Impact of nano-TiO2 addition on the reduction of net CO2 emissions of cement pastes after CO2 curing
Introduction
The cement industry is one of the main carbon dioxide (CO2) emitters, with around 8% of the total emissions each year [1]. However, cementitious materials are well-known to react with CO2. Xi et al. [2] claimed that cementitious materials captured 43% of the total CO2 emitted by the cement industry in the period 1930–2013. Therefore, cementitious composites could be an important carbon sink that should be considered due to the sequestration of almost half of their total CO2 emissions.
The carbonation of cementitious materials can be classified into two different types (passive or active) depending on several properties (e.g., exposure time or CO2 concentration) [3]. The passive carbonation involves the exposure of the cementitious materials for a long time in a low CO2 concentration environment. All cementitious materials experiment this type of carbonation during their lifetime since the atmosphere possesses a low CO2 concentration (0.0415% or 415 ppm) [4]. During the exposure, atmospheric CO2 reacts with the hydration products (e.g., calcium hydroxide (CH or Ca(OH)2) or calcium silica hydrate (C–S–H or (CaO)x·(SiO2)y·(H2O)z)) of the cement paste producing calcium carbonate as shown in Eqs. (1), (2)) [[5], [6], [7], [8]].
Nevertheless, while the worldwide cement production has been increasing very fast over the last decades [9], the passive or natural carbonation of cementitious materials is a slow process. Therefore, those materials do not have enough time to partially compensate the CO2 emitted by the cement industry. An acceleration of the carbonation process may help to reduce the net CO2 emissions of cementitious composite materials in a shorter time.
The active or accelerated carbonation consists of increasing the CO2 concentration for a short time to promote the quicker reaction of the different phases of the cement paste with the CO2 present [6,10]. Thus, in addition to the hydration products, the principal phases of anhydrous cement (alite, C3S or 3(CaO)·SiO2, and belite, C2S or 2(CaO)·SiO2) could react with the CO2 as exhibited in Eqs. (3), (4)) [10].
Previous research showed the beneficial effects of the accelerated (or CO2) curing [[11], [12], [13], [14]]. Cementitious materials may improve the microstructure [[15], [16], [17], [18]], the strength [[19], [20], [21], [22], [23], [24]], or the durability [12,13,25] after CO2 curing. Some authors argued that the refinement of the microstructure and the reduction of porosity could be the reason beyond the enhancement of strength [16,17] and durability [12,13,25]. The accelerated curing promotes the carbonation of the hydration products [8] and, therefore, the porosity is reduced due to the higher molar volume of the CaCO3 in comparison to other phases (e.g., CH or C–S–H) [26]. However, the bigger pores (micropores and mesopores) may increase due to the leaching of the C–S–H gel [25,27]. Another potential explanation for the compressive strength improvement could be the rapid cement dissolution [22,23]. Either way, the carbonation of cementitious materials could decrease total net CO2 emissions while improving the material properties. Thus, it would be a potential double positive effect.
Traditionally, the hydration products content and the porosity have been identified as the major factors contributing to the compressive strength of cement pastes [28]. Nonetheless, the porosity affects the compressive strength to a greater extent [28]. Numerous studies showed that the use of TiO2 nanoparticles may decrease the total porosity of cementitious materials [[29], [30], [31], [32]]. Besides, the nanoparticles may enhance the interfacial transition zone (ITZ) of mortars due to the particle packing density [33,34]. Nano-TiO2 may restrict the CH size increasing its reactivity and leading to a denser microstructure [29,31,35] and, therefore, improving both strength and durability [29,[36], [37], [38], [39], [40]]. In terms of CO2 sequestration, recent studies showed that nano-TiO2 addition modifies the CO2 capture of hardened cement pastes [30,41]. Francioso et al. [30] suggested that the use of 0.5% of nano-TiO2 (by the total weight of cement) may promote natural carbonation (carbonation due to the exposure of the sample to the atmospheric CO2) of powdered mortar samples at 7 days. In addition, a recent investigation found that nano-TiO2 addition increased the CO2 capture of hardened cement paste after a weathering carbonation [41].
However, the effect of nano-TiO2 on CO2 sequestration by CO2 curing was not studied. Previous studies focused on CO2 sequestration of plain cement pastes showed the benefits of CO2 curing compared to the carbonation of hardened cement paste [10,12,42]. Besides, there is a lack of investigation regarding the effect of TiO2 nanoparticles on cementitious materials properties after CO2 curing. Therefore, the main objective of this investigation is to evaluate the potential effect of nano-TiO2 addition on net CO2 emissions related to CO2 cured cement pastes. It also studies the impact of the curing procedure on both hydration products and compressive strength of cement pastes made with different levels of TiO2 nanoparticles.
Section snippets
Materials and mix procedure
Commercial nano-TiO2 (85% anatase and 15% rutile), supplied by Sigma-Aldrich (St. Louis, MO), was employed in this investigation. According to the supplier, the nanoparticles have a particle size of 21 nm (TEM - transmission electron microscopy), a formula weight 79.87 g/mol, and a surface area of 35–65 m2/g. The chemical and phase composition (XRD of the cement is shown in the supplementary material) of the Portland cement type I (CEM I 52,5 N-CP2) used are shown in Table 1.
