# Effect of raw and calcined sediments on mechanical and rheological behaviour of grout cement paste

Mohamed Afif Zeggar, Nassim Sebaibi, Walid Maherzi, Nor-Edine Edine

Abriak, Mahfoud Benzerzour

► To cite this version:

Mohamed Afif Zeggar, Nassim Sebaibi, Walid Maherzi, Nor-Edine Edine Abriak, Mahfoud Benzerzour. Effect of raw and calcined sediments on mechanical and rheological behaviour of grout cement paste. Journal of Building Engineering, 2026, 121, pp.115594. 10.1016/j.jobe.2026.115594. hal-05532849

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# Effect of raw and calcined sediments on mechanical and rheological behaviour of grout cement paste

Mohamed Afif Zeggar<sup>a,b,c,*</sup>, Nassim Sebaibi<sup>c</sup>, Walid Maherzi<sup>a,b</sup>, Nor-Edine Abriak<sup>a,b</sup>, Mahfoud Benzerzour<sup>a,b</sup>

## ARTICLE INFO

### Keywords:

Raw and calcined sediment

Rheology

Structural build-up

Hydration rate

Grout

## ABSTRACT

Construction consume enormous amounts of materials, the manufacture of which generates significant greenhouse gas emissions. The use of municipal waste from street sweeping as a component of backfill grout offers the potential to reduce both environmental impacts and costs. However, its heterogeneity and organic matter content can negatively affect rheology in the fresh state and hydration. In this study, the effects of raw (S2) and calcined (S1) sediments used as substitutes for sand and partial cement, respectively, on the flow, structure formation and early hydration of compactible grouts were investigated. Class F fly ash (FA) was added to optimise particle settling. A fly ash content of 10% by weight and a water/binder ratio of 0.27 minimised both the static elasticity limit and apparent viscosity through improved settling due to a ball-bearing effect.

Calcined sediments used as partial cement substitutes increased structural strength (Athix) and immediate bearing index (IBI) up to a substitution level of 15%, and then decreased to levels around 40% due to the dilution effect. Isothermal calorimetry, thermogravimetry, and X-ray diffraction indicated increased formation of early ettringite and carboaluminate phases with calcined sediments, as well as a reduction in portlandite content as substitution increased. The results demonstrate a practical window for low-carbon grouts that meet the objectives of fluidity and early bearing capacity.

## 1. Introduction

Today's structures present increasingly complex geometries, resulting in major challenges for concrete construction and placement. Conventional concrete must be vibrated during placement and is typically poured in layers of limited height to prevent horizontal movement over long distances. These constraints have led to the development of highly self-compacting materials. [1]. Indeed, these materials easily adapt to complex frame shapes and can withstand drop heights of up to 5 m as well as horizontal flow distances in the formwork of 5 to 10 m without segregation or loss of homogeneity. Indeed, self-compacting grouts (SCG), which behave similarly to self-compacting concrete (SCC), are designed to be easily placed without vibration, but their composition and application differ [2]. Unlike SCC, whose fluidity is mainly controlled by the binder and superplasticiser, grout has similar rheological characteristics

<footnote><sup>a</sup> IMT Nord Europe, Institut Mine-Télécom, Centre for Materials and Process, F-59000, Lille, France</footnote>

<footnote><sup>b</sup> LGCgE, Laboratoire de Génie Civil et GéoEnvironnement, F-59000, Lille, France</footnote>

<footnote><sup>c</sup> BUILDERS Ecole D'ingénieurs, Research Unity "Builders Lab", Campus Caen, ComUE NU, Epron, France</footnote>

<footnote>* Corresponding author. IMT Nord Europe, Institut Mine-Télécom, Centre for Materials and Process, F-59000, Lille, France.</footnote>

<footnote>E-mail addresses: nassim.sebaibi@builders-ingenieurs.fr (N. Sebaibi), walid.maherzi@imt-nord-europe.fr (W. Maherzi).</footnote>
without the use of chemical additives. The main components are usually sand, fly ash, cement and fibres [3], each component contributing to adequate workability and mechanical performance. This guarantees their workability and ease of flow in trenches during on-site implementation, as well as sufficient bearing capacity after 24 h, characterised by an immediate bearing index (IBI) greater than 10, and a compressive strength between 0.7 and 2 MPa required after 28 days [4].

The incorporation of raw sediments can alter these properties significantly, particularly rheological behaviour [5]. It is therefore necessary to analyse the parameters that determine the grout's performance during placement, such as its viscosity and yield stress [5, 6]. [7].

Consequently, two key rheological parameters must be considered. The static yield stress represents the force required to initiate flow. This threshold is determined by increasing the applied stress until it reaches a value that initiates flow in the material. It reflects the internal friction within the granular structure of the material, such as cement grains and ultrafine particles, on a larger scale. The second parameter is the viscosity, which characterises the material's resistance to flow, the lower the viscosity, the higher the fluidity of the material. It can also be defined as the ratio between shear stress and shear rate, according to Bingham's model [8].

Several methods can be used to characterise the rheological behaviour of grout [9]. The type of rheometer is chosen according to the design of the experiment and the specific observations to be made, such as the effect of particle size distribution or the evaluation of hydration. The evolution of this parameter over time reflects the rate of structuring, mainly influenced by reversible or irreversible intergranular bonds, thus indicating the hydration of the binder [10,11].

Therefore, rheological testing is essential to assess the impact of mineral additives on the fluidity of the grout. [8]. Different studies have shown that the incorporation of certain minerals additions can improve the flow of self-compacting cementitious materials by modifying their particle packing characteristics [12,13]. Class F fly ash, due to its small size and spherical shape, can fill interstitial spaces and increase the density of the material. [14,15]. As a result, the gradual release of water from the granular pores improves the workability of the material and reduces water consumption. In contrast, class C fly ash is characterised by its irregular and porous structure and tends to produce the opposite effect [8].

The use of silica fume can improve workability at low substitution levels; higher substitution levels may have the opposite effect. However, due to their extremely small diameter, their specific surface area increases considerably, resulting in higher water demand [16,17]. Limestone fillers are widely used in self-compacting materials for their filling, nucleation and dilution effects. Their moderate water demand allows adequate flow to be maintained in cementitious materials at moderate dosages without significant impact [18, 19].

In addition, in order to measure their effect on hydration kinetics, different models can also be used to measure their effect on hydration kinetics. [20]. Roussel proposed a model that links rheological parameters to bond formation levels through a linear progression. [10]. Perrot developed a power law that allows the exponential evolution of hydration to be evaluated [21].

The analysis of hydration kinetics is important in the study of cementitious materials. The introduction of untreated sediments containing organic matter can disrupt the cement hydration process, adversely affecting the mechanical properties and, consequently, the hardened phase and subsequent applications of the material [5]. However, this study used thermally treated sediments to optimise the short-term mechanical properties.

