Keywords

1 Introduction

The possibility of achieving high strength and durability has made concrete a well-established structural material for use in roads, tunnels and bridge infrastructure [1,2,3,4]. Various studies have proposed alternative materials (supplementary cementitious materials (SCMs)) to partially replace cement to reduce the CO2 emissions caused by its production [5,6,7,8]. Using SCMs such as fly ash, slag, silica fume, etc. as partial replacement of cement is effective in making concrete more cost-effective, and high performing in terms of strength and durability requirements [9,10,11,12,13,14,15,16].

The carbonation of concrete caused by the presence of CO2 in the atmosphere (≈380 ppm) acts as an environmental load and contributes significantly to the deterioration of concrete structures [17], because it causes the alkalinity of the concrete to decrease (pH 8). If the pH decreases below 12, the risk of corrosion is increased for any fixed steel components present within the concrete elements [18, 19]. Carbonation increases the porosity of the concrete, resulting in reduced compressive strength and impermeability in the carbonated zone of the concrete. The results of several experimental studies have revealed that the carbonation of slag-based concrete is highly dependent on the water/cement ratio, cement replacement ratio, curing method, and in-situ environmental conditions [20,21,22,23,24]. Because the carbonation process is long term in its manifestation, many researchers have used accelerated carbonation tests by adding pressurized CO2 or increased the temperature to increase CO2 diffusivity in the pore solution to shorten the experimental time [25,26,27,28].

Depending on the cement replacement ratio and the type of SCM, the properties of SCM-based concrete change over time as a result of both the hydration process and the carbonation behavior [29,30,31]. After carbonation, the total capillary porosity volume of the slag-based concrete increases, which has a negative impact on the durability of the concrete by creating a larger pore structure and thus increasing the permeability coefficients for the concrete [32,33,34].

To control this behavior, nondestructive techniques such as micro- and nanotomography can be used to monitor changes in the pore structure of slag-based concretes [35]. Han et al. demonstrated how these modern technologies are useful for analyzing the depth of concrete carbonation [36].

Recent advancements in micro- and nanoscale techniques give insight into forecasting and simulating concrete durability, cracking potential, and steel depassivation behaviors. We present the main advances in these techniques in investigating carbonated concrete behavior. Although several studies were reviewed there is still limited information published about the use of tomographic techniques for better understanding of the carbonation behavior of slag-based concretes.

2 Tomography Techniques

Tomography, in the broadest sense, is any technique that uses sectional views as an intermediary stage before reassembling a three-dimensional (3D) object. This characterization technique is useful for identifying the richness of the microstructure in three dimensions rather than only presenting two-dimensional projections. 3D micro- and nanoscale views are required to fully comprehend and monitor the behavior of the concrete. Materials scientists have used X-ray tomographic techniques for decades to discover the behavior of 3D microstructures [36,37,38,39,40]. FIB/SEM tomography (focused-ion beam scanning electron microscopy), electron tomography, X-ray micro-computed tomography (micro-CT), and X-ray nano-computed tomography (nano-CT) are nondestructive 3D imaging techniques that are useful for investigating the interior structure of a wide range of materials [34, 35, 38,39,40,41].

Tomographic techniques can give new insights into concrete deterioration by providing precise information on the changed layers of concrete affected by carbonation, corrosion, leaching or sulfate attack [42]. Carbonation as a durability concern in concrete is strongly influenced by microstructural and pore network characteristics. The porosity parameters obtained from the tomographic data can successfully indicate the internal pore network geometry and microstructural features [37]. X-ray nano- and micro-CT techniques are useful for monitoring slag-based concrete behavior subjected to carbonation [43, 44].

