# Effect of supercritical carbonation on porous structure and mechanical strength of cementitious materials modified with bacterial nanocellulose

Juan Cruz Barría, Diego Manzanal, Siavash Ghabezloo, Jean-Michel Pereira

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## To cite this version:

Juan Cruz Barría, Diego Manzanal, Siavash Ghabezloo, Jean-Michel Pereira. Effect of supercritical carbonation on porous structure and mechanical strength of cementitious materials modified with bacterial nanocellulose. Materials and structures, 2023, 56 (10), pp.180. 10.1617/s11527-023-02264-z. hal-04393067

HAL Id: hal-04393067

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Submitted on 14 Jan 2024

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Distributed under a Creative Commons Attribution 4.0 International License
# Effect of supercritical carbonation on porous structure and mechanical strength of cementitious materials modified with bacterial nanocellulose

Juan Cruz Barría, Diego Manzanal, Siavash Ghabezloo, Jean-Michel Pereira

► To cite this version:

Juan Cruz Barría, Diego Manzanal, Siavash Ghabezloo, Jean-Michel Pereira. Effect of supercritical carbonation on porous structure and mechanical strength of cementitious materials modified with bacterial nanocellulose. Materials and structures, 2023, 56 (10), pp.180. 10.1617/s11527-023-02264-z. hal-04393067

HAL Id: hal-04393067

https://hal.science/hal-04393067v1

Submitted on 14 Jan 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
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Effect of supercritical carbonation on porous structure and mechanical
strength of cementitious materials modified with bacterial nanocellulose.

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Juan Cruz Barría¹,³, Diego Manzanal¹,², *, Siavash Ghabezloo³, Jean-Michel Pereira³

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¹ Facultad de Ingeniería, Universidad Nacional de la Patagonia San Juan Bosco, 9004 Comodoro Rivadavia, Chubut, Argentina.

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² E.T.S.I. Caminos, Universidad Politécnica de Madrid, Prof. Aranguren 3, 28040 Madrid, Spain.

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³Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, Marne-la-Vallée, France.

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*Correspondence to d.manzanal@upm.es

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Abstract

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The effect of wet supercritical CO₂ (90 °C and 20 MPa) on the performance of cement paste (PC) modified with bacterial nanocellulose (BNC) was investigated. The pore structure of carbonated cement shows a clogging over the outer rim of the samples. In contrast, near the sample core, the characteristic peak of pore size distribution shifted towards smaller pores analyzed by mercury intrusion porosimetry. The effect of the carbonation overtime on mechanical properties shows increasing alteration. XRD results show more crystalline phases of hydrated cement in the BNC samples before carbonation. Cement reinforced with BNC shows lower density, a reduction in its porosity, and experiences fewer porosity changes at the cement core. Furthermore, its mechanical performance was less affected by the carbonation process.

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Keywords

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Cement paste – Bacterial nanocellulose – Cement carbonation – Porosity – Mechanical
Strength

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1. Introduction

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Global carbon dioxide (CO₂) emissions to the atmosphere are growing year after year [1]. Although new technologies are being developed to replace fossil fuels, humankind is still dependent on them. During this transition, reducing greenhouse gas emissions will be
1 **Effect of supercritical carbonation on porous structure and mechanical**
2 strength of cementitious materials modified with bacterial nanocellulose.

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4 **Juan Cruz Barría¹,³**, Diego Manzanal¹,², *, Siavash Ghabezloo³, Jean-Michel**Pereira³**

6 ¹ Facultad de Ingeniería, Universidad Nacional de la Patagonia San Juan Bosco, 9004
7 Comodoro Rivadavia, Chubut, Argentina.

8 ² E.T.S.I. Caminos, Universidad Politécnica de Madrid, Prof. Aranguren 3, 28040 Madrid,
9 Spain.

10 ³Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, Marne-la-Vallée, France.

11 *Correspondence to d.manzanal@upm.es

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13 **Abstract**

14 The effect of wet supercritical CO₂ (90 °C and 20 MPa) on the performance of cement paste
15 (PC) modified with bacterial nanocellulose (BNC) was investigated. The pore structure of
16 carbonated cement shows a clogging over the outer rim of the samples. In contrast, near
17 the sample core, the characteristic peak of pore size distribution shifted towards smaller
18 pores analyzed by mercury intrusion porosimetry. The effect of the carbonation overtime
19 on mechanical properties shows increasing alteration. XRD results show more crystalline
20 phases of hydrated cement in the BNC samples before carbonation. Cement reinforced
21 with BNC shows lower density, a reduction in its porosity, and experiences fewer porosity
22 changes at the cement core. Furthermore, its mechanical performance was less affected
23 by the carbonation process.

24 **Keywords**

25 Cement paste – Bacterial nanocellulose – Cement carbonation – Porosity – Mechanical
26 Strength

27 **1. Introduction**

28 Global carbon dioxide (CO₂) emissions to the atmosphere are growing year after year [1].
29 Although new technologies are being developed to replace fossil fuels, humankind is still
30 dependent on them. During this transition, reducing greenhouse gas emissions will be
crucial. Nowadays, there are several ways to increase the amount of carbon sinks in the world [2,3]. One of these methods relies on CO₂ geological storage [4], by which significant amounts of CO₂ can be stored in geological reservoirs [5,6].

Inside injection well, a cement sheath is placed between the casing and the formation rocks during the well completion to ensure stability, protect the casing against corrosion, and, most importantly, ensure wellbore sealing and zonal isolation. Along with the caprock, the cement sheath constitutes the hydraulic barrier against CO₂ leakage when injection is finished [7]. However, in presence of CO₂, carbonation of the cementitious materials is enhanced by the high pressure and high temperature conditions. The carbonation process affects the mechanical properties and permeability of the cement paste and may alter its role in ensuring the sealing and zonal isolation [8].

When injected in a geological reservoir, CO₂ first dissolves in the aqueous phase, generating carbonic acid (H₂CO₃). It then dissociates into bicarbonate (HCO₃⁻) and carbonate ions (CO₃²⁻), which in turn react with the hydrated cement compounds:

$$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^- \quad (1)$$

$$HCO_3^- \leftrightarrow H^+ + CO_3^{2-} \quad (2)$$

The two main cement components react chemically with CO₂. The carbonation of saturated cement samples is considered to be diffusion-dominated processes [9]. Both reactions lead to the precipitation of calcium carbonate (CaCO₃) in its various forms (calcite, aragonite, or vaterite). The first cement component to react is calcium hydroxide (Portlandite):

$$Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \quad (3)$$

The drop in pH then allows the hydrated calcium silicate (C-S-H) to be decalcified in contact with CO₂. New studies have demonstrated that C-S-H carbonation does not release water, and the only effect is C-S-H decalcification and amorphous silica (SiO₂·zH₂O(am)) formation [10]:

$$C_{1.7} - S - H_z + 1.7 CO_2 \rightarrow 1.7 CaCO_3 + SiO_2 \cdot zH_2O(am) \quad (4)$$
crucial. Nowadays, there are several ways to increase the amount of carbon sinks in the world [2,3]. One of these methods relies on CO₂ geological storage [4], by which significant amounts of CO₂ can be stored in geological reservoirs [5,6].

Inside injection well, a cement sheath is placed between the casing and the formation rocks during the well completion to ensure stability, protect the casing against corrosion, and, most importantly, ensure wellbore sealing and zonal isolation. Along with the caprock, the cement sheath constitutes the hydraulic barrier against CO₂ leakage when injection is finished [7]. However, in presence of CO₂, carbonation of the cementitious materials is enhanced by the high pressure and high temperature conditions. The carbonation process affects the mechanical properties and permeability of the cement paste and may alter its role in ensuring the sealing and zonal isolation [8].

When injected in a geological reservoir, CO₂ first dissolves in the aqueous phase, generating carbonic acid (H₂CO₃). It then dissociates into bicarbonate (HCO₃⁻) and carbonate ions (CO₃²⁻), which in turn react with the hydrated cement compounds:

$$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^- \quad (1)$$

$$HCO_3^- \leftrightarrow H^+ + CO_3^{2-} \quad (2)$$

The two main cement components react chemically with CO₂. The carbonation of saturated cement samples is considered to be diffusion-dominated processes [9]. Both reactions lead to the precipitation of calcium carbonate (CaCO₃) in its various forms (calcite, aragonite, or vaterite). The first cement component to react is calcium hydroxide (Portlandite):

$$Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \quad (3)$$

The drop in pH then allows the hydrated calcium silicate (C-S-H) to be decalcified in contact with CO₂. New studies have demonstrated that C-S-H carbonation does not release water, and the only effect is C-S-H decalcification and amorphous silica (SiO₂·zH₂O(am)) formation [10]:

$$C_{1.7} - S - H_z + 1.7 CO_2 \rightarrow 1.7 CaCO_3 + SiO_2 \cdot zH_2O(am) \quad (4)$$
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Nevertheless, it has been found that dissolution of portlandite (CH) and C-S-H occurs simultaneously [10,11].