However, several additives can accelerate setting of cementitious materials. Calcium chloride and calcium nitrate reduce setting time by accelerating hydrate formation [22,23], but these accelerators may also reduce the available sulphates, which can negatively affect the long-term strength of the material [24]. In addition, the use of binary cements such as calcium aluminate and calcium sulfoaluminate cements in combination with ordinary Portland cement can further promote hardening. Nevertheless, these additional binders can have negative environmental and economic impacts [24,25].

However, several studies have examined the use of recycled materials [26], identifying the optimal quantities needed to improve the properties of self-compacting materials. It has been shown that the incorporation of limestone can improve mechanical properties at an early stage due to the filling effect. This addition can also induce a secondary chemical effect, leading to an increase in hydrate volume. Kadr et al. [25] examined the influence of various mineral sources on the cement hydration process and mechanical strength, highlighting the significant impact of limestone fillers. Rizwan et al. [27] studied various formulations containing limestone and fly ash fillers. The synergy between these two mineral additives improved mechanical strength, enhanced fluidity and significantly reduced water absorption.

Also, Pailyn et al. [28] reported a higher acceleration effect than that of ordinary Portland cement by adding 5 µm limestone particles and fly ash to the systems. Tests also indicated the formation of monocarboaluminates, while ettringite showed increased stability, confirming that fly ash and limestone were actively involved in the chemical reactions. Additionally, Dongbing et al. [29] also analysed the effects of adding limestone and fly ash to cement mixtures on their mechanical properties. The results showed that incorporating 10% limestone into a mixture containing 20% fly ash resulted in a compressive strength of 45 MPa after 28 days, compared to 38 MPa for cement containing 30% fly ash alone. The formation of carboaluminates contributed to a dense and compact microstructure and increased strength, thus explaining the improvement in short-term mechanical properties.

The incorporation of fibres is also a valid option to a certain extent, as several studies have demonstrated their ability to improve the flexural strength and durability of cementitious materials, depending on the type of fibres used. However, it is essential to carefully control the fibre content, an a excessive amounts can adversely affect the fluidity of self-compacting materials, potentially reducing their spreadability. [30]. Other studies have also shown that sand modified with sludge ash and polypropylene fibres exhibits significant improvements in compressive strength. Finally, mixed formulations based on nano-silica and fly ash have been proposed in order to better balance fluidity and stability in the fresh state [31–33].

Other studies have demonstrated the advantages of using other recycled materials such as calcined sediments [34–36], With an optimal calcination temperature, the mechanical properties of cementitious materials can be improved by removing organic matter
that hinders the cement hydration process. Unlike dredged sediments, whose effect on cementitious materials has been widely studied [5,36–46], sweep sediments, due to their specific characteristics, have been the subject of only a few studies devoted to their recovery [47,48]. This is why this study aims to provide greater visibility on the subject.

This study aimed to evaluate the potential for recycling this sediments obtained from street sweeping activities using a two-step protocol. First, the sediments were used in their raw state as sand substitutes after adjusting the particle size by incorporating class F fly ash and their feasibility was studied through a rheological study. Next, the mechanical properties were improved by partially replacing the cement with 15%, 30% and 40% calcined sediments. This study focused on the effect of cement replacement on mechanical characteristics at different stages of hardening. A calorimetric analysis was performed to evaluate the contribution of calcined sediments as a source of CaCO₃. A microstructural characterisation of the samples was carried out using X-ray diffraction (XRD) and thermogravimetric analysis (TGA), as well as SEM analysis.

## 2. Materials and methods

### 2.1. Materials

Three sources of aluminosilicate were used in this study: Class F fly ash (FA), cement (OPC), and calcined sediments (S). The calcined sediments were ground using a Retsch RS200 vibrating disc mill operating at 750 rpm for 1 min and 30 s. This grinding process aimed to increase the number of nucleation sites, thereby enhancing the reactivity and the overall effect of calcination. The calcination temperature, determined from thermogravimetric analysis (TGA), was set at 880 °C for 1 h.

For the sediments collected from a street-sweeping and suction company, two sediments were chosen for the grout composition: a calcined sediment (S1), used as a partial substitute for cement and raw sediment (S2) used as an aggregate.

For the calcined and raw sediments (Figs. 1 and 2), the average particle sizes were 10 µm and 4 mm, respectively, with corresponding densities of 2.625 and 2.72 g/cm³ (Table 1). The oxide compositions of the various minerals were analysed by using X-ray fluorescence (XRF) spectroscopy. The results showed that the FA had a very high SiO₂ content, accounting for nearly half of its composition (51%). For the raw sediment (S2), the SiO₂ content was approximately 52.05%, whereas for the calcined sediment (S1), CaO was the dominant oxide (43.20 %). Similar to OPC, CaO was the most dominant, component for approximately 59%.

According to Table 1, the actual density of fly ash is 2.25 g/cm³ with an average particle size of about 18.1 µm, whereas the cement have a density of about 3.19 g/cm³ and an average particle size of 11.83 µm.

The scanning electron microscopy images (SEM) (Fig. 4) reveal the presence of small elongated particles, identified as organic materials of plant origin, as reported by Hung et al. [42]. In addition, the organic matter present in the sediment exhibited porous structures and irregular shapes, which increased the water demand. Similarly, the particle size distribution (Fig. 3) and the irregular grain morphology hindered the workability of the paste to some extent, thereby influencing the rheological behaviour of the grout paste.

Therefore, it is important to improve the particle size distribution of the raw sediment by adding a mineral component in order to mitigate the granulometric effect, which is detrimental to grout application. Consequently, Class F fly ash was selected for this study due to its fine particle size, spherical morphology, and low water demand, which are known to enhance the packing density and rheological behavior of cementitious systems. The incorporation of fly ash thus represents a pragmatic formulation-based strategy to compensate for the unfavorable characteristics of raw street sweeping sediments. Furthermore, it should be noted that other approaches could also be considered, such as physical pre-treatment methods aimed at reducing organic matter content or improving particle morphology. However, these pre-treatments were not considered in this study, which focuses primarily on assessing the feasibility of valorization through the optimization of mixture design.

Partial decarbonation can also be observed, as shown by the results of XRD and TGA analyses (Figs. 5 and 6). Consequently, calcined sediments should react more readily in the cement–fly ash system, suggesting increased reactivity compared to their raw state.

