Characterization of Mortars Incorporating Concrete Washing Fines: Impact on Mechanical Properties, Microstructure and Carbon Footprint

Abstract

This study examines the potential use of wash fines, a waste product from concrete plant cleaning, as supplementary cementing materials (SCMs) in mortars. The main objective is to assess the feasibility and benefits of this incorporation in terms of technical performance and environmental impact. Extensive tests were carried out on different mortar formulations, incorporating varying rates of washing fines (0%, 10%, 20%, 30%) as a partial replacement for cement. This choice of replacement is prompted by the fineness of washing fine particles. The properties studied included compressive and flexural strength, porosity, density, water absorption, shrinkage and fire resistance. The results show that the incorporation of washing fines increases porosity and decreases mortar density. There was also a decrease in mechanical strength and fire resistance as the substitution rate increased. However, the use of washing fines enables a significant reduction in the mortar’s carbon footprint, reaching up to 29% for the formulation with 30% substitution. This study demonstrates the potential of washing fines as an alternative SCM, as part of a circular economy approach to reducing the environmental impact of the concrete industry. However, it underlines the need to optimize formulations to maintain acceptable technical performance.

1. Introduction

Concrete is the world’s most widely used construction material, with annual production exceeding 10 billion tons [1]. This ubiquity is due to its exceptional durability, remarkable strength and unrivaled versatility. Composed mainly of cementitious binder, aggregates, water and chemical admixtures, concrete offers a unique combination of properties that make it the preferred choice for a multitude of construction applications.

However, despite its countless advantages, the production of its main component, cement, generates around 7% of the world’s carbon dioxide (CO₂) emissions, making a significant contribution to global warming, one of the greatest environmental challenges of our time [2]. The cement manufacturing process is not only energy-intensive, requiring substantial quantities of fossil energy, but also releases considerable volumes of CO₂ during the calcination of limestone, the crucial stage in the production of clinker, the key component of cement.

Faced with these growing environmental concerns and the urgent need to act to mitigate the impacts of climate change, intensive efforts have been made within the concrete industry to develop alternative cementitious materials and optimize production processes. The main objective is to significantly reduce the carbon footprint associated with concrete manufacture while maintaining its outstanding technical performance and durability.

Supplementary cementing materials (SCMs) are at the heart of this innovative strategy to reduce the environmental impact of concrete [3,4,5,6,7,8,9,10,11,12,13]. By partially replacing Portland cement clinker, the main but energy-intensive component of cement, with SCMs, it is possible to significantly reduce the CO₂ emissions associated with concrete production [14,15]. This partial substitution not only reduces clinker demand and related emissions but also improves concrete performance.

The use of cement substitute materials (SCMs) plays a crucial role in promoting the circular economy by adding value to industrial by-products that would otherwise be considered waste. By incorporating these materials into concrete formulations, we not only reduce the quantities of waste to be disposed of but also make more efficient use of available resources, thus contributing to a more sustainable future for the construction sector.

Among the most commonly used SCMs are fly ash, silica fume and metakaolin. These materials offer significant environmental benefits by reducing CO₂ emissions and waste generation, while improving the mechanical properties and durability of concrete thanks to their pozzolanic properties and beneficial effects on the microstructure of cement paste. Fly ash, derived from coal combustion, is widely recognized for its ability to improve the workability and long-term strength of concrete. Its incorporation into concrete formulations reduces the need for Portland cement, while increasing the strength and durability of the final material [16]. Silica fume, a by-product of silicon production, stands out for its significant effect on concrete strength and durability. Due to its extreme fineness and high reactivity, it contributes to the formation of an additional hydrated calcium silicate gel (C-S-H), thus improving the density and durability of concrete [17]. Metakaolin, obtained by calcining kaolinite clay, improves not only the mechanical strength of concrete but also its resistance to chemical attack. Its fine granulometry and pozzolanic reactivity make it an excellent reinforcement for concrete mixes, increasing their longevity and performance [18,19].

