1. Introduction
Global warming arising from climate change is a real global concern. The greenhouse gases, e.g., CO
2 and other gases like methane, have been identified as the worst culprits of this menace [
1,
2,
3]. Several efforts have been made to mitigate global warming through the reduction of greenhouse gas emissions. Such efforts centre around carbon capture and storage (CCS) in deep geological aquifers, depleted oil reservoirs, ocean beds and abandoned coal seams [
4,
5,
6,
7,
8]. Recent evidence reveals that the carbonation of Portland cement concrete is a more promising approach to carbon storage than the geological carbon storage approach. Cement can serve as carbon sink from the reactions of CO
2 with the hydrates of tri- and di-calcium silicates (C
3S, C
2S), which are generally present in cement (during hydration process), as well as the reaction of CO
2 with Ca(OH)
2, which is a by-product of cement hydration [
9,
10,
11]. The process of carbonation in cement and/or concrete often takes place in various stages. However, the dominant reaction in fresh cement carbonations is as expressed in Equation (1) while Equations (2) and (3) show the early-stage carbonation of hydrated concrete [
9].
Further, the presence of AFm phases in cement offers additional routes to carbonate formation with myriads of other products. AFm are a family of hydrated calcium aluminates based on the hydrocalumite-like structure [
12],
[
13]. Yaseen et al. [
13] express the decomposition of AFm phases during carbonation with graphene oxide (GO) as follows:
Equation (4) shows that, in addition to carbonates from calcium hydroxide (CH) and calcium-silicate-hydrate (C-S-H) phases, the carbonation of AFm phases generates more carbonates and produces gypsum and alumina gel. This increased productivity of carbonates contributes substantially to CaCO
3 polymorph formation, and it is frequently mentioned that the formed CaCO
3 amount far exceeded that which could be obtained from the entire CH dissolution [
14,
15].
The manufacturing of cement follows a process whereby CO
2 emissions are generated into the atmosphere. Besides the emissions from fuel combustion in the cement manufacturing process, the chemistry of the reactions is responsible for almost two-thirds (64%) of the CO
2 emissions emanating from the Portland cement industry [
16,
17]. Eventually, the cement is used in the making of concrete for building houses, bridges, road constructions, etc. However, these structures made with cement can reabsorb some of those emissions over time, reversing a portion of the calcination reaction in a process referred to as carbonation [
18].
Among the factors expatiated in the literature, the quantity of carbonation in concrete is strongly connected to the available microstructural space in the matrix of the concrete. Galan et al. [
19] state that available pore space decreases as the carbonation process continues over time, owing to the formation of calcium carbonate, which contains larger molecules than calcium hydroxide, thus decreasing the available pore space with time. Additionally, if the humidity is too high, the porous system can be blocked off by condensed water, so the carbonation could be obstructed [
20]. Zhang [
21] reports that the fastest carbonization rate will be reached in the relative humidity of 50–70%, while carbonization will stop when the relative humidity reaches 100% (or in water) or when it is less than 25% (or in the dry environment). Wang et al. [
22] also acknowledge the importance of moisture in efficient carbonation. Thus, an effective management of concrete humidity and microstructural space will enhance the carbonization of concrete.
Apart from humidity, the pore size of the concrete can also be affected by the precipitation of the carbonation reaction products owing to the fact that calcium carbonate is formed, which is a larger molecule than calcium hydroxide, thereby reducing the available pore microstructure, making it difficult for the diffusion of carbon dioxide into the sample core as time progresses [
19]. This behaviour makes carbonization a surface-based reaction that discontinues after a layer of calcium carbonate has covered the concrete, while calcium hydroxide lies underneath [
21]. Therefore, the task of improving carbonation efficiency in concrete involves exposing the available calcium hydroxide as well as the hydrates of tri- and di-calcium silicates (C
3S, C
2S) for further reaction with incoming CO
2. This can be achieved in various ways, including the flow mixing of the concrete with CO
2 before the concrete is set. In a semi-batch system, this involves turning the concrete continuously while CO
2 is being poured in. Increasing the surface area of the cement aggregate phase is another viable means. Since the reactions take place at the exposed surface, the use of particles with larger surface areas can bring about an improvement in carbonation efficiency.