TGA (Fig. 5) was performed to identify the predominant mineralogical phase in the raw sediment fraction smaller than 500 µm. The

<figure><img src="image_2.png" /><figcaption>Fig. 1. Graphical abstract.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2. Particle size distribution of cement (OPC), calcined sediment (S1), and fly ash (FA).</figcaption></figure>

<table><caption>**Table 1**

Chemical composition (wt.%) and physical characteristics of OPC, S1, S2 and FA.</caption><thead><tr><th>Chemical composition (%)</th><th>calcined sediment (S1)</th><th>OPC</th><th>Fly ash</th><th>Raw sediment (S2)</th></tr></thead><tbody><tr><td>CaO</td><td>43.20%</td><td>59.00%</td><td>3.62%</td><td></td></tr><tr><td>SiO2</td><td>36.01%</td><td>17.38%</td><td>42.77%</td><td>29.28%</td></tr><tr><td>Fe2O3</td><td>7.29%</td><td>2.84%</td><td>17.44%</td><td>52.05%</td></tr><tr><td>MgO</td><td>4.40%</td><td>0.85%</td><td>2.50%</td><td>5.74%</td></tr><tr><td>Al2O3</td><td>4.42%</td><td>4.87%</td><td>19.47%</td><td>3.40%</td></tr><tr><td>K2O</td><td>1.65%</td><td>1.00%</td><td>8.70%</td><td>5.71%</td></tr><tr><td>SO3</td><td>1.38%</td><td>4.35%</td><td>1.64%</td><td>1.70%</td></tr><tr><td>TiO2</td><td>0.74%</td><td>0.39%</td><td>2.44%</td><td>0.92%</td></tr><tr><td>Cl</td><td>0.24%</td><td>-</td><td>0.36%</td><td>0.74%</td></tr><tr><td>MnO</td><td>0.14%</td><td>-</td><td>0.24%</td><td>0.08%</td></tr><tr><td>Cr2O3</td><td>0.10%</td><td>-</td><td>0.09%</td><td>0.10%</td></tr><tr><td>SrO</td><td>0.09%</td><td>-</td><td>0.10%</td><td>-</td></tr><tr><td>ZnO</td><td>0.08%</td><td>-</td><td>0.06%</td><td>-</td></tr><tr><td><strong>Physical characteristics</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Density (g/cm³)</td><td>2.62</td><td>3.19</td><td>2.25</td><td>2.72</td></tr><tr><td>BET Area(m2/g)</td><td>1.87</td><td>1.48</td><td>2.60</td><td>1.38</td></tr><tr><td>D50(µm)</td><td>10</td><td>11.83</td><td>18.1</td><td>203.263</td></tr></tbody></table>

heating rate was set at 3 °C/min under an inert argon gas flowing at 75 mL/min. The analysis revealed, a weight loss around 785 °C and 850 °C, primarily attributed to the decarbonation of calcite (Fig. 5).

## 2.2. Jet grouting preparation

The grout was prepared following a well-defined mixing protocol to ensure homogeneity and repeatability (Fig. 7). First, the fly ash and sediment were mixed by adding one-third of the total water. Subsequently, fibres were added and the friction between aggregates facilitated their uniform dispersion.
<figure><img src="image_4.png" /><figcaption>Fig. 3. Particle size distribution of a raw sediment (S2).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Scanning Electron Microscopy observation of raw sediment below 500 µm (S2).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. XRD characterization of the materials: a) raw sediment, b) calcined sediment.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6. Thermogravimetric analysis of raw sediment (S2) and calcined sediment (S1).</figcaption></figure>

This mixture was then followed to rest for 10 min to enables water absorption and particle saturation. In the third step, cement and the remaining water were added to produce a grout that can then be characterised in the fresh state, with the objective of obtaining a material that meets performance standards in both fresh and hardened conditions (Table 2).

## 2.3. Mechanical characteristics

Unconfined compression strength (UCS) tests were conducted on 4 × 4 × 16 cm³ samples in accordance with the NF EN 196-1 standard, using an Instron 5500R mechanical press with a capacity of 150 kN. The purpose of these tests was to assess the evolution of the samples mechanical performance over time. specimens were removed from the standard curing room and tested for compressive strength after 28 and 60 days of curing. All tests were performed in compliance with current standards, and the strength values represent the average of at least three reproducible specimens.

Notably, results with average standard deviations greater than 10% were excluded, as the testing methods employed aimed to provide precise and reliable evaluations of the mechanical properties. This approach facilitated the selection of the most appropriate formulation to ensure optimal mechanical performance.

## 2.4. Mineralogical and microstructural characterisation

### 2.4.1. Differential thermogravimetric analysis (ATG)

Differential thermogravimetric analysis (DTG) curves were obtained from TGA data to quantity the amount of hydration products. The samples were first ground in a fine powder and then subjected to a controlled temperature increase up to 1000 °C at a heating rate of 3 °C/min using a thermal analyser. The corresponding mass loss of the samples was continuously recorded at 0.1 s intervals.

### 2.4.2. X-ray diffraction (XRD) and EDS

The hydration products were identified using XRD analysis. After being ground to a fine powder with an adequate particle size, the samples were examined with an X-ray diffractometer equipped with a CuKα radiation source. The scans were performed over a 2θ angles of 5°–80°. This approach provided detailed information on the mineralogical composition, which is crucial for understanding their characteristics and reactivity.

Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was used to characterise the hydration products in the grout samples. The analyses were performed using a Schottky field emission electron microscope (model S-4300Se/N). After fragmentation, the samples were coated with a thin carbon layer to ensure conductivity. Semi-quantitative chemical analyses were as then performed using EDS to characterise the composition of the hydrated phases.

## 2.5. Granulometry

The particle size distributions of all the minerals components were evaluated by laser granulometry in accordance with ISO 13320:2020. The instrument measures the diffraction of a laser beam by suspended particles, enabling the determination of particle sizes within a range of 0.04–2000 µm.

## 2.6. Rheometer

A modular compact rheometer (MCR 102, Anton Paar) equipped with a vane geometry was used for the rheological measurements. Each sample was placed in a cylindrical cup, and a pre-shearing step was applied to eliminate the stress history of each formulation, ensuring optimal comparability and repeatability. The sample was then subjected to an increasing shear rate ramp, ranging from 0 s⁻¹ to 0.2 s⁻¹, over a period of 200 s. The rheometer measured both the shear stress curve and apparent viscosity. The experiment was paused after each measurement to allow the material to reach equilibrium, with a resting period of 10 min between successive measurements over a total testing duration of 60 min.

## 2.7. Method experimental design phase 1 and phase 2

In this study, raw sediment (S2) was used as an aggregate, while calcined sediment (S1) was employed as a partial cement substitute. The objective of the first phase is to quantify the influence of particle size distribution on the rheology of self-compacting grout (Table 3), with a specific focus on the respective roles of raw and calcined street-sweeping sediments under confined conditions where the grout must flow through narrow pathways. To ensure unobstructed flow, particles larger than 4 mm were excluded from the grout
<figure><img src="image_8.png" /><figcaption>Fig. 7. Mixing process.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. (continued).</figcaption></figure>