In addition, other SCMs, such as granulated blast furnace slag (GGBFS), are gaining importance in research and practical applications. Granulated blast furnace slag, derived from the steel-making process, is a by-product rich in silica and alumina, which can replace part of the Portland cement in concrete mixes. It improves the durability of concrete, notably its resistance to sulfates and chloride penetration, and reduces the carbon footprint of concrete [20,21].

SCMs offer promising opportunities for reducing the carbon footprint of cementitious matrices. For example, a 22% reduction in CO₂ emissions has been observed for a 40% replacement of cement by fly ash in a cementitious paste [22]. Similarly, replacing 15% of cement with silica fume in concrete results in a 12% reduction in CO₂ emissions [23]. In the case of metakaolin, a 20% substitution results in around 17% reduction [24]. GGBS can reduce emissions by up to 47% with a substitution rate of 70% in concrete [25]. These results demonstrate the potential of SCMs to significantly reduce the environmental impact of cementitious materials.

At present, research is focusing on optimizing the substitution rates of these SCMs, assessing their impact on the mechanical and durability properties of concrete and analyzing the life cycle of the materials. However, major gaps remain, notably concerning the variability of the chemical composition of these wastes and their long-term effects on concrete structures. Further study is needed to better understand these effects and improve SCM design and application practices in the concrete industry.

By judiciously exploiting SCMs, the concrete industry can reduce its carbon footprint while optimizing the technical performance of its products, thus contributing to a more sustainable future for the construction sector. The use of concrete industry waste as SCM is a rapidly expanding field. The current research focuses on optimizing substitution rates, assessing impacts on mechanical and durability properties and life-cycle analysis. However, there are still significant gaps concerning the variability of the chemical composition of these wastes and their long-term effects on concrete structures. Some of these studies have explored recycling concrete industry waste as SCM. For example, the reuse of fines from washing ready-mix concrete has been studied, showing a reduction in compressive strength but an improvement in durability [26]. Similarly, recycled concrete fines obtained by mechanosynthesis have been valorized for construction material production [27].

Among the promising new sources of SCMs, washing fines represent a particularly interesting and innovative opportunity. These fines are waste products from the cleaning operations of concrete plants and truck-mixers after the production of ready-mix concrete. Composed mainly of fine silica and calcium particles, these wash fines are often considered non-value-added waste and simply disposed of, generating additional costs and undesirable environmental impacts.

However, their incorporation into mortar and concrete formulations as a partial substitute for Portland cement offers a significant double advantage. On the one hand, it would reduce cement consumption and, consequently, the CO₂ emissions associated with its energy-intensive production. On the other hand, it would transform a previously unused waste product into a valuable resource, reclaiming these washing fines and reducing the environmental impact associated with their management and disposal.

The use of washing fines as an alternative SCM fits perfectly with the concrete industry’s strategy to reduce its carbon footprint while promoting a more sustainable circular economy. The judicious recovery of this waste not only reduces waste management costs but also reduces demand for Portland cement, the production of which is particularly energy-intensive and emits greenhouse gases.

This innovative approach represents a further step towards a more environmentally friendly concrete industry by fully exploiting the potential of all available by-products and waste as valuable alternative resources. Wash fines, once considered a worthless waste product, could play a key role in the transition to more sustainable construction.

A great deal of research has been carried out into the use of washing fines as a substitute for sand. The results of this research indicate that washing fines can be used as a partial substitute for fine aggregates (sand) in concrete, sometimes even improving certain properties of the material [26,27,28]. However, studies focusing specifically on their use as a partial cement substitute remain limited [29].

The main objective of this study is to assess the feasibility and potential benefits of using wash fines as SCMs in mortars. A series of extensive tests were carried out to examine the impact of incorporating these fines on various key mortar properties, such as compressive strength, flexural strength, porosity, bulk density, water absorption, shrinkage and fire resistance. These rigorous tests make it possible to determine the extent to which wash fines can improve or modify mortar characteristics, and thus to assess their viability as promising SCMs.