Furthermore, concrete being exposed to a higher CO
2 concentration in the environment might accelerate the carbonation process [
23]. Besides cement variety, the water–cement ratio and the relative humidity, an important external factor that can limit carbonization is the concentration of CO
2 [
21]. Haselbach and Thomas [
21] investigated the carbonation of decades-old concrete sidewalk samples. They found the carbonation level decreasing with increasing thickness from the surface of the sample. Up to 80% carbonation was calculated for surface samples. The effects of curing and the time of carbonation were demonstrated on carbonation efficiency by [
24], with up to 24% carbonation found for an initial curing time of 18 h, but 8.5% efficiency observed in the absence of initial curing. The authors reported up to 35% efficiency for 4-day carbonation time, when using slag in the concrete. For 2 h of carbonation after 1 h of cooling, [
9] recorded only 15% efficiency in hollow-core concrete slab.
Furthermore, the carbonation process improves the mechanical properties of concrete [
25,
26]. This indicates that mankind will benefit immensely from the practice of concrete carbonation. The quality of calcium carbonate generated by the carbonation process and the developed mechanical strength are both affected by the internal water content [
27].
On the strength of the above reports, it can be argued that the simultaneous influences of particle sizes and operating pressures have not been involved in the earlier investigations. The particle sizes, distribution and arrangement will definitely influence the effective microstructural spaces in concrete. This will further determine the pore-scale diffusivity of the CO2, in order to improve the carbonation yield. This work presents a scientific effort to distinguish the effects of various particles sizes on carbonation efficiency under various operating parameters of pressure, carbonation time, the hydration stage and so on. Gravimetric and pyrolysis analyses were used to determine carbonation levels, while the microstructures of the carbonated and uncarbonated concrete samples were examined with the X-ray diffraction (XRD) instrument.
4. Discussion
The graphical representations of the results from the experiments were described earlier. As a way of affirming the reliability and repeatability of the results,
Figure 3 shows the comparisons between the analyses conducted on similar samples using gravimetric and pyrolysis methods. Under similar conditions, the results from the two analyses (gravimetric and pyrolysis (Grav./Pyro.)) were comparable. The figure shows coarse-sand concrete with 6.19% carbonation by gravimetric analysis, while it shows 6.81% carbonation by pyrolysis. In fine-sand concrete, the results show 8.27% carbonation by gravimetric analysis while it shows 8.66% carbonation by pyrolysis. These are close results, and the trends show the reliability of either of the methods.
As shown in
Figure 4, following 4 h of carbonation at 5 bar and 25 °C as well as 40 h of curing, approximately 6.4 and 8.2% carbonations were achieved in the coarse-sand and fine-sand mortar samples, respectively. The results showed higher carbonation in fine-sand mortar. Ordinarily, owing to its higher porosity, it is expected that the coarse-sand based mortar will have more interstitial pore spaces to allow CO
2 penetration and carbonation reaction to take place. However, the results show that the higher surface area present in fine-sand based concrete is a stronger factor to be considered.
Under similar conditions to that in
Figure 4, but with 64 h of carbonation, the carbonation of the mortar increased to 16.6% and 18.3% for coarse-sand and fine-sand based mortar samples, respectively. This also gives higher carbonation in fine-sand based mortar than the coarse-sand type, thereby confirming the influence of particle sizes on the carbonation reactions in mortar. It further shows the influence of carbonation time on the reaction. Thus, under suitable conditions, longer exposure of concrete to CO
2 plume will improve the carbonation reaction. The plot of these results is shown in
Figure 5.
After 104 h of curing and 6 h of carbonation time, similar levels of carbonation were observed as after 4 h of carbonation (
Figure 4), while other conditions were similar. The results show that carbonation efficiency is reduced as curing time increases. This is depicted in
Figure 6. In the figure, there are 6.2% and 8.3% carbonations in coarse-sand and fine-sand based concrete samples, respectively, which, again, shows higher carbonation potential in fine-sand mortar.
