Performance of Alkali-Activated Slag Concrete Masonry Blocks Subjected to Accelerated Carbonation Curing

Abstract

This study investigates the effect of accelerated carbonation curing on the carbon sequestration potential, performance, and microstructure of alkali-activated slag mixes representing concrete masonry blocks (CMBs). The carbonation curing process parameters varied, including initial curing duration, carbonation curing duration, and carbonation pressure. Research findings showed that a maximum CO₂ uptake of 12.8%, by binder mass, was attained upon exposing concrete to 4 h initial curing and 20 h carbonation curing at a pressure of 5 bars. The compressive strength and water absorption capacity improved with longer initial and carbonation curing durations and higher pressure. Upon subjecting to salt attack, the mass and strength of 28-day concrete samples increased, owing to the formation of Friedel’s salt and Halite. All mixes could be used as non-load-bearing CMB, with a 1-day strength greater than 4.1 MPa. Based on the global warming potential index, the carbon footprint of carbonation-cured, alkali-activated slag concrete masonry units was up to 46% lower than non-carbonation-cured counterparts. Research findings offer valuable information on the production of carbonation-cured, cement-free concrete masonry blocks to replenish natural resources, recycle industrial waste, and mitigate CO₂ emissions.

1. Introduction

The construction industry has a detrimental impact on the environment, with the consumption of natural resources, generation of construction waste, and emission of carbon dioxide gas. Concrete, mortar, and paste are mainly produced with ordinary Portland cement. The production of 1 ton of cement requires up to 3.7 MJ of energy, consumes up to 1.6 kg of raw materials, and emits 1 ton of CO₂ [1]. As a result, it contributes to around 5–8% of the total carbon dioxide emissions annually, causing an increase in the global warming potential and leading to catastrophic natural incidents, such as flooding, wildfires, and droughts [2]. With the increase in population and structures reaching the end of their service lives, more infrastructure and superstructures, i.e., concrete, will be needed. As such, scientists have been examining different methods to reduce the anthropogenic emissions induced by cement manufacture, including alternative sustainable binding materials for concrete and carbon dioxide sequestration [3,4].

Accelerated carbonation curing of concrete has been developed as a means of reducing carbon emissions [5,6]. This curing method captures and stores CO₂ while enhancing the mechanical and durability properties of cement-based materials [5,7]. The carbonation curing process involves exposing concrete samples to CO₂ gas for a pre-determined duration and under specific environmental conditions in a closed chamber. Thus, it is ideal for precast concrete products, such as masonry blocks, paving stones, and others [4]. In conventional Portland cement concrete, CO₂ reacts with the calcium ions (Ca²⁺) of decalcified hydration products, including calcium silicate hydrates (C-S-H) and calcium hydroxide (CH), to form calcium carbonate polymorphs (CaCO₃). Past literature has examined the effect of carbonation process parameters on reaction efficiency and concrete performance. An increase in the pressure from 0.1 to 10 bars accelerated the carbonation reaction and led to higher CO₂ uptake and improved the mechanical and durability properties of cement-based concrete [6]. In addition, carbonation curing was found to be less energy consuming than steam curing of concrete masonry units and less time consuming than water curing, which decelerates the hardness rate of concrete [6]. From a durability standpoint, Rostami et al. [8] reported higher acid resistance when concrete was carbonation cured rather than steam cured. Conversely, from an environmental perspective, if carbonation curing were adopted, for instance, by the concrete blocks and bricks and concrete paver block industries, they would sequester around 900 million and 1.07 million tons of CO₂ per year globally, respectively [9]. Thus, implementing this technique on precast concrete elements serves to mitigate greenhouse gas emissions induced by the construction industry.

Other studies have aimed to reduce the carbon emissions of the construction industry by replacing cement with alkali-activated materials or geopolymers as the main binder in concrete. The alkali-activated or geopolymeric binder mainly consists of aluminosilicates and calcium-based waste materials, such as fly ash (FA), ground granulated blast furnace slag (i.e., slag), ladle slag, metakaolin, waste perlite, and rice husk ash, among others [10]. Recycling these by-products rather than disposing of them in stockpiles or landfills is crucial in reducing environmental pollution. With higher mechanical and durability properties than cement-based counterparts, geopolymers, and alkali-activated materials have proven to be excellent candidates for construction applications [10,11,12].