The results of this research provide a detailed overview and in-depth understanding of the valorization of wash fines in cement-based construction materials. They demonstrate how more sustainable waste management can be seamlessly integrated into concrete production, contributing to a greener, more efficient construction industry. This study paves the way for innovative industrial practices that valorize waste while meeting the growing need for sustainability and superior performance in the construction sector. It represents an important step towards the transition to a more environmentally friendly circular economy in the concrete industry.

2. Materials and Methods

This section presents a detailed description of the materials used in this study, based on various methods.

2.1. Cement

The binder used in this study is Portland cement type CEM I 52.5R, known for its rapid hardening and high strength. This cement is commonly used in building construction where high mechanical performance is required.

Table 1. Cement characteristics as given in technical sheet.

Data	CEM I 52.5R–SR5 CE
SO3	2.1%
MgO	0.6%
Na2O	<0.3%
Cl−	<0.04%
Loss on ignition at 950 °C over time	1.1%
Compressive strength at 1 day	21–27 MPa
Compressive strength at 2 days	40–48 MPa
Compressive strength at 28 days	66–76 MPa

shows the cement’s chemical composition and its key mechanical properties, including compressive strengths at different hardening ages.

Cement is a fine powder. Figure 1 illustrates the particle size distribution of the cement used in this study.

2.2. Sand

Natural washed sand with a particle size distribution conforming to fine aggregate standards was used in this study. It is clean, free of impurities and has a density of 2.63. The properties described include density, water absorption and other relevant characteristics.

For this study, washed natural sand complying with aggregate standards was used. The sand is clean and free of impurities. Its particle size distribution complies with the normative requirements applicable to fine aggregates.

Table 2. Sand physical properties as given in technical sheet.

Physical Properties	0/4 Sand
Density [kg/m3]	2630
Water absorption [%]	0.1
Sand equivalent	81.3
Fineness modulus	2.3

presents the key properties of this sand relevant to its use in mortar and concrete formulations.

2.3. Fresh Concrete Washing Fine

In this study, fresh concrete washing fine comes from a precast concrete plant washing process. Preparation of washing fines for use as supplementary cementing materials (SCMs) begins with their collection from production sites and storage in hermetically sealed containers at room temperature. The fines are then dried at 65 ± 5 °C to a constant mass, ensuring complete dewatering. The dried sludge is then ground by hand to obtain a fine powder, which is sieved to 100 μm to remove the coarsest particles; see Figure 2. Wash fines can be used without heating, but heating is chosen to make mixing easier.

To characterize the thermal stability and composition of these washing fines, thermogravimetric analyses (TGA) were carried out. Measurements were carried out in an inert nitrogen atmosphere, varying the temperature from ambient to 1000 °C, in a Netzsch STA449 F3 Jupiter^® furnace (Netzsch Gerätebau GmbH, Selb, Germany). Several critical phases are revealed at different temperature ranges (see Figure 3). Up to 120 °C, a decrease in mass is observed, corresponding to the evaporation of residual free water. Between 120 and 600 °C, another significant mass loss is identified, probably linked to the release of chemically bound water during the decomposition of calcium hydroxide (Ca(OH)₂), with a peak between 400 and 500 °C. Finally, a further decrease in mass occurs between 600 and 1000 °C, suggesting the release of carbon dioxide (CO₂) resulting from the decomposition of calcium carbonate (CaCO₃), with a notable peak between 800 and 900 °C.

In summary, washing fine TGA reveals mass loss processes associated with three main phenomena: residual free water evaporation up to 120 °C, chemically bound water release between 120 and 600 °C (probably related to calcium hydroxide decomposition) and calcium carbonate decomposition between 600 and 1000 °C with carbon dioxide release. It provides crucial information for understanding the chemical composition and thermal stability of washing fines, guiding the appropriate processing and recovery strategies for these materials.

In addition, washing fines were examined using laser granulometry. The results in Figure 4 show a distribution similar to that of cement but with certain differences. Wash fines contain finer particles, probably resulting from additions to the concrete mix, such as filler. At the same time, coarser particles are present, originating from the fines in the sand used in the concrete. This particle distribution depends on the nature of the concrete produced. Indeed, a formulation containing more sand, cement or supplementary cementing materials, as well as admixtures, may affect the results. Studies on several sources could be of interest for future work.