<table><caption>**Table 2**

Required properties of a self-compacting material.</caption><thead><tr><td>Aspect of behavior considered</td><td>Mechanical characterization</td></tr></thead><tbody><tr><td>Immediate bearing index</td><td>IBI &gt; 10</td></tr><tr><td>Compressive strength</td><td>0,5&lt;Rc &lt; 1 MPA</td></tr><tr><td>Slump-fow</td><td>h &gt; 15 mm</td></tr><tr><td>Water-immersion resistance</td><td>RcI/Rc60 &gt; 0,8</td></tr></tbody></table>

preparation. For rheological investigations a finer fraction (S2) under 500 µm was selected from the fraction 4 mm to characterise paste behaviour to ensure representative paste behaviour, particularly static yield stress and apparent viscosity, which govern flow without vibration. The study also aims to identify the optimal combination of FA dosage and water-to-solid ratio corresponding as the ratio of water mass to the sum of fly ash and raw sediment mass. At appropriate levels, FA can enhance workability through packing and ball-bearing effects, however, excessive fine content can densify the matrix and increase resistance to flow. Ordinary Portland cement (OPC), Class-F FA, raw sediment (<500 µm), and calcined sediment (880 °C, 1 h) were used in this study. In Phase 1, the FA (0, 10, 20 %) and ratio (0.25, 0.27, 0.29) were varied in raw-sediment pastes to identify the rheological optimum. In Phase 2, the FA content was fixed at 10 wt% while calcined sediment replaced cement at 0, 15, 30, and 40 wt% with W/S = 0.27–0.29. Mixing followed the standard protocol shown in Fig. 6. A vane-in-cup rheometer (MCR-102) was employed to measure static yield stress and apparent viscosity under a controlled stress ramp with 10-min rest intervals over a total 60 min. Structural build-up (Athix) was quantified from the slope of τ₀(t) during rest periods. Early hydration was monitored by isothermal calorimetry, while TGA/DTG analyses quantified mass losses associated with C–S–H and AFt phases (≤200 °C), portlandite (400–500 °C) and calcite (600–800 °C). XRD was identified crystalline hydrates and AFm phases at 24h and 28 days, SEM/EDS analyses were conducted to examine particle morphology and
<table><caption>**Table 3**
Mix design of grout's paste for the rheology study phase 1.</caption><thead><tr><th colspan="5">Content formulation by wt(%)</th></tr><tr><th>Formulation</th><th>Raw Sediment (S2)</th><th>Fly ash</th><th>OPC</th><th>W/S</th></tr></thead><tbody><tr><td>A1</td><td>100%</td><td>-</td><td>-</td><td>0.25</td></tr><tr><td>B1</td><td>90%</td><td>10%</td><td>-</td><td>0.25</td></tr><tr><td>C1</td><td>80%</td><td>20%</td><td>-</td><td>0.25</td></tr><tr><td>A2</td><td>100%</td><td>-</td><td>-</td><td>0.27</td></tr><tr><td>B2</td><td>90%</td><td>10%</td><td>-</td><td>0.27</td></tr><tr><td>C2</td><td>80%</td><td>20%</td><td>-</td><td>0.27</td></tr><tr><td>A3</td><td>100%</td><td>-</td><td>-</td><td>0.29</td></tr><tr><td>B3</td><td>90%</td><td>10%</td><td>-</td><td>0.29</td></tr><tr><td>C3</td><td>80%</td><td>20%</td><td>-</td><td>0.29</td></tr><tr><td>A1*</td><td>100%</td><td>-</td><td>7%</td><td>0.25</td></tr><tr><td>B1*</td><td>90%</td><td>10%</td><td>7%</td><td>0.25</td></tr><tr><td>C1*</td><td>80%</td><td>20%</td><td>7%</td><td>0.25</td></tr><tr><td>A2*</td><td>100%</td><td>-</td><td>7%</td><td>0.27</td></tr><tr><td>B2*</td><td>90%</td><td>10%</td><td>7%</td><td>0.27</td></tr><tr><td>C2*</td><td>80%</td><td>20%</td><td>7%</td><td>0.27</td></tr><tr><td>A3*</td><td>100%</td><td>-</td><td>7%</td><td>0.29</td></tr><tr><td>B3*</td><td>90%</td><td>10%</td><td>7%</td><td>0.29</td></tr><tr><td>C3*</td><td>80%</td><td>20%</td><td>7%</td><td>0.29</td></tr></tbody></table>

Note: The water-to-binder ratio (W/S) was calculated as the ratio of water mass to the sum of fly ash and raw sediment mass.

partial FA dissolution. Mechanical performance was evaluated through the immediate bearing index (IBI, 24 h) and compressive strengths at 28 and 60 days on 4 × 4 × 16 cm.

The second phase of the investigation will focus on assessing the effects of raw and calcined sediments on cement hydration using a predefined perrot model according to the following equation:

$$ \tau_0(t) = A_{thix} t_c (e^{t_{rest}} - 1) + \tau_{0,0} \quad (2) $$

where $A_{thix}$ is structural build up, $t_c$ is a characteristic time constant, $t_{rest}$ is resting time and $\tau_{0,0}$ is initial yield stress

The objective is to develop a methodology that enhance both the flow characteristics of the grout and its early age mechanical properties (IBI), thereby optimising the material quantitie for improved performance. (Table 4). This sequential multi-criteria approach provides a transparent rationale for selecting the investigated mixtures and for identifying the optimal formulation for the targeted application. However, other methods exist to optimise the experimental approach, which allows trials to be structured and reduces the number of experiments required [49].

## 3. Results and discussion

### 3.1. Effect of particle size

Pastes made from raw sediments with a water content between 0.27 (A2) and 0.29 (A3) showed a constant yield stress throughout the test (Fig. 8). This behaviour can be attributed to the lubricating effect of water at the intergranular level. Moreover, the amount of added water was sufficient to satisfy the sediment's water demand.

The sediment pastes with a water content of 0.25 (A1) showed a gradual increase in the yield stress, reflecting the progressive water absorption process. Upon the addition of Class F fly ash (B1, B2, and B3), the initial $\tau_0$ values for each sample was lower than that of the

<table><caption>**Table 4**
Mix proportioning of grout's paste for the rheology study phase 2.</caption><thead><tr><th colspan="5">Content formulation by wt(%)</th><th>W/S</th></tr><tr><th>Mix</th><th>Raw Sediment (S2)</th><th>Fly ash</th><th>Calcined sediment (S1)</th><th>OPC</th><th></th></tr></thead><tbody><tr><td>A2*</td><td>100%</td><td>-</td><td>-</td><td>7%</td><td>0.25</td></tr><tr><td>B2*</td><td>90%</td><td>10%</td><td>-</td><td>7%</td><td>0.25</td></tr><tr><td>B2*15%S1</td><td>90%</td><td>10%</td><td>1%</td><td>6%</td><td>0.25</td></tr><tr><td>B2*30%S1</td><td>90%</td><td>10%</td><td>2.1%</td><td>4.9%</td><td>0.27</td></tr><tr><td>B2*40%S1</td><td>90%</td><td>10%</td><td>2.8%</td><td>4.2%</td><td>0.27</td></tr><tr><td>A3*</td><td>80%</td><td>-</td><td>-</td><td>7%</td><td>0.27</td></tr><tr><td>B3*</td><td>100%</td><td>10%</td><td>-</td><td>7%</td><td>0.29</td></tr><tr><td>B3*15%S1</td><td>90%</td><td>10%</td><td>1%</td><td>6%</td><td>0.29</td></tr><tr><td>B3*30%S1</td><td>90%</td><td>10%</td><td>2.1%</td><td>4.9%</td><td>0.29</td></tr><tr><td>B3*40%S1</td><td>90%</td><td>10%</td><td>2.8%</td><td>4.2%</td><td>0.29</td></tr></tbody></table>

Note: The water-to-binder ratio (W/S) was calculated as the ratio of water mass to the sum of fly ash and raw sediment mass.
<figure><img src="image_10.png" /><figcaption>Fig. 8. Flow curve for granular mixtures of raw sediment.</figcaption></figure>

corresponding reference pastes mixes (A1, A2, and A3). This observation suggests that the rheological properties of fly ash-sediment is established during the mixing phase. Several studies have shown that the rheological characteristics are influenced by the incorporation of fly ash. Vance et al. [19] demonstrated that the addition of Class F fly ash improves fluidity through the filler effect. The water retained in the flocs of raw sediment particles decreased, making more water available to improve the fluidity of the paste.

