Mechanical performance and water resistance of biochar 2024 Science of The T

Science of the Total Environment 912 (2024) 168773

Available online 24 November 2023

0048-9697/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Mechanical performance and water resistance of biochar admixture lightweight magnesium oxychloride cement

Yuekai Xie

^a,*

, Hongxu Wang

^a

, Yingying Guo

^a,b

, Chenman Wang

^c

, Hanwen Cui

^a,d

, Jianfeng Xue

^a

aSchool of Engineering and Technology, The University of New South Wales, Canberra, ACT 2612, Australia

bCivil Branch, Infrastructure Delivery Partner, Major Projects Canberra, Canberra, ACT 2606, Australia

cGuangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, Guangdong, China

dQueensland Department of Transport and Main Roads, South Coast Region, Nerang, QLD 4211, Australia

H I G H L I G H T S G R A P H I C A L A B S T R A C T

•Effect of biochar on strength of magne- sium oxychloride cement (MOC) was tested.

•56-day strength retention ratio of MOC incorporated with biochar was tested.

•Biochar addition promoted formation of phase 5 and decreased unreacted MgO.

•Compressive strength of immersed MOC during drying was described by a new formula.

A R T I C L E I N F O Editor: Daniel CW Tsang Keywords:

Magnesium oxychloride cement Biochar

Water resistance Compressive strength Degree of saturation

A B S T R A C T

The applications of magnesium oxychloride cement (MOC) have been extensively studied recently due to its eco- friendly and high-strength nature. However, one of the significant limitations of MOC is its poor water resistance.

To address this limitation, this study explored the prospect of incorporating biochar particles (up to 25 % of the dry mass of MgO) to form lightweight MOC with improved water resistance. The compressive (fc) and flexural (ff) strengths were investigated after 28-day curing and under 56-day water attack. The fc of MOC after immersion was determined under both wet (directly after immersion) and dry (air-dried to constant weights) conditions.

The results indicated that the inclusion of 5 % and 10 % biochar increased the 28-day fc, while the addition of biochar decreased ff regardless of its dosage. Microscopic examination uncovered that the increase in strength resulted from the promoted production of phase 5 (5 Mg(OH)2⋅MgCl2⋅8H2O) and the reduction in unreacted MgO. The inclusion of 5 % and 10 % biochar increased the compressive and flexural strength retention ratios after 56-day immersion. The ff with 5 % biochar inclusion after immersion was higher compared to that of pure MOC. Moreover, the inclusion of biochar had minimal effects on the thermal degradation of MOC. The above results suggest that biochar can be a potential additive to enhance the mechanical behaviour and water resistance

* Corresponding author.

E-mail address: yuekai.xie@adfa.edu.au (Y. Xie).

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Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

https://doi.org/10.1016/j.scitotenv.2023.168773

Received 29 August 2023; Received in revised form 14 November 2023; Accepted 19 November 2023

of MOC. As fc of immersed MOC increased during air-drying, a new equation was developed to describe varia- tions in fc of MOC subject to different degrees of saturation during drying.

1. Introduction

The construction industry has been one of the key sources of CO2

emissions and waste generation (Xie, 2022; Ma et al., 2022). Compared to conventional Ordinary Portland Cement (OPC), magnesium oxy- chloride cement (MOC) is more environmentally friendly due to low carbon emissions during its manufacture (Guo et al., 2018). MOC has been extensively used in construction materials due to its quick hard- ening rates, high mechanical performance and durability, low carbon footprints and thermal conductivities, and satisfying abrasion resistance (Hao and Li, 2021; Li et al., 2021).

Nevertheless, an important problem associated with MOC is its low water resistance (Gu et al., 2021). The strength of MOC-based materials is mainly attributed to phase 5 (5 Mg(OH)2⋅MgCl2⋅8H2O) and phase 3 (3 Mg(OH)2⋅MgCl2⋅8H2O). The phase 3 (P3) and phase 5 (P5), however, are readily converted to brucite (Mg(OH)2) upon exposure to water (Guo et al., 2021). Therefore, this notable deficiency in water resistance of MOC significantly limits its practical application (Yu et al., 2020a; Ye et al., 2021).

Various experiments have been carried out to enhance the water stability of MOC by incorporating different additives or modifiers, including acids (e.g., hydroxyacetic acid, phosphoric acid), acid salts (e.

g., sodium monofluorophosphate, potassium dihydrogen phosphate), and active silicon dioxide (e.g., pulverized fuel ash, fly ash, silica fume, blast-furnace slag) (Li et al., 2020; Luo et al., 2020; Guo et al., 2021, 2022). Although those additives slightly decreased the compressive strength, significant improvement in the water stability was achieved.

For example, the addition of 50 % risk husk ash increased MOC's 14-day compressive strength retention ratio by 24 % compared to pure MOC (Nie et al., 2022). This improvement was attributed to the changes in growth orientations and morphologies of P5 crystals. However, only around 50 % of compressive strength (after air-drying) was retained after 56-day water immersion. Therefore, further studies need to be conducted to enhance the water resistance of MOC.

Another limitation regarding the understating of water resistance of MOC is the moisture content. The strength of MOC could be low if directly measured after water immersion. To avoid this problem, immersed MOC was air-dried for 24 h prior to measurement (Guo et al., 2021). However, due to various pore shapes and diameters, the exclu- sion rates of moisture from MOC with different additives can be different. The variable moisture contents in MOC may contribute to variations in its compressive strength (Guo, 2020). In field conditions, MOC can be partially saturated instead of fully saturated or even fully air-dried. It is, therefore, indispensable to understand the strength variation of immersed MOC at different degrees of saturation.