Of these various applications, geopolymers may be used as the main binder in concrete masonry blocks (CMBs), as they provide fast setting times [11] and early strength gains [13], thereby helping in accelerating the production and use of CMB. Such blocks are extensively produced and used by the construction and building sector as loading and non-loading insulating walls. They are mainly known for their strength, durability, and fire resistance properties. By the end of 2020, the United States of America and the United Kingdom alone have produced approximately 552.6 billion and 62.13 million cement-based CMB, respectively [14]. With an annual growth rate of 5%, it is forecasted that CMB production will reach nearly 3.1 trillion units by 2030 [15]. ASTM C129 [16] and C90 [17] categorize a CMB based on its compressive strength as non-load bearing and load bearing, with a minimum average compressive strength of 4.14 MPa and 13.8 MPa, respectively. Recently, several studies have studied the valorization of alkali-activated or geopolymer-based material to be used as CMBs [18,19,20,21,22]. It was found that fly ash- and slag-based geopolymer blocks had superior mechanical properties to blocks produced with cement [18,19]. In another study, Petrillo et al. [21] stated that the production of geopolymeric CMBs consumed less energy and raw materials than conventional CMB. Ban et al. [22] produced a moist-cured, load-bearing geopolymer CMB based on high calcium wood ash and pulverized fuel ash with a compressive strength of over 25 MPa after 90 days of curing.

The combination of these two initiatives, i.e., geopolymers and carbonation curing, may be a promising technique to reduce the carbon footprint of cement manufacture further while beneficially recycling industrial wastes. However, this scheme has seen little investigation [23]. In fact, current studies have assessed the carbonation resistance of geopolymer composites as a form of a durability test rather than a curing regime. Law et al. [24] found that the pH of fly ash-based geopolymer concrete after carbonation varied between 11 to 12, whereas that of ordinary Portland cement concrete ranged between 9 and 13. Furthermore, another study observed that the pH of fly ash- and slag-based geopolymer concrete was higher than 10.2 at the depth of the steel reinforcement [25]. Thus, carbonated geopolymer or alkali-activated composites were deemed suitable for use in reinforced concrete elements, as the steel does not corrode until the pH drops below 10.0 [25]. Moreover, Li et al. [26] found that the mix proportions affected the carbonation resistance of fly ash- and slag-based geopolymer concrete, as a higher content of alkaline activator produced with sodium hydroxide and sodium silicate resulted in higher carbonation resistance. In another study [27], it was found that alkali-activated slag concrete possessed a higher ability to sequestrate CO₂ compared to FA-based geopolymer and cement-based counterparts. The absorption of CO₂ was mainly due to the presence of higher micro-cracks inside the microstructure of the alkali-activated slag concrete and the low ratio of Ca/Si found in calcium silicate hydrate gel (C-S-H) compared to the cement-based concrete C-S-H gel. Additionally, Mei et al. [28] pointed out that carbonation curing resulted in the formation of new compounds, primarily calcite and vaterite, through the decalcification of the C-S-H gel. This process led to a weakening of the sample matrix. On the effect of water content on the properties of a carbon-cured, slag-based, alkali-activated material, it was found that the water-to-binder ratio had a limited impact on the microstructure before and after carbonation, as well as on the formation of carbonates [29]. Moreover, Jun et al. [30] developed a carbonation-cured, slag-based geopolymer material suitable for salty and humid environments, where samples achieved most of its strength after 3 days of carbonation curing. However, its strength showed slight improvement with further emersion in water bath or in a sea water [30]. Summarizing the literature, it is clear that the synergic effect of utilizing accelerated carbonation curing and cement replacement by an alkali-activated binder on the properties of mixes resembling concrete masonry blocks (CMBs) has not been investigated yet. Thus, this paper aims to evaluate the CO₂ uptake, mechanical and durability properties, microstructure characteristics, and environmental impact of alkali-activated slag concrete subjected to accelerated carbonation curing. The carbonation curing process parameters were varied, including initial curing durations (0, 2, 4, and 20 h), carbonation curing durations (4, 20, and 24 h), and carbonation pressure (1 and 5 bars). The performance was characterized by compressive strength, water absorption, volume of permeable voids, pH, and resistance to salt attack. The microstructure of carbonated and hydrated CMB mixes pre- and post-exposure to salt attack was evaluated using X-ray diffraction (XRD) analysis. Furthermore, the environmental impact of so-produced CMB mixes was assessed based on global warming potential indices.

2. Research Significance

Past research papers discussed the performance of alkali-activated materials after exposure to carbonation curing. Yet, none have aimed to produce carbonation-cured, alkali-activated materials for concrete masonry block applications while assessing their behavior in saline environment and analyzing their environmental impact. Accordingly, this paper assesses the impact of accelerated carbonation curing on the mechanical properties, durability, microstructure characteristics, and environmental impact of alkali-activated slag. Different process parameters were included, such as initial curing duration, carbonation curing duration, and carbonation pressure. The findings of this study provide crucial insights into the potential for producing carbonation-cured, cement-free concrete masonry blocks to replenish natural resources, recycle industrial wastes, and mitigate CO₂ emissions.