2.4. Porosity and Density

Mortar samples were tested for water-accessible porosity and density in accordance with standard NF P18-459 [30]. This test, commonly known as the “vacuum water test”, measures both sample open porosity and bulk density. Samples are first oven-dried to a constant mass, and then placed in a desiccator connected to a vacuum pump. A high vacuum is applied for 24 h to extract the air contained in the open pores of the samples. The desiccator is then filled with demineralized water, allowing for complete impregnation of the pores under vacuum for a further 24 h. After resaturation, the immersed mass of the samples is measured by hydrostatic weighing. The samples are then superficially wiped with a damp cloth to determine their saturated mass. The combination of these different mass measurements gives the water-accessible porosity and the apparent density according to the formulas prescribed by standard NF P18-459.

2.5. Water Absorption by Immersion

The water absorption of mortars was measured using a simple direct immersion protocol. Samples of hardened mortar were first oven-dried to constant mass to remove any residual water. Their dry mass was then carefully recorded. The samples were then fully immersed in demineralized water for a period of 24 h to allow for total saturation by capillary absorption. After this immersion period, the samples were removed from the water, superficially wiped with a damp cloth, and their saturated mass was measured. The water absorption rate, expressed as a percentage, was calculated as the increase in sample mass due to water absorption, relative to their initial dry mass.

2.6. Flexural Strength

Mortar flexural strength was assessed using a three-point bending test in accordance with the French standard NF EN 196-1 [31]. Prismatic specimens measuring 40 mm × 40 mm × 160 mm were cast for each mortar formulation. After a standardized curing period, these specimens were subjected to a three-point bending test, in which the specimen is simply placed on two lower supports and loaded at the center by a third upper support descending at constant speed until failure. The maximum force applied at the moment of flexural failure is recorded. The flexural strength is then calculated according to the formula prescribed by the standard, depending on the breaking load, the span between lower supports and the cross-sectional dimensions of the specimen.

2.7. Compressive Strength

Following the three-point bending tests, the two resulting prismatic half-tests were subjected to compression tests in accordance with NF EN 196-1 [31]. The compression machine’s plate allows for testing of a dimension of 40 mm × 40 mm × 40 mm. An increasing compression load was applied at a constant speed of 2.4 kN/s in accordance with the standard. The maximum force recorded at the moment of crushing failure was used to calculate the compressive strength Rc according to the standardized formula, taking into account the dimensions of the support section. This standardized test, which complements the bending test, provides a complete characterization of the mechanical performance of the different mortar formulations developed.

2.8. Total Shrinkage

Total shrinkage of the various mortar formulations was assessed in accordance with the French standard NF P 15-433 [32], which describes the method for measuring dimensional variations on hardened mortar specimens. Prismatic specimens measuring 40 mm × 40 mm × 160 mm were cast in molds and demolded after 24 h. These specimens were fitted with metal pins at the ends to facilitate accurate measurement of length variations. After demolding, the specimens were stored in an air-conditioned room at 50% relative humidity and 20 °C for curing under controlled conditions.

At different time intervals, the dimensional variations of the specimens were measured using a high-precision length comparator. Total shrinkage was then calculated as the ratio of length variation to initial specimen length.

2.9. Fire Resistance

The fire resistance of the various mortar formulations was assessed on the basis of their residual compressive strength after exposure to high temperatures, using a procedure derived from current investigations [33,34]. For each mix, three 40 mm × 40 mm × 160 mm prismatic specimens were cast and stored under standard conditions for 28 days for complete curing. These specimens were then placed in a preheated muffle furnace and held for 1 h at target temperatures of 200 °C, 400 °C and 900 °C, respectively. The choice of these test temperatures was guided by the results of prior thermogravimetric analyses.

After exposure to the target temperature for 1 h, the samples were removed from the furnace and cooled naturally to room temperature. Once cooled, they were visually inspected for cracks, spalling or other degradation. Their mass was also measured and compared with the initial mass to quantify any losses. Finally, the samples were subjected to a flexion and compression test to determine their strength after thermal aggression.