2.1 X-ray Micro-CT Imaging

X-ray micro-CT is a radiographic imaging technique that generates a series of cross-sectional images to identify the internal structure of materials without causing damage to the specimen [45,46,47]. In general, micro-CT provides a series of reconstructed images represented by a pixel, either 8- or 16-bit. Micro-CT processing transforms the 8 or 16-bit images into binary or segmented images, which are useful for examining the features of the specimen. A 3D microstructural image can be created by stacking 2D segmented images to analyze volumetric, multi-directional, and other advanced sample features. The pore structure of the concrete can affect the qualities of the concrete, including strength, durability, and permeability. Using micro-CT images can assist in discovering more information about these properties [48,49,50,51]. Although several scales are used, such as macro-, micro- and nano-CT, the spatial resolution of micro-CT can be used for microscopic CT scanning [45]. The micro-scale CT technique can scan a range from 1 to 10 µm in size, which is also a good range for investigating the size of capillary pores inside the cement paste [45, 52, 53]. For covering the whole range of cement paste pore sizes, nano-CT images can further scan a range of 10 µm to < 10 nm [48, 54]. In order to obtain high-quality nano-CT data it requires meticulous sample preparation. Measuring 3D morphology at < 10 nm can give unique information on transportation characteristics and the mechanical and durability behavior of the concrete [55].

2.2 Micro-CT and Nano-CT to Investigate Carbonated Concrete

During the hydration process, the cement paste interacts with CO2. Carbonated cement paste contributes to steel reinforcement corrosion, causing major and long-term durability issues for concrete structures. Several studies have noted that as carbonation develops, porosity reduces due to calcite formation filling the pore microstructure [56,57,58,59]. Phenolphthalein is a well-known technique for determining carbonation depth in concrete, but it necessitates the destruction of the sample, and the findings vary based on the sampling location [60, 61]. In contrast, X-ray micro- and nano-CT are nondestructive techniques that can identify and analyze the depth of carbonation and the microstructure of the concrete during the carbonation process [62,63,64,65,66,67]. In addition, they can continually monitor the evolution of the carbonated area of the same sample at different ages [68].

Some researchers have used micro-CT to determine the microstructural development of the cement paste during the carbonation process, particularly the distribution of porosity and the effective pore width [69, 70]. In both studies, the results of average porosities revealed that the porosity reduces with additional carbonation time, which infers that calcite (calcium-bearing phase) forms during the carbonation process, and that the porosity distribution may validate the pore microstructural change (e.g., reduction in porosity caused by carbonation). Because the porous structure of concrete extends from the nano- to the macroscopic scale [71], X-ray micro- and nano-CT with good resolution are in high demand for characterizing a wide range of behavior [74]. Particle size and shape, interfacial topology, particle structure, pore structure, carbonation depth, and morphology of distinct solid phases in concrete have all been studied by these methods [72,73,74,75,76].

X-ray micro-CT results show that microcracks form from the surface to the inside of the cement paste after carbonation. Furthermore, the carbonated area increases in depth with increasing carbonation time. Moreover, cracks form during the carbonation process and reduce the density [77]. A new generation of laboratory-based nano-CT with high resolution [45] can provide 3D images for measuring different properties of the concrete such as durability, cracking potential, and steel depassivation behavior. Dimensional and transitional stability is necessary for generating quality data by any instrument involved in sub-micron imaging. It has been demonstrated that nano-CT with high-resolution scanning is a well-established and mature method [78] for gaining insight of the porous and hierarchical structure of the concrete at the sub-micron scale [55]. The ability to perform nano-CT scanning under ambient conditions keeps test samples in their natural form under normal and accelerated conditions [55, 71, 79,80,81,82,83]. Han et al. [83] reported that by using nano-CT, the solid-phase composition, pore structure, damage degradation, and nano-mechanical characteristics of the concrete can be measured at different accelerated carbonation ages. Their results confirmed that without any prior drying preparation, X-ray CT is a suitable technique for obtaining 3D images of concrete to assess the degree of microstructural damage.