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Since C-S-H is the compound with most important influence on the cement mechanical characteristics, its loss will reduce cement mechanical strength and could compromise the wellbore integrity, while the reduction of pH may induce corrosion of the steel casing.

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Cement carbonation produces a leaching front, which is characterized by an increase in porosity, followed by a CaCO₃ precipitation zone of low porosity [12], and finally, near the exposed front, a degraded zone of low pH, high porosity, and low strength is generated (brown color zone) [13]. These fronts are of variable penetration and thickness, depending on the cement curing conditions [14] and density [15].

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The pressure and temperature in the geological reservoirs considered for CO₂ storage are above the critical point for CO₂ (31.6 °C and 7.3 MPa). These conditions result in a high CO₂ content per unit volume and a gas-like viscosity, which accelerate the carbonation process of the cement. This barrier constituted by calcium carbonates can also be dissolved by the CO₂-rich environment surrounding the cement sheath, which would increase its porosity again [13,16–18].

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The porosity development of cement during hydration depends mainly on the water to cement ratio and the curing conditions. Cement with a higher hydration degree has lower porosity for a given curing temperature, and therefore lower advection and diffusion parameters [19,20].

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Previous studies have shown that the incorporation of nanocellulose modifies the mechanical properties, thermal stability, viscosity, and hydration degree of cement [21–25].

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Bacterial nanocellulose (BNC) is a bio-polymer non-soluble in aqueous solutions, with high mechanical properties, and thermally resistant. Furthermore, BNC is produced more efficiently through new culture media [26]. Studies show that nanofibers act as bridges over cracks creating reinforcement mechanisms, which prevents crack propagation at the nano level [27,28]. Overall, these properties make this material to be considered as an admixture for cement-based materials [29,30], including in the oil and gas industry [30,31].

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Nevertheless, it has been found that dissolution of portlandite (CH) and C-S-H occurs simultaneously [10,11].

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Since C-S-H is the compound with most important influence on the cement mechanical characteristics, its loss will reduce cement mechanical strength and could compromise the wellbore integrity, while the reduction of pH may induce corrosion of the steel casing.

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Cement carbonation produces a leaching front, which is characterized by an increase in porosity, followed by a CaCO$_3$ precipitation zone of low porosity [12], and finally, near the exposed front, a degraded zone of low pH, high porosity, and low strength is generated (brown color zone) [13]. These fronts are of variable penetration and thickness, depending on the cement curing conditions [14] and density [15].

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The pressure and temperature in the geological reservoirs considered for CO$_2$ storage are above the critical point for CO$_2$ (31.6 °C and 7.3 MPa). These conditions result in a high CO$_2$ content per unit volume and a gas-like viscosity, which accelerate the carbonation process of the cement. This barrier constituted by calcium carbonates can also be dissolved by the CO$_2$-rich environment surrounding the cement sheath, which would increase its porosity again [13,16–18].

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The porosity development of cement during hydration depends mainly on the water to cement ratio and the curing conditions. Cement with a higher hydration degree has lower porosity for a given curing temperature, and therefore lower advection and diffusion parameters [19,20].

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Previous studies have shown that the incorporation of nanocellulose modifies the mechanical properties, thermal stability, viscosity, and hydration degree of cement [21–25].

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Bacterial nanocellulose (BNC) is a bio-polymer non-soluble in aqueous solutions, with high mechanical properties, and thermally resistant. Furthermore, BNC is produced more efficiently through new culture media [26]. Studies show that nanofibers act as bridges over cracks creating reinforcement mechanisms, which prevents crack propagation at the nano level [27,28]. Overall, these properties make this material to be considered as an admixture for cement-based materials [29,30], including in the oil and gas industry [30,31].

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There is currently limited information on the effects of bacterial nanocellulose on cement [28,32], and there are no studies on carbonating this cement under supercritical conditions. Recently, authors have utilized BNC as an additive. They observed an improvement in mechanical strength, hydration degree, and thermomechanical properties of cement samples [33]. Therefore, the study of bacterial nanocellulose as an additive for oil well class G cement is a promising alternative to improve cement performance in petroleum engineering applications, especially in the context of geological storage of CO2.

The present work aims to study the porous structure and mechanical properties of cement modified with bacterial nanocellulose after carbonation under supercritical CO₂ conditions similar to geological storage of CO₂. The determination of the temperature and pressure to be used was based on literature, and the fact that carbonation was carried out under the most extreme conditions that could be obtained in the laboratory. Specimens with BNC addition were cured for 28 days at 20°C and then subjected to supercritical CO₂ in an environment at 20 MPa of pressure with a temperature of 90 °C. These conditions are to be expected in deep wells and several authors have worked with similar conditions [9].

Mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and uniaxial compressive strength (UCS) tests were performed on the carbonated samples. The results of this experimental program are presented and discussed hereafter. Scarce information exists on the subject, so this study allows to expand the current knowledge on the use of this type of additives.

## 2. Materials and methods

### 2.1. *Materials*

The clinker and calcium sulfate were dosed to satisfy the chemical requirements of the American Petroleum Institute (API) Standard 10A for class G oil well cement [34]. The initial chemical composition obtained by X-ray fluorescence (XRF) show 62.39% of CaO, 21.23% of SiO₂, 2.22% of MgO, 3.84% of Al₂O₃, 5.07% of Fe₂O₃, 2.04% of SO₃, and 0.64% of total alkali equivalent.
There is currently limited information on the effects of bacterial nanocellulose on cement [28,32], and there are no studies on carbonating this cement under supercritical conditions. Recently, authors have utilized BNC as an additive. They observed an improvement in mechanical strength, hydration degree, and thermomechanical properties of cement samples [33]. Therefore, the study of bacterial nanocellulose as an additive for oil well class G cement is a promising alternative to improve cement performance in petroleum engineering applications, especially in the context of geological storage of CO2.

The present work aims to study the porous structure and mechanical properties of cement modified with bacterial nanocellulose after carbonation under supercritical CO₂ conditions similar to geological storage of CO₂. The determination of the temperature and pressure to be used was based on literature, and the fact that carbonation was carried out under the most extreme conditions that could be obtained in the laboratory. Specimens with BNC addition were cured for 28 days at 20°C and then subjected to supercritical CO₂ in an environment at 20 MPa of pressure with a temperature of 90 °C. These conditions are to be expected in deep wells and several authors have worked with similar conditions [9].

Mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and uniaxial compressive strength (UCS) tests were performed on the carbonated samples. The results of this experimental program are presented and discussed hereafter. Scarce information exists on the subject, so this study allows to expand the current knowledge on the use of this type of additives.

## 2. Materials and methods

### 2.1. *Materials*

The clinker and calcium sulfate were dosed to satisfy the chemical requirements of the American Petroleum Institute (API) Standard 10A for class G oil well cement [34]. The initial chemical composition obtained by X-ray fluorescence (XRF) show 62.39% of CaO, 21.23% of SiO₂, 2.22% of MgO, 3.84% of Al₂O₃, 5.07% of Fe₂O₃, 2.04% of SO₃, and 0.64% of total alkali equivalent.
112 Bacterial nanocellulose (BNC) is a membrane with fiber diameters between 20 and 50 nm
113 and lengths of 1 µm [26], it differs from other polymers by not having free macromolecules,
114 but is formed by micrometric fibers with nanometric thickness. These fibers have a high
115 mechanical strength necessary to improve the properties of cement [35]. The BNC
116 membranes are approximately 98% water and 2% bacterial nanocellulose. The dispersion
117 method used was ultrasound on previously cut and wet-grinded BNC membranes. Its
118 treatment to obtain an adequate additive for cement requires several steps described in
119 previous works [33]. The final mix of water and BNC contains 0.46% of BNC by mass, and
120 the rest is distilled water. This additive is added to the cement mixture taking into account
121 the already added water.

122 **2.2. Samples preparation**

123 Samples were prepared according to the American Petroleum Institute Standard 10A [34],
124 with a water to cement ratio of 0.44. Three types of mixtures were prepared; Portland
125 cement (PC), cement with the addition of 0.05% BNC (BNC05), and cement with the
126 addition of 0.15% BNC (BNC15). The slurry was poured into 38 mm diameter, 76 mm high
127 cylindrical molds and compacted 27 times in two layers with a puddling rod. The samples
128 were cured for 24 hours in a 20 °C batch and unmolded. After demolding, they were kept
129 under lime-saturated water for 28 days.

130 **2.3. Supercritical CO₂ testing conditions**

131 The carbonation tests were carried out in a titanium vessel of 16 cm in diameter and 20 cm
132 high under static conditions. PC and BNC-modified cement samples were carbonated
133 under wet supercritical CO2 at 20 MPa and 90 °C, for 30 days and 120 days. Before
134 carbonation, the container with the samples underwater was placed inside a vacuum vessel
135 to remove the air bubbles possibly trapped inside the samples.