The fine particles of fly ash contributed to the overall organisation of the matrix by filling the voids between sediment particles, thereby increasing the compactness of the mixture. As a result, less water was trapped within these voids, leaving more free water available to enhance the fluidity of the paste [50].

The addition of fly ash led to changes the particle size distribution, generated new nucleation sites, and enhanced the surface activity of the grains, resulting in improved workability. The smooth and rounded surface of fly ash particles significantly reduced interparticle friction leading to lower rheological parameters in the samples containing fly ash [51].

At higher fly ash dosages (C1), the excess of fine particles led to a compact matrix. Restricting particle mobility and consequently increasing both viscosity and static yield stress. Several studies have shown that fly ash content can hinder fluidity [52,53]. However, at water content of 0.27 and 0.29, this trend was reversed with a noticeable decrease in the yield stress. At a 0.29 water content, the rheological parameters were similar to those observed for mixtures containing 10% (B3) and 20% (C3) FA.

Based on these results, the optimum combination selected for Phase 2 was FA = 10 wt% and W/S = 0.27, respectively. Based on the rheological optimization obtained in Phase 1, Phase 2 focused on the effect of calcined sediments on hydration kinetics and early strength.

## 3.2. Study of binder kinetics (phase 2)

### 3.2.1. Structural build-up of pure cement paste

To investigate the effect of sediments on cement hydration, the same formulations used to study the influence of particle size distribution on grout rheology were applied to assess cement hydration in the presence of raw sediments and fly ash. Accordingly, the cement dosage was kept constant at 7% (Fig. 9).

The yield stress (τ0) of the reference sample (A1*, A2*, and A3*) showed a stable linear growth pattern, attributed to the local formation of a C-S-H structure by C3S (tricalcium silicate) through pseudo-contact points within the particle network. Cement
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Evolution of static yield stress of a grout past at different resting time. **Note:** (C1*C2*C3* 80%S2;20%FA;7%OPC with 0.27 0.27 0.29 W/S), (B1*B2*B3* 90%S2;10%FA;7%OPC with 0.27 0.27 0.29 W/S), (A1*A2*A3* 100%S2;0%FA;7%OPC with 0.27 0.27 0.29 W/S).</figcaption></figure>

hydration produces reaction products such as C-S-H and calcium hydroxide (CH), which progressively strengthen the interparticle bonds leading to a denser and more rigid microstructure.

These observations confirm that the effect of cement hydration can be observed through raw sediment particles, which is important for the continuation of our study.

In general, a reduction in the water-to-cement ratio (W/C) leads to an increase in yield stress [48, 49]. This makes the paste more thixotropic as the closer cement particles enhances interparticle interactions and accelerates structural build-up. A low W/C ratio can therefore promote the rapid formation of hydration bonds between cement particles, contributing to a quicker structural recovery after shear stress (A2* with Athix 4.29 Pa/min and A3* with Athix 3.83 Pa/min) [54]. However, an excessive reduction in the W/C ratio,

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Correlation between rheological parameters and empirical measurements.</figcaption></figure>
without considering the water demand of the sediment, hinder cement hydration (A1* with Athix 2.05 Pa/min).

Conversely, increasing the W/C ratio reduced the concentration of cement particles in the paste in the paste, thereby reducing both the initial viscosity and thixotropy [5]. A high W/C ratio indicates more free water within the system; which weakens interparticle interactions and can delay the formation of a continuous hydrated network (A3*), ultimately resulting in slower structural recovery of the paste after the application of shear stress.

The incorporation of 10% FA effectively improved the accessibility of residual anhydrous cement particles to water, leading to an increase in the degree of structuring with increasing water content.

However, when the FA content exceeded 10%, hydration was delayed. This behaviour may be attributed to the altered hydration kinetics of the cement incorporating fly ash as. Ca²⁺ ions in the pore solution tend to be adsorbed onto the fly ash surface due to its inherently low Ca/Si ratio. This extended the time required for the Ca²⁺ ions to reach supersaturation, thereby hindering the nucleation and crystallisation of C-S-H. As a result, a gradual increase in the microscopic rigidity was observed in the samples containing fly ash [55].

### 3.2.2. Spread measurement, flow time, and entrapped air

According to the results presented in Fig. 10, the rheological measurements showed strong agreement with the empirical observations. A reduction in the spread was accompanied by a decrease in static yield stress, indicating improved workability. This improvement is attributed to the spherical morphology and the filler effect of FA which reduce intergranular friction, limit the amount of water trapped within interparticle pores and providing additional water [12]. This improvement resulted in a decrease in the apparent viscosity. However, beyond a critical dosage, the addition of fly ash increased the rigidity of the paste, resulting in higher rheological parameters [10,46]. The reduction in entrapped air was associated to the increased compactness of the granular matrix, caused by the excessive addition of fly ash (Fig. 10).

Moreover, the incorporation of fly ash improves the mechanical properties of the material in both the short and long term (Table 5), which helps to avoid increasing the cement content and thus reduces the overall cost of the formulation. However, these mechanical properties can be further enhanced by partially substituting cement with calcined sediments rich in calcite.

Also, sediments are rich in silica, which can help improve matrix density. Studies have already shown that nanosilica can densify the cement matrix, increase paste rigidity and promote hydrate nucleation, which could improve early-age mechanical strength and limit the propagation of a critical network of fissures [56,57].

The presence of interstitial water also plays a significant role in the mechanical response under load. In saturated material, water can delay the formation of cracks through a local damping effect. However, under cyclic loading, excess water can reduce mechanical strength and accelerate crack accumulation. Optimising the water content is therefore essential for this type of application [49,58].

## 3.3. Microstructural properties of formulations

Partially dissolved fly ash particles with needle-shaped phases rich in aluminium and silicon, were observed on their surfaces (Fig. 11). These phases are likely associated with mullite crystals, in which iron may substitute for aluminium within the crystal structure.

The presence of mullite containing iron substitutions indicates a modification of the crystalline structure, which affected the properties of the material. The substitution of aluminium by iron in mullite phase can affect the reactivity of fly ash when used as an additive in cementitious applications, potentially enhancing the mechanical strength or altering hydration kinetics.

The partial dissolution of these particles further suggests active reaction with the surrounding medium, likely as part of the hydration process.