3. Materials and Methods

3.1. Materials

Locally available ground granulated blast furnace slag (referred to hereafter as slag), supplied by Super Cement Manufacturing, was used as the binder material in the alkali-activated CMB. Its chemical composition is presented in

Table 1. Chemical composition of slag and crushed limestone sand.

Oxide	Slag	Crushed Limestone
CaO, %	59.7	89.3
SiO2, %	27.0	8.5
Al2O3, %	7.5	-
SO3, %	2.5	-
Fe2O3, %	1.2	1.5
Loss on ignition, %	1.1	-
Others, %	1.0	0.7

. Its specific gravity and Blaine fineness were 2.5 and 4350 g/cm², respectively. Slag was selected as the sole binder in this study due to its high calcium oxide content and calcium-rich reaction products, which readily reacted with CO₂ gas to produce various carbonation products [9,27,31]. Indeed, previous studies found that the microstructures of alkali-activated slag materials were richer in calcium-based gels, such as calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) gels, compared to those of fly ash- or perlite-based geopolymer concrete [32,33].

Crushed limestone, with a maximum particle size of 9.5 mm, served as the combined aggregates (fine and coarse). They were particularly selected due to their local abundances and superior mechanical and durability properties compared to other aggregates, such as dune and river sands [11,34]. Figure 1 shows the particle size distribution of the crushed limestone aggregates, with 95% of the particles possessing a size below 4.5 mm. Their dry-rodded density and specific gravity were 1640 kg/m³ and 2.65, respectively. According to the X-ray fluorescence (XRF) results of Table 1, the crushed limestone aggregates were mainly composed of calcium oxide (89.3%).

Figure 2a,b presents the XRD patterns of slag and crushed limestone sand. Slag has a pure glassy texture and does not exhibit any crystalline phase, owing to its amorphous composition, with a prominent hump observed between 25 and 35° 2θ. On the other hand, crushed limestone sand mainly comprises crystalline phases of dolomite and calcite.

Grade N sodium silicate (SS) and sodium hydroxide (SH) solutions were used to formulate the alkaline activator solution (AAS). The SH solid flakes (98% purity) were mixed with tap water to formulate an SH solution with 8 M. Such molarity was selected to produce the alkali-activated slag mixes with sufficient setting time for CMB production [11,35]. Indeed, previous work stated that decreasing SH molarity from 14 to 8 M increased the workability and setting time of alkali-activated materials [36,37]. However, reducing the molarity to less than 8 M reduced the effectiveness of the activation reaction [38]. The SS solution had a density of 1506 kg/m³ and was composed of 26.3% SiO₂, 10.3% Na₂O, and 53.4% H₂O.

3.2. Mix Design

The alkali-activated slag mixes were prepared following the commercial CMB mix design as follows:

-

Binder percentage: 13% of the total mass [4];

-

Binder-to-aggregate ratio (B/A): 1:6;

-

Alkaline solution-to-binder ratio (AAS/B): 0.6;

-

SS-to-SH ratio (SS/SH): 1.5.

These values were selected to ensure adequate mechanical performance while creating a porous microstructure to allow CO₂ penetration [7,11,18].

After casting the alkali-activated concrete, different curing schemes were adopted, as presented in

Table 2. Carbonation curing schemes.

Scheme No.	Mix Designation	Initial Curing (h)	Carbonation Curing (h)	Pressure (bars)
1	0a-0c-0b	-	-	-
2	0a-4c-1b	0	4	1
3	0a-24c-1b	0	24	1
4	2a-4c-1b	2	4	1
5	4a-4c-1b	4	4	1
6	4a-20c-1b	4	20	1
7	20a-4c-1b	20	4	1
8	0a-4c-5b	0	4	5
9	4a-4c-5b	4	4	5
10	4a-20c-5b	4	20	5
11	20a-4c-5b	20	4	5

. Each curing regime was divided into two parts, initial curing (denoted as ‘a’) and carbonation curing (denoted as ‘c’). The total curing time frame was less than or equal to 24 h to promote the adoption of these regimes by the CMB industries. In the first part, the concrete samples were left to rest in the open air after demolding for a designated duration (0, 2, 4, or 20 h). Afterward, they were placed in a sealed carbonation chamber for a specific time (4, 20, or 24 h) and at constant pressure (1 or 5 bars), as shown in the schematic diagram of Figure 3. The curing schemes are labelled Ya-Zc-Pb, where Y denotes the initial curing time in hours, Z describes the carbonation curing time in hours, and P represents the carbonation pressure of 5 bars. Scheme 1 (0a-0c-0b) is the hydrated control regime and is used for comparison purposes. Schemes 2/3, 5/6, and 9/10 were compared to evaluate the effect of carbonation duration on the performance and reaction efficiency of fresh and semi-hardened (4-h) concrete. Conversely, comparing schemes 2, 4, 5, and 7 aims to examine the impact of initial curing duration on said characteristics. Finally, the influence of carbonation pressure was evaluated using schemes 2/8, 5/9, 6/10, and 7/11. Several schemes were compared to ensure that the effect of each process parameter did not change while varying another parameter.