136 The saturated specimens were identified, measured, weighed, and placed on a container
137 grid. A thin water layer was poured at the bottom of the vessel for maintaining humidity
138 (500 ml). The grid was placed inside the cell, avoiding contact between the samples and
139 water layer. The cell was then sealed and connected to the CO2 pressure line. CO2
112 Bacterial nanocellulose (BNC) is a membrane with fiber diameters between 20 and 50 nm
113 and lengths of 1 µm [26], it differs from other polymers by not having free macromolecules,
114 but is formed by micrometric fibers with nanometric thickness. These fibers have a high
115 mechanical strength necessary to improve the properties of cement [35]. The BNC
116 membranes are approximately 98% water and 2% bacterial nanocellulose. The dispersion
117 method used was ultrasound on previously cut and wet-grinded BNC membranes. Its
118 treatment to obtain an adequate additive for cement requires several steps described in
119 previous works [33]. The final mix of water and BNC contains 0.46% of BNC by mass, and
120 the rest is distilled water. This additive is added to the cement mixture taking into account
121 the already added water.

122 **2.2. Samples preparation**

123 Samples were prepared according to the American Petroleum Institute Standard 10A [34],
124 with a water to cement ratio of 0.44. Three types of mixtures were prepared; Portland
125 cement (PC), cement with the addition of 0.05% BNC (BNC05), and cement with the
126 addition of 0.15% BNC (BNC15). The slurry was poured into 38 mm diameter, 76 mm high
127 cylindrical molds and compacted 27 times in two layers with a puddling rod. The samples
128 were cured for 24 hours in a 20 °C batch and unmolded. After demolding, they were kept
129 under lime-saturated water for 28 days.

130 **2.3. Supercritical CO2 testing conditions**

131 The carbonation tests were carried out in a titanium vessel of 16 cm in diameter and 20 cm
132 high under static conditions. PC and BNC-modified cement samples were carbonated
133 under wet supercritical CO2 at 20 MPa and 90 °C, for 30 days and 120 days. Before
134 carbonation, the container with the samples underwater was placed inside a vacuum vessel
135 to remove the air bubbles possibly trapped inside the samples.

136 The saturated specimens were identified, measured, weighed, and placed on a container
137 grid. A thin water layer was poured at the bottom of the vessel for maintaining humidity
138 (500 ml). The grid was placed inside the cell, avoiding contact between the samples and
139 water layer. The cell was then sealed and connected to the CO2 pressure line. CO2
pressure was increased to 8 MPa. The temperature was brought to 90° with the heating system and the pressure regulated at 20 MPa with the leak valve, removing the remaining air on the top of the vessel. This were the initial conditions for the test in static conditions. The ratio of sample volume/vessel volume was 0.38, and the ratio of sample volume/CO₂ volume was 0.79. A diagram of the carbonation system is presented in Fig. 1. After carbonation, the samples were weighed, and photographed. The pH level of the remaining water inside the cell was measured after carbonation.

In the first stage, a total of 18 samples were carbonated for 30 days, of which 6 were Portland Cement (PC), 6 had bacterial nanocellulose content of 0.05% (BNC05), and 6 of 0.15% (BNC15). In the second stage, another group of 18 samples (6 PC; 6 BNC05 and 6 BNC15) were carbonated for 120 days.

After carbonation, three samples of each BNC percentage were used for compressive strength tests, while other samples were cut radially and longitudinally to observe the penetration depth for the different cement types. The remaining cut pieces were used to perform porosimetry tests. These were taken at the middle of each sample, either at the core or near the outer surface.

## 2.4. Mercury intrusion porosimetry (MIP)

MIP allows to characterize the pore structure of a sample in terms of porosity and pore size distribution (PSD). The basic assumption to interpret MIP data is that pores are cylindrical and interconnected, thus allowing a simple calculation through the Jurin or Washburn equation:

$$p = \frac{4Y\cos(\theta)}{d} \qquad (5)$$

where Y = mercury surface tension = 0.485 N/m, θ = mercury contact angle = 130°, p = mercury pressure, d = pore diameter [36].

Eq. 5 permits the determination of the pore entry diameter for each pressure step applied.
pressure was increased to 8 MPa. The temperature was brought to 90° with the heating system and the pressure regulated at 20 MPa with the leak valve, removing the remaining air on the top of the vessel. This were the initial conditions for the test in static conditions. The ratio of sample volume/vessel volume was 0.38, and the ratio of sample volume/CO₂ volume was 0.79. A diagram of the carbonation system is presented in Fig. 1. After carbonation, the samples were weighed, and photographed. The pH level of the remaining water inside the cell was measured after carbonation.

In the first stage, a total of 18 samples were carbonated for 30 days, of which 6 were Portland Cement (PC), 6 had bacterial nanocellulose content of 0.05% (BNC05), and 6 of 0.15% (BNC15). In the second stage, another group of 18 samples (6 PC; 6 BNC05 and 6 BNC15) were carbonated for 120 days.

After carbonation, three samples of each BNC percentage were used for compressive strength tests, while other samples were cut radially and longitudinally to observe the penetration depth for the different cement types. The remaining cut pieces were used to perform porosimetry tests. These were taken at the middle of each sample, either at the core or near the outer surface.

## 2.4. Mercury intrusion porosimetry (MIP)

MIP allows to characterize the pore structure of a sample in terms of porosity and pore size distribution (PSD). The basic assumption to interpret MIP data is that pores are cylindrical and interconnected, thus allowing a simple calculation through the Jurin or Washburn equation:

$$p = \frac{4Y\cos(\theta)}{d} \qquad (5)$$

where Y = mercury surface tension = 0.485 N/m, θ = mercury contact angle = 130°, p = mercury pressure, d = pore diameter [36].

Eq. 5 permits the determination of the pore entry diameter for each pressure step applied.
According to the amount of mercury that penetrated the sample, it is possible to calculate capillary porosity and the PSD. However, the technique does not allow the characterization of isolated pores or pore sizes less than 0.010 µm [37].

Before the MIP tests, the samples were freeze-dried. This method was performed by immersing the 1 cm-side cubic samples in liquid nitrogen for 5 minutes, lowering the pressure, and vacuuming for 24 hours (more details are available in [38]). Samples were then kept sealed in containers with silica-gel to prevent rehydration until the MIP test was performed. The equipment utilized was a Micromeritics AutoPore IV 9500 with a maximum pressure of 230 MPa. All non-carbonated MIP samples were taken from a representative zone of the 28 days cured samples (NC). In the carbonated cement case, samples were taken from the core (Int) and near the outer rim (Ext). Two tests were performed on these same zones to analyze repeatability.

The list of tests carried out is given in Table 1. The samples reference is: cement type (PC, BNC05, or BNC15), days of carbonation (30 or 120), and location (Int or Ext).
If it was obtained from the interior or near the exterior of the cylindrical sample.

## 2.5. X-ray diffraction (XRD)

XRD analysis was performed on a Philips 3020 diffractometer using CuKα radiation with a Ni-filter (35 kV, 40 mA). Scanning was performed between 3° and 70° 2θ, with a step of 0.04° and a count time of 2 s/step. No monochromator was used, and openings were 1° for divergence, 0.2° for the reception, and 1° for dispersion. The identification of the mineral phases in the material was performed using the X'Pert High Score program. The procedures described by Moore and Reynolds [39] were followed to identification, while quantification was based on the work of Biscaye [40]. The samples analyzed were taken from non-carbonated specimens (NC) and after 120 days of carbonation. For carbonated specimens, samples were obtained from the interior (120Int) and near the exposed edge (120Ext).

## 2.6. Uniaxial compression test (UCS).
According to the amount of mercury that penetrated the sample, it is possible to calculate capillary porosity and the PSD. However, the technique does not allow the characterization of isolated pores or pore sizes less than 0.010 µm [37].

Before the MIP tests, the samples were freeze-dried. This method was performed by immersing the 1 cm-side cubic samples in liquid nitrogen for 5 minutes, lowering the pressure, and vacuuming for 24 hours (more details are available in [38]). Samples were then kept sealed in containers with silica-gel to prevent rehydration until the MIP test was performed. The equipment utilized was a Micromeritics AutoPore IV 9500 with a maximum pressure of 230 MPa. All non-carbonated MIP samples were taken from a representative zone of the 28 days cured samples (NC). In the carbonated cement case, samples were taken from the core (Int) and near the outer rim (Ext). Two tests were performed on these same zones to analyze repeatability.

The list of tests carried out is given in Table 1. The samples reference is: cement type (PC, BNC05, or BNC15), days of carbonation (30 or 120), and location (Int or Ext).
If it was obtained from the interior or near the exterior of the cylindrical sample.