Biochar is a carbon-rich, lightweight, and carbon-negative material produced by pyrolysis and gasification of biomass in anaerobic condi- tions (Wang and Wang, 2019; Zhang et al., 2022). The sources of biochar are generally wood and food wastes. Historically, biochar has been used for adsorbing harmful gases (Braghiroli et al., 2019), carbon captures (Dissanayake et al., 2020), soil amendments (Wong et al., 2022; Xie et al., 2023), and immobilizations of contamination elements (Meng et al., 2020). Recently, the applications of biochar in construction ma- terials have attracted extensive attention owing to its porous structure, low bulk density, and diverse surface functional groups (Zhang et al., 2022). Biochar can be applicable to manufacture lightweight and energy-efficient materials (Praneeth et al., 2021). For instance, after being mixed with biochar, the degrees of hydration and porosity of cementitious materials in the vicinity of biochar particles were increased significantly (Zhu et al., 2023). This potentially contributed to higher compressive strength and fracture toughness of cement-based materials

(Tayyab et al., 2023). Furthermore, the inclusion of biochar could enhance the hydration processes, thereby improving the mechanical performance of cementitious materials. This was attributed to the in- ternal curing effects and the high water retention capacity of angular, flaky, and needle-like biochar particles (Gupta and Kua, 2019; Wang et al., 2019; Chen et al., 2022a).

Furthermore, the inclusion of biochar increased the water resistance of MOC after 7 days of immersion, owing to (1) the physical filling ca- pacity of biochar to stabilize dispersed particles and create strong structures to resist water penetration, (2) improved cohesion strength due to interactions between active functional groups of biochar and Mg²⁺, preventing from water erosion, and (3) core-shell structures with a waterproof barrier produced by P5, enhancing water stability (Han et al., 2022). The core-shell structures were formed due to the nucleation sites provided by the porous structures of biochar particles. The dense and rod-like P5 was generated within and on the surface of biochar particles, forming a multi-phase barrier to resist water attack. However, a few limitations should be noted. The immersion duration of 7 days was short compared to 56 days in other studies (e.g., Guo et al., 2021). The development of cracks or a significant reduction in strength may occur after 7 days of immersion (Guo, 2020). Additionally, the flexural strength of biochar-incorporated MOC has not been well investigated.

Further studies should be conducted to gain a comprehensive under- standing of the performance of MOC with water immersion.

This paper investigated the impact of biochar admixture on the mechanical behaviour and water resistance of MOC. The biochar con- tents varied from 5 % to 25 % (w/w) of MgO. The compressive and flexural strengths of MOC with different biochar contents were tested under water attack after 14, 28, and 56 days. Additionally, a new empirical equation was developed to characterize the compressive strength of immersed MOC during air-drying. To reveal the mechanisms of effects induced by biochar on the performance of MOC, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy/energy dispersive spectroscopy (SEM- EDS), and mercury intrusion porosimetry (MIP) analysis were per- formed. The thermal properties of MOC were evaluated by thermogra- vimetric analysis/derivative thermogravimetry (TGA/DTG).

2. Materials and methodology 2.1. Raw materials

The light-burnt magnesia powders (MgO) with purity over 90 % and magnesium chloride hexahydrate (MgCl2⋅6H2O) with purity of 97 % were used. The compositions of MgO and MgCl2 are summarized in Tables S1 and S2, respectively. To prepare the MOC pastes, a concen- trated MgCl2 solution (28.4 %) was obtained by dissolving MgCl2⋅6H2O into demineralized water. The prepared solution was stabilized for at least 24 h prior to mixing.

The biochar was generated by pyrolysis of Eucalyptus woods over 450 ^◦C. The temperatures between 300 and 500 ^◦C were beneficial to the high yields, large surface areas, and excellent absorption capacities of biochar produced from Eucalyptus wood (Zeng et al., 2021). The further increase in pyrolysis temperatures could lead to increased energy con- sumption for biochar production. The compositions of biochar were determined by the combustion technique, revealing the main chemical elements as C (67 %), O (27 %), H (4 %), and N (2 %). The biochar was screened by a 425 μm sieve prior to mixing. The particle size distribution of MgO and biochar is presented in Fig. S1 (a). Fig. S1 (c) presents the patterns of biochar particles in SEM. The dry density, specific gravity, and porosity of biochar were 0.36 g/cm³, 1.32, and 72 %, respectively.

Fig. S1 (b) presents the FTIR results of the prepared biochar. The peaks can be observed at 2921, 2851, 1567, 1416, 1031, and 873 cm⁻¹. Those peaks were associated with the asymmetric C–H stretching, symmetric C–H stretching, C=C/C=O/C=N bonds, C––C stretching/

CH₂ bending, symmetric C–O stretching, and C–H bending, respec- tively (Keiluweit et al., 2010; Cantrell et al., 2012).

2.2. Mix design and sample preparation

The contents of biochar added in cement-based materials varied with binders used and products desired. The addition of biochar in previous studies ranged from 0.8 % to 2.5 % (Ahmad et al., 2020) and 5 % to 20 % (Han et al., 2022) by mass of MgO for cement paste, and 169 % to 565 % (saturated surface dry condition) for particleboards (Chen et al., 2022b).

Therefore, the biochar contents varying from 5 % to 25 % mass of MgO were selected in this study. The mix designs used in this study are summarized in Table 1. The molar ratio between MgO, MgCl2, and H2O was maintained constant at 5:1:13. The high ratio between H2O and MgO allowed biochar to absorb sufficient moisture and served as an internal curing component to enhance the hydration processes. The biochar equivalent to 5 %, 10 %, 15 %, and 25 % mass of MgO was added to the mixture for different designs. The binder included MgO and solid MgCl2, while biochar served as the filler. The water/binder ratio (w/b) was held constant at 0.73 for all groups.