3.3. Sample Preparation

Alkali-activated slag CMB mixes were prepared based on the proportions highlighted in the previous section. The alkaline activator solution was first prepared by mixing SH flakes with water and left to rest to dissipate the heat from the exothermic reaction, after which the SS solution was added. After reaching room temperature, the alkaline activator solution was added to the premixed slag and limestone aggregates and mixed for 3 min to ensure homogeneity. Freshly mixed concrete was cast into 50 mm cubic molds and compacted on a vibrating table for 10 s.

3.4. Performance Evaluation

The CO₂ uptake was determined using the mass gain method (Equation (1)) by measuring the increase in mass of each sample after carbonation curing as a function of the binder mass [4,7]. In this method, the CO₂ absorbed was measured as the change in mass of the concrete samples between before (initial mass) and after (final mass) the carbonation process. As the system was closed, the water lost during the exothermic carbonation reaction was collected and added to the final mass (water loss). The mass of binder represented 13% of the total mass of the concrete sample, as per the mix design proportions stated earlier.

After the accelerated carbonation curing process, phenolphthalein solution was used to assess the average carbonation depth and identify the carbonated and uncarbonated regions of the carbonated and hydrated samples. Indeed, phenolphthalein is a pH indicator that turns pink-fuchsia when the pH is above 10 and colourless when the pH falls below 8.3 [32]. After the solution was sprayed on the freshly cut surfaces of the 28-day specimens, it would turn violet/pink if the surface were uncarbonated, whereas it would remain colorless if it were in contact with a carbonated surface.

$$\mathrm{Carbon} \mathrm{uptake} \left(\%\right)=\frac{(\mathrm{Final} \mathrm{mass} - \mathrm{Initial} \mathrm{mass})+\mathrm{Water} \mathrm{loss}}{\mathrm{Mass} \mathrm{of} \mathrm{binder}} \times 100$$

(1)

The 28-day pH of the surface (0–1 cm) [pH_(s)] and in-depth (1–2.5 cm) [pH_(d)] of non-carbonated and carbonated samples were evaluated using the leaching solution method described by Plusquellec et al. [39]. Before the measurement, powder from two different depths (0–1 cm and 1–2 cm) was collected and sieved to a diameter of less than 75 μm. Then, the obtained powder was mixed in distilled water for ion dispersion. Subsequently, the pH of the solution was determined using the LAQUA PH2000 pH meter, Horiba, Kyoto, Japan. An average of five readings was taken per mix.

The 1-, 7-, and 28-day compressive strengths (f_c) of alkali-activated slag concrete specimens were assessed using 50 mm cube samples and following the ASTM C109 standard [40]. The 28-day water absorption and volume of permeable voids were measured as per ASTM C642 [41]. Furthermore, to evaluate the resistance of alkali-activated slag CMB mixes to salt attack, the procedure of ASTM C267 [42] was followed. Samples were placed in 3 M sodium chloride solution (NaCl) after 28 days of casting. In addition, the mass variation and compressive strength were measured after 7, 14, 21, and 28 days of the salt attack. The average readings of three samples were considered for all described testing.

X-ray diffraction analysis was conducted to determine the change in the crystal structure of the alkali-activated slag matrix after carbonation curing and salt attack using a Rigaku Diffractometer (Cu, Kα radiation, X’celerator proportional detector, scan interval 10–60°, 0.02°, and 0.5 s per step). Alkali-activated slag concrete was crushed to a powder and sieved to have a maximum particle size of 75 μm. Figure 4 shows the various testing procedures carried out in this work.

The environmental impact of the carbonated and non-carbonated alkali-activated slag CMB mixes was assessed based on the global warming potential (GWP), which represented the total mass of CO₂ associated with the materials used in the production of 1 m³ of masonry blocks (kg CO_{2 eq}/m³). As such, the GWP of the alkali-activated slag CMB mixes subjected to carbonation curing was calculated by subtracting the CO₂ uptake (in kg CO₂ eq/m³) from the GWP of the mix. The GWP indices (

Table 3. Global warming potential (GWP) of used materials.

Material	GWP (kg CO2 eq/kg)	Reference
Slag	0.0416	[39]
Cement	0.8980	[39]
Crushed limestone	0.0052	[39]
Sodium silicate	0.4240	[40]
Sodium hydroxide	0.8293	[41]
Water	0.0126	[39]

) for the materials used in this study were obtained from previously published studies on the life cycle assessment of geopolymer and cement-based composites.