## 2.5. X-ray diffraction (XRD)

XRD analysis was performed on a Philips 3020 diffractometer using CuKα radiation with a Ni-filter (35 kV, 40 mA). Scanning was performed between 3° and 70° 2θ, with a step of 0.04° and a count time of 2 s/step. No monochromator was used, and openings were 1° for divergence, 0.2° for the reception, and 1° for dispersion. The identification of the mineral phases in the material was performed using the X'Pert High Score program. The procedures described by Moore and Reynolds [39] were followed to identification, while quantification was based on the work of Biscaye [40]. The samples analyzed were taken from non-carbonated specimens (NC) and after 120 days of carbonation. For carbonated specimens, samples were obtained from the interior (120Int) and near the exposed edge (120Ext).

## 2.6. Uniaxial compression test (UCS).
The uniaxial compression test was performed on a 100 kN testing machine with a 0.5 mm/min velocity rate. Vertical displacement and applied force were monitored during the compression tests. The compressive strength and Young's modulus were calculated for all samples. The averages of strength values and Young's modulus were calculated from 3 tested cylindrical samples.

## 3. Results and discussion

Carbonation conditions are critical to obtaining more or less penetration. There have been reports on how carbonation effects are affected by temperature or pressure variations by maintaining one of these parameters constant [41]. It was shown that, by increasing pressure and keeping the temperature constant, the penetration depth was lower and the compressive strength was higher. On the other hand, when the pressure was maintained constant and the temperature increased, cement was more affected with a larger penetration depth and a further reduction in compressive strength.

After 120 days of carbonation, the specimens have not been completely carbonated. Therefore, the results shown here are of a cement not totally carbonated in the interior. The carbonation conditions of the current work are similar to Barlet-Gouedard and Fabbri [17,42], where larger penetrations or complete penetration have been observed after 30 days of carbonation. However, the carbonation effect in our samples is different, similar to the small penetrations observed by Kutchko and Gu [12,43]. The slow advancement rate in our samples is explained by their low capillary porosity (19%) compared to the aforementioned authors [17,42] (33% - 41%).

The samples have a thin brown outer ring, and a thinner darker brown ring was observed surrounding the interior part of the brown ring in some samples. The change in color in the altered zones might result from the release of Fe³⁺ ions, which react chemically and form iron hydroxides [44]. This comes from the fact that the brownmillerite begins to carbonate, due to a decrease in the amount of crystals in the XRD results in Table 3.
The uniaxial compression test was performed on a 100 kN testing machine with a 0.5 mm/min velocity rate. Vertical displacement and applied force were monitored during the compression tests. The compressive strength and Young's modulus were calculated for all samples. The averages of strength values and Young's modulus were calculated from 3 tested cylindrical samples.

## 3. Results and discussion

Carbonation conditions are critical to obtaining more or less penetration. There have been reports on how carbonation effects are affected by temperature or pressure variations by maintaining one of these parameters constant [41]. It was shown that, by increasing pressure and keeping the temperature constant, the penetration depth was lower and the compressive strength was higher. On the other hand, when the pressure was maintained constant and the temperature increased, cement was more affected with a larger penetration depth and a further reduction in compressive strength.

After 120 days of carbonation, the specimens have not been completely carbonated. Therefore, the results shown here are of a cement not totally carbonated in the interior. The carbonation conditions of the current work are similar to Barlet-Gouedard and Fabbri [17,42], where larger penetrations or complete penetration have been observed after 30 days of carbonation. However, the carbonation effect in our samples is different, similar to the small penetrations observed by Kutchko and Gu [12,43]. The slow advancement rate in our samples is explained by their low capillary porosity (19%) compared to the aforementioned authors [17,42] (33% - 41%).

The samples have a thin brown outer ring, and a thinner darker brown ring was observed surrounding the interior part of the brown ring in some samples. The change in color in the altered zones might result from the release of Fe³⁺ ions, which react chemically and form iron hydroxides [44]. This comes from the fact that the brownmillerite begins to carbonate, due to a decrease in the amount of crystals in the XRD results in Table 3.
As shown in Fig. 2, the degraded zone has a slow penetration rate, and no change can be seen at the core of the specimens. However, in the top part, a substantial penetration is observed. BNC05 cement was less degraded than PC. Lower amounts of calcium hydroxides characterize this zone and a lower Ca/Si ratio in the C-S-H structure [45].

The top degraded zone observed in Fig. 2 might result from sedimentation during the cement setting, which leaves a more porous area at the top of the sample. This zone measured with caliper penetrates 8.7 mm inwards from the top of the PC sample, while a value of 5.6 mm is measured for BNC05. The porosity of BNC05 in the top part was lower than for PC, due to BNC's ability to diminish cement sedimentation by reducing the free water content [33], thus allowing this part to have more cement hydrates and less porosity.

Bacterial nanocellulose is known for enhancing the creation of more cement hydrates for prolonged curing times [46].

After carbonation, the fluid pH was reduced from 13 to 7, similar to studies of Barlet-Gouédard [47]. Unfortunately, the pH could not be measured during the carbonation test.

In fact, the pH during the carbonation process could be reduced to 2.9 [12] or 3.2 [48], depending on CO₂ solubility in water. According to Duan and Carey's model [48,49], the solubility of CO₂ in our experiment was 1.1 mol/kg.

## 3.1 Density variation

Table 2 shows the density variation of samples over the 30 and 120 days of carbonation. These values are the averages of all the samples tested, and the deviation does not exceed 2% of the averages.

PC increases its bulk density by 1.4% and 2.5% at 30 and 120 days, respectively. BNC05 was the lightest compared to the other two cement types and showed an increase of its density by 1.5% and 2%, respectively. BNC15 had the greatest initial densities and increased 1.3% and 3.6% at 30 and 120 days, respectively.

The stoichiometric relation between the solid phases CH and CaCO₃ is one. As the density of CH is 2.23 g/cm³, and CaCO₃ is 2.71 g/cm³ (or 2.93 g/cm³ for aragonite or 2.54 g/cm³ for vaterite), bulk density will increase. C-S-H is an amorphous material that have different
As shown in Fig. 2, the degraded zone has a slow penetration rate, and no change can be seen at the core of the specimens. However, in the top part, a substantial penetration is observed. BNC05 cement was less degraded than PC. Lower amounts of calcium hydroxides characterize this zone and a lower Ca/Si ratio in the C-S-H structure [45].

The top degraded zone observed in Fig. 2 might result from sedimentation during the cement setting, which leaves a more porous area at the top of the sample. This zone measured with caliper penetrates 8.7 mm inwards from the top of the PC sample, while a value of 5.6 mm is measured for BNC05. The porosity of BNC05 in the top part was lower than for PC, due to BNC's ability to diminish cement sedimentation by reducing the free water content [33], thus allowing this part to have more cement hydrates and less porosity.

Bacterial nanocellulose is known for enhancing the creation of more cement hydrates for prolonged curing times [46].

After carbonation, the fluid pH was reduced from 13 to 7, similar to studies of Barlet-Gouédard [47]. Unfortunately, the pH could not be measured during the carbonation test.

In fact, the pH during the carbonation process could be reduced to 2.9 [12] or 3.2 [48], depending on CO₂ solubility in water. According to Duan and Carey's model [48,49], the solubility of CO₂ in our experiment was 1.1 mol/kg.

## 3.1 Density variation

Table 2 shows the density variation of samples over the 30 and 120 days of carbonation. These values are the averages of all the samples tested, and the deviation does not exceed 2% of the averages.

PC increases its bulk density by 1.4% and 2.5% at 30 and 120 days, respectively. BNC05 was the lightest compared to the other two cement types and showed an increase of its density by 1.5% and 2%, respectively. BNC15 had the greatest initial densities and increased 1.3% and 3.6% at 30 and 120 days, respectively.

The stoichiometric relation between the solid phases CH and CaCO₃ is one. As the density of CH is 2.23 g/cm³, and CaCO₃ is 2.71 g/cm³ (or 2.93 g/cm³ for aragonite or 2.54 g/cm³ for vaterite), bulk density will increase. C-S-H is an amorphous material that have different
phases with different densities, ranging from 2.6 g/cm³ to 2.86 g/cm³ [50]. The C-S-H chemical balance of Eq. 4 indicates that density will also increase due to the formation of CaCO₃, with a relation of 1.7 mol of CaCO₃ per 1 mol of C-S-H carbonated. Less variation in density is an indicator of a lower degree of carbonation.

## 3.2 MIP: non-carbonated cement

MIP tests are shown in Fig. 3, Fig. 4, and Fig. 5. Adding bacterial nanocellulose led to lower porosity, and the characteristic pore size peak shifted to smaller pores (from 0.050 to 0.045 µm). Cellulose nanocrystals have been reported to reduce porosity in cement for pores smaller than 0.100 µm [51]. The hydrophilic properties of bacterial nanocellulose induce the precipitation of larger quantities of hydration products during hydration [33,52], thus accumulating more of them in the pores previously filled with water [53]. Furthermore, the fibers of the bacterial nanocellulose have substantial mechanical strength characteristics [35], which during cement shrinkage decrease the probability of having micro-cracks [54]. PC has an average of 19% capillary porosity, while BNC05 has an average of 12% and BNC15 of 16% (blue dots). Nanocellulose distribution within the samples is crucial in obtaining homogeneous properties, but it cannot be easily measured. An uneven distribution can lead to scattering in measurements [55]. The ultrasonic scrubber allowed us to have a better distribution of BNC in the mix [56]. However, in Fig. 6, MIP results for the non-carbonated cement with bacterial nanocellulose show some scatter certainly due to heterogeneity.

