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
Introduction
Over the last thirty years, the United Nations Framework Convention on Climate Change (UNFCCC) has been working in preventing “dangerous anthropogenic interference with the climate system” [1]. Despite some agreements, like the Kyoto Protocol [2] or the Paris Agreement [3] the measures taken were not enough to reduce climate change or even mitigating its increase [4], [5]. In a recent report, the World Meteorological Organization claimed that the concentration of greenhouse gases (GHG) was the highest of the last 3–5 million years. Carbon dioxide (CO2) represents a huge percentage of the total amount; in fact, in 2014, the Intergovernmental Panel on Climate Change (IPCC) estimated its contribution as a 65% of the total GHG emissions. On the other hand, the World Health Organization reported 4 million premature deaths around the world due to the pollution in 2016 [6]. In addition to the clear effect of high CO2 emissions on global warming and health issues, those levels also promote the ocean acidification. The oceans absorb CO2 from the atmosphere, that reacts with the water and the carbonate ions. The consumption of carbonate ions impedes calcification and produced an increase in acidity of 30% from the last industrial revolution [7]. The reduction of CO2 emissions is vital for the future of our planet and our health.
Regrettably, the cement industry is one of the principal sources of CO2 emissions. Around 8% of the total CO2 emissions were produced by the cement manufacturing in 2016 [8]. Between 40% and 50% of these emissions come from the limestone decomposition during the production of the clinker (the main component of cement), while another 40%-50% of the cement industry emissions are caused by the use of fossil fuels in the kiln to achieve the 1450 °C required to produce the clinker [9], [10]. Finally, a small percentage of this 8% is related to the transportation of materials [9], [10]. However, a study published [11] concluded that, between 1930 and 2013, cementitious materials have captured (through natural carbonation) the 43% of the CO2 emitted during the cement production due to the limestone decomposition for the same period of time. This study concluded that the carbonation of cement hydration products represents a substantial carbon sink that is not currently considered in emissions inventories.
The theoretical basis is that carbonation of concrete occurs due to the reaction of two compounds: (i) CO2 from the atmosphere, and (ii) the calcium hydroxide (Ca(OH)2, also designated with shorthand notation as CH) from cement paste. First, CO2 dissolves in the water of the pores forming carbonate ions [12], [13], [14]. Then, these carbonate ions can react with the Ca ions of the pore solution leading to calcium carbonate (CaCO3) precipitation. The carbonation reaction is shown in Eq.1 [13], [15].
Likewise, the calcium silicate hydrate (C-S-H) gel can also be carbonated though a complex decalcification–polymerization process where C-S-H can be decalcified and the liberated lime (CaO) can react with the carbonate ions previously created forming calcium CaCO3 [12]. Eq. 2 exhibits an overview of the decalcification–polymerization reaction.
Two facts need to be highlighted: (i) the carbonation is a very slow process, (it could take decades), and (ii) the world annual cement production per capita during the last 30 years has been multiplied by 3 [16] while the population has been multiplied by 2 in the same period. Just as an illustrative example, China has used more cement between 2011 and 2013 than the U.S.A. in the whole 20th century [17]. Considering these two facts jointly, the debt created with the environment (CO2 emissions) is too high and it is being paid so slow (CO2 uptake).
Previous research showed two different carbonation processes (active and passive) [18]. The passive or weathering carbonation consists in exposing cementitious materials to CO2 in the atmosphere. The active or accelerated carbonation lies in exposing fresh cementitious composites to a high CO2 concentration to speed up the carbonation process.
Cementitious composites are exposed to weathering or natural carbonation during their lifetime. This type of carbonation is a slow process since the atmosphere has a low CO2 concentration (0.0415% or 415 ppm) [19]. The weathering carbonation is usually analyzed by the carbonation depth test [20]. Previous research showed that the carbonation depth is increased when cementitious materials are exposed to CO2 [21], [22].
