1. Introduction
Over the past decade, the concentration of CO
2 in the atmosphere has increased significantly [
1], resulting in associated environmental pollution problems. As the binder of reinforcement concrete, cement has been widely used across the world. However, the production of cement generates a huge amount of anthropogenic CO
2 [
2,
3,
4]. The cement industry as a whole accounts for about 5–8% of global CO
2 emissions. Thus, extending the service life of concrete is a viable strategy to reduce the demand of cement, which in turn will effectively cut the carbon footprint.
It is well-known that mature concrete has a tendency to react with the atmospheric CO
2 in natural environments. The carbonation of concrete is more prone to occurring upon exposure to a high concentrated CO
2 level, because CO
2 can readily diffuse into the pore solutions through a normally porous surface structure of concrete. The precipitation of CaCO
3 depletes Ca
2+ in the pore solution, resulting in the release of Ca
2+ from hydration products to maintain the dissolution equilibrium of calcium in the pore solution. This inevitably leads to changes in the hydration products. Among the hydration products, calcium hydroxide (CH) and calcium silicate hydrate (C–S–H) are the main reactants in the carbonation process (Equations (1) and (2)). For C–S–H, the depletion of Ca
2+ tends to cause decalcification and decrease the Ca/Si ratio, forming decalcified C–S–H and eventually amorphous silica gel.
Another problem caused by the carbonation of concrete is the drop in the pH of the pore solution due to the neutralization reaction between carbonic acid and alkali. Under such a circumstance, the passive film on the steel surface has a propensity to undergo chloride-induced corrosion [
5,
6]. This poses a great threat to the durability of reinforcement concrete. Therefore, enhancing the carbonation resistance of reinforcement concrete represents an effective way to improve its durability and extend the service life.
In recent years, surface protective materials (SPMs) have attracted increasing attention as a feasible and economic way to prevent the diffusion of CO
2 into the inner parts of concrete, especially the vicinity of the reinforced steel [
7,
8]. On the other hand, supplementary cementitious materials (SCMs), such as fly ash and slag, are reported to be effective in augmenting the resistance of concrete to carbonation and chloride ingress [
9]. At the same time, new cementitious binder materials composed of a high content of SCMs were referred to as lower carbon emission in cement manufacture [
10]. Some researchers even found that some sorts of blast furnace slag can provide a better performance than the reference concrete [
11]. This reasonably raises the question of whether the combination of SCMs with SPMs can synergically enhance the carbonation resistance of reinforcement concrete. However, one downside of the addition of SCMs in cementitious material is the negative effect on the early strength [
12]. NanoSiO
2 (NS) has been extensively investigated to evaluate its ability to compensate for the SCMs-induced adverse influence on the early-age properties [
13,
14]. As an inorganic agent for modifying cement-based materials, NS holds great potential for commercial applications due to its low cost and excellent properties. For example, NS can elicit a filling effect because it can act as a micro aggregate to fill the aperture between the cement particles and thereby densify the microstructures [
15,
16]. Moreover, NS also has a seeding effect by adsorbing calcium ions and serving as nucleation sites of C–S–H due to the large specific surface area [
17]. Furthermore, NS can function as a pozzolana because it can react with CH to form C–S–H [
18,
19].
The above three effects of NS work together to improve the early mechanical properties of reinforcement concrete, such as flexural strength and compressive strength [
20,
21]. As nano particles, however, NS is prone to agglomeration in the pore solution in the presence of various positive ions. In contrast, hybrid nanoSiO
2 (HNS) is a novel nanoparticle which has better dispersion than traditional NS. For example, Gu et al. [
22] synthesized a core-shell nanoparticle and found that the obtained hybrid NS was well dispersed in water and remained stable. Collodetti et al. [
23] found that the siloxane modified nanoSiO
2 densified the nanostructure of Portland cement pastes, and coating siloxane on the surface enhanced the nanoSiO
2 stability in pore solutions. Mora et al. [
24] developed novel hybrid silica particles functionalized with n-dodecyl groups (–C
12H
25) and found that they both had better dispersion and hydrophobicity. Therefore, hybrid nanoSiO
2 with an organic-inorganic core shell structure seems to be an effective additive to promote microstructure and durability of cement-based materials. Since surface is the first barrier to resist carbonation, coating the surface with SPMs is a cost-effective way to enhance the carbonation resistance of concrete.
In this paper, surface protective materials (SPMs) were prepared with the incorporation of two types of SCMs, i.e., fly ash and slag. In order to compensate for the adverse effect of SCMs on the early-age properties, a kind of hybrid nanoSiO2 (HNS) with a core-shell structure was also added to the SPM. The carbonation depth of the SPM was measured to assess its carbonation resistance ability. X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and fourier-transform infrared spectroscopy (FTIR) were employed to characterize the changes in the chemical compositions of the SPM before and after carbonation. Moreover, mercury intrusion porosimeter (MIP) was conducted to investigate the changes in the pore structures before carbonation in different blends. Finally, thermodynamic modeling was used to simulate the changes in phase assemblage, pH value, and Ca/Si ratios upon interacting with CO2.