## 3.3 MIP: Carbonated cement paste

The variation in the shape of MIP curves means that the characteristic peak of PSD tends to disappear, and smaller pores (less than 0.010 µm) begin to appear after 30 days. As carbonation continues towards 120 days, the smaller pores are continuously reduced. While this was happening, the CH and C-S-H were being dissolved, thus increasing the smallest pores at 0.020 µm.

In Fig. 3 to Fig. 5 after 30-days of carbonation, PC, BNC05, and BNC15 samples show an increase in pores' population having a diameter close to 0.100 µm. That population was
phases with different densities, ranging from 2.6 g/cm³ to 2.86 g/cm³ [50]. The C-S-H chemical balance of Eq. 4 indicates that density will also increase due to the formation of CaCO₃, with a relation of 1.7 mol of CaCO₃ per 1 mol of C-S-H carbonated. Less variation in density is an indicator of a lower degree of carbonation.

## 3.2 MIP: non-carbonated cement

MIP tests are shown in Fig. 3, Fig. 4, and Fig. 5. Adding bacterial nanocellulose led to lower porosity, and the characteristic pore size peak shifted to smaller pores (from 0.050 to 0.045 µm). Cellulose nanocrystals have been reported to reduce porosity in cement for pores smaller than 0.100 µm [51]. The hydrophilic properties of bacterial nanocellulose induce the precipitation of larger quantities of hydration products during hydration [33,52], thus accumulating more of them in the pores previously filled with water [53]. Furthermore, the fibers of the bacterial nanocellulose have substantial mechanical strength characteristics [35], which during cement shrinkage decrease the probability of having micro-cracks [54]. PC has an average of 19% capillary porosity, while BNC05 has an average of 12% and BNC15 of 16% (blue dots). Nanocellulose distribution within the samples is crucial in obtaining homogeneous properties, but it cannot be easily measured. An uneven distribution can lead to scattering in measurements [55]. The ultrasonic scrubber allowed us to have a better distribution of BNC in the mix [56]. However, in Fig. 6, MIP results for the non-carbonated cement with bacterial nanocellulose show some scatter certainly due to heterogeneity.

## 3.3 MIP: Carbonated cement paste

The variation in the shape of MIP curves means that the characteristic peak of PSD tends to disappear, and smaller pores (less than 0.010 µm) begin to appear after 30 days. As carbonation continues towards 120 days, the smaller pores are continuously reduced. While this was happening, the CH and C-S-H were being dissolved, thus increasing the smallest pores at 0.020 µm.

In Fig. 3 to Fig. 5 after 30-days of carbonation, PC, BNC05, and BNC15 samples show an increase in pores' population having a diameter close to 0.100 µm. That population was
quasi inexistent in pre-carbonated samples. Some authors have confirmed the occurrence
of nano-cracks and micro-cracks after carbonation shrinkage [57], probably induced by
calcite crystallization in pores [58]. However, this specific increment of the pore volume
could be due to CH dissolution or C-S-H decalcification. In fact, studies on the dissolution
of CH and C-S-H indicate a similar porosity increment at 0.100 µm [59]. Once the pH drops
in the pore solution due to the CH leaching, the dissolution of ettringite (pH < 10.7) and C-
S-H (pH < 10.5 or 8.8 depending on the study [60]) will begin.

Regardless of this pore size growth of 0.100 µm in **Fig. 3** to **Fig. 5** after 30 days of
carbonation, these pore sizes of 0.100 µm disappear in samples carbonated for 120 days.

C-S-H decalcification is the one that produces more CaCO₃ that further clogs the pores
after 120 days of carbonation, according to Morandeau studies [10]. As samples were not
underwater, dissolved ions could not be released out of the specimens. Therefore, the
CaCO₃ precipitation has been refilling these carbonation-induced cracks from the hydrated
material of the core, and blocking further carbonation inwards. This remains to be confirmed
by analyzing SEM images.

PC results in **Fig. 7** show a porosity decrease at the cement core (blue dots), suggesting
that chemical reactions of carbonation have reached this point. The carbonation process
becomes clearer here by looking at the PSD variation in **Fig. 8**. Indeed, MIP results suggest
that carbonation exists all over the sample, at a relatively slow rate at the center and a
faster one near the outer rim, where a carbonation front can visually be identified. This was
observed first by Rimmelé [18], who reported that CH depletion and CaCO₃ precipitation
occurred at the cement core, ahead of the carbonation front. Later, Adeoye [61] noted that
the Vickers hardness increases throughout the material after carbonation.

As in non-carbonated samples, we can observe a shift of the characteristic peak at the core
of carbonated samples. The characteristic peak is centered on 0.015 µm for BNC and 0.020
µm for PC. This shows that the effects of nanocellulose remain during cement carbonation,
maintaining its characteristic peak at lower values than for PC.
275 quasi inexistent in pre-carbonated samples. Some authors have confirmed the occurrence
276 of nano-cracks and micro-cracks after carbonation shrinkage [57], probably induced by
277 calcite crystallization in pores [58]. However, this specific increment of the pore volume
278 could be due to CH dissolution or C-S-H decalcification. In fact, studies on the dissolution
279 of CH and C-S-H indicate a similar porosity increment at 0.100 µm [59]. Once the pH drops
280 in the pore solution due to the CH leaching, the dissolution of ettringite (pH < 10.7) and C-
281 S-H (pH < 10.5 or 8.8 depending on the study [60]) will begin.

282 Regardless of this pore size growth of 0.100 µm in Fig. 3 to Fig. 5 after 30 days of
283 carbonation, these pore sizes of 0.100 µm disappear in samples carbonated for 120 days.
284 C-S-H decalcification is the one that produces more CaCO₃ that further clogs the pores
285 after 120 days of carbonation, according to Morandeau studies [10]. As samples were not
286 underwater, dissolved ions could not be released out of the specimens. Therefore, the
287 CaCO₃ precipitation has been refilling these carbonation-induced cracks from the hydrated
288 material of the core, and blocking further carbonation inwards. This remains to be confirmed
289 by analyzing SEM images.

290 PC results in Fig. 7 show a porosity decrease at the cement core (blue dots), suggesting
291 that chemical reactions of carbonation have reached this point. The carbonation process
292 becomes clearer here by looking at the PSD variation in Fig. 8. Indeed, MIP results suggest
293 that carbonation exists all over the sample, at a relatively slow rate at the center and a
294 faster one near the outer rim, where a carbonation front can visually be identified. This was
295 observed first by Rimmelé [18], who reported that CH depletion and CaCO₃ precipitation
296 occurred at the cement core, ahead of the carbonation front. Later, Adeoye [61] noted that
297 the Vickers hardness increases throughout the material after carbonation.

298 As in non-carbonated samples, we can observe a shift of the characteristic peak at the core
299 of carbonated samples. The characteristic peak is centered on 0.015 µm for BNC and 0.020
300 µm for PC. This shows that the effects of nanocellulose remain during cement carbonation,
301 maintaining its characteristic peak at lower values than for PC.
According to Kutchko [12], the carbonation causes a decrease in CH amount at the inner part of cement, and a decrease in porosity over the carbonate barrier. Although $CaCO_3$ precipitation is seen, a large number of pores smaller than 0.020 µm are observed.

## 3.4 X-ray diffraction (XRD)

Fig. 9 shows the XRD patterns of PC and BNC15, while Table 3 indicates the relative percentage of the crystalline phases. The increase in the amount of portlandite and decrease in brownmillerite in samples before carbonation reinforces the results obtained in previous studies showing that BNC increases the degree of cement hydration [33]. XRD curves are similar to other types of G-cements [62], showing similar crystals. The lack of ettringite and monosulfoaluminates is due to the low content of $C_3A$ and gypsum in cement [63, 64]. Katoite and silicious hydrogarnet can be form from $C_2S$ and $C_4AF$ and they have a similar chemical formula depending on fluid condition, sulfate activity, and temperature.

Here, both are listed as katoite in Table 3. Additionally, the lack of crystals related to C-S-H is due to the insufficient intensity of reflection of this amorphous material [65].

The post-carbonation XRD results on the exposed surface indicate almost complete carbonation. A large amount of aragonite in this area denotes higher chemical instability due to its high solubility. The core is less affected by carbonation, but the percentages of minerals have changed. The increase in katoite is probably a combination of temperature effect during carbonation, as this material is more stable at higher temperatures, and $C_4AF$ hydration. In fact, katoite was found to increase significantly upon hydration of $C_4AF$.