The active or CO2 curing allows to speed up the CO2 capture in fresh cementitious composite. Even though the use of CO2 for curing cementitious materials was suggested in the 1970s, recent research found that this type of carbonation may possess significant benefits [23]. Many researchers have examined the potential applicability of curing fresh cement composites with CO2 [15], [23], [24], [25], [26], [27], [28], [29]. The chemistry beyond that process is the carbonation of the dicalcium silicate (C2S) and tricalcium silicate (C3S) [15], [24], [30]. This mechanism would promote CO2 sequestration of cementitious materials, for both OPC concrete and concrete with supplementary cementitious materials [27], [31], [32]. Besides, it may increase concrete durability without steel reinforcement [33], [34], [35], [36] and the compressive strength compared to specimens with standard curing [15], [30], [37], [38]. Rostami et al. [35] showed an increase in either the sulfate and freeze-thaw resistance or the electrical resistivity after the early carbonation. They suggest that this result could be related to the modification of the microstructure of cement pastes. Another study found that the chloride penetration was reduced after the carbonation curing [34]. Therefore, this would help to prevent issues in cementitious composites exposed to marine environments.
Even though, this would highly reduce the environmental impact of cementitious composites such as concrete (the most widely used construction material in the world), nowadays, it is not technically feasible for many construction field applications.
Since hardened cementitious composites still have the ability of absorbing CO2 during their whole service life through the carbonation process [11] but, the carbonation process is very slow, and the production of cement is increasing year by year. Accelerating the process of CO2 sequestration of hardened cementitious composites may have important environmental benefits.
Previous research suggested that mortars with 0.5% of nano-TiO2 (by the total weight of cement) exhibited a higher carbonation than plain mortars during the grinding of the samples for thermogravimetric analysis sample preparation [39]. Despite this observation, no research was found that study the effect of nano-TiO2 on CO2 sequestration of hardened cementitious materials. Based on that, the main objective of this paper is to study how nano-TiO2 may affect the CO2 uptake of hardened cement paste as a function of its w/c ratio and pore structure.
Section snippets
Materials and mix procedure
In this study, the cement used was Portland cement type I (CEM I 52,5 N-CP2). Table 1 presents the chemical and phase composition of the cement used. The nano-TiO2 (85% anatase and 15% rutile, >99.5% of trace metal basis) employed, with a particle size of 21 nm (TEM - transmission electron microscopy), was provided by Sigma-Aldrich (St. Louis, MO). Its formula weight and surface area were 79.87 g/mol and 35–65 m2/g, respectively.
Twelve cement paste mixtures were made with four different
TGA results: CH consumption and CaCO3 formation
Fig. 2.a and .b present the CH consumption due to the CO2 exposure on the studied pastes at two different ages (14 and 28 days), calculated from the TGA results. Fig. 2.c and .d present the relative CH consumption due to CO2 exposure calculated using Eq. (8). Besides, a table with CH contents of non-exposed and exposed samples is included in the Supplementary material.
At 14 days (Fig. 2.a), the use of nano-TiO2 up to 1% promotes
Conclusions
The present paper was designed to study the effect of nano-TiO2 on CO2 uptake of cement paste.
Based on the thermal analysis results, three remarkable conclusions can be drawn: (i) the effect of nano-TiO2 on the CH consumption, CaCO3 formation and CO2 uptake due to the CO2 exposure are highly influenced by both exposure age and water-to-cement ratio of the cement pastes; (ii) the use of nanoparticles is beneficial for CO2 uptake until a certain percentage from which the improvement starts to
Funding
The authors gratefully acknowledge start-up funding from Purdue University (VF), (MV-L), and (CM). The experiments reported in this study were performed in the Pankow Materials Laboratories at Lyles School of Civil Engineering (Purdue University).
CRediT authorship contribution statement
Carlos Moro: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Validation, Visualization, Writing - original draft, Writing - review & editing. Vito Francioso: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Mirian Velay-Lizancos: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project
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.
Acknowledgments
We acknowledge 3D X-Ray Microscope Facility in the Department of Physics and Astronomy and Chven Mitchell who assisted us on running the scans on a Zeiss Xradia 510 Versa 3D X-ray Microscope that was supported by the Executive Vice President for Research and Partnerships, Major Multi-User Equipment Program 2017 at Purdue University.
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