2.3 Microstructural Characteristics and Properties of Slag-Based Concrete During Carbonation

SCMs play a critical role in controlling and improving the mechanical and durability properties of the concrete [14,15,16]. Han et al. [83] described how they used micro-CT to track the progression of carbonation-induced fractures and how the carbonation depth increased with exposure duration for concrete containing a slag content up to 70% of the total binder content. Figure 1 shows several cracks produced during the carbonation process, which demonstrates how micro-CT can be utilized to categorize carbonation behavior over time [81, 83]. The micro-CT results revealed that the width and length of microcracks significantly affects the carbonation behavior of concrete [83]. More CO2 penetration causes an increase in crack length. When the fracture width is < 10 µm at 1 year, CO2 diffusivity around the crack is nearly equal to that in the surrounding concrete. However, with fracture width > 10 µm, the CO2 diffusivity in the concrete increases. When the crack width is > 100 µm, the CO2 diffusivity is somewhat further increased [80, 81, 83, 84].

Fig. 1
Eight cross-sectional images for 50% and 70% slag at 0, 3, 7, and 14 days. They indicate several cracks produced during the carbonation process. A color chart and its range are given near each image.

Cross-sectional images of the carbonation front for 50% and 70% slag with increasing accelerated carbonation testing time, 3D voxel size 0.086 mm3 [67, 81]

Full size image

In another study, Han et al. [67] analyzed the carbonation depth of the cement paste with different slag addition from 0 to 70% under 0–14 days of accelerated carbonation testing (after 3 months of curing). Because of slag’s cementitious and pozzolanic properties, the hydration reaction of the slag-based cement will improve the pore microstructure. Large pores will eventually transition into smaller pores with the pozzolanic reaction, which significantly reduces the CO2 diffusion coefficient and therefore the rate of carbonation will also decrease. However, slag-based concrete generates a large amount of calcium hydroxide during the hydration process, and calcium hydroxide is an important chemical component in carbonation. The results from Han et al. demonstrated that the ideal slag addition to the binder to mitigate carbonation is < 50%. Figure 2 shows the specimen's carbonation front and depth of penetration with 50% and 70% slag during 14 days of accelerated carbonation. The carbonation front can be identified by micro-CT, which is seen to increase for 50% slag addition with the carbonation zone steadily expanding with increasing curing time. However, for 70% slag addition, the specimens are totally carbonated in only 7 days. These findings demonstrate that the ideal slag addition, which mitigates carbonation, is < 50%. Figure 3 shows that the carbonation depth estimated from micro-CT appears to be the same as with the phenolphthalein method during 14 days of accelerated carbonation without the addition of slag. These results prove that micro-CT is a reliable and appropriate technique for characterizing the carbonation depth of the concrete [67]. Table 1 is a summary of micro-and nano-CT techniques and the test conditions of different samples for identifying carbonation behavior.

Fig. 2
Eight views of micro-C T images for 50% and 70% slag at 0, 3, 7, and 14 days. They indicate the carbonation front and depth of penetration with 50% and 70% slag during 0, 3, 7, and 14 days of accelerated carbonation.

Views of the carbonation depth with 50% and 70% slag during 14 days of accelerated carbonation testing, 3D voxel size 0.086 mm3 [67]

Full size image
Fig. 3
Two views of micro-C T and phenolphthalein method images without the addition of slag indicate carbonation depth during 14 days of accelerated carbonation.

Comparable results for micro-CT and phenolphthalein method for determining carbonation depth, 3D voxel size 0.086 mm3 [67]

Full size image
Table 1 Use of micro- and nano-CT with accelerated carbonation conditions
Full size table

3 Conclusions

Tomographic techniques for assessing carbonation behavior in concrete are still in the early stage of development, but advances made in the previous decade have been significant. As has been demonstrated, micro-and nano-CT can efficiently and nondestructively investigate micro- and nanostructural behaviors. These techniques are suitable for analyzing micro- and nanoscale topologies and morphologies, and studying the porosity network. The advantage and benefit of using micro- and nano-CT 3D imaging to assess the pore network of the concrete is the volumetric insight into the interactions between different phases and pores. The techniques may be useful for characterizing the carbonation behavior of slag-based concretes, with r providing greater insight into accelerated carbonation testing and the impact on the pore structure. However, the need to use small-sized samples necessitates meticulous sample preparation.