2.10. Formulation

Four different mortar formulations were prepared, incorporating varying levels of washing fines as a partial replacement for cement. An unadded control formulation (T), as well as three formulations containing, respectively, 10% (L10), 20% (L20) and 30% (L30) washing fines to replace cement were studied. The proportions of each component in each mixture are detailed in

Table 3. Mortar incorporating washing fines formulations.

Component	Ref	L10	L20	L30
Cement	450	405	360	315
Washing fine	0	45	90	135
Sand	1350	1350	1350	1350
Water	225	225	225	225

.

3. Results

3.1. Open Porosity and Density

The physical properties of different mortar formulations incorporating washing fines are presented in

Table 4. Physical properties of mortar incorporating washing fine.

Formulation	Porosity [%]	Density [kg/m3]
Ref	19.3	2008
L10	23.2	1937
L20	24.1	1898
L30	25.8	1835

. An increase in porosity is observed with the increasing incorporation of washing fines as a partial replacement for cement. Indeed, the porosity of the reference control mortar is 19.3%, while L10, L20 and L30 formulations have porosities of 23.2%, 24.1% and 25.8%, respectively. This tendency for porosity to increase with the rate of substitution can be explained by the less efficient filling effect of wash fines compared to cement in the cementitious matrix, leaving more voids [35].

At the same time, mortar bulk density decreases with the incorporation of washing fines. The reference mortar has a density of 2008 kg/m³, while the densities of the L10, L20 and L30 formulations are 1937, 1898 and 1835 kg/m³, respectively. This decrease in density is consistent with the observed increase in porosity, and can also be attributed to the intrinsically lower density of wash fines compared to Portland cement.

These results indicate that the incorporation of washing fines as a partial replacement for cement has a significant impact on the microstructure of mortars, increasing their total porosity and reducing their compactness.

3.2. Water Absorption by Immersion

The water absorption results for the different mortar formulations incorporating washing fines are presented in

Table 5. Water absorption of mortars incorporating washing fines.

Formulation	Water Absorption [%]
Ref	4.6
L10	5.9
L20	5.3
L30	10.3

.

The results indicate an increase in water absorption with a higher incorporation of wash fines. The reference mortar (Ref) showed a water absorption rate of 4.6%. Formulations L10 and L20 showed slightly higher absorption rates of 5.9% and 5.3%, respectively. However, formulation L30, which contained the highest amount of washing fines, showed a significant increase in water absorption to 10.3%. This increase in water absorption with the addition of washing fines can be attributed to several factors. Firstly, higher porosity: as observed in the porosity results, mortars with wash fines have higher porosity. This increased porosity creates more capillary pores capable of absorbing and retaining water, resulting in higher water absorption rates. Secondly, particle compaction density: the less efficient compaction of washing fines compared to cement particles results in a more open microstructure, facilitating water entry because of their irregular shape [29,36]. Thirdly, surface characteristics: washing fines may have different surface characteristics to cement, possibly with higher surface roughness or specific surface, which enhances water absorption. Finally, hydration products: the incorporation of washing fines can alter the type and quantity of hydration products used. This may be due in part to the fact that washing fines contain far fewer active ingredients than cement, as previous studies on this topic have pointed out [29].

3.3. Total Shrinkage

The total shrinkage of the various mortar formulations incorporating wash fines was monitored over time, with measurements taken at 1, 3, 28 and 70 days. Shrinkage values expressed in “μm/m” are shown in

Table 6. Total shrinkage of mortar incorporating washing fine.

Total Shrinkage [µm/m]	Ref	L10	L20	L30
1 day	509	1054	1203	1637
3 days	603	1451	1504	2162
28 days	713	1473	1624	2222
70 days	800	1608	1684	2362

.

It can be seen that the increasing incorporation of washing fines as a partial replacement for cement leads to a significant increase in total shrinkage at all ages. After 70 days, the shrinkage of the reference mortar without addition reaches 800 μm/m. In contrast, the L10, L20 and L30 formulations show significantly higher shrinkages of 1608, 1684 and 2362 μm/m, respectively.