BNC15 shows a decrease in portlandite, suggesting that CH dissolution has started. However, this is not observed in PC cement. Some aragonite traces are detected at the core of the PC sample. Carbonation and/or hydration during the test produced a change in the amount of katoite and brownmillerite, while portlandite and carbonates do not appear to have changed. This seems to indicate that the core did not undergo carbonation, differing from the MIP and total porosity results.

## 3.5 Uniaxial compression tests (UCS)
According to Kutchko [12], the carbonation causes a decrease in CH amount at the inner part of cement, and a decrease in porosity over the carbonate barrier. Although $CaCO_3$ precipitation is seen, a large number of pores smaller than 0.020 µm are observed.

### 3.4 X-ray diffraction (XRD)

Fig. 9 shows the XRD patterns of PC and BNC15, while Table 3 indicates the relative percentage of the crystalline phases. The increase in the amount of portlandite and decrease in brownmillerite in samples before carbonation reinforces the results obtained in previous studies showing that BNC increases the degree of cement hydration [33]. XRD curves are similar to other types of G-cements [62], showing similar crystals. The lack of ettringite and monosulfoaluminates is due to the low content of $C_3A$ and gypsum in cement [63, 64]. Katoite and silicious hydrogarnet can be form from $C_2S$ and $C_4AF$ and they have a similar chemical formula depending on fluid condition, sulfate activity, and temperature.

Here, both are listed as katoite in Table 3. Additionally, the lack of crystals related to C-S-H is due to the insufficient intensity of reflection of this amorphous material [65].

The post-carbonation XRD results on the exposed surface indicate almost complete carbonation. A large amount of aragonite in this area denotes higher chemical instability due to its high solubility. The core is less affected by carbonation, but the percentages of minerals have changed. The increase in katoite is probably a combination of temperature effect during carbonation, as this material is more stable at higher temperatures, and $C_4AF$ hydration. In fact, katoite was found to increase significantly upon hydration of $C_4AF$.

BNC15 shows a decrease in portlandite, suggesting that CH dissolution has started. However, this is not observed in PC cement. Some aragonite traces are detected at the core of the PC sample. Carbonation and/or hydration during the test produced a change in the amount of katoite and brownmillerite, while portlandite and carbonates do not appear to have changed. This seems to indicate that the core did not undergo carbonation, differing from the MIP and total porosity results.

### 3.5 Uniaxial compression tests (UCS)
In supercritical conditions, some authors have reported an increment in compressive strength [41,66], while more recent researchers report a decrease in mechanical performance [42,67,68]. This contradiction stems from the different hydration conditions before carbonation [69] and the subsequent carbonation conditions [41]. For the same water/cement ratio before the carbonation test, the advance of carbonation is dependent on the hydration level. A well-hydrated cement will probably only experience mechanical degradation, while an early-carbonated cement specimen will also show the effects of ongoing hydration [70].

In Fig. 10, supercritical carbonation is adversely affecting the mechanical performance. Several reasons can explain this: micro-cracks due to pressure variation in the carbonation cell, the high porosity and softening of the outer layer due to CaCO₃ dissolution [71], the degradation of C-S-H [72], possible cracks due to increased crystallization stresses of CaCO₃ in the pores [42,73], or the preferential paths of the cracks through the different interfaces of the rings of the carbonated material [42]. Pore crystallisation is the overgrowth of CaCO₃ and begins to generate tensile stresses from within the pore.

C-S-H decalcification is mainly responsible for the strength loss during carbonation [74], and Young's modulus in leached zones can decrease up to 80% [75]. It was observed by Hidalgo [76] that CH is depleted rapidly and carbonation products surround the remains.

The discontinuities between the different rings of the carbonation zones generate stress concentrations and can initiate failures during compression. Some of our samples broke parallel to the applied force and near the rim, as observed by other authors [42].

CaCO₃ crystallization can produce tensile stresses around the pores [77,78]. However, it is difficult to see this after the carbonation process. Once the confinement pressure is removed, the pore increases its volume again, and the crystals can be re-solubilized in the fluid. A work carried out by Lesti highlighted the problems of pore crystallization [73]. In that report, even though the carbonation front did not advance deeply, cracks were visually appreciated, which enhanced the failure probability of the sample.
In supercritical conditions, some authors have reported an increment in compressive strength [41,66], while more recent researchers report a decrease in mechanical performance [42,67,68]. This contradiction stems from the different hydration conditions before carbonation [69] and the subsequent carbonation conditions [41]. For the same water/cement ratio before the carbonation test, the advance of carbonation is dependent on the hydration level. A well-hydrated cement will probably only experience mechanical degradation, while an early-carbonated cement specimen will also show the effects of ongoing hydration [70].

In Fig. 10, supercritical carbonation is adversely affecting the mechanical performance. Several reasons can explain this: micro-cracks due to pressure variation in the carbonation cell, the high porosity and softening of the outer layer due to CaCO₃ dissolution [71], the degradation of C-S-H [72], possible cracks due to increased crystallization stresses of CaCO₃ in the pores [42,73], or the preferential paths of the cracks through the different interfaces of the rings of the carbonated material [42]. Pore crystallisation is the overgrowth of CaCO₃ and begins to generate tensile stresses from within the pore.

C-S-H decalcification is mainly responsible for the strength loss during carbonation [74], and Young's modulus in leached zones can decrease up to 80% [75]. It was observed by Hidalgo [76] that CH is depleted rapidly and carbonation products surround the remains.

The discontinuities between the different rings of the carbonation zones generate stress concentrations and can initiate failures during compression. Some of our samples broke parallel to the applied force and near the rim, as observed by other authors [42].

CaCO₃ crystallization can produce tensile stresses around the pores [77,78]. However, it is difficult to see this after the carbonation process. Once the confinement pressure is removed, the pore increases its volume again, and the crystals can be re-solubilized in the fluid. A work carried out by Lesti highlighted the problems of pore crystallization [73]. In that report, even though the carbonation front did not advance deeply, cracks were visually appreciated, which enhanced the failure probability of the sample.
Nanocellulose has been reported incrementing mechanical properties when added to cement mixtures [28,54,79]. Results in this work show an initial increment in strength due to the addition of bacterial nanocellulose before carbonation. They were also less affected by carbonation. After 120 days of carbonation, the PC strength is 36 MPa, while BNC05 and BNC15 are 48 MPa and 42 MPa. Young's modulus was also affected in PC to a value of 16.5 GPa, while BNC and BNC15 had 22 and 21 GPa. The increased level of cement hydration from the hydrophilic characteristics of BNC probably reduced the progression of carbonation. Indeed, the lower capillary porosity due to the higher degree of hydration slows down the diffusion rate [20].

### 3.6 Porosity development in time

Fig. 11 shows the pore size diameter variation at a material point near the external surface of PC from the initial state (no carbonation) to 120 days of carbonation where: Time 1 is before carbonation, Time 2 is 30 days of carbonation and Time 3 is 120 days of carbonation.

The corresponding porosities are 19%, 14% and 5% for PC. All of our cement types behave in this manner with their corresponding capillary porosities. As seen before, the CaCO₃ growth in pores shifts the characteristic peak from 0.050 µm to 0.010 µm after 30 days. At this time, an increment of 0.100 µm pores is observed. After 120 days of carbonation, C-S-H depletion continues but towards smaller pores, and CaCO₃ continues growing inside the smallest pores. At this point, no cracks are observed, the ones that appeared previously having been filled by CaCO₃ precipitation.

## 4. Conclusions

A supercritical carbonation test was performed on cement modified with bacterial nanocellulose for 30 and 120 days. Density variation was measured. Capillary porosity was measured by mercury intrusion porosimetry on different zones. X-ray diffraction was used to characterize the crystalline phases. Compressive strength values and Young's modulus were calculated from uniaxial compressive strength tests. Results are summarized below:
Nanocellulose has been reported incrementing mechanical properties when added to cement mixtures [28,54,79]. Results in this work show an initial increment in strength due to the addition of bacterial nanocellulose before carbonation. They were also less affected by carbonation. After 120 days of carbonation, the PC strength is 36 MPa, while BNC05 and BNC15 are 48 MPa and 42 MPa. Young's modulus was also affected in PC to a value of 16.5 GPa, while BNC and BNC15 had 22 and 21 GPa. The increased level of cement hydration from the hydrophilic characteristics of BNC probably reduced the progression of carbonation. Indeed, the lower capillary porosity due to the higher degree of hydration slows down the diffusion rate [20].

### 3.6 Porosity development in time

Fig. 11 shows the pore size diameter variation at a material point near the external surface of PC from the initial state (no carbonation) to 120 days of carbonation where: Time 1 is before carbonation, Time 2 is 30 days of carbonation and Time 3 is 120 days of carbonation.

The corresponding porosities are 19%, 14% and 5% for PC. All of our cement types behave in this manner with their corresponding capillary porosities. As seen before, the CaCO₃ growth in pores shifts the characteristic peak from 0.050 µm to 0.010 µm after 30 days. At this time, an increment of 0.100 µm pores is observed. After 120 days of carbonation, C-S-H depletion continues but towards smaller pores, and CaCO₃ continues growing inside the smallest pores. At this point, no cracks are observed, the ones that appeared previously having been filled by CaCO₃ precipitation.