## 3.4. Structural build-up of fresh cement pastes with calcined sediments (phase 2)

In this part of the study, formulations containing 10% fly ash (B2* and B3*) were selected for water content of 0.27 and 0.29 (Fig. 12). These mixtures were subsequently used to investigate the effects of incorporating calcined sediment.

During cement hydration, calcined sediment was incorporated at substitution levels of 15%, 30%, and 40%. The evolution of the Athix for different formulations was predicted using the previously applied Perrot model. With an increasing resting time, it was possible to identify the optimal calcined sediment content corresponding to the maximum structuration rate. below 15%.

For formulations with 29% water, the incorporation of 15% calcined sediment promoted the Athix structuring rate, reaching a value of 19.37 Pa/min. However, this rate decreased to 11.44 Pa/min when the calcined sediment content reached 40% by weight

<table><caption>**Table 5**

Mechanical properties of grouts.</caption><thead><tr><th>Mechanical characterisation</th><th>(A2*)</th><th>(B2*)</th><th>(C2*)</th></tr></thead><tbody><tr><td>IBI</td><td>4.58</td><td>10.52</td><td>13.62</td></tr><tr><td>R<sub>c28d</sub> (MPa)</td><td>2.01</td><td>2.11</td><td>4.22</td></tr><tr><td>R<sub>c60d</sub> (MPa)</td><td>3.13</td><td>3.22</td><td>6.21</td></tr><tr><td>Imrg 32<sub>d</sub> (MPa)</td><td>3.09</td><td>3.14</td><td>6.01</td></tr><tr><td>Rci/Rc60</td><td>0.99</td><td>0.98</td><td>0.97</td></tr></tbody></table>
<figure><img src="image_13.png" /><figcaption>Fig. 11. SEM observation of grouted mortars.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 12. Athix measurement for different formulations at different water contents.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 13. Effect of calcined sediment content on heat flow and hydration heat of cement.</figcaption></figure>
(Fig. 12). These observations indicate that the increase in Athix is not entirely due to the contribution of calcined sediments to hydration and structuring. This suggests that, in addition to hydration, other factors influence the structuring of cement paste, such as the dilution effect, which enhances particle interactions. This is evident in the formulations with 0.27 water, where the addition of calcined sediments increased the friction between the grains. The amount of water added was insufficient to minimise the particle size effect.

### 3.4.1. Hydration heat of cement with calcined sediments

The results of the heat of hydration analysis in the presence of calcined sediments revealed their significant influence on the formation of the cement paste microstructure during the early stage of hardening (Fig. 13). Isothermal calorimetry measurements revealed the impact of adding calcined sediments on the thermal flux generated during cement hydration. An addition of 15% calcined sediments stimulated the cement hydration process at an early stage by promoting the microcrystalline nucleation of hydration products such as calcium silicate hydrate [55]. However, when the sediment content increased to 30 wt%, this beneficial effect diminished to the dilution effect. At a 40 wt% substitution level, the total heat of hydration was lower than that of the reference cement paste.

### 3.4.2. Measuring mechanical properties

A relationship was observed between the degree of structuring of the various formulations in the fresh state and their immediate bearing index (IBI) values (Figs. 14 and 15).

This observation implies that IBI values are partly influenced by the interaction between the binder (composed of cement and calcined sediment) and filling effect, which improves the density of the granular mixture. Increasing the water/sediment (W/S) ratio reduced the impact of the particle size distribution (friction between particles), thereby highlighting the influence of the binder. The B2*15%S1 and B2*30%S1 formulations demonstrated a higher level of structuring than those without the calcined sediment. However, for the B2*40%S1 formulation, the particle size effect strongly influenced the thixotropy. Increasing the W/S ratio caused a reduction in thixotropy, which differed from the behaviour observed in the B2*15%S1 and B2*30%S1 formulations.

Table 6 shows that the formulations with 15% (B2*15%S1) and 30% (B2*30%S1) cement replacement by calcined sediments exhibited higher IBI values (19 and 15, respectively) compared to the control formulation B2* (10.52). This indicates better cohesion and an enhanced binder hydration resulting from the incorporation of calcined sediment. However, when the replacement level reached 40% (B2*40%S1, IBI = 10.03), the chemical effect provided by the calcined sediment diminished, suggesting the existence of a threshold beyond which additional calcined sediments no longer improve mechanical performance. Various TGA curves were obtained for the different formulations containing varying proportions of calcined sediment and fly ash (Fig. 16).

A clearly observable peak above 100 °C, partially attributable to the decomposition of ettringite (AFt), was more pronounced in the presence of calcined sediment. This indicates that addition of calcined sediments enhances ettringite formation. However, B2* formulation, a shoulder was observed around 170 °C. This phenomenon was caused by the addition of fly ash helped fill the inter-granular voids, preventing water from being trapped in these pores. Consequently, the availability of water increased, promoting more efficient cement hydration and increasing the formation of hydration products. This observation was absent in formulations based solely on calcined sediments, as the reduction in the clinker-containing cement content led to a decrease in the formation of calcium silicate hydrate (C-S-H).

A mass loss is observed around 400 °C due to the formation of portlandite, and another loss was observed around 840 °C due to the calcite present in the raw and calcined sediments. The mass loss between 400 °C and 840 °C is more pronounced for certain compositions, such as B2*30%S1 and B2*15%S1. This implies that the addition of fly ash and calcined sediments alters the formation process of hydration products and the proportion of phases such as portlandite and calcite.

Based on the IBI and TGA results, it can be concluded that the increase in early strength (IBI) resulting from the addition of the calcined sediment can only be partially explained to improved hydration. Although the B2* formulation exhibited more hydration product formation than the B2*40%S1 formulation, their IBI values were similar. To fully explain this phenomenon, the physical action (filler effect) of the calcined sediment must also be considered. The B2*30%S1 and B2*15%S1 formulations exhibited more pronounced formations of C-S-H and ettringite, which likely accounted for the improvement in their IBI values. Fig. 17 presents XRD spectra of various formulations, including a reference sample made of pure cement and formulations in which cement has been partially replaced by 15%, and 40% calcined sediment (85C15%S1, 70%OPC30%S1, and 60%OPC40%S1, respectively.

Analyses (Fig. 17) reveal that, the portlandite (CH) peaks progressively decrease with increasing calcined sediment content, indicating a reduced availability of clinker for hydration.; This reduction is particularly pronounced at 28 days for the mixture containing 40% sediment. On the other hand, ettringite forms as early as 24 h in all formulations. At 28 days, it is still present in those containing calcined sediment, to the simultaneous presence of carbonated AFm phases notably monocarboaluminate which contribute to its stabilization. Finally, these same carbonated AFm phases appear early, from 24 h, in formulations rich in calcite, and then evolve into more stable forms, mainly monocarbonate, by 28 days due to the excess $CaCO_3$ introduced by the calcined sediment [59,60].

The TGA (Fig. 18). Performed at 28 days on four grout binder formulations highlights the effect of progressively substituting cement with calcined sediment rich in $Ca^{2+}$ and calcite on the hydration products.

