, is primarily due to the lower reactivity of CDW relative to cement, which limits the generation of hydration products and, consequently, a diminished contribution to strength development. The results obtained corroborate the data reported in prior investigations [86,90,91]. Kim et al. [90] reported that multiple recycling cycles of construction and demolition waste (CDW) grinding can lead to a deterioration in the mechanical strength of composites containing CDW powder, with the extent of degradation being directly proportional to the number of recycling iterations. Therefore, optimization of the CDW grinding process could significantly improve the results presented. However, it is crucial to consider that the results obtained for the flexural strength of mortars incorporating CDW are identical, and in the case of compressive strength, even superior to the results
obtained using fly ash, a supplementary cementitious material extensively documented and employed in cement-based composites. The observed behavior supports the potential use of CDWs as an SCM. Several studies have reported a reduction in compressive strength associated with the incorporation of fly ash, which is attributed to its lower pozzolanic reactivity and the reduction in clinker content [92,93].

<figure><img src="image_16.png" /><figcaption>**Figure 13.** Compressive strength of the mortars.</figcaption></figure>

An analysis of the compressive strength results of the mortars at 28 days reveals that all formulations exhibit values exceeding 6 MPa. This performance is particularly noteworthy, considering their applicability in the construction industry according to the Portuguese standard NP EN 998-1 [94], since they present the maximum classification expected of CS IV.

Transport properties can influence several properties of mortars, with a relationship between capillarity water absorption and the mechanical performance of mortars [89]. Accordingly, the mechanical strength results correlate with the observed capillary water absorption behavior. Mortars incorporating CDW that demonstrate reduced mechanical strength also exhibit increased capillary water absorption, which is attributed to increased microporosity. These findings are in line with other studies [83,95,96], also reporting that higher incorporation contents of demolition waste powder led to increased water absorption and decreased compressive strength, as a consequence of the high open porosity of the recycled particles.

## 4.6. Strength Activity Index

In line with the European standard EN 450-1 [68], several studies [97,98] have adopted a 75% threshold for the fly ash activity index as a reference criterion. Figure 14 illustrates the strength activity index for mortars incorporating CDW and fly ash. It was possible to confirm that none of the mortars developed reached the normative limit established for 28 days. However, an important and promising result was achieved, since the mortar incorporating 25% CDW powder showed a performance approximately 10% higher, when compared to the mortar incorporating 25% fly ash. This behavior can be attributed to the finer particle size and higher specific surface area of the CDW powder in comparison to the fly ash particles (Table 1 and Figure 3).
<figure><img src="image_17.png" /><figcaption>**Figure 14.** Strength activity index of the mortars.</figcaption></figure>

4.7. *Carbonation Depth*

Figure 15 presents the average carbonation depth of the developed mortars measured at 7, 14, and 28 days. Based on these results, it was possible to observe that the reference mortar presents a lower carbonation depth than the mortars incorporating supplementary cementitious materials (fly ash and CDW) at all ages, reflecting enhanced resistance to CO₂ penetration.

<figure><img src="image_18.png" /><figcaption>**Figure 15.** Carbonation depth of the mortars.</figcaption></figure>

At 7 days of age, the mortar with incorporation of 25% CDW (M_CDW25) presents a greater carbonation depth compared to the other mortars, by approximately 0.7 mm compared to the mortars with incorporation of 25% fly ash and by approximately 2.5 mm compared to the reference mortar. Usually, supplementary cementitious materials develop their hydration reaction more slowly over time, and at early ages they may present a more porous microstructure [90].

At 14 and 28 days of age, the mortar with 25% CDW incorporation (M_CDW25) presents a trend contrary to that previously presented, showing a higher resistance to CO₂ penetration in comparison to the mortars with incorporation of 25% fly ash, showing a lower carbonation depth, approximately 0.7 mm at 7 days and 1.2 mm at 28 days. This behavior is essentially associated with the greater absorption of water by immersion and consequent open porosity of the mortars (Figure 9). At 14 and 28 days of age, with the progressive curing of the samples, more hydration products were formed, creating a denser
microstructure with respect to larger pores [99]. On the other hand, these results are also associated with the filler effect of CDW powder, which promotes improved particle packing [46,54]. However, it continues to present a greater carbonation depth in comparison to the reference mortars, which present lower water absorption through immersion and capillarity, suggesting lower open porosity (Figures 9 and 11). These results are consistent with those obtained by Cantero et al. [100]. Rocha et al. [99] reported that carbonation depth generally increases with both the content of incorporated CDW and the age of testing. Mixtures with cement replacement levels above 30% exhibited higher carbonation depths, whereas those with replacement levels below 30% showed comparatively lower values.