## 4. Conclusions

A supercritical carbonation test was performed on cement modified with bacterial nanocellulose for 30 and 120 days. Density variation was measured. Capillary porosity was measured by mercury intrusion porosimetry on different zones. X-ray diffraction was used to characterize the crystalline phases. Compressive strength values and Young's modulus were calculated from uniaxial compressive strength tests. Results are summarized below:
383

* Density has increased in all samples, but cement with 0.05% of BNC was the lightest. A substantial degraded zone was seen at the top of the samples, but cement was less damaged radially.

384

385

386

* A clear reduction in capillary porosity was noted radially over the exterior of the samples during the 120 days of carbonation.

387

388

* At the core, capillary porosity was reduced in PC and BNC15 samples but not for BNC05. The pore distribution from MIP results indicates the existence of 0.100 µm microcracks after 30 days of carbonation, but they disappear after 120 days of carbonation.

389

390

391

392

* XRD results show more crystalline phases of hydrated cement in the BNC samples before carbonation. A large amount of aragonite in the outer rim indicate that dissolution of the CaCO₃ minerals can take place if external conditions allowed it.

393

394

395

* Carbonation generates a reduction in the mechanical strength and Young's modulus of all samples. Non-modified cement shows a continuous decline in strength, while samples with BNC maintain the same strength from 30 to 120 days. Samples with BNC show higher values of their mechanical properties in absolute and relative terms before and after the carbonation process.

396

397

398

399

400

* Cement with 0.05% of bacterial nanocellulose visually presents a lesser degraded zone and is not affected at the center of the sample as the other mixtures. The average ultimate strength of this cement is 48 MPa, 33% higher than the unmodified cement, which was 36 MPa. However, there are no significant variations in porosity and XRD results.

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## Acknowledgments

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The first author gratefully acknowledges the fellowship granted by the National Scientific and Technical Research Council of Argentina (CONICET) and to the EIFFEL fellowship program of Excellence granted by the Ministère de l'Europe et des Affaires étrangères of France. The authors acknowledged the financial support of the European Union's Horizon 2020 research and innovation program under the Grant Agreement N° 101007851
383

* Density has increased in all samples, but cement with 0.05% of BNC was the lightest. A substantial degraded zone was seen at the top of the samples, but cement was less damaged radially.

384

385

386

* A clear reduction in capillary porosity was noted radially over the exterior of the samples during the 120 days of carbonation.

387

388

* At the core, capillary porosity was reduced in PC and BNC15 samples but not for BNC05. The pore distribution from MIP results indicates the existence of 0.100 µm microcracks after 30 days of carbonation, but they disappear after 120 days of carbonation.

389

390

391

392

* XRD results show more crystalline phases of hydrated cement in the BNC samples before carbonation. A large amount of aragonite in the outer rim indicate that dissolution of the CaCO₃ minerals can take place if external conditions allowed it.

393

394

395

* Carbonation generates a reduction in the mechanical strength and Young's modulus of all samples. Non-modified cement shows a continuous decline in strength, while samples with BNC maintain the same strength from 30 to 120 days. Samples with BNC show higher values of their mechanical properties in absolute and relative terms before and after the carbonation process.

396

397

398

399

400

* Cement with 0.05% of bacterial nanocellulose visually presents a lesser degraded zone and is not affected at the center of the sample as the other mixtures. The average ultimate strength of this cement is 48 MPa, 33% higher than the unmodified cement, which was 36 MPa. However, there are no significant variations in porosity and XRD results.

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## Acknowledgments

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The first author gratefully acknowledges the fellowship granted by the National Scientific and Technical Research Council of Argentina (CONICET) and to the EIFFEL fellowship program of Excellence granted by the Ministère de l'Europe et des Affaires étrangères of France. The authors acknowledged the financial support of the European Union's Horizon 2020 research and innovation program under the Grant Agreement N° 101007851
411 (H2020 MSCA-RISE 2020 Project DISCO2-STORE), the Universidad Nacional de la
412 Patagonia San Juan Bosco -Project UNPSJB PI1614 80020190200006 IP, Res. R/9Nº207-
413 2020 CRD1365 FI004/ 17-, the Agency of Scientific and Technological Promotion from the
414 Argentine Republic. (Projects PICT 2016-4543, PICT 0843 2016), and the Institutional
415 project ITPN PUE 0034 (CONICET).

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680 Table Captions

681 Table 1. MIP tests

682 Table 2. Average values of density and mass absorption after carbonation

683 Table 3. Relative percentage of the crystalline phases before and after exposure

684 Figure Captions

685 Fig. 1. Carbonation system

686 Fig. 2. PC (Left longitudinal and top radial samples) and BNC05 (right longitudinal and
687 inferior radial) samples after 120 days of carbonation

688 Fig. 3. Cumulative intrusion volume and PSD of PC

689 Fig. 4. Cumulative intrusion volume and PSD of BNC05
662 the chemo-mechanical behaviour of leached cement-based materials: Interactions
663 between damage and leaching, Cem. Concr. Res. 40 (2010) 1226–1236.
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668 https://doi.org/10.1007/s10853-008-2521-5.

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678 https://doi.org/10.1016/j.matdes.2016.10.050.

680 Table Captions

681 Table 1. MIP tests

682 Table 2. Average values of density and mass absorption after carbonation

683 Table 3. Relative percentage of the crystalline phases before and after exposure

684 Figure Captions

685 Fig. 1. Carbonation system

686 Fig. 2. PC (Left longitudinal and top radial samples) and BNC05 (right longitudinal and
687 inferior radial) samples after 120 days of carbonation

688 Fig. 3. Cumulative intrusion volume and PSD of PC

689 Fig. 4. Cumulative intrusion volume and PSD of BNC05
**Fig. 5.** Cumulative intrusion volume and PSD of BNC15

**Fig. 6.** Capillary porosity of non-carbonated samples and 30-days carbonated samples (PC, BNC05 and BNC15)

**Fig. 7.** Capillary porosities at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15).

**Fig. 8.** PSD at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15)

**Fig. 9.** XRD patterns at the core (Int) and near the outer rim (Ext) of PC and BNC15 samples exposed to scCO2 for 120 days.

**Fig. 10.** Development of the compressive strength (red) and Young's modulus (blue) of PC (a), BNC05 (b) and BNC15 (c) over the carbonation time.

**Fig. 11.** PSD variation in time for a point near the exposed surface of PC samples. Non-carbonated (Time 1), 15 days of carbonation (Time 2), 30 days of carbonation (Time 3) and 120 days of carbonation (Time 4).

<table><caption>Table 1. MIP Tests</caption><thead><tr><th>Test</th><th>Reference</th><th>BNC [%]</th><th>Carbonation days</th></tr></thead><tbody><tr><td>1</td><td>PCNC</td><td>0</td><td>0</td></tr><tr><td>2</td><td>PC30Ext</td><td>0</td><td>30</td></tr><tr><td>3</td><td>PC120Int</td><td>0</td><td>120</td></tr><tr><td>4</td><td>PC120Ext</td><td>0</td><td>120</td></tr><tr><td>5</td><td>BNC05NC</td><td>0.05</td><td>0</td></tr><tr><td>6</td><td>BNC0530Ext</td><td>0.05</td><td>30</td></tr><tr><td>7</td><td>BNC05120Int</td><td>0.05</td><td>120</td></tr><tr><td>8</td><td>BNC05120Ext</td><td>0.05</td><td>120</td></tr><tr><td>9</td><td>BNC15NC</td><td>0.15</td><td>0</td></tr><tr><td>10</td><td>BNC1530Ext</td><td>0.15</td><td>30</td></tr><tr><td>11</td><td>BNC15120Int</td><td>0.15</td><td>120</td></tr><tr><td>12</td><td>BNC15120Ext</td><td>0.15</td><td>120</td></tr></tbody></table>

<table><caption>Table 2. Average values of density and mass absorption after carbonation</caption><thead><tr><th rowspan="2">Samples<br>(I and II)</th><th rowspan="2">Initial density<br>[g/cm³]</th><th colspan="2">30 Days of carbonation</th><th colspan="2">120 Days of carbonation</th></tr><tr><th>Mass<br>Uptake [g]</th><th>Density<br>variation [%]</th><th>Mass<br>Uptake [g]</th><th>Density<br>variation [%]</th></tr></thead><tbody><tr><td>PC</td><td>1.99</td><td>2.7</td><td>+1.4</td><td>4</td><td>+2.5</td></tr><tr><td>BNC05</td><td>1.93</td><td>2.8</td><td>+1.5</td><td>3.3</td><td>+2</td></tr><tr><td>BNC15</td><td>1.97</td><td>2.1</td><td>+1.3</td><td>7.5</td><td>+3.6</td></tr></tbody></table>
**Fig. 5.** Cumulative intrusion volume and PSD of BNC15

**Fig. 6.** Capillary porosity of non-carbonated samples and 30-days carbonated samples (PC, BNC05 and BNC15)

**Fig. 7.** Capillary porosities at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15).

**Fig. 8.** PSD at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15)

**Fig. 9.** XRD patterns at the core (Int) and near the outer rim (Ext) of PC and BNC15 samples exposed to scCO2 for 120 days.

