Impact of nano-TiO2 addition on the reduction of net CO2 emissions of cement pastes after CO2 curing

https://doi.org/10.1016/j.cemconcomp.2021.104160Get rights and content

Highlights

  • This study evaluates the effect of nano-TiO2 addition on CO2 uptake during CO2 curing.

  • Four nano-TiO2 dosages and two curing types (normal and CO2 curing) are analyzed.

  • Optimum nano-TiO2 level regarding compressive strength is lower in CO2 cured samples.

  • Nano-TiO2 increases the early CO2 uptake of cement paste cured with CO2.

  • Nano-TiO2 decreases the net CO2 emissions of both normal and CO2 cured cement paste.

Abstract

This investigation aims to study the effect of nano-TiO2 addition on CO2 uptake of cement paste during CO2 curing. Cement paste mixtures with different percentages of nano-TiO2 (0%, 0.5%, 1%, 2%), and two different types of curing (normal curing (NC) at 23 °C and 50% RH, and CO2 curing (CC) that includes a 12-h curing in a chamber with 20% of CO2) were studied. Compressive strength, X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and an estimation of the net CO2 emissions were performed for each studied mixture and curing condition. All samples were tested at 48 h. Results showed that the optimum percentage of nano-TiO2 in terms of compressive strength is lower for CO2 curing samples than for NC samples. XRD and TGA results exhibited that the use of nano-TiO2 promotes the CO2 capture during the CO2 curing. Since nano-TiO2 reduces the size of CH, it increases the CH surface area. This may enhance the CH reactivity with the CO2, producing an increase in the CO2 uptake. Results also showed that not considering the effect of nano-TiO2 on compressive strength and promotion of CO2 capture would highly overestimate the net CO2 emissions of cementitious materials with nano-TiO2. It highlights the importance of selecting an adequate functional unit for the analysis.

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]].Ca(OH)2+CO2CaCO3+H2 O(CaO)x(SiO2)y(H2 O)z+xCO2xCaCO3+y(SiO2)(H2O)t

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].3CaO · SiO2+(3- x)CO2+yH2 xCaO · SiO2 · yH2+(3- x)CaCO3CaO · SiO2+(2- x)CO2+yH2 xCaO · SiO2 · yH2+(2- x)CaCO3

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.

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