Cement paste
Compressive strength
Fig. 1 displays the compressive strength results of normal curing (NC) and CO2 curing (CC) samples. The shaded areas represent the dispersion of the results for all studied specimens. The homogeneity through the samples (dispersion between the two compressive strength values of the same sample) was also checked, being lower than 1.8 MPa for all the samples.
In NC samples, the use of nano-TiO2 was beneficial in terms of compressive strength. The higher the nanoparticles percentage, the higher the
Conclusions
The optimum percentage of nano-TiO2 in terms of compressive strength depends on the curing process (normal curing vs. CO2 curing). CO2 cured samples showed a lower optimum percentage of nano-TiO2 than samples cured under normal conditions in terms of compressive strength in terms of compressive strength. The initial reduction of the porosity produced by the nano-TiO2 addition could be the reason behind that. The lower the initial porosity before CO2 curing, the lower the maximum potential
Funding
The authors gratefully acknowledge start-up funding from Purdue University (CM), (VF), and (MV-L). The experiments reported in this study were performed in the Pankow Materials Laboratories at Lyles School of Civil Engineering (Purdue University).
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 (51)
- et al.
Carbonation of cement paste: understanding, challenges, and opportunities
Construct. Build. Mater.
(2016) - et al.
Investigation of the carbonation front shape on cementitious materials: effects of the chemical kinetics
Cement Concr. Res.
(2007) - et al.
Microstructure of cement paste subject to early carbonation curing
Cement Concr. Res.
(2012) - et al.
Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics, microstructure changes and moisture properties, Cem
Concr. Res.
(2014) - et al.
Review on carbonation curing of cement-based materials
J. CO2 Util.
(2017) - et al.
Durability of concrete pipes subjected to combined steam and carbonation curing
Construct. Build. Mater.
(2011) - et al.
Effect of early carbonation curing on chloride penetration and weathering carbonation in concrete
Construct. Build. Mater.
(2016) - et al.
Accelerated carbonation curing of cement mortars containing cement kiln dust: an effective way of CO2 sequestration and carbon footprint reduction
J. Clean. Prod.
(2018) - et al.
Effect of carbonation curing regime on strength and microstructure of Portland cement paste
J. CO2 Util.
(2019) - et al.
Accelerated carbonation of hardened cement pastes: influence of porosity
Construct. Build. Mater.
(2019)
The effects of the early carbonation curing on the mechanical and porosity properties of high initial strength Portland cement pastes
Construct. Build. Mater.
Microstructure changes of waste hydrated cement paste induced by accelerated carbonation
Construct. Build. Mater.
Accelerated curing of cementitious systems by carbon dioxide
Cement Concr. Res.
Carbonation curing for wollastonite-Portland cementitious materials: CO2 sequestration potential and feasibility assessment
J. Clean. Prod.
Microstructural changes caused by carbonation of cement mortar
Cement Concr. Res.
Influences of nano-TiO2 on the properties of cement-based materials: hydration and drying shrinkage
Construct. Build. Mater.
Hydration and properties of nano-TiO2 blended cement composites
Cement Concr. Compos.
Nano-core effect in nano-engineered cementitious composites
Compos. Part A Appl. Sci. Manuf.
Influence of nano-TiO2 on physical and hydration characteristics of fly ash–cement systems
Construct. Build. Mater.
Multifunctional cementitious composites modified with nano titanium dioxide: a review
Compos. Part A Appl. Sci. Manuf.
Single and combined effects of nano-SiO2, nano-Al2O3 and nano-TiO2 on the mechanical, rheological and durability properties of self-compacting mortar containing fly ash
Construct. Build. Mater.
Effects of fly ash and TiO2 nanoparticles on rheological, mechanical, microstructural and thermal properties of high strength self compacting concrete
Mech. Mater.
Enhancements and mechanisms of nanoparticles on wear resistance and chloride penetration resistance of reactive powder concrete
Construct. Build. Mater.
Modification of CO2 capture and pore structure of hardened cement paste made with nano-TiO2 addition : influence of water-to-cement ratio and CO2 exposure age
Construct. Build. Mater.
Early age carbonation curing for precast reinforced concretes
Construct. Build. Mater.
Cited by (31)
Multi-scale experimental studies on mechanical properties of three-dimensional porous graphene cementitious composite
2024, Cement and Concrete CompositesSynergic effect of metal-organic frameworks and process parameters on the properties of concrete subjected to accelerated carbonation
2024, Construction and Building MaterialsCarbonate binders: Historic developments and perspectives
2024, Cement and Concrete ResearchEffect of different aggregates on the properties of carbonated self-pulverized low-calcium clinker mortar
2023, Construction and Building MaterialsAdvancing waste-based construction materials through carbon dioxide curing: A comprehensive review
2023, Results in EngineeringEarly-stage performance enhancement of concrete via commercial C-S-H seeds: From lab investigation to field implementation in Illinois, US
2023, Case Studies in Construction Materials