**Fig. 10.** Development of the compressive strength (red) and Young's modulus (blue) of PC (a), BNC05 (b) and BNC15 (c) over the carbonation time.

**Fig. 11.** PSD variation in time for a point near the exposed surface of PC samples. Non-carbonated (Time 1), 15 days of carbonation (Time 2), 30 days of carbonation (Time 3) and 120 days of carbonation (Time 4).

<table><caption>Table 1. MIP Tests</caption><thead><tr><th>Test</th><th>Reference</th><th>BNC [%]</th><th>Carbonation days</th></tr></thead><tbody><tr><td>1</td><td>PCNC</td><td>0</td><td>0</td></tr><tr><td>2</td><td>PC30Ext</td><td>0</td><td>30</td></tr><tr><td>3</td><td>PC120Int</td><td>0</td><td>120</td></tr><tr><td>4</td><td>PC120Ext</td><td>0</td><td>120</td></tr><tr><td>5</td><td>BNC05NC</td><td>0.05</td><td>0</td></tr><tr><td>6</td><td>BNC0530Ext</td><td>0.05</td><td>30</td></tr><tr><td>7</td><td>BNC05120Int</td><td>0.05</td><td>120</td></tr><tr><td>8</td><td>BNC05120Ext</td><td>0.05</td><td>120</td></tr><tr><td>9</td><td>BNC15NC</td><td>0.15</td><td>0</td></tr><tr><td>10</td><td>BNC1530Ext</td><td>0.15</td><td>30</td></tr><tr><td>11</td><td>BNC15120Int</td><td>0.15</td><td>120</td></tr><tr><td>12</td><td>BNC15120Ext</td><td>0.15</td><td>120</td></tr></tbody></table>

<table><caption>Table 2. Average values of density and mass absorption after carbonation</caption><thead><tr><th rowspan="2">Samples<br>(I and II)</th><th rowspan="2">Initial density<br>[g/cm³]</th><th colspan="2">30 Days of carbonation</th><th colspan="2">120 Days of carbonation</th></tr><tr><th>Mass<br>Uptake [g]</th><th>Density<br>variation [%]</th><th>Mass<br>Uptake [g]</th><th>Density<br>variation [%]</th></tr></thead><tbody><tr><td>PC</td><td>1.99</td><td>2.7</td><td>+1.4</td><td>4</td><td>+2.5</td></tr><tr><td>BNC05</td><td>1.93</td><td>2.8</td><td>+1.5</td><td>3.3</td><td>+2</td></tr><tr><td>BNC15</td><td>1.97</td><td>2.1</td><td>+1.3</td><td>7.5</td><td>+3.6</td></tr></tbody></table>
706

<table><caption>Table 3. Relative percentage of the crystalline phases before and after exposure</caption><thead><tr><th rowspan="2">Crystalline phase (%)</th><th colspan="3">PC</th><th colspan="3">BNC15</th></tr><tr><th>NC</th><th>120Int</th><th>120Ext</th><th>NC</th><th>120Int</th><th>120Ext</th></tr></thead><tbody><tr><td>Portlandite</td><td>50</td><td>52</td><td>5</td><td>63</td><td>55</td><td>1</td></tr><tr><td>Katoite</td><td>16</td><td>23</td><td>-</td><td>14</td><td>26</td><td>-</td></tr><tr><td>Gypsum</td><td>-</td><td>Traces</td><td>Traces</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Brownmillerite</td><td>16</td><td>11</td><td>9</td><td>12</td><td>10</td><td>5</td></tr><tr><td>Tobermorite</td><td>-</td><td>-</td><td>-</td><td>Traces</td><td>Traces</td><td>-</td></tr><tr><td>Brucite</td><td>-</td><td>3</td><td>-</td><td>-</td><td>Traces</td><td>-</td></tr><tr><td>Okenite</td><td>5</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Calcite</td><td>13</td><td>10</td><td>12</td><td>10</td><td>8</td><td>9</td></tr><tr><td>Aragonite</td><td>-</td><td>Traces</td><td>73</td><td>-</td><td>-</td><td>85</td></tr></tbody></table>

707

<figure><img src="image_3.png" /></figure>

708

709

<figure><img src="image_4.png" /><figcaption>Fig. 1. Carbonation system</figcaption></figure>

710
706

<table><caption>Table 3. Relative percentage of the crystalline phases before and after exposure</caption><thead><tr><th rowspan="2">Crystalline phase (%)</th><th colspan="3">PC</th><th colspan="3">BNC15</th></tr><tr><th>NC</th><th>120Int</th><th>120Ext</th><th>NC</th><th>120Int</th><th>120Ext</th></tr></thead><tbody><tr><td>Portlandite</td><td>50</td><td>52</td><td>5</td><td>63</td><td>55</td><td>1</td></tr><tr><td>Katoite</td><td>16</td><td>23</td><td>-</td><td>14</td><td>26</td><td>-</td></tr><tr><td>Gypsum</td><td>-</td><td>Traces</td><td>Traces</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Brownmillerite</td><td>16</td><td>11</td><td>9</td><td>12</td><td>10</td><td>5</td></tr><tr><td>Tobermorite</td><td>-</td><td>-</td><td>-</td><td>Traces</td><td>Traces</td><td>-</td></tr><tr><td>Brucite</td><td>-</td><td>3</td><td>-</td><td>-</td><td>Traces</td><td>-</td></tr><tr><td>Okenite</td><td>5</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Calcite</td><td>13</td><td>10</td><td>12</td><td>10</td><td>8</td><td>9</td></tr><tr><td>Aragonite</td><td>-</td><td>Traces</td><td>73</td><td>-</td><td>-</td><td>85</td></tr></tbody></table>

707

<figure><img src="image_2.png" /></figure>

708

709

<figure><img src="image_3.png" /><figcaption>Fig. 1. Carbonation system</figcaption></figure>

710
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** PC (Left longitudinal and top radial samples) and BNC05 (right longitudinal and inferior radial) samples after 120 days of carbonation</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Cumulative intrusion volume and PSD of PC.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** PC (Left longitudinal and top radial samples) and BNC05 (right longitudinal and inferior radial) samples after 120 days of carbonation</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Cumulative intrusion volume and PSD of PC.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Cumulative intrusion volume and PSD of BNC05</figcaption></figure>

718

719

720
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Cumulative intrusion volume and PSD of BNC05</figcaption></figure>

718

719

720
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Cumulative intrusion volume and PSD of BNC15</figcaption></figure>

721

722

723
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Cumulative intrusion volume and PSD of BNC15</figcaption></figure>

721

722

723
724

<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Capillary porosity of non-carbonated samples and 30-days carbonated samples (PC, BNC05 and BNC15)</figcaption></figure>

725

726

727

728

<figure><img src="image_12.png" /></figure>

729
724

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Capillary porosity of non-carbonated samples and 30-days carbonated samples (PC, BNC05 and BNC15)</figcaption></figure>

725

726

727

728

<figure><img src="image_11.png" /></figure>

729
730

**Fig. 7.** Capillary porosities at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15).

731

<figure><img src="image_13.png" /></figure>

732

733

**Fig. 8.** PSD at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15).

734

<figure><img src="image_14.png" /></figure>

735

736

**Fig. 9.** XRD patterns at the core (Int) and near the outer rim (Ext) of PC and BNC15 samples exposed to scCO₂ for 120 days.

737
730

**Fig. 7.** Capillary porosities at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15).

731

<figure><img src="image_12.png" /></figure>

732

733

**Fig. 8.** PSD at the core (Interior) and near the exposed surface (Exterior) of samples after 120-days of carbonation (PC, BNC05 and BNC15).

734

<figure><img src="image_13.png" /></figure>

735

736

**Fig. 9.** XRD patterns at the core (Int) and near the outer rim (Ext) of PC and BNC15 samples exposed to scCO₂ for 120 days.

737
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 10.** Development of the compressive strength (red) and Young's modulus (blue) of PC (a), BNC05 (b) and BNC15 (c) over the carbonation time.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 11.** PSD variation in time for a point near the exposed surface of PC samples. Non-carbonated (Time 1), 30 days of carbonation (Time 2) and 120 days of carbonation (Time 3).</figcaption></figure>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 10.** Development of the compressive strength (red) and Young's modulus (blue) of PC (a), BNC05 (b) and BNC15 (c) over the carbonation time.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 11.** PSD variation in time for a point near the exposed surface of PC samples. Non-carbonated (Time 1), 30 days of carbonation (Time 2) and 120 days of carbonation (Time 3).</figcaption></figure>