To prepare the pastes, the saturated surface dry biochar particles and MgO powder were firstly mixed thoroughly by a mixer. The stabilized MgCl2 solution was then mixed with the powders for 5 min. The fluidity and setting time were measured by the flow test and Vicat needle method (Guo et al., 2021). To prepare samples for compressive strength (fc) determination, pastes were cast in cubic moulds (50 ×50 ×50 mm).

To prepare samples to determine the flexural strength (ff), pastes were cast in prisms (40 ×40 ×160 mm). Finally, the prepared samples were demoulded after 24 h and cured under the temperature of 22 ±2 ^◦C and relative humidity (RH) of 64 ±4 % for 28 days. It is worth noting that the RH during curing was crucial for the mechanical behaviour of MOC (Guo et al., 2018). Previous studies suggested that the prepared MOC samples were cured in an RH range between 50 % and 70 % to achieve optimal mechanical properties (Guo et al., 2021; Nie et al., 2022).

2.3. Mechanical properties

The development of fc of MOC after 28 days of curing was measured under a constant loading rate of 0.33 MPa/s (Yu et al., 2020b). The 28- day ff of MOC was obtained by three-point bending tests at a fixed displacement rate of 0.1 mm/min.

To assess the water resistance of MOC with different biochar con- tents, cured samples were soaked in water at a temperature of 22 ±2 ^◦C for 56 days. The fc and ff of MOC under water immersion after 14, 28, and 56 days were measured in both dry and wet conditions. The wet condition was defined as testing with MOC samples immediately after their removal from water, with surface water wiped away. In contrast, the dry condition was achieved by air-drying the samples at 30 ±2 ^◦C for up to 5 days, until their weights remained unchanged. The fc (after 56 days of water attack) during the drying processes was recorded. The water resistance of MOC could be expressed as strength retention ratios

defined by Eqs. (1) and (2), respectively (Sun et al., 2022; Jin et al., 2023).

Wn,c=fcn

fc (1)

Wn,f=ffn

ff (2)

where Wn,c, Wn,f = strength retention ratio of MOC after n days of immersion,

fcn, ffn =compressive or flexural strength of MOC after n days of immersion,

f_c, f_f =compressive or flexural strength of MOC after curing of 28 days.

After water immersion, the moisture retained within MOC samples can be evaporated by air-drying. However, the evaporation rates from immersed MOC samples depend on biochar contents and environmental conditions, such as temperature and relative humidity. For example, Guo (2020) determined Wn,c at dry conditions by drying samples at 40 ^◦C for 24 h, resulting in potentially higher evaporation rates compared to those in this study. The drying durations may not be appropriate to describe compressive strength retention ratios (Wn,c) during drying. Therefore, W_n,c is correlated with degrees of saturation, as expressed in Eq. (3) (Xie and Xue, 2023). The saturated moisture content is the moisture content of MOC samples after the water attack for 56 days. Whereas the residual moisture contents correspond to the moisture contents of MOC samples after drying for 5 days, when no further moisture can be excluded.

Se= θ− θr

θs− θr (3)

where Se =degrees of saturation,

θ =actual moisture contents at a given time, θ_s =saturated moisture contents,

θ_r =residual moisture contents.

The volumetric changes during drying were found to be neglectable (within 0.5 %), and the loss in mass was attributed to the decrease in the free water contents. Therefore, Eq. (3) can be written as Eq. (4).

Se=mt− mr

ms− mr (4)

where mt =mass of MOC at a given time, ms =mass of MOC after water attack, mr =mass of MOC after air-drying.

2.4. Water absorption and voids

The measurement of water absorption, bulk density, and voids of MOC followed test procedures presented by Praneeth et al. (2021).

Firstly, cured samples were oven-dried under a constant temperature of 110 ±2 ^◦C for 24 h. The weights of dried samples (mA) were recorded post-drying. Subsequently, these dried samples were immersed in water for 48 h at a constant temperature of 22 ±2 ^◦C. After eliminating surface moisture, the total weights of samples (mB) were determined. The samples were then placed in water and boiled for 5 h, followed by 16 h of cooling to room temperatures at 22 ± 2 ^◦C. The weights of boiled samples (mC) were measured after wiping water on the surfaces. Finally, processed samples were suspended by a wire, and immersed apparent weights (mD) were obtained. The water absorption after immersion (wa) and permeable voids (ep) of MOC can be calculated by Eqs. (5) and (6), respectively.

wa=mB− mA

mA (5)

Table 1

Mixture designs of the MOC pastes.

Sample ID Molar ratios Weight ratios (based on MgO)

MgO/MgCl2 H2O/MgCl2 Biochar/MgO

BA0 5 13 0 %

BA5 5 13 5 %

BA10 5 13 10 %

BA15 5 13 15 %

BA25 5 13 25 %

ep=mC− mA

mC− mD (6)

2.5. Microstructure analysis

The samples used for microscopic analysis were prepared using fragments obtained after compression tests. Small pieces of MOC paste (typically 5 to 10 mm) from each mix design were used to analyse the morphology of hydration products of MOC by SEM. Simultaneously, EDS was conducted together with SEM to determine the elemental compo- sitions of MOC. To facilitate this, the samples were coated with a plat- inum layer of 15 nm thick. Additionally, the fine fragments of MOC (characterized by sizes within 5 mm and volumes around 0.1 cm³) were used for MIP tests (Guo et al., 2021).

To prepare samples for XRD, FTIR, and TGA/DTG tests, fragments after compression tests were ground by a ball mill and screened by a 425 μm sieve, ensuring that all biochar particles were retained within the

samples for analysis. The XRD analysis was performed at 2θ ranging from 10^◦to 90^◦with a scanning speed of 0.13^◦/s (Cu Kα radiation). The FTIR tests were implemented to investigate the functional groups at wavelengths ranging from 650 to 4000 cm⁻¹. Besides, the thermal behaviour of MOC specimens was studied via TGA/DTG analysis at temperatures between 40 and 1100 ^◦C under a constant heating rate of 10 ^◦C/min.