This tendency for shrinkage to increase with the substitution rate can be explained by several factors. Firstly, the greater porosity of mixes with washing fines favors water movement during drying, thus amplifying shrinkage. Secondly, the very nature of wash fines, rich in fine silica and calcium particles, may result in higher water demand for hydration, resulting in a larger pore network after drying.

3.4. Mechanical Strength

The flexural strength test results for the different mortar formulations incorporating wash fines are presented in

Table 7. Flexural strength of mortar incorporating washing fines.

Formulation	Flexural Strength
	14 Days [MPa]	28 Days [MPa]
Ref	6.4 ± 0.4	6.7 ± 0.2
L10	5.4 ± 0.2	5.8 ± 0.3
L20	4.7 ± 0.3	5.1 ± 0.4
L30	2.5 ± 0.3	2.9 ± 0.2

. The values correspond to the average strength measured for three specimens at 14 and 28 days, with the associated standard deviation.

A decrease in flexural strength is observed as the rate of substitution of cement by washing fines increases. At 28 days, the unadded reference mortar had a flexural strength of 6.7 MPa. The incorporation of 10% washing fines results in a slight drop to 5.8 MPa. This decrease becomes more pronounced for P20 and P30 formulations, with strengths of only 5.1 and 2.9 MPa, respectively.

This downward trend in flexural performance with the incorporation of washing fines can be attributed to several factors. Firstly, the greater porosity of these mixes [35], as highlighted above, weakens the compactness of the cementitious matrix and creates stress concentration sites. Secondly, the very nature of wash fines, composed mainly of fine particles of low reactivity, does not allow for the development of a microstructure as dense and resistant as Portland cement [29,37,38].

On the other hand,

Table 8. Compressive strength of mortar incorporating washing fines.

Formulation	Compressive Strength
	14 Days [MPa]	28 Days [MPa]
Ref	47 ± 1.8	51.1 ± 1.2
L10	33.3 ± 1.1	36.1 ± 1.2
L20	26.9 ± 1.3	29.3 ± 1.0
L30	17.9 ± 1.2	21.5 ± 1.3

presents the results of compressive strength tests carried out on the various mortar formulations incorporating the washing fines. The values correspond to the average strength measured for three cubic specimens at 14 and 28 days.

A significant decrease in compressive strength is observed as the rate of substitution of cement by washing fines increases. The reference mortar with no added fines shows a strength of 51 MPa after 28 days of curing. The incorporation of 10% washing fines (L10) results in a moderate decrease to 36 MPa. However, this reduction becomes more marked for formulations L20 and L30, with strengths of just 29 MPa and 21 MPa, respectively, at 28 days.

This drop in compressive performance with an increasing substitution ratio can be attributed to several factors. Firstly, as previously observed, the incorporation of washing fines increases the total porosity of the mortar, thus weakening its compactness and density [35]. This more porous and less dense microstructure does not allow the mortar to develop as high mechanical strength as the reference mortar. Secondly, the very nature of the wash fines, composed mainly of fine particles of low reactivity, does not contribute significantly to the development of a dense, resistant microstructure [29,37,38].

These results of a compressive strength decrease with increasing substitution rate are consistent with those of Wu et al. (2023) [29], who observed a similar trend in their study on waste powders as cement mortar components.

3.5. Fire Resistance

The results of fire resistance tests reveal a worrying trend in the incorporation of washing fines in mortars. All formulations containing washing fines (L10, L20, L30) showed a significant reduction in flexural (

Table 9. Flexural strength of mortar incorporating washing fines after exposure to fire.

Formulation	Flexural Strength [MPa]
	200 °C	400 °C	900 °C
Ref	5.8	4.8	0.6
L10	5.1	3.3	0.4
L20	3.7	2.7	0.3
L30	1.1	0.9	0.1

) and compressive strength (

Table 10. Compressive strength of mortar after exposure to fire.