The results show that the carbonation time and particle sizes are stronger factors in carbonation efficiency while the curing time has a reverse influence. However, the slightly longer carbonation time (6 h) with the slightly lower carbonation yield and longer curing time compared to
Figure 4 implies that the carbonation potential reduces as the curing of the mortar progresses. This means that as silicates are consumed during the hydration of concrete, the carbonation potential reduces [
9,
32]. Early-age carbonation of the concrete has been reported to improve the mechanical strength of concrete composite [
25]. Thus, there is a higher advantage in concrete carbonation at a shorter curing period. Further, since the concrete samples used here are not rewetted with water after preparation, the result might imply that as carbonation progresses, concrete loses water, which reduces its potential for reaction with CO
2 under dry conditions [
21]. It should be noted that the works of [
9] as well as [
24] employed rewetting of the mortar, which might have enhanced carbonation, while [
24] compensates for the water loss during initial curing and carbonation curing with water spray, which was applied to restore the original water content. The early carbonation approach was also employed by [
9] by conducting the process after about 2 h of curing. This approach of compensating water loss is also expected to improve carbonation efficiency by making use of the moderate moisture in the mortar. This practice of compensating water loss or rewetting obviously explains the differences in the results of the current work and earlier investigations by [
24] as well as by [
9]. Adding liquid water into samples at appropriate time intervals could enhance carbonation reactions effectively, with a maximum improvement of 34.1% previously recorded [
22].
Figure 7 shows the results of experiments conducted at a lower pressure of 2 bar in order to test the influence of pressure on the carbonation yield in the mortar. The results show that the carbonation yield became lower than before, ostensibly owing to the lower pressure of the experiment. Similar to the earlier observations, the influence of carbonation time is reflected in the results, with the yield at 24 h carbonation time (1.9%) obviously higher than that at 7 h (1.4%).
Figure 8 shows the effect of pressure on the carbonation yields in fine sand samples under the similar conditions of the curing and carbonation periods. The results show the clear positive influence of pressure on the carbonation efficiency under similar conditions.
Figure 9 shows another interesting dimension to the previous discussions. In this case, experiments were conducted at 5 bar but at a longer curing period of 164 h. These experiments took place for longer carbonation times of 20 h and 26 h. Despite the higher carbonation time, the yields here (6.5% for fine and 6.4% for coarse) are even lower than those for fine-sand mortar at 4 h carbonation time (8.2% in
Figure 4). These results defy the influence of the carbonation time, as propounded earlier. However, the ostensible reason for this can be traced to the longer curing time of 164 h. Owing to this lengthy period of curing, much of the original silicates in the mortar have been consumed by the hydration reaction and much moisture was lost as the hydration progressed. Thus, the hydration stage accounts for the degree of carbonation in concrete. Furthermore, the influence of particle size is evident in the results. The coarse-sand concrete had a lower carbonation yield than fine-sand concrete. This was despite the longer number of hours involved for the carbonation of coarse-sand concrete (26 h) compared to the time involved for the fine-sand concrete (20 h). As stated earlier, a longer curing time often leads to reduced water content in the sample, which hampers the efficiency of the carbonation process. Carbonization will stop when the relative humidity reaches 100% (or in water) or is less than 25% (or in the dry environment) [
21].
Figure 10 shows the experimental results obtained at a higher pressure in concrete using 20 bar gas pressure in the chamber. Approximately 4.0% carbonation was achieved in the coarse-sand mortar, while the first and second fine-sand mortars had 4.4% and 4.6% carbonation yields, respectively. This again shows the influence of particle size in the carbonation efficiency of concrete. However, compared to the earlier results (see, e.g.,
Figure 6), the expected influence of higher pressures is missing in this case (
Figure 10). In
Figure 6, a more than 8% carbonation yield was obtained at 5 bar, 104 h curing time and 6 h carbonation time. The low carbonation yield in
Figure 10 (e.g., 4.6%), was ostensibly as a result of a higher curing period of 200 h, which reduces the carbonation efficiency. Thus, it can be inferred that the curing or hydration stage has great influence on the carbonation in the mortar. After days of hydration, the carbonation effectiveness reduces. The stage of hydration at which this effectiveness starts to decline needs to be investigated in the future. However, rewetting the mortar can reduce the effect of moisture loss on the carbonation potential after a long period of curing. Furthermore, as shown in
Figure 10, the results of the repeat experiments for the two fine samples (4.4% and 4.6%) show the repeatability of the results in these investigations. The expected influence of higher pressures can be seen between the performances of the carbonation processes in
Figure 6 and
Figure 7, under similar curing conditions. In
Figure 6, at a pressure of 5 bar, fine-sand concrete recorded 8.27% carbonation at a 104 h curing period and 6 h of carbonation time, while in
Figure 7, at a pressure of 2 bar, fine-sand concrete recorded 1.4% carbonation at a 110 h curing period and 7 h of carbonation time. This performance, despite the higher carbonation time in the latter, shows the obvious influence of pressure (5 bar in the former and 2 bar in the latter).