3. Results and discussion 3.1. Fluidity and setting time

Fig. 1 (a) presents the fluidity and setting time of fresh MOC pastes with different biochar contents. The percentage of biochar significantly affected the setting time and fluidity of MOC (p <0.05). Notably, as the percentage of biochar increased from 0 % to 25 %, the fluidity of MOC pastes reduced from 348 to 252 mm. This decrease in fluidity was

Fig. 1. Fluidity, setting time, water absorption, voids, bulk density, and pore diameters of MOC with various biochar contents: (a). Fluidity and setting time, (b).

Water absorption and voids, (c). Bulk density after water immersion, (d). Pore size distribution, (e). Cumulative pore volume, (f). Percentage of pore size (unit: μ_m).

attributed to the biochar inclusions, which reduced the water contents in mixtures due to their high water retention capacity. Thus, the moisture in the pastes could be retained in the porous structure of biochar par- ticles. Moreover, the surface contact areas between solids and liquids were increased due to the large surface areas of added biochar granules, resulting in enhanced inner cohesions and thereby reduced fluidity of the fresh pastes.

The setting time of MOC was associated with phase-formation pro- cesses and significantly impacted by the dosages and particle sizes of additives (Guo, 2020). The incorporation of biochar increased the initial and final setting time, signifying a slowdown in the hydration processes.

For instance, the inclusion of 25 % biochar increased both the initial and final setting time from 292 to 388 min and 343 to 440 min, respectively.

The intervals between the initial and final setting times remained almost constant with various biochar contents, ranging from 40 (BA5) to 52 (BA25) minutes. This suggested that the retarded hydration process occurred in both the initial and final setting stages, as a result of the dilution effects of biochar addition. Additionally, the ratios between water and powder in the mix were decreased by biochar addition due to its high porosity and water retention capacity. Owing to the large sur- face area and high porosity of biochar particles, components like Mg²⁺, Cl⁻, OH⁻, and H2O were retained and aggregated on the biochar surface (Nie et al., 2022). These mechanisms contributed to the deceleration of the hydration processes. On the other hand, the hydrophilic nature and filling effects of biochar increased the stickiness of the mixtures while decreasing water secretion (Gupta et al., 2018a; Jia et al., 2023). This contributed to the reduction in the gaps between the initial and final setting. Therefore, the influences of biochar incorporation on the setting time might depend on the balance between the dilution effects and filling effects.

3.2. Bulk density, water absorption, and voids

Water absorption and the presence of permeable voids are key fac- tors affecting the water resistance of MOC (Praneeth et al., 2021). Fig. 1 (b) shows the water absorption rates and permeable voids of MOC with different biochar contents. The biochar content exhibited a significant influence on the water absorption and permeable voids of MOC (p <

0.05). More specifically, the water absorption increased with the per- centage of biochar incorporated. For example, the water absorption of BA25 (16.9 %) was considerably higher compared to BA0 (5.9 %). This difference was attributed to the high porosity and water absorption capacity of biochar particles. The inclusion of 5 % biochar (BA5) slightly decreased permeable voids from 14.5 % to 12.6 %. However, further addition of biochar increased the permeable voids, up to 33.5 % in BA25.

The bulk densities of MOC before and after immersion are shown in Fig. 1 (c). The incorporation of lightweight biochar resulted in a reduction in the bulk density of MOC by up to 16.4 % (1.365 g/cm³) compared to pure pastes (1.633 g/cm³) after 28-day curing. This reduction was attributed to the low-density and high-porosity nature of biochar. After being immersed in water, the bulk densities of pure MOC (BA0) decreased because of the decomposition of major hydration products such as P3 and P5. In the first 28 days of water immersion, the bulk densities of MOC with low biochar additions decreased (BA5) or remained stable (BA10), while those with higher biochar contents increased (i.e., BA15 and BA20). The final bulk densities of MOC with biochar inclusions after 56 days of water attack were higher than that of unexposed MOC. The increases in bulk densities after water immersion were more pronounced in MOC with higher biochar contents. This was attributed to the high water retention capacity of highly porous biochar particles, which allowed the water absorbed by biochar particles to be retained during the test.

The incorporation of biochar decreased the bulk density of MOC after curing. This suggested the reduced usage of MgO and MgCl2 to produce the same volume of MOC. The corresponding reductions in the usage of

MgO and MgCl2⋅6H2O were 5.2 %, 11.1 %, 14.6 %, and 24.1 % for biochar additions of 5 %, 10 %, 15 %, and 25 %, respectively. Based on the production cost data from Li et al. (2017) and Ma et al. (2022), the inclusion of 5 %, 10 %, 15 %, and 25 % biochar had reduced the cost of MOC by 1.8 %, 4.8 %, 5.5 %, and 10.9 %, respectively. In addition, together with biochar particles with a negative carbon footprint (− 0.49 kg CO2/kg biochar) (Praneeth et al., 2021), the decreased consumption of MgO (0.60 kg CO2/kg MgO) and MgCl2⋅6H2O (0.20 kg CO2/kg MgCl2⋅6H2O) (Ma et al., 2022) contributed to a lower carbon footprint.

The corresponding carbon footprint was decreased by 9.0 %, 18.3 %, 24.8 %, and 39.1 % with 5 %, 10 %, 15 %, and 25 % biochar contents, respectively, compared to that of pure MOC. Fig. S2 compares the car- bon footprint of MOC with different additives, including fly ash (FA), silica fume (SF), gypsum, rice husk ash (RHA) (Ma et al., 2022; Nie et al., 2022) and biochar in this study. The lowest carbon footprint can be observed with 25 % biochar (331 kg CO2/m³). This suggested that biochar can be a potential additive to produce low-carbon cement materials.