Three main regions of mass loss were identified, each hydration phase. The first region, between 50 and 200 °C, is associated with the dehydration of hydration products such as C-S-H gel and hydrated sulfoaluminate phases, notably ettringite. A distinct shoulder between 190 and 200 °C was also observed, corresponding to hydrated monocarbonate particularly pronounced in the 85C15%S1 formulation indicating a higher quantity of these hydrated phases, likely due to the additional aluminat and calcite supplied by the sediment [61].
<figure><img src="image_16.png" /><figcaption>Fig. 14. IBI value as a function of calcined sediment dosage.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 15. Relationship between IBI values and Athix.</figcaption></figure>

<table><caption>**Table 6**

Mechanical properties of grouts.</caption><thead><tr><th></th><th>B2*</th><th>B2*15%S1</th><th>B2*30%S1</th><th>B2*40%S1</th></tr></thead><tbody><tr><td>IBI</td><td>10.52</td><td>19</td><td>15</td><td>10.03</td></tr><tr><td>R<sub>c28d</sub> (MPa)</td><td>2.11</td><td>2.01</td><td>1.87</td><td>0.98</td></tr><tr><td>R<sub>c 60d</sub> (MPa)</td><td>3.23</td><td>3.13</td><td>2.98</td><td>1.87</td></tr><tr><td>Imrg 32<sub>d</sub> (MPa)</td><td>3.14</td><td>3.01</td><td>2.83</td><td>1.57</td></tr><tr><td>R<sub>ci/Rc60</sub></td><td>0.97</td><td>0.96</td><td>0.95</td><td>0.84</td></tr></tbody></table>

Note: (B2* = 90%S2 + 10%FA with 7%OPC and 0.27 W/S), (B2*15%S1, B2*30%S1, B2*40%S1 = B*2 with 1% S1 and 6% OPC; 2.1% S1 and 4.9% OPC; 2.8% S1 and 4.2% OPC.

The mass loss observed between 400 and 500 °C is attributed to portlandite (Ca(OH)<sub>2</sub>), a direct product of clinker hydration, whose presence is predominant in the OPC paste. However, its intensity gradually decreases as the substitution rate increases.

Finally, the last mass-loss region, between 600 and 800 °C, corresponds to the decarbonation of calcite. The mass loss in this range increases markedly with the content of calcined sediment. This trend confirms that calcined sediment S1 is naturally rich in carbonate. Such behavior is typical of formulations incorporating mineral additions high in CaCO<sub>3</sub>.

In summary, substituting cement with calcined sediment alters the thermal profile of the binders: it promotes active hydration through the formation of hydrated sulfate phases, leads to a progressive reduction in portlandite, and results in a significant increase in mineral derived carbonates. These findings confirm the structural impact of calcined sediment on the matrix, underscoring its chemical compatibility with cement.
<figure><img src="image_18.png" /><figcaption>Fig. 16. TGA patterns of the blended cement pastes for 24h</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 17. X-ray diffraction patterns of the blended cement pastes for 24h (a) and 28 days (b).</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Fig. 18. ATG measurement of blended cement pastes with calcined sediment after 28 days.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>Fig. 19. ATG measurement of blended cement pastes with calcined sediment after 28 days.</figcaption></figure>

### 3.5. Scanning electron microscopy (SEM)

SEM observations provide additional microstructural evidence confirming the identification of 15% substitution in calcined sed- iments as the optimal rate. As illustrated in Fig. 19 (Fig. 19a), the microstructure appears relatively dense and homogeneous, with a uniform distribution of hydration products forming a compact matrix. This microstructural organisation suggests that at this substitution rate, the calcined sediments mainly act as nucleation sites, promoting hydration and densification of the matrix.

Conversely, the microstructure corresponding to a substitution rate of 40% (Fig. 19b) is significantly more heterogeneous and porous. The matrix is less compact, with weaker interconnections between hydration products and a higher proportion of unreacted or weakly bound particles. The discontinuity of the hydrated phases reflects the predominance of the dilution effect, limiting the supply of reactive cementitious phases and leading to a less cohesive microstructure. The increased presence of poorly bonded areas and microcracks is also observed, which is consistent with the degradation of mechanical performance measured at this high substitution rate.

### 3.6. Leaching measurement

The table below (Table 7) shows how pollutant concentrations vary depending on the different mixtures and in recycled materials (raw and calcined sediments). Several significant trends can be observed.

For heavy metals such as arsenic (As), cadmium (Cd), lead (Pb), antimony (Sb), selenium (Se) and zinc (Zn), concentrations remain low in all cases, regardless of the mixture. This low mobilisation of heavy metals is beneficial to the environment.

However, for barium (Ba), chromium (Cr), copper (Cu) and nickel (Ni), the trend is quite different. Barium has its lowest concentration in the reference mixture (1.44 mg/L), with a marked increase at 20% replacement with calcined sediments (7 mg/L), followed by a decrease at 40% (3.92 mg/L). This is explained by a greater release of barium, which becomes more soluble after the calcination of sediments. Chromium is not detected in the mixture with 20% replacement, but reappears at 40% (0.42 mg/L).

Finally, the results indicate that the use of these sediments in this formulations is from an environmental point of view, feasible. The formulations meet the criteria for non-hazardous waste, which corresponds to the acceptable level for standard fillings.

Table 8 presents an environmental analysis of the reference formulation B2\* and three mixtures incorporating calcined sediments (15%, 30% and 40%). For the calculations, raw sediments were considered equivalent to river sand, while calcined sediments were approximated as metakaolin, a representative calcined aluminosilicate material.