The influence of the curing period on the carbonation yield is comparatively shown in
Figure 11, with yields depicted for 7 h of curing (5 bar, 4 h carbonation), 40 h of curing (5 bar, 4 h carbonation) and 200 h of curing (20 bar pressure, 6 h carbonation). After 4 h carbonation time, 12.5% carbonation was achieved in the fine-sand mortar cured for 7 h, while around one-third of the yield (4.56%) was obtained for 200 h-cured concrete. This clearly depicts the strong influence of the curing period on the carbonation efficiency in concrete.
Summary analysis of the gravimetric results shows that for a curing period of 7 h (
Figure 11), more than 12% carbonation was achieved. The carbonation reduced to 8.2% at the 40 h curing period (
Figure 4 and
Figure 11). On the pressure effect, for comparable curing conditions, 2 bar at 7 h carbonation time gave a 1.4% yield (
Figure 5) while 5 bar at 6 h carbonation time gave more than 8.2% (
Figure 6). Furthermore, analysing the effect of carbonation time for comparable conditions shows that 4 h of carbonation time gave up to a 8.2% yield, while 64 h of carbonation gave up to 18.5%. The carbonation time is effective in ensuring that CO
2 diffusion takes place at the surface and core parts of the concrete. Thus, the longer the carbonation time, the further the reach of the gas in the concrete structure.
In comparison, [
9] record up to 14.5% carbonation in 2 h of carbonation following 2 h of heat curing. This carbonation yield is comparable to the 64 h of carbonation and 40 h of curing in this work (
Figure 5). It may appear that the efficiency is higher in the work of [
9], but a consideration of the different experimental processes and conditions explains some underlying factors. In [
9], they used a flow-through experiment where CO
2 was made to pass through the microstructural pores in the mortar, unlike in this work, where CO
2 had to overcome surface inhibition or pore blockage to diffuse further into the mortar matrix. That explains why the authors were able to report the carbonation yields at the top, core and bottom of the carbonated mortar. Furthermore, the carbonation by [
9] took place immediately following 2 h of curing. This enhances the performance of the process, unlike in the current case, where carbonation took place after 40 h of curing, which resulted in more loss of moisture. Furthermore, the cement content in the mortar used by [
9] was 15% compared to 12.7% used in this work. They used a water content of 0.3 compared to 0.6 in this work. Thus, the mortar samples in [
9] had more pore space owing to the lower water content and more binder cement, both of which contributed to their better carbonation performance.
Ref [
24] reported up to 24% carbonation with initial curing of up to 18 h. The effect of carbonation time was also reported, with up to 35% carbonation based on 4-day carbonation time (96 h) recorded. Again, this seems to show a better carbonation than recorded in this work. However, it should be noted that the authors used slag in their concrete mixture, which might have enhanced the carbonation efficiency. Pozzolana cement and blast-furnace slag cement readily undergo carbonization [
21]. Similar to the current findings, their work corroborated the fact that carbonation increases with CO
2 exposure time (carbonation time). The observation that carbonation increases with time is similar to the findings of [
18] on the carbonation of decades-old concrete sidewalk samples.