Formulation	Compressive Strength [MPa]
	200 °C	400 °C	900 °C
Ref	27.7	25.9	4.4
L10	18.7	16.9	3.6
L20	15.6	10.6	2.5
L30	6.2	4.9	1.3

) compared with the reference formulation (Ref) at all temperatures tested (200 °C, 400 °C, 900 °C). This drop in performance became more pronounced as the rate of substitution of cement by washing fines increased. At 900 °C, all formulations, including the reference, show a drastic loss of strength, which is consistent with the typical behavior of cementitious materials at high temperatures. However, the incorporation of wash fines seems to exacerbate this loss of strength.

This heat-induced degradation of mechanical properties could be explained by several factors. Firstly, the increased porosity of mortars containing washing fines, as observed in the porosity results, could facilitate heat penetration and accelerate the degradation of the material’s internal structure. Secondly, the partial substitution of cement by washing fines could reduce the amount of cement hydrates formed, which are primarily responsible for the mortar’s mechanical strength. Finally, potential differences in thermal behavior between washing fines and Portland cement could induce internal stresses and microcracks when exposed to high temperatures, further weakening the mortar’s structure.

These results underline the importance of considering fire resistance when using washing fines as a partial cement substitute, particularly in applications where fire safety is paramount. Although the incorporation of washing fines offers environmental and economic benefits, as demonstrated in the previous sections, their negative impact on fire resistance could limit their use in certain contexts. However, it is important to note that the results obtained are preliminary and require more detailed analysis and research to determine the feasibility and effectiveness of these mortars in practical applications. Further studies will be required to comprehensively assess the fire performance of these materials, taking into account various parameters such as processing conditions and variations in composition.

3.6. Embodied Carbon

Cement is the factor with the greatest carbon impact. On the other hand, sand, used as a fine aggregate, although less impactful than cement, also contributes to CO₂ emissions through its extraction and transportation. Wash fines, considered in this study as a partial substitute for cement, are by-products of concrete plant cleaning operations, the use of which reduces the demand for Portland cement and, therefore, the emissions associated with its production.

To assess the carbon impact of mortar manufacturing, several assumptions were made. Firstly, it was assumed that the integrated carbon emissions for each raw material were taken from the ICE (Inventory of Carbon and Energy) database [39]. Secondly, washing powder was dried for 8 hours in a laboratory oven, followed by sieving. As the product is sinterable, no grinding was required. To assess the carbon impact of this process, it is essential to analyze the energy consumption of production stages.

Carbon impact calculations for each kg of powder produced have taken into account two key stages in the process: drying the waste in an oven and sieving. These two operations require specific energy consumption, which varies according to production parameters and depending on the type of energy source used (low-carbon electricity or coal-based electricity).

• Energy consumption:
  • Oven drying: For a batch of 50 kg of powder, a laboratory oven consuming 1000 W [40] was used for 8 h. The total energy consumption for this stage was 8 kWh for 50 kg, or 0.16 kWh per kg of powder.
  • Sieving: Each kilogram of powder was then sieved using a sieve shaker consuming 360 W [41] for 20 min. The energy consumption for this stage is 0.12 kWh per kg.
  • The total energy consumption to prepare fine wash is 0.28 kWh per kg.
• CO₂ emissions: CO₂ emissions vary according to the source of electricity used [42]:
  • With low-carbon electricity (e.g., nuclear in France, 50 g CO₂/kWh), the total carbon footprint is 14 g CO₂ per kg of powder.
  • With coal-based electricity (1000 g CO₂/kWh), the total carbon footprint is 280 g CO₂ per kg of powder.
  • As this study was located in France, it is estimated that 1 kg of fine washing causes an emission equivalent to 14 g of CO₂.

These results cover the energy consumed in drying and screening the waste but do not include the transport of raw materials or finished products. With regard to transport distance, it was assumed that all mortar constituents are located within 30 miles of the manufacturing plant, thus minimizing the carbon impact associated with transport. These assumptions make it possible to standardize the calculation conditions and provide a consistent estimate of the carbon footprint of different mortar formulations.