3.3. Mechanical properties

3.3.1. Compressive and flexural strength

Fig. 2 (a) presents the development of fc with various biochar con- tents at curing ages of 7, 14, and 28 days, while the ff of MOC with different additions of biochar is shown in Fig. 3 (a). As illustrated in the figures, the biochar contents led to a significant impact on both fc and ff

of MOC (p <0.05). Specifically, the 5 % and 10 % dosages of biochar (BA5 and BA10) increased the fc by 14.1 % and 10.9 %, respectively, compared to the fresh MOC (BA0). The fc of MOC incorporated with 5 % and 10 % biochar reached 36.0 and 35.0 MPa, in comparison to 31.6 MPa for pure MOC. However, a reduction in f_c was observed in MOC with 15 % (27.2 MPa) and 25 % biochar (19.8 MPa), corresponding to 13.7 % and 37.2 % reductions compared to pure MOC (31.6 MPa). In terms of flexural strength, the inclusion of biochar decreased f_f regard- less of its dosage. The ff was 7.41, 6.80, 6.46, 6.19, and 4.81 MPa for biochar contents of 0 %, 5 %, 10 %, 15 %, and 25 %, respectively. The corresponding reductions were 8.3 %, 12.9 %, 16.9 %, and 35.1 % for biochar contents of 5 %, 10 %, 15 %, and 25 %, respectively.

The reduction in water/MgCl2 ratio was one of the contributory factors to the improvement in fc and ff of MOC (Nie et al., 2022). In this study, the high water absorption capacity of biochar resulted in the decrease in water/MgCl2 ratios in MOC mixtures, thereby contributing to the increased fc of MOC. However, a boost in biochar dosages implied a reduction in MgO contents and an increase in porosity. The reduction in the proportion of MgO may result in the decrease of the major strengthening component, i.e., P5, whereas the high porosity could lead to a loose and dispersed structure, and consequently the reduction in f_c and ff.

The effects of biochar on fc and ff were different. The incorporation of a small quantity of biochar (e.g., 5 %) could increase fc, whereas the addition of biochar consistently decreased ff regardless of its dosage.

This phenomenon might be attributed to the generation of pores in the tensile plane (Gupta et al., 2018b). Although the porous nature of bio- char particles increased the water retention capacity of MOC, biochar particles also trapped air in the porous structures. The trapped air resulted in the formation of pores in MOC. These pores had a negligible impact on f_c because of the closure of pores and voids under compressive loading, ultimately leading to a shear-dominated failure (Gupta and Kua, 2019). On the other hand, ff was sensitive to these pores on the tension side under bending due to the extension of defects in tension.

These defects could serve as weak points for the development of cracks.

3.3.2. Water resistance

The fc values of MOC after water immersion in both dry and wet conditions are presented in Fig. 2 (c) and (e), respectively. The corre- sponding Wn,c is shown in Fig. 2 (d) and (f). Fig. 3 (a) illustrates the ff of

Fig. 2. Compressive strength of MOC with various biochar contents: (a). Strength development, (b). Relative compressive strength retention ratios after immersion, (c). Residual compressive strength (dry condition), (d). Compressive strength retention ratio (dry condition), (e). Residual compressive strength (wet condition), (f).

Compressive strength retention ratio (wet condition).

Fig. 3. Flexural strength of MOC with various biochar contents: (a). Flexural strength (dry condition), (b). Flexural strength retention ratio (dry condition).

MOC after different durations of water immersion. The corresponding W_n,f is shown in Fig. 3 (b). After immersion in water for 14, 28, and 56 days, fc in both dry and wet conditions decreased with immersion pe- riods. Such reductions were attributed to the hydration of MgO, degra- dation of P3 and P5, and dissolution of MgCl₂ (Guo et al., 2022).

Although the strength of MOC decreased significantly due to water attack, the strength retention ratios (Wn,c and Wn,f) were improved because of appropriate percentages of biochar incorporation. After 28 days, Wn,c in dry conditions (0.89 to 0.92) of biochar-incorporated MOC was higher than that of pure MOC (0.77). After 56 days, Wn,c (dry conditions) ranged from 0.74 to 0.88 with different biochar contents.

Only 5 % (BA5) and 10 % (BA10) biochar additions showed higher Wn,c

compared to pure MOC. In wet test conditions, the corresponding Wn,c of MOC showed similar results, but the Wn,c with 5 % and 10 % biochar addition was slightly higher than that of BA0 (0.49).

As presented in the figures, the addition of biochar increased the Wn,f

of MOC despite its dosage. The Wn,f of MOC with biochar addition varied from 0.67 to 0.80, higher than that of pure MOC (0.60). Although bio- char reduced initial ff after curing, it increased ff and Wn,f after water immersion with appropriate addition (5 %). The increase in strength retention with water attack resulted from the reduction in MgO pro- portion in the matrix. The inclusion of biochar potentially reduced the unreacted MgO and thereby decreased the risk of cracking (Nie et al., 2022).

3.3.3. Strength development during air-drying after water immersion Fig. 4 (a) presents the changes in mass of MOC during air-drying. The final masses of MOC cubes ranged from 94 % to 96 % compared to that before drying. The fc of immersed MOC increased during air-drying (Fig. 4 (b)). The Wn,c was correlated with the degrees of saturation, as

shown in Fig. 4 (c). The Wn,c decreased with degrees of saturation, i.e., increased during drying. The W_n,_c can be estimated with Eq. (7), and the fitted results were presented as lines in Fig. 4 (c). The predicted Wn,c

fitted well with the test results. The corresponding strength degradation ratios (λ) with different biochar contents are shown in Fig. 4 (d).