4. Results and Discussion

4.1. CO₂ Uptake

The degree of carbonation reaction is characterized by the CO₂ uptake. Figure 5 shows the effect of different carbonation curing parameters, i.e., carbonation curing duration, initial curing duration, and carbonation pressure, on the CO₂ uptake of alkali-activated slag CMB mixes. The CO₂ uptake varied between 9.6% and 12.8%, by mass of the binder. Figure 5a shows that the CO₂ uptake increased with an increase in the carbonation duration, regardless of the initial curing period. In fact, mixes exposed to immediate carbonation of 4 h and 24 h without any initial curing (0a-4c-1b and 0a-24c-1b) experienced a 16% increase in CO₂ uptake, from 9.6% and 11.2%, respectively. Similarly, mixes initially cured for 4 h and carbonation cured for 4 and 20 h had respective CO₂ uptakes of 9.9% and 10.7%, representing an increase of 8%. This increase in CO₂ uptake with extended carbonation curing durations was owed to the more prolonged contact between CO₂ and calcium-rich components in the slag, which promoted the carbon storage capacity of slag-based geopolymers [4].

Figure 5b presents the impact of extending the initial curing duration on the CO₂ uptake of alkali-activated slag mixes. In these mixes, the carbonation curing duration and pressure were set to 4 h and 1 bar, respectively. The immediate carbonation curing with no initial air curing (0a-4c) resulted in a CO₂ uptake of 9.6%. After 2, 4, and 20 h of initial air curing, the CO₂ uptake increased to 9.7%, 9.9%, and 10.2%, respectively. Longer initial air curing drew more water from the mix through evaporation, forming more voids within the matrix and causing an increase in the carbon sequestration capacity. Compared to cement-based concrete, the effect of initial curing on the carbonation efficiency was less prominent in alkali-activated slag concrete [4]. Such a limited impact of initial curing was owed to the difference in the reaction mechanism between the hydration of cement particles and the alkali-activation of calcium-rich precursor. Actually, water evaporated in the latter at a faster rate than the former, where alkali-activated slag binders experienced accelerated water loss during the casting and curing stages [6,42].

Figure 5c evaluates the effect of increasing the carbonation pressure from 1 bar to 5 bars. With an increase in the carbonation pressure, the CO₂ uptake increased by up to 15% and 19% for 4- and 20 h carbonation-cured mixes, respectively. This enhancement in CO₂ uptake can be attributed to more carbon dioxide penetrating the sample at higher pressure [43].

4.2. Compressive Strength

The 1-, 7-, and 28-day compressive strengths of alkali-activated slag CMB mixes are presented in Figure 6. Generally, carbonation curing either maintained or reduced the compressive strength of alkali-activated slag CMB. This was owed to the fact that the carbonation of such material led to the decalcification of C-S-H, C-A-S-H, and C-N-A-S-H gels and the formation of calcium carbonate polymorphs [44,45]. Nevertheless, a compressive strength greater than 4.1 MPa would suffice to use this material in non-load-bearing CMB applications, as per ASTM C129 [16]. Such a strength requirement was met by all mixes regardless of the curing scheme adopted. Therefore, all mixes could be employed as non-load-bearing CMBs. Yet, further analysis is carried out herein to maximize the CO₂ uptake without compromising the mechanical strength.

The 1-, 7-, and 28-day compressive strengths varied between 5.9–11.9 MPa, 6.5–12.3 MPa, and 7.0–12.5 MPa, respectively. The strength development from 1 to 28 days was observed. The results show that 85 to 98% of the 28-day compressive strength was attained after 1 day. Such a rapid strength gain was owed to the accelerated and high degree of reactivity of calcium-bearing components in the slag particles [11,46]. This phenomenon can help in the early service and use of alkali-activated slag CMBs by the construction and building industry.

As the trends in the 1-, 7-, and 28-day compressive strengths were similar, the focus of the analysis was on the 28-day results. Figure 6a highlights the effect of carbonation curing duration on the compressive strength of alkali-activated slag concrete mixes. An increase in the carbonation curing duration from 4 to 24 h (0a-4c-1b and 0a-24c-1b) and 4 h to 20 h (4a-4c-1b and 4a-20c-1b) led to 8% and 33% losses in strength, respectively. The larger strength reduction in the latter comparison may be due to the higher loss of the activator solution in the alkali-activated slag paste before carbonation, resulting in a low degree of activation reaction, void formation, fast and deep penetration of CO₂ into the matrix, and advanced decalcification of calcium-rich components [23]. Similar results were found elsewhere [28], where the compressive strengths of carbonation-cured, alkali-activated materials dropped by 50% after 1 day of exposure.

