Incorporating hollow natural fiber (HNF) to enhance CO2 sequestration and mechanical properties of reactive magnesia cement (RMC)-based composites: Feasibility study
Introduction
Reactive magnesia cement (RMC) is an emerging alternative concrete binder to Portland cement (PC) [1,2]. The manufacture of unit mass of RMC via the dry route, i.e., calcining magnesite, can cause higher CO2 emission than the that of Portland cement (PC) (1.7 vs. 1.1 t/tonne) [3]. However, if completely carbonated RMC can sequestrate significant amount of ambient CO2 reducing the total CO2 emission to appromixately 0.5−0.6 tonne/tonne [1]. Besides, RMC can also be acaquired from the wet route, i.e., extracting it from reject brine [4], or from recycling of RMC-concrete [5]. These features make RMC potentially a more sustainable binder than PC. Specifically, the reactive MgO can hydrate in water forming brucite (Mg(OH)2), which provides little strength yet but can further react with dissolved CO2 (CO32− or HCO3- ions) to form a variety of hydrated magnesium carbonates (HMCs) e.g., hydromagnesite (4MgCO3⋅Mg(OH)2⋅4H2O), nesquehonite (MgCO3⋅3H2O), dypingite (4MgCO3⋅Mg(OH)2⋅5H2O), etc. [6,7]. These HMCs have mechanical strength comparable to or even higher than PC hydrate. Theoretically, one ton of CO2 can be sequestrated by approximately one ton of RMC. However, the application of RMC-based concrete is still limited, especially for structural components. This is attributed to the low hydration degree of MgO, the low solubility of CO2, and the inadequate CO2 penetration through the dense RMC matrix [1]. Recently, MgO hydration has been improved by adding minor hydration agent [8]; CO2 dissolution has been improved by adjusting the alkalinity of the RMC matrix with sodium bicarbonates [9,10] or ureolytic bacteria [11]. Elevated or the flow CO2 gas was also applied to accelerate the carbonation of RMC [[12], [13], [14]]. These approaches improve the compressive strength of small-sized RMC-based concrete; for example, the strength of 50-mm cubes has reached 70 MPa [9]. However, they cannot increase and even decrease CO2 penetration as they densify the microstructure of RMC matrix. Additionally, mesoporous biochar has been incorporated in MgO-PC binary composite (MP) and RMC to enhance the carbonation degree and mechanical properties, resulting in noticeable enhancement for the compressive strength for MP and RMC composites, respectively [15,16]. So far the size of RMC-based concrete is still limited by the carbonation depth, e.g., Pu and Unluer [17] reported the carbonation depth was only 35 mm or less even in a porous concrete (w/c = 0.8) cured under high CO2 concentration (10 %) for 27 days.
Natural fibers have been used as reinforcement for PC-based composite, making fiber cement board [18,19] or fiber-reinforced concrete [20,21]. As the organic constituents of the fiber, e.g., lignin, hemicellulose, pectin, etc., would undergo severe degradation in the alkaline PC matrix (pH = 12.5), pre-treatment of the fiber is often required to improve the environmental durability of these products. Many natural fibers, e.g., sisal, jute, and curauá, have longitudinal hollow lumens inside to transport nutrients [22]. The lumen opening (1−10 μm [22]) is larger than the capillary pores in RMC matrix (∼10−100 nm [14]) by several orders; more importantly, it is significantly larger than the mean free path of CO2 molecule (45 nm [23]), which means the collision between the CO2 molecules and the lumen wall is negligible and thus the CO2 should move faster. Therefore, incorporating HNFs into the RMC-based composites may significantly enhance the CO2 penetration in RMC matrix; meanwhile, the relatively low alkalinity of RMC matrix (pH = 9–10.5 [24,25]) may mitigate the HNF degradation. In this paper, we report the effect of adding HNFs on the carbonation and strength development of RMC-based composite for the first time; the protective effect of RMC matrix to the HNF is also demonstrated.
Section snippets
Specimen preparation
Four groups of RMC-based composites were made in this study. Their mix proportions are given in Table 1. Besides the two HNF-reinforced composites (sisal fibers), the plain RMC composite was the control group to study the effect of incorporating HNFs; the polyvinyl alcohol (PVA)-fiber reinforced RMC composite was the control group to compare the fiber with or without lumens.