Wn,c=Wn,de^−λSe (7)

where Wn,d =compressive strength retention ratios at dry conditions, λ =strength degradation ratios.

3.4. Microscale analysis 3.4.1. SEM

Fig. 5 presents the SEM results of BA0, BA5, and BA25, together with EDS. The EDS was performed with a rectangular mapping (dotted liners in yellow), as shown in the figures. The typical shape of the main hy- dration products (P5) was like rods or needles, as can be seen from Fig. 5 (a, c, e). The EDS analysis results shown in Fig. 5 (d) present the typical elemental composition of P5. Additionally, a small quantity of unreacted MgO can be found in the well-cured samples, as shown in Fig. 5 (a). The corresponding EDS results are presented in Fig. 5 (b). The outside part of the MgO agglomeration was reacted with MgCl2 solution and formed some intermediate hydration products. Therefore, Cl can be detected in the EDS spectrum. Compared to P5 and other components, the sizes and pores of biochar particles were larger.

As shown in Fig. 5 (c) and (e), needle-like P5 crystals grew on the surface of biochar particles, but the structure and interlock between P5 and biochar particles were loose and scattered. The high contents of biochar indicated that a significant quantity of solutions can be absorbed by the biochar particles. Fine MgO powders in the paste can react with

Fig. 4. Compressive strength of MOC during air-drying: (a). Relative mass, (b). Compressive strength retention ratios with time, (c). Compressive strength retention ratios with degrees of saturation, (d). Strength degradation ratios.

Fig. 5. SEM results of MOC with various biochar contents: (a). SEM of BC0, (b). EDS of MgO, (c). SEM of BC5, (d). EDS of P5, (e). SEM of BC25, (f). EDS of biochar.

the solution absorbed by biochar slowly and thus randomly form P5 crystals near or along the channels or pores of biochar particles. The biochar particles can also retain moisture twice as much as their self- mass. Although those solutions can be discharged into the MOC pastes, the high moisture contents or RH near biochar particles may convert P5 to P3 or brucite. This inhibited the formation of P5 in the pastes after curing and resulted in loose and discrete P5 structures. The loose and discontinuous structure of P5 can lead to lower cohesive strength and load-transferring capacity (Han et al., 2022). This ulti- mately resulted in the reduction in fc and ff.

Fig. 5 (c) shows a denser distribution of P5 in BA5 compared to that in BA0 (Fig. 5 (a)). Biochar particles absorbed the MgCl2 solution during mixing. Those absorbed ions and moisture can be gradually released into the pastes and consume the unreacted MgO. This enhanced the hydra- tion processes, especially near or within the pores in the biochar parti- cles. This indicated that the addition of a small amount of biochar could potentially form a denser and continuous surface, thereby contributing to higher strength. Higher energy was required to break carbon particles compared to other hydration products (Praneeth et al., 2021). The ex- istence of biochar may contribute to ductile failure instead of surface fractures with increased strength (Ahmad et al., 2015). Similarly, Han et al. (2022) suggested that the incorporation of biochar reduced the macro-cracks generated within the MOC matrix and enhanced structure compactness. The produced P5 and P3 can form a protective core shell structure and increase the stability of the hydration components. How- ever, a large quantity of biochar additions resulted in loose and dispersed microstructures, which decreased the cohesion between par- ticles and generated weakened interfacial areas. Similar phenomena have been observed in other studies (Muthukrishnan et al., 2019; Pra- neeth et al., 2021).

3.4.2. XRD

Fig. 6 (a) presents the effects of biochar on XRD analytical results of MOC. The corresponding phase compositions are summarized in Table S3. The addition of biochar (5 % and 10 %) increased the for- mation of P5 and decreased the unreacted MgO contents in the matrix.

The increase in P5 directly enhanced the strength of MOC. This was consistent with the mechanical properties shown in Figs. 2 and 3. For

MOC with a small amount of biochar, the liquid phase (MgCl2 solution) can be retained in the pores of biochar particles. Initially, the pores were filled with moisture, and the pastes were fully saturated with the RH of 100 %. Subsequently, the pastes gradually lost moisture due to hydra- tion processes and evaporation. This contributed to self-desiccation (Gupta and Kua, 2018). The reduction in internal RH of MOC pastes may retard the hydration processes. The moisture and ions in biochar particles can be gradually released into the matrix, and thus supplement the hydration processes (Gupta and Kashani, 2021). This directly resulted in the reduction in unreacted MgO, an increase in the strengthening component, i.e., P5, and a dense structure of the pastes (Han et al., 2022). The reduction in MgO potentially decreased micro- cracks caused by expansion during water immersion (Nie et al., 2022).

The further incorporation of biochar promoted the generation of P3 and reduced the formation of P5 with the decrease in the strength of MOC.

The biochar with high proportions absorbed Mg²⁺and prevented the reaction between Mg²⁺and MgO. Therefore, the surface of unreacted MgO can be surrounded by biochar particles. Those particles can perform as a barrier and reduce the interaction between MgO solids and MgCl2 solution. This decreased further hydration processes in both air- curing and water immersion (Han et al., 2022).