The influence of the initial curing duration on 4 h carbonation-cured, alkali-activated slag CMBs is highlighted in Figure 6b. It can be seen that 2 h initial curing seems to have the least impact on the compressive strength, with similar strength to the control mix (12.4 MPa). However, the strength dropped by 16%, 8%, and 10% when the concrete was initially cured for 0, 4, and 20 h, respectively. Similar results were found by El-Hassan et al. [4] in the production of carbonation-cured, cement-based lightweight CMBs, where the highest compressive strength was reported upon initially curing mixes for 4 h in the open air. Furthermore, it is worth noting that the highest drop in strength in mix 0a-4c-1b may be due to the weak bond of calcium-based gels at such an early stage. It is also possible that the fresh mix had an excess solution, thereby decelerating the penetration of CO₂ into the matrix, reducing the CO₂ uptake, decreasing the density of the concrete mix, and, consequently, causing a loss in strength. Additionally, samples 0a-24c-1b, 4a-20c-1b, and 20a-4c-1b were compared to study the impact of carbonation curing on the compressive strengths of alkali-activated slag CMBs cured for a total of 24 h. These samples achieved 28-day strengths of 9.6 MPa, 7.7 MPa, and 11.2 MPa, respectively. To promote strength development, it was favorable to either not expose the concrete to any initial air curing (0a) or to extensive initial air curing (20a). It seems that immediate carbonation curing advanced the formation of carbonation products at early age while extending initial air curing followed by carbonation allowed for the co-existence of activation and carbonation products, thereby resulting in higher compressive strength.

The influence of an increase in carbonation curing pressure from 1 to 5 bars on the compressive strength of alkali-activated slag CMBs is shown in Figure 6c. Upon increasing the pressure, the strength decreased by 33%, 9%, 10%, and 14% in mixes 0a-4c, 4a-4c, 4a-20c, and 20a-4c, respectively. Clearly, the highest strength loss was in the mix without initial curing. Meanwhile, the incorporation of initial curing reduced this strength loss. The performances of the carbonation-cured, alkali-activated slag CMBs were impacted by two factors: the decalcifications of the activation reaction products and the formations of the calcium carbonate polymorphs. While the former causes a loss in strength due to the breakage of the bonds and weakening of the matrix, the latter enhances the strength by filling the voids in the matrix. Yet, the influence of the former phenomenon on compressive strength seems to be more dominant than the latter.

4.3. Water Absorption and Volume of Permeable Voids

Figure 7 presents the water absorption (WA) and volume of permeable voids (VPV) of the control and carbonation-cured, alkali-activated slag CMB mixes. Generally, the two properties followed a similar trend. The control mix (0a-0c-0b) possessed the highest WA and VPV values of 9.8% and 22.9%, respectively. Among the carbonated samples, mix 0a-4c-1b, exposed to direct carbonation for 4 h without initial curing, had the highest WA and VPV, with values of 9.3% and 22.9%. In comparison, samples exposed to initial curing for 4 h followed by 20 h carbonation curing under a pressure of 5 bars absorbed the least amount of water and had the lowest VPV (7.2% and 20.5%), respectively. Clearly, carbonation curing had a positive impact on the WA and VPV. A detailed discussion of the influence of the process parameters on these two properties follows.

The duration of carbonation curing positively affected the WA and VPV. In fact, the WA and VPV dropped from 9.8% to 9.3% and from 22.9% to 21.9% upon increasing the carbonation duration from 4 to 24 h (without initial curing), respectively. In parallel, the WA and VPV values were reduced from 8.4% to 8.1% for mixes 4a-4c-1b and 4a-20c-1b, respectively. These two properties slightly decreased with an increase in the carbonation curing duration, which also caused an increase in CO₂ uptake. Apparently, the newly formed carbonation products within the alkali-activated matrix filled the voids and densified the concrete structure, causing a reduction in the WA and VPV. Indeed, such extension of the carbonation curing led to longer contact time between the matrix and CO₂, thereby inducing more precipitation of calcium carbonate and filling the internal pores and voids of alkali-activated slag CMBs [23].

The effect of initial curing on WA and VPV is examined in Figure 7b. The WA and VPV decreased by up to 18 and 7% compared to the control mix upon the implementation of carbonation curing, respectively. This is aligned with the increase in CO₂ uptake. In fact, extending the initial curing duration reduced the free water and promoted CO₂ gas diffusion, calcium carbonate precipitation, and structural densification [4].

Figure 7c illustrates the impact of carbonation pressure on WA and VPV. With the increase in pressure from 1 to 5 bar, the WA/VPV values decreased by 17/7%, 10/2%, 11/4%, and 8/4% for mixes 0a-4c, 4a-4c, 4a-20c, and 20a-4c, respectively. Such reductions in these properties were aligned with the increase in CO₂ uptake. Similar conclusions were highlighted by Ahmad et al. [47], where the increase in carbonation pressure resulted in the densification of the matrix due to calcium carbonate precipitation, leading to a drop in concrete water absorption and permeability. Nevertheless, this densification was not associated with a strength increase, as the decalcification of the activation reaction products caused a strength loss higher than the strength gain due to the formation of the calcium carbonate polymorphs.