Table 5 presents the accelerated carbonation coefficients of the different mortars, calculated from linear regression based on the carbonation depth/square root of time curve and Fick's first law, based on Equation (9) [100].

$$C_d = d_0 + K_{ac} \times \sqrt{t}, \qquad (9)$$

where

$C_d$—carbonation depth at time t, mm;

$K_{ac}$—carbonation coefficient under the experimental conditions, mm/year$^{0.5}$;

$d_0$—carbonation depth at time = 0, mm;

t—time (years).

<table><caption>**Table 5.** Carbonation coefficient ($K_{ac}$) in mortars.</caption><thead><tr><th>Mortar</th><th>Carbonation Coefficient<br>(K<sub>ac</sub>) (mm/year<sup>0.5</sup>)</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>M_REF</td><td>16.99</td><td>0.96</td></tr><tr><td>M_FA25</td><td>33.57</td><td>0.98</td></tr><tr><td>M_CDW25</td><td>20.35</td><td>0.99</td></tr></tbody></table>

Through an analysis of Table 5, it is possible to verify that the accelerated carbonation coefficients obtained a good approximation (R$^2$), all being greater than 0.96. It is also possible to observe that the reference mortar presented the lowest accelerated carbonation coefficient, with the incorporation of CDW resulting in an increase of approximately 20% and the incorporation of fly ash resulting in an increase of approximately 98%. These results demonstrate that mortars incorporating fly ash have a superior carbonation rate. Regarding the incorporation of 25% CDW, Cantero et al. [100] obtained higher values only with the use of recycled concrete as a binder replacement.

Using the model developed by Sisomphon and Franke [100,101], the correlation between accelerated and natural carbonation was established, as presented in Equation (10).

$$\frac{K_n}{\sqrt{c_n}} = \frac{K_{ac}}{\sqrt{c_{ac}}}, \qquad (10)$$

where

$K_n$—natural carbonation coefficient, mm/year$^{0.5}$;

$K_{ac}$—accelerated carbonation coefficient, mm/year$^{0.5}$;

$c_n$—CO$_2$ concentration under natural conditions (~0.04%) [100];

$c_{ac}$—CO$_2$ concentration in the accelerated carbonation tests, %.

Based on the natural carbonation coefficient, it is possible to classify the mortars produced according to the study published by Greve-Dierfeld and Gehlen [102] (Table 6). Cantero et al. [100] applied this methodology to concretes produced with recycled cement
and recycled aggregates, having obtained the same classification for compositions without the addition of supplementary cementitious materials and higher classes for compositions with CDW additives.

<table><caption>**Table 6.** Carbonation resistance classes [100,102].</caption><thead><tr><th colspan="2">Carbonation Resistance Class</th></tr></thead><tbody><tr><td>RC2</td><td>1 &lt; K<sub>n</sub> ≤ 2</td></tr><tr><td>RC3</td><td>2 &lt; K<sub>n</sub> ≤ 3</td></tr><tr><td>RC4</td><td>3 &lt; K<sub>n</sub> ≤ 4</td></tr><tr><td>RC5</td><td>4 &lt; K<sub>n</sub> ≤ 5</td></tr><tr><td>RC6</td><td>5 &lt; K<sub>n</sub> ≤ 6</td></tr><tr><td>RC7</td><td>6 &lt; K<sub>n</sub> ≤ 7</td></tr><tr><td>RC2</td><td>1 &lt; K<sub>n</sub> ≤ 2</td></tr></tbody></table>