3.4.3. FTIR

Fig. 6 (b) shows the FTIR spectrum of MOC samples with different biochar contents. The peaks at 3690, 3641, and 3610 cm⁻¹ were induced by Mg-OH stretching in P5. The peaks at 3392 and 1601 cm⁻¹ resulted from O–H stretching of water in crystalline P5. The weak peaks at 1434 and 873 cm⁻¹ were relevant to C––O bond of CO²⁻₃ . The peak at 717 cm⁻¹ indicated CH2 groups were connected, as a result of biochar incorporation (Ren et al., 2021). There was no significant difference with the FTIR results shown in Fig. 6 (b). The FTIR patterns in a narrow range of wavelength between 1600 and 800 cm⁻¹ suggested the exis- tence of symmetric C–O stretching, and C–H bending (Fig. S3). The increase in peak intensity at 3690, 3641, and 3610 cm⁻¹ with 5 % and 10 % biochar addition indicated the enhanced production of P5, as consistent with XRD results.

Fig. 6. XRD, FTIR and TG-DTG results of MOC with various biochar contents: (a). XRD, (b). FTIR, (c). TGA, (d). DTG.

3.4.4. TGA/DTG

Fig. 6 (c, d) illustrates the thermal performance of MOC specimens.

The mass loss of MOC samples during heating is presented in Fig. 6 (c).

The corresponding mass loss rates are shown in Fig. 6 (d). The thermal degradation of P5 and P3 can be expressed into two processes, i.e., dehydration and decomposition. The dehydration involved two stages, i.

e., Eq. (8) at temperatures between 40 and 110 ^◦C and Eq. (9) between 110 and 230 ^◦C for P5 (Luo et al., 2020), and Eq. (10) at temperatures between 40 and 125 ^◦C and Eq. (11) at temperatures between 125 and 215 ^◦C for P3 (Lojka et al., 2020). The decomposition processes of P3 and P5 were similar and can be divided into two steps, i.e., Eqs. (12) and (13), and Eq. (14) at temperatures between 350 and 500 ^◦C. After 500 ^◦C, the mass loss was mainly induced by the decomposition of MgCO3 (Eq. (15)) (Guo et al., 2021).

5Mg(OH)2•MgCl₂•8H2O→5Mg(OH)2•MgCl₂•4H2O+4H2O (8) 5Mg(OH)2•MgCl₂•4H2O→5Mg(OH)2•MgCl₂+4H2O (9) 3Mg(OH)2•MgCl₂•8H2O→3Mg(OH)2•MgCl₂•4H2O+4H2O (10) 3Mg(OH)2•MgCl₂•4H2O→3Mg(OH)2•MgCl₂+4H2O (11) 5Mg(OH)2•MgCl₂→MgCl₂•H2O+4H2O+5MgO (12) 3Mg(OH)2•MgCl₂→MgCl₂•H2O+4H2O+3MgO (13)

MgCl₂•H2O→MgO+2HCl (14)

MgCO₃→MgO+CO2 (15)

The DTG curves during the heating processes are presented in Fig. 6 (d). There was no additional peak observed due to the addition of bio- char. This suggested that the incorporation of biochar did not signifi- cantly affect the hydration products and phase formations. As the dehydration processes of P3 and P5 occurred at similar temperatures, the peaks at around 150 ^◦C were attributed to the dehydration of both P3 and P5. This might cause the higher peak of BA25 at 150 ^◦C than that of BA15.

The relative mass, mass loss in each stage, and accumulated loss are summarized in Table 2. The addition of biochar did not have a signifi- cant effect on the decomposition process of MOC. The final mass ranged from 43.1 % to 45.3 % of the initial mass, suggesting that biochar was thermal stable in an oxygen-free environment, and the incorporation of biochar did not result in further mass loss after heat exposures. The total mass loss at 1100 ^◦C varied from 54.6 % to 56.9 %.

3.4.5. MIP

The pore size distribution of MOC with 0 %, 5 %, and 25 % biochar additions is shown in Fig. 1 (d). The cumulative volume is plotted accordingly in Fig. 1 (e). The addition of 5 % biochar decreased the cumulative pore volume, whereas the incorporation of 25 % biochar increased the cumulative pore volume. The pores in the MOC can be classified as macro-pores (>5 μm), large capillary pores (0.1 to 5 μm), middle capillary pores (0.05 to 0.1 μm), mesopores (0.01 to 0.05 μm), and gel micropores (<0.01 μm) (Guo et al., 2021). The existence of macro- and large capillary pores can be destructive to the mechanical properties of MOC. There were almost no macro-pores after the incor- poration of 5 % biochar. The peak of capillary pores (0.07 μm) of BA5 slightly shifted to the left compared to that of BA0 (0.09 μm), indicating the reduction in the average sizes of capillary pores. The inclusion of 5 % biochar increased the amount of middle capillary pores and mesopores compared to pure MOC. This suggested that the addition of 5 % biochar decreased the dimensions of pores in MOC, and a denser structure could be formed.

The percentage of each type of pore is presented in Fig. 1 (f). There were almost no gel micropores in BA0 (0.1 %). The percentages of macro-pores, large capillary pores, middle capillary pores, and meso- pores in BA0 were 11.8 %, 41.4 %, 40.0 %, and 6.7 %, respectively. The incorporation of 5 % biochar (BA5) increased the percentages of middle capillary pores (54.6 %), mesopores (12.6 %), and gel micropores (5.9

%) and decreased the proportions of macro-pores (8.0 %) and large capillary pores (18.9 %). The high dosage of biochar (25 %) increased the percentages of macro-pores (14.3 %), large capillary pores (74.0 %), and gel micropores (1.5 %) compared to BA0, and decreased the con- tents of middle capillary pores (3.7 %) and mesopores (6.5 %). The re- sults suggested that the incorporation of a small amount of biochar (5 %) decreased the pore sizes and porosity, and increased the structural compactness, as consistent with the SEM results shown in Fig. 5 and permeable voids shown in Fig. 1 (b).