Based on the foregoing, the WA and VPV of alkali-activated slag CMB mixes exposed to carbonation curing were related to the CO₂ uptake. Figure 8 plots the relationships between each of the WA and VPV with the CO₂ uptake. Clearly, an increase in the CO₂ uptake led to a decrease in the WA and VPV, where new carbonation products filled the internal pores of concrete matrix. However, it should be noted that, although a good correlation existed among these properties, the case was different with compressive strength. It seems that the carbonation reaction products precipitating in the matrix did not contribute to the strength as much as the activation products, resulting in a loss in mechanical properties. Nevertheless, they had pore-filling capabilities, thereby reducing WA and VPV and indirectly improving their durability performances.

4.4. pH

The results of pH(s) and pH(d) are highlighted in Figure 9a. For the same sample, the pH values did not vary significantly between the surface and in-depth measurement. The limited variations were because the mixes were generally porous, and the CO₂ seemed to have reached the internal parts of the samples as well as the surfaces. This is evidenced by the brighter and uniform color of the surface of a typical carbonated sample (Figure 9b) after spraying with phenolphthalein compared to the control mix (Figure 9c). Meanwhile, the pH(d)s of CMB mixes dropped from 11.4 to 9.1 (20%) due to carbonation. Such a drop in pH was owed to the consumption of alkaline compounds during carbonation and the formation of calcium carbonate [23]. The results also show that the pH was directly impacted by the CO₂ uptake. In fact, as the CO₂ uptake increased, the pH decreased. Such a correlation is presented in Figure 10. In another study, the depth of carbonation of slag geopolymer paste, tested with phenolphthalein solution, increased with longer carbonation duration [29].

4.5. Salt Attack

The mass gain values of alkali-activated slag CMBs after exposure to a 28-day salt attack are plotted in Figure 11. Generally, the mass of all samples increased with salt exposure. The control mix (0a-0c-0b) showed the highest mass gain by 0.91% after 28 days. Similar results were found in another study [48], where the masses of geopolymer concrete mixes prepared with fly ash and slag increased by 1.0% to 1.8% after nine months of NaCl exposure. This was owed, initially, to the penetration of the chemical compounds of the solution and, secondly, to the expansion of some compounds and/or the formation of new ones within the concrete matrix [48]. Comparing the control mix to other mixes, carbonation curing affected the mass gain of CMB. For instance, mix 4a-4c-1b achieved a mass gain of 0.48%, whereas, with further carbonation exposure for 24 h, the mass of 4a-20c-1b showed the lowest mass gain by only 0.07% after 28 days of salt exposure.

On the effect of initial carbonation duration, mix 2a-4c-1b possessed the highest mass gain of 0.67%. This was corroborated by the 28-day compressive strength values of CMB samples, where it seemed that alkali-activation reaction products responsible for strengthening the concrete matrix reacted with NaCl solution, leading to an increase in the mass of the concrete sample [49]. Similarly, the mass gain of carbonation-cured CMB samples exposed to a pressure of 5 bars was shown to be lower than that exposed to a lower pressure of 1 bar, except for the sample 4a-20c. The mass gain of mixes 0a-4c, 4a-4c, and 20a-4c dropped from 0.41%, 0.48%, and 0.46% to 0.40%, 0.42%, and 0.21% when the pressure increased from 1 to 5 bars, respectively. It seemed that, as the degree of carbonation increased, lesser activation reaction products were available to react with the NaCl, thereby reducing the mass gain.

Figure 12 shows the strength evolution of alkali-activated slag CMB mixes after 28 days of exposure to salt. It is worth noting that samples exposed to salt were tested 56 days after casting, whereas the unexposed counterparts were 28 days of age. This comparison was still valid, as the strength increase between 28 and 56 days was negligible in all non-exposed samples. The control mix (0a-0c-0b) experienced the highest strength increase of 28%, with a value of 16 MPa. This was associated with the highest mass gain, as reported in Figure 10. Similar findings were found by Jiao et al. [50], where the strength of geopolymer mortar increased after immersion in NaCl solution for 80 days, owing to an increase in the crystalline phases. The strength of carbonation-cured mixes increased by up to 20%, except for mix 4a-20c-5b, which showed a reduction in strength of 4%. In another study, Jun et al., [30] found a slight increase in the strength of carbonation-cured, alkali-activated slag paste samples after immersion in seawater. In fact, the strength profiles of alkali-activated slag CMB mixes after a 28-day salt attack followed a similar trend to that of the unexposed counterparts. Apparently, the effect of the carbonation process parameters did not change after the salt attack. Furthermore, it can be noted that the higher the CO₂ uptake, the lower the percent increase in the compressive strength due to salt attack. Similar results were found in Sakr and Bassuouni [49], where accelerated carbonation reduced the salt attack resistance of Portland cement and slag-blended concrete. This was owed to the decalcification of alkaline gels in the concrete matrix upon being subjected to carbonation curing [30].