Some authors [100,103] have established that natural carbonation coefficients lower than 4 mm/year<sup>0.5</sup> indicate quality materials, also highlighting the critical value of 6 mm/year<sup>0.5</sup>. Table 7 shows the natural carbonation coefficient and the carbonation resistance class of the different mortars developed. Based on the performance obtained (Table 7), it was observed that the reference mortar presented the lowest natural carbonation coefficient value and, consequently, the lowest carbonation resistance class. The incorporation of 25% CDW and fly ash as cement replacement caused an increase in the natural carbonation coefficient of approximately 20% and 97% in comparison to the reference mortar, respectively. Nonetheless, it should be emphasized that the carbonation resistance class remained unchanged with the incorporation of 25% CDW, maintaining the minimum resistance class (RC2), as in the reference mortar. Meanwhile, the mortar with incorporation of 25% fly ash obtained a higher carbonation resistance class (RC3).

<table><caption>**Table 7.** Natural carbonation coefficient ($K_n$) and carbonation resistance class of the mortars.</caption><thead><tr><th>Mortar</th><th>Carbonation Coefficient ($K_n$)<br>(mm/year<sup>0.5</sup>)</th><th>Carbonation Resistance<br>Class</th></tr></thead><tbody><tr><td>M_REF</td><td>1.52</td><td>RC2</td></tr><tr><td>M_FA25</td><td>3.00</td><td>RC3</td></tr><tr><td>M_CDW25</td><td>1.82</td><td>RC2</td></tr></tbody></table>

## 4.8. Chloride Migration Test

Figure 16 displays the non-steady-state migration coefficient ($D_{NSSM}$) results for the developed mortars, emphasizing the influence of CDW and fly ash incorporation on chloride ion penetration resistance. The results revealed an improvement in chloride resistance with the presence of CDW and fly ash. It was possible to verify that the incorporation of 25% fly ash and CDW resulted in a decrease in the chloride diffusion coefficient of approximately 37% and 51%, respectively. The enhanced resistance to chloride ion penetration in CDW mortars can be attributed to the filling effect of the CDW powder [99] (Figure 3) and to their lower porosity (Figure 9) [100]. Rocha et al. [99] indicate that CDW composites are generally characterized by a lower content of hydration products, as well as increased pore size and overall porosity. Nevertheless, the adverse effects associated with their low pozzolanic activity may be mitigated through the enhanced fineness of the CDW particles, which improves particle packing and contributes to a denser microstructure.
<figure><img src="image_19.png" /><figcaption>Figure 16. Chloride diffusion coefficient of mortars.</figcaption></figure>

Although fly ash exhibits a lower maximum particle size relative to CDW powder (Figure 4), its particle size distribution is characterized by a higher proportion of coarser particles (Figures 2 and 3), which contributes to increased porosity in mortars (Figure 9) and, consequently, to a higher chloride diffusion coefficient. In this way, CDW has a positive impact on chloride ion penetration resistance, which is also in line with its higher pozzolanic activity (Figure 14).

## 4.9. *Electrical Resistivity*

Figure 17 shows the electrical resistivity of mortars over time, with an increasing trend in all mixtures and a more pronounced increase after 28 days of age being observed, as already reported by Cantero et al. [75]. This behavior is associated with the gradual hydration process of the binder, which allowed densification and refinement of the capillary pore system, reducing the connectivity of the system and hindering the transport of ions [75]. It was also possible to observe that the reference mortar presented the worst performance, with the mortars incorporating CDW and fly ash presenting a higher electrical resistivity. At 90 days, the electrical resistivity of the mortars incorporating CDW and fly ash was higher than the electrical resistivity of the reference mortars by approximately 42% and 242%, respectively. The higher resistivity of mortars incorporating fly ash, compared to mortars incorporating CDW and reference mortars, can be justified by the continuous development of the hydration reactions of the mortars, since the hydration process of mortars incorporating fly ash is generally slower, resulting in a denser structure of the mortars for curing ages greater than 28 days. According to Gagan and Singh [76], electrical resistivity can be influenced by several factors, such as pore saturation, chloride diffusion potential, moisture levels, pore system continuity, and cement paste electrical resistivity. The same authors [76] indicate that the risk of corrosion is negligible if the electrical resistivity is greater than 20 k.Ω.cm, low when between 10 and 20 k.Ω.cm, high when between 5 and 10 k.Ω.cm, and very high when less than 5 k.Ω.cm. Therefore, we can conclude that none of the mortars developed present a high risk of corrosion.
<figure><img src="image_20.png" /><figcaption>**Figure 17.** Electrical resistivity of the mortars.</figcaption></figure>