The total porosity of BC0, BC5, and BC25 was 24.4 %, 20.1 % and 46.7 %, respectively. This was consistent with the permeable voids shown in Fig. 1 (b). The MgCl2 solution can be retained in biochar particles. The retained solution can be gradually discharged into the solid matrix, promoting the hydration process. The generated hydration products like P5 could fill the voids of biochar particles. This potentially counterpoised the increase in the porosity due to biochar addition.

Therefore, the pore sizes and porosity of BA5 can be lower than those of BA0. On the other hand, a high percentage of biochar (25 %) can absorb a large quantity of solution and dilute the contents of MgO in the pastes.

This inhibited the hydration processes, contributing to the formation of P3 instead of P5, as consistent with the XRD and FTIR results.

The pore sizes and mechanical properties of MOC could be signifi- cantly affected by the sizes of biochar particles. The increase in the particle sizes and contents of biochar may lead to an increase in the porosity and pore size distribution of MOC. Such an increase in the porous structure may result in a reduction in the mechanical properties of MOC. On the other hand, the increase in the surface area of biochar due to finer particle sizes can enhance the hydration processes and heat release (Gupta and Kua, 2019). This could contribute to the increase in the mechanical performance of MOC.

Based on the microstructural analysis presented above, the enhanced strength and water resistance of MOC due to biochar incorporation were mainly attributed to the internal curing effects of biochar. Due to the high water retention capacity, the incorporation of biochar decreased the H2O/MgCl2 ratios in the MOC matrix, increasing the strength.

During curing, the moisture and ions absorbed by biochar can be gradually released into the matrix, providing moisture and MgCl2 for the hydration processes. Therefore, more P5 can be formed, and the struc- tural compactness can be enhanced, increasing the strength of MOC. The enhanced formation of P5 was consistent with the XRD results. The enhanced structural compactness can be observed from SEM and MIP analysis. With the reduction in pore sizes (MIP results shown in Fig. 1 (d- Table 2

Relative mass and mass loss of MOC with different biochar contents during heating.

Temperature (^◦C) BA0 BA5 BA10 BA15 BA25

Relative mass (%)

40 100.0 100.0 100.0 100.0 100.0

110 93.6 94.2 93.5 92.9 93.9

230 72.4 72.5 72.2 72.1 73.5

350 68.6 68.7 68.1 67.7 69.6

500 46.9 48.7 48.2 48.2 50.2

1100 44.4 44.4 44.6 43.1 45.3

Mass loss (%)

40–110 6.4 5.8 6.5 7.1 6.1

110–230 21.3 21.7 21.3 20.8 20.4

230–350 3.7 3.8 4.2 4.4 3.9

350–500 21.8 19.9 19.9 19.5 19.4

500–1100 2.5 4.3 3.6 5.0 5.0

Accumulated mass loss (%)

110 6.4 5.8 6.5 7.1 6.1

230 27.6 27.5 27.8 27.9 26.5

350 31.4 31.3 31.9 32.3 30.4

500 53.1 51.3 51.8 51.8 49.8

1100 55.6 55.6 55.4 56.9 54.7

f)), promoted P5 formation, and reduction in unreacted MgO (XRD re- sults shown in Fig. 6), the water resistance of MOC can be enhanced. On the other hand, high percentages of biochar decreased the concentration of MgO in the matrix, inhibiting the hydration processes to produce P5.

Although the production of P3 can be promoted, the reduction of P5 directly resulted in the reduction in the strength of MOC. The excessive addition of biochar also increased the pore sizes in the matrix, decreasing the structural compactness and strength of MOC.

4. Conclusions

This paper presents a comprehensive laboratory investigation of the mechanical properties, water resistance, and micro-scale performance of magnesium oxychloride cement (MOC) incorporated with different percentages of biochar. The results indicated that the addition of biochar retarded the hydration processes and reduced the fluidities. The addi- tion of 5 % biochar slightly decreased the permeable voids and formed a denser and continuous structure. The small dosage of biochar (5 % and 10 %) can promote the production of phase 5 (P5) and increase the compressive strength (fc) before and after water immersion. Although the flexural strength (ff) decreased with increased biochar contents, ff

after immersion was higher compared to pure MOC. Considering the difference between fc of immersed MOC before and after air-drying, a new equation was developed to describe fc during drying. The inclusion of biochar did not have significant impacts on the degradation of MOC under elevated temperatures. Therefore, the incorporation of a small percentage of biochar can be a viable approach to improve the me- chanical properties and water resistance of MOC. However, this study only focused on the mechanical behaviour of biochar-incorporated MOC under water attack. As MOC can be exposed to different environments, further study should be conducted to investigate its performance when immersed in different types and concentrations of liquids.

CRediT authorship contribution statement

Yuekai Xie: Conceptualization, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing, Visualization. Hongxu Wang: Conceptualization, Writing – review &

editing, Funding acquisition. Yingying Guo: Conceptualization, Formal analysis, Methodology, Writing – original draft, Writing – review &

editing. Chenman Wang: Conceptualization, Formal analysis, Meth- odology, Writing – review & editing. Hanwen Cui: Conceptualization, Formal analysis, Writing – review & editing. Jianfeng Xue: Conceptu- alization, Methodology, Writing – review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgements

This research was financially supported by Industrial Transformation Training Centres (IC230100015) from the Australian Research Council.

The authors would like to thank Dr. Huixin Wang and Dr. Brit David from X-ray Fluorescence Laboratory, Dr. Ruoming Tian and Dr. Yu Wang from X-ray Diffraction Laboratory, Ms. Katie Jean Levick and Dr. Arnab Chakraborty from Electron Microscope Unit, and Dr. Anne Rich from Spectroscopy Facility, Mr. Damian Buck from School of Science, Mr. Jim Baxter and Mr. Umesh Kaini from School of Engineering and

Technology, University of New South Wales.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.scitotenv.2023.168773.

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