<table><caption>**Table 7**
Leaching test results.</caption><thead><tr><th>Metals mg/kg dry</th><th>S1</th><th>S2</th><th>B2*15SedC</th><th>B2*40SedC</th><th>B2*</th><th>Inert</th><th>Non hazardous</th><th>Hazardous</th></tr></thead><tbody><tr><td>As</td><td>&lt;0.1</td><td>&lt;0.1</td><td>&lt;0.1</td><td>&lt;0.1</td><td>&lt;0.1</td><td>25</td><td>2</td><td>25</td></tr><tr><td>Ba</td><td>9.5</td><td>0.099</td><td>7</td><td>3.92</td><td>1.44</td><td>300</td><td>100</td><td>300</td></tr><tr><td>Cd</td><td>&lt;0.009</td><td>&lt;0.009</td><td>&lt;0.009</td><td>&lt;0.009</td><td>&lt;0.01</td><td>5</td><td>1</td><td>5</td></tr><tr><td>Cr</td><td>0.59</td><td>0.008</td><td>0</td><td>0.42</td><td>0.42</td><td>70</td><td>10</td><td>70</td></tr><tr><td>Cu</td><td>0.02</td><td>0.783</td><td>2</td><td>1.77</td><td>1.4</td><td>100</td><td>50</td><td>100</td></tr><tr><td>Mo</td><td>0.53</td><td>&lt;0.09</td><td>0</td><td>0.48</td><td>0.43</td><td>30</td><td>10</td><td>30</td></tr><tr><td>Ni</td><td>&lt;0.05</td><td>0.049</td><td>1</td><td>0.38</td><td>0.44</td><td>40</td><td>10</td><td>40</td></tr><tr><td>Pb</td><td>&lt;0.03</td><td>&lt;0.03</td><td>&lt;0.03</td><td>&lt;0.03</td><td>0.03</td><td>50</td><td>10</td><td>50</td></tr><tr><td>Sb</td><td>0.06</td><td>&lt;0.06</td><td>&lt;0.06</td><td>&lt;0.06</td><td>0.06</td><td>5</td><td>0.7</td><td>5</td></tr><tr><td>Se</td><td>&lt;0.08</td><td>&lt;0.08</td><td>&lt;0.08</td><td>&lt;0.08</td><td>0.08</td><td>7</td><td>0.5</td><td>7</td></tr><tr><td>Zn</td><td>0.01</td><td>0.024</td><td>&lt;0.02</td><td>&lt;0.01</td><td>&lt;0.01</td><td>200</td><td>50</td><td>200</td></tr><tr><td>Fluorures</td><td>19</td><td>16</td><td>39</td><td>38</td><td>41</td><td>10</td><td>150</td><td>500</td></tr><tr><td>Sulfates</td><td>104</td><td>335</td><td>52</td><td>51</td><td>50</td><td>100</td><td>20 000</td><td>50 000</td></tr><tr><td>Chlorures</td><td>17</td><td>76</td><td>49</td><td>88</td><td>42</td><td>800</td><td>15 000</td><td>25 000</td></tr></tbody></table>
<table><caption>**Table 8**
Material impacts on carbon emissions.</caption><thead><tr><td>Formulation</td><td>IPI = 10 obtained (h)</td><td>CO<sub>2</sub> (kg/m<sup>3</sup>)</td><td>Resources (kg/m<sup>3</sup>)</td></tr></thead><tbody><tr><td>B2*15S1</td><td>13h</td><td>145.41</td><td>2478.18</td></tr><tr><td>B2*30S1</td><td>17h</td><td>134.97</td><td>2460.46</td></tr><tr><td>B*2</td><td>24h</td><td>123.12</td><td>2433.53</td></tr><tr><td>B2*40S1</td><td>24h</td><td>115.83</td><td>2421.17</td></tr></tbody></table>

### 3.7. Carbon input for different mixing ratios

The results indicate a decrease in environmental impacts as the level of cement replacement increases. This trend is mainly attributed to the reduction in OPC content, which is the main factor contributing to overall impacts. In terms of carbon footprint, formulation B2* reaches 145.41 kgCO₂e/m³; the incorporation of calcined sediments reduces this value to 134.97 kgCO₂e/m³ (−7.2%) at 15%, 123.12 kgCO₂e/m³ (−15.3%) at 30% and 115.83 kgCO₂e/m³ (−20.3%) at 40%, while fly ash and sediments contribute much less according to the selected impact factors. However, when we take into account the operational constraint linked to the time needed to achieve sufficient bearing strength (t IBI = 10), the interpretation becomes more nuanced: the 15% formulation reaches IPI = 10 much more quickly (13 h) than reference (24 h), while 40% does not reduce this time (24 h). In terms of eco-efficiency on construction sites, where the aim is to reduce impact while minimising immobilisation time, the 15% formulation therefore appears to be the best compromise, as it combines a reduction in CO₂/m³ with significantly faster deployment.

### 4. Conclusion

In order to reduce the environmental impact of cement-based construction materials and promote the circular use of resources, this study assessed the feasibility of recycling street sweepings into backfill grout using a two-step approach. The first stage involves incorporating raw sediments as a total replacement for natural sand, after optimising the fresh mix formulations. The second stage involves partially replacing cement with calcined sediments, with the aim of improving short-term mechanical performance while maintaining adequate workability.

Sediment from street sweeping is heterogeneous and may contain organic matter and irregular particles, which increase water demand and can impair flow. The first step therefore focused on controlling the workability of the grout when large quantities of raw sediment are used. Rheological optimization showed that adjusting the water/binder ratio to achieve an effective water content of approximately 0.27 allowed for stable flow despite the high absorption capacity of the sediments, confirming the functional role of the granular skeleton. In addition, the incorporation of class F fly ash at an optimal dosage of 10% significantly improved workability by reducing friction between particles, due to their fineness and spherical morphology, which reduced the static yield stress.

The second stage consisted of studying the added value of calcined sediments as partial cement substitutes in order to improve short-term mechanical performance. Hydration and structuring were analysed using Athix rheological analysis based on the Perrot model, which revealed distinct patterns: cement-dominant mixtures showed an almost linear evolution of structuring, while systems containing fly ash showed a parabolic trend consistent with slower initial hydration. The introduction of calcined sediments increased the structuring rate to an optimal substitution level of 15%. At this level, calcined sediments offer a favourable balance between nucleation and filling effect, resulting in improved performance at an early age, which is relevant for backfill grouts where rapid development of bearing capacity is essential. Conversely, at higher replacement levels (≥30-40%), the dilution effect becomes predominant. Microstructural and mineralogical evidence corroborates this interpretation: the combined trends from calorimetry/TGA-DTG/XRD indicate changes in the hydrate assembly and the formation of additional AFm phases containing carbonate, while SEM observations show a denser and more homogeneous matrix at 15% replacement compared to a more heterogeneous and porous microstructure at 40%, with less continuous hydrates and weaker bonded areas. These observations provide direct microstructural support for the identified optimum and confirm that excessive cement replacement reduces the integrity of the matrix.

Overall, this work demonstrates that there is a realistic path towards low-carbon backfill grouts by simultaneously replacing natural sand with raw sediments from street sweeping and reducing cement demand through the incorporation of calcined sediments. From an environmental perspective, increasing the level of cement substitution reduces CO₂ emissions; however, when initial operational requirements are taken into account, replacing 15% of cement with calcined sediments appears to be the most attractive option. Future work should extend the current results to long-term durability (shrinkage, creep, sulfate resistance, freeze-thaw) and field-scale validation under realistic hardening and placement conditions. Finally, given the strong influence of calcined sediments on early structuring and setting, their potential for 3D printing applications deserves further study.

### CRediT authorship contribution statement

**Mohamed Afif Zeggar:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Conceptualization.
**Nassim Sebaibi:** Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Conceptualization.
**Walid Maherzi:** Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.
**Nor-Edine Abriak:** Software, Resources, Project administration, Methodology, Investigation.
**Mahfoud Benzerzour:** Visualization, Validation, Supervision, Resources, Project administration,
Methodology, Investigation, Conceptualization.

## Code availability

Custom code used for data reduction and figure generation is available upon reasonable request.

## Ethics statement

No human participants or live vertebrates were involved.

## Declaration of competing interest

The authors declare that they have no financial or non-financial conflicts of interest related to the work presented in this manuscript.

## Acknowledgements

This work was co-financed by BUILDERS École d'ingénieurs and IMT Nord Europe under a PhD grant. The authors thank both institutions for their support.

## Data availability

Data will be made available on request.

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