The results in Figure 5 show a significant reduction in carbon footprint with the gradual incorporation of washing fines. Mortar formulations L10, L20 and L30, with 10%, 20% and 30% cement substitution by washing fines, respectively, show carbon footprint reductions of 9%, 19% and 28% compared to the reference formulation (Ref), whose integrated carbon is 436 kg CO₂/m³. Thus, integrated carbon values decrease to 395 kg CO₂/m³, 355 kg CO₂/m³ and 313 kg CO₂/m³, respectively, for L10, L20 and L30 formulations.

This reduction is due to the lower quantity of cement required, with the cement saved translating into a significant reduction in CO₂ emissions. Consequently, using wash fines as a cement substitute in mortar manufacture is an effective strategy for reducing carbon impact by valorizing an industrial by-product while reducing dependence on Portland cement. This approach not only reduces overall CO₂ emissions but also promotes a more sustainable, environmentally friendly construction industry.

4. Discussion

The incorporation of washing fines into mortars offers significant environmental benefits but also presents technical challenges that need to be analyzed in depth. On the one hand, the results show that the addition of washing fines can considerably reduce the carbon footprint of mortars. In particular, a partial substitution of cement by these fines can reduce CO₂ emissions by up to 28% for a 30% substitution rate, which is in line with the construction industry’s current objectives to minimize the environmental impact of construction materials [43]. This reduction is mainly due to a reduction in the use of cement, an energy-intensive material, which supports a circular economy approach by recovering industrial waste.

However, the incorporation of washing fines has negative effects on certain technical properties of the mortar, in particular, an increase in porosity and a decrease in mechanical and heat resistance. Tests have shown that the higher the rate of substitution of cement by washing fines, the greater the increase in porosity, resulting in a loss of density and a reduction in flexural and compressive strengths. For example, at a 30% substitution rate, compressive strength falls by almost 50% compared to the reference mortar without the addition of fines. This behavior is consistent with other studies on the use of secondary cementitious materials (SCM), which show that increased porosity due to poor compaction of fine particles affects mechanical strength [22,44].

Wash fines, mainly composed of silica and calcium particles, have limited chemical reactivity compared to traditional Portland cement, which reduces their ability to form a dense network of hydration products. In addition, increased porosity promotes water absorption, increasing susceptibility to shrinkage and reducing durability [45]. These results suggest that simply replacing cement with washing fines is not enough to guarantee acceptable mechanical performance in demanding applications. It would therefore be appropriate to explore combinations with other SCMs, such as fly ash or blast furnace slag, to offset these negative effects while maintaining the environmental benefits [46,47].

Finally, although the environmental benefits are undeniable, further research is needed to improve the formulation of mortars incorporating washing fines. Pre-treatment of the fines, such as thermal or chemical activation, could increase their reactivity and thus improve their performance. In addition, the addition of specific additives could improve compaction and limit the increase in porosity. Future studies should also assess the long-term impact of these formulations on the durability of the structures, in particular, their fire resistance, which showed a significant drop at high temperatures in this study.

5. Conclusions

This study on the use of washing fines as supplementary cementing materials (SCMs) in mortars offers interesting prospects for the concrete industry while highlighting some of the challenges ahead.

The incorporation of washing fines as a partial replacement for cement offers significant environmental benefits, including a substantial reduction in the carbon footprint of mortars, up to 28% for a substitution rate of 30%. This approach fits in perfectly with a circular economy approach by transforming an industrial waste product into a recoverable resource.

However, the results also highlight trade-offs to be considered. The increase in porosity and the reduction in mechanical and fire resistance with the increasing incorporation of washing fines raise questions about the limits of use of this material in certain structural applications or those subject to specific constraints.

These observations pave the way for future research aimed at optimizing formulations to strike a balance between environmental performance and technical properties. Avenues to be explored could include combining washing fines with other supplementary cementing materials, optimizing fine pre-treatment processes, or developing methods to improve their reactivity.

In conclusion, this study demonstrates the promising potential of wash fines as an alternative SCM, while highlighting the need for further research to overcome the technical challenges identified. It thus makes a significant contribution to the advancement of knowledge in the field of sustainable building materials and paves the way for future innovations in the concrete industry.