The RMC was acquired from Shanghai Yunajiang Chemical Co., Ltd; it is composed of 95 wt.% of reactive MgO and minor
Mechanical properties
Fig. 2a shows the compressive strength fc of the four tested groups. At all the ages of testing (3d, 7d, and 14d), the four groups rank as the same orders: RMC < RMC-PVA < RMC-Sisal < RMC/FA-Sisal. Expectedly, the inclusion of hollow sisal fibers significantly improved the strength of RMC matrix, and the improvement became even larger as the CO2 curing extended; after 14 days, the fc of RMC-Sisal and RMC/FA-Sisal had become 1.88 and 2.10 times of the RMC. On the other hand, inclusion of the
Conclusions
This work investigated the feasibility of adding hollow natural fiber (herein sisal fibers) into RMC matrix to improve the CO2 penetration and mechanical properties of this class of composites. The experimental results show that both the carbonation depth and the compressive strength of RMC-based composites were multiplied by the sisal fibers, while adding the same amount of finer and stronger PVA fibers hardly improved those properties. It was the more efficient CO2 penetration through the
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.
References (38)
Carbon Dioxide Sequestration in Magnesium-based Binders, Carbon Dioxide Sequestration in Cementitious Construction Materials
(2018)- et al.
Sequestration of CO2 in reactive MgO cement-based mixes with enhanced hydration mechanisms
Constr. Build. Mater.
(2017) - et al.
Investigation of the properties of MgO recovered from reject brine obtained from desalination plants
J. Clean. Prod.
(2018) - et al.
Recycling and reuse of reactive MgO cements-A feasibility study
Constr. Build. Mater.
(2017) - et al.
Development of MgO concrete with enhanced hydration and carbonation mechanisms
Cem. Concr. Res.
(2018) - et al.
Hydration, carbonation, strength development and corrosion resistance of reactive MgO cement-based composites
Cem. Concr. Res.
(2020) - et al.
Influence of nucleation seeding on the performance of carbonated MgO formulations
Cem. Concr. Compos.
(2017) - et al.
Performance of reactive MgO concrete under increased CO2 dissolution
Cem. Concr. Res.
(2019) - et al.
Influence of aqueous carbonate species on hydration and carbonation of reactive MgO cement
J. Co2 Util.
(2020) - et al.
Use of microbial carbonation process to enable self‑carbonation of reactive MgO cement mixes
Cem. Concr. Res.
(2021)
Influence of CO2 concentration on the performance of MgO cement mixes
Cem. Concr. Compos.
Accelerated carbonation of reactive MgO and Portland cement blends under flowing CO2 gas
Cem. Concr. Compos.
Carbonated binder systems containing reactive MgO and Portland cement: strength, chemical composition and pore structure
J. Clean. Prod.
Biochar as green additives in cement-based composites with carbon dioxide curing
J. Clean. Prod.
Investigation of carbonation depth and its influence on the performance and microstructure of MgO cement and PC mixes
Constr. Build. Mater.
Microstructural and chemical effects of wet/dry cycling on pulp fiber-cement composites
Cem. Concr. Res.
Introducing a curauá fiber reinforced cement-based composite with strain-hardening behavior
Ind. Crops Prod.
Design of strain hardening cement-based composites with alkali treated natural curauá fiber
Cem. Concr. Compos.
Influence of natural fibers characteristics on the interface mechanics with cement based matrices
Compos. Part B Eng.
Cited by (15)
CO<inf>2</inf>-sequestering ability of lightweight concrete based on reactive magnesia cement and high-dosage biochar aggregate
2024, Journal of Cleaner ProductionAutogenous healing of the interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix
2024, Construction and Building MaterialsModification of magnesium hydroxide for improved performance in CO<inf>2</inf> sequestration
2024, Cement and Concrete ResearchInfluence of natural fibers on hydration and carbonation of reactive magnesium oxide cement (RMC)
2024, Construction and Building MaterialsProperties and high-temperature resistance of tailing-based magnesium oxysulfate (MOS) cement affected by phosphogypsum and water-binder ratio
2023, Construction and Building MaterialsA new fiber-bridging constitutive model for quantifying the matrix carbonation and fiber-to-matrix interface healing in PVA fiber-reinforced SHCC
2023, Construction and Building Materials