In
Figure 12, at 2θ =14.9, 27.5, 29.6, 32.3, 32.6, 34.3 and 41.3, the peaks of tricalcium silicate (C
3S) were conspicuously stronger in the uncarbonated mortar samples than in the carbonated ones. The reason for this is that the carbonation reactions reduced the amount of remaining silicates in the concrete [
19]. At 2θ = 36.1, 43.3, 47.5 and 48.6, CaCO
3 peaks were stronger in the carbonated mortar samples, which shows evidence for carbonation. At 2θ = 18.1 and 34.2, stronger peaks appear for Ca(OH)
2 in uncarbonated samples than in the carbonated ones. This observation is similar to the findings by [
19]. This observation may be accounted for by the fact that Ca(OH)
2 growth is inhibited by carbonation in carbonated mortar while its growth is less-restrained in uncarbonated mortar samples.
Figure 13 shows further evidence of carbonation in mortar samples. In the figure, peaks for Ca(OH)
2 and C
3S are conspicuously stronger in the uncarbonated samples than in the carbonated ones. This implies that these compounds were consumed by the carbonation process, leading to their reduced quantities in the carbonated sample. Evidence of carbonation is further indicated in the carbonated sample at 2θ = 39.6, with a stronger peak for CaCO
3 in the carbonated mortar. Meanwhile, it should be noted that the reaction of CO
2 with Ca(OH)
2 during the carbonation process leads to reduced pH. Thus, acidity increases in carbonated mortar, which may have a detrimental effect on the steel in reinforced concrete. Therefore, carbonation may be more suitable for unreinforced concrete.
Furthermore,
Figure 14 shows relative carbonation in two carbonated mortar samples with CaCO
3 peaks. Sample 1 (red line) was 6.55% carbonated while sample 2 (black line) was 1.94% carbonated. At every point considered, there are relatively stronger peaks in sample 1 (6.55%) than sample 2 (1.94%). This indicates the increase of carbonate content in carbonated samples and may be taken as evidence of relative percentages of carbonation.
Micrographs of scanning electron microscopy (SEM) for the carbonated and uncarbonated concrete samples are shown in
Figure 15. The uncarbonated sample (
Figure 15A) displays the amorphous C-S-H phase with Ca(OH)
2 background. The ettringite needles are hardly visible in the uncarbonated sample owing to the early stage curing of the samples (120 h or 5 days). In [
9], they did not observe ettringite needles in heat-cured concrete until after 28 days. Therefore, the faint and scarce needle-like structures at the background of the samples may indicate the early formation of the product. In their work, [
33] observed a small amount of ettringite at the early stage but more at a later age.
Figure 15B shows a uniform denser and cloudy mass of carbonated concrete with a crispy look and without the elements of connecting needles or a nebulous connection as in
Figure 15A. The absence of the needle-like connection in the carbonated concrete can be attributed to the early stage of curing as well as the inhibition of ettringite formation by the presence of CO
2 and CaCO
3 formation [
19,
34]. According to [
34], early-age carbonation curing decomposed this ettringite into calcium carbonates. In the figure, the CaCO
3 has been seamlessly integrated into the C-S-H phase to give a more solid and denser structure. As concrete is carbonated, it becomes denser, because CaCO
3 occupies a greater volume than the Ca(OH)
2, which it replaces in the concrete [
35,
36]. Thus,
Figure 15B reflects the evidence of carbonation. The darker appearance of the micrograph in
Figure 15B is also evidence of carbonation in the sample. The production of crystalline CaCO
3 under carbonation had a filling effect that refined the pore sizes [
34], thus leading to a uniform denser and cloudy mass of carbonated concrete with a crispy look.
Figure 16 shows another set of carbonated and counterpart uncarbonated concrete samples. In
Figure 16A, widespread distribution of the needle form of C-S-H (calcium-silicate-hydrate) is visible together with the background Ca(OH)
2 phase. These patterns disappeared in the carbonated concrete, leading to denser-looking structure (
Figure 16B). This dense look can mean increased strength in the carbonated concrete. Carbon dioxide reacts with tricalcium silicate (C
3S) and this accelerates the setting and early strength development [
37].
Similarly, a change in appearance is also noticeable. The carbonated concrete has a darker appearance than the uncarbonated sample. This is likely the effect of carbonation on the sample. The un-hydrated cement particles appear brightest [
38], while the carbonated concrete has more grey to dark grey areas [
39].