### 4.10. *Research Limitations*

The incorporation of supplementary cementitious materials offers both environmental and economic advantages. When these materials are derived from construction industry waste, their contribution becomes even more significant. This practice not only reduces the consumption of natural raw materials and associated carbon dioxide emissions but also enhances the valorization of industrial waste. Consequently, it supports a more circular construction economy and supports the mitigation of the sector's environmental footprint. However, this study has some limitations, namely, because all the materials used are produced in Portugal, and mainly because the CDW in their composition represents the construction materials used locally. Nevertheless, the conclusions of this study highlight both the potential and the relevance of promoting the use of CDW as a partial cement replacement. Thus, research teams will be able to use this study to include sustainable solutions in construction practices.

## 5. Conclusions

This study presents a complete evaluation of the processing of CDW through mechanical grinding, alongside an assessment of the physical, mechanical, and durability properties of mortars containing 25% CDW as supplementary cementitious material. Additionally, a comparative analysis was conducted between mortars incorporating CDW and mortars incorporating 25% fly ash, a conventional supplementary cementitious material. The outcomes of this investigation are outlined in detail below:

1. The mechanical grinding of CDW was found to be an effective method for producing a powder with particle size characteristics appropriate for partial cement replacement. Although the resulting powder exhibited slightly lower density and average particle size compared to Portland cement, it presented a significantly higher specific surface area, which may contribute to enhanced pozzolanic reactivity. Furthermore, the chemical composition of the CDW powder satisfied several criteria reported in the literature for identifying potential pozzolanic materials, comparable to those observed in fly ash.

2. Water absorption by immersion tests revealed that mortars incorporating CDW as a partial cement replacement exhibited lower water absorption than the reference mortar and the mortar containing fly ash as cement replacement. This reduction in absorption is associated with the filler effect of the finely ground CDW, which enhances particle packing and contributes to a denser microstructure. These findings
are particularly noteworthy from a durability standpoint, as the CDW-based mortars also showed reduced carbonation depths and lower chloride diffusion coefficients, indicating superior performance in comparison to mortars with fly ash incorporation.

3. Mechanical behavior tests revealed a decrease in flexural and compressive strength with the incorporation of 25% CDW, with identical behavior being observed for mortars with the incorporation of 25% fly ash. This behavior can be attributed to the lower reactivity of CDW compared to cement, resulting in reduced formation of hydration products and, consequently, a diminished contribution to strength development. However, it is important to note the potential of CDW, since at 28 days of age, the mortars presented a higher strength activity index than that recorded for mortars with fly ash.

Future studies will aim to deepen the understanding of the findings presented in this work. First, alternative CDW grinding processes and their influence on the behavior of mortars will be studied. And, finally, the long-term behavior of the mortars, namely, the evaluation of mechanical properties and durability at advanced ages, will also be studied.

**Author Contributions:** Conceptualization, S.C., K.K. and J.A.; methodology, S.C., K.K. and J.A.; software, S.C.; validation, S.C. and J.A.; formal analysis, S.C., K.K. and E.H.; investigation, S.C., K.K., E.H. and J.A.; resources, S.C. and J.A.; data curation, S.C. and E.H.; writing—original draft preparation, S.C. and K.K.; writing—review and editing, S.C., K.K. and J.A.; visualization, S.C., K.K., E.H. and J.A.; supervision, S.C., K.K. and J.A.; project administration, S.C., K.K. and J.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by FCT/MCTES through national funds (PIDDAC) under the R&D Unit Centre for Territory, Environment and Construction (CTAC) under reference UIDB/04047/2025 and UIDP/04047/2025.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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