As noted earlier, the mass gain (MG) and strength gain (SG) after 28 days of immersion in a saline environment can be related. Figure 13 highlights an acceptable proportional correlation between the two properties, with an R² value of 0.71. The described relation is shown in Equation (2).

$$\mathrm{SG}=14.147 \mathrm{MG} - 4.975; {\mathrm{R}}^2=0.71$$

(2)

4.6. X-ray Diffraction

Figure 14 shows the X-ray diffractograms of selected alkali-activated slag CMB samples. The control (0a-0c-0b) and 4a-20c samples exposed to 1 and 5 bars of CO₂ gas pressure were chosen to study the effect of carbonation process parameters on the microstructures of the CMBs, as they exhibited impressive carbon sequestration potentials. Generally, all three mixes revealed crystalline phases related to the raw material used to produce the CMB mixes under study, such as dolomite and calcite, as highlighted in Figure 2. In addition, an amorphous hump was detected, representing the alkaline activation of slag [13]. Most of the products detected were calcium based, contributing majorly to the early strength development of alkali-activated slag composites [11]. Upon carbonation, peaks related to calcite, representing the crystalline phase of CaCO₃, increased. Similarly, Mei et al. [28] found an increase in the intensity of calcite peaks after carbonation curing of alkali-activated slag materials. In addition, the intensities of peaks representing the C-S-H gel were reduced, owing to the decalcification of this hydration gel upon exposure to CO₂ gas [23]. Such results explain the loss in strength reported earlier.

The effect of salt exposure on the microstructures of these three mixes is also presented in Figure 14. New chloride-based materials were identified, including Friedel’s salt (C-S-Cl) and Halite (NaCl). These crystalline phases filled in the voids and densified the concrete matrix, thereby improving its mechanical properties, as noted in Figure 12 [51]. Furthermore, with carbonation exposure and pressure, peaks related to newly formed amorphous phases decreased, which explained the decrease in the strength development of samples after the salt attack. In addition, the XRD pattern of mix 4a-20c-5b showed a decrease in the intensity of the peaks of the crystalline phases. This provided evidence of the strength loss after salt exposure. According to Anderson [52], CaCO₃, the by-product of the carbonation reaction, may react with the saline solution to form hydrogen carbonate and calcium ions. In contrast, the C-S-H gel was converted to chloride-based products (C-S-Cl and NaCl), densifying the matrix and improving the mechanical properties of alkali-activated slag CMB mixes.

4.7. Environmental Impact

The amount of CO₂ produced for one cubic meter (kg CO₂ eq/m³) of CMB is presented in Figure 15 (GWP, global warming potential). The reported values represent the difference between the GWP and sequestered CO₂. The GWP was calculated using the data in Table 3 and the mixture proportions. Meanwhile, the amount of CO₂ sequestrated by the alkali-activated slag CMB mixes was determined by multiplying the CO₂ uptake by the mass of the binder. The results show that the GWP of the control alkali-activated slag CMB mix (0a-0c-0b) was 85.2 kg CO₂ eq/m³. Upon carbonation, the value decreased to up to 46.1 kg CO₂ eq/m³. This represented a decrease of up to 46% compared to the control mix. Given that all mixes satisfied the compressive strength of non-load-bearing concrete masonry units, the mix with the lowest GWP 4a-20c-5b was deemed most suitable among the alkali-activated slag CMB mixes developed herein, as it was associated with the lowest carbon footprint, i.e., global warming potential.

5. Conclusions

This paper evaluated the fresh, mechanical, durability, and microstructure properties of carbonation-cured, non-load-bearing alkali-activated slag mixes representing concrete masonry blocks (CMBs), complying with the ASTM C129 requirements. The effects of the acceleration of the carbonation curing process parameters, including the initial curing duration, carbonation curing duration, and carbonation pressure, were assessed. The following conclusions were drawn based on the findings of this study:

• The CO₂ uptake of alkali-activated slag CMB mixes increased with an increase in the carbonation duration and carbonation pressure. This was owed to the longer contact time between the calcium-rich material of the binder matrix and CO₂ and the deeper penetration of CO₂ into the samples at higher pressure, respectively. Longer initial curing duration resulted in higher CO₂ uptake due to the removal of water from the matrix, facilitating the penetration of CO₂.
• All alkali-activated slag CMB mixes achieved a 1-day strength higher than 4.1 MPa, rendering them acceptable for use as non-load-bearing CMBs, as per ASTM C129. This high-strength development could help in the early service of alkali-activated slag CMBs by the construction and building industry.
• The compressive strength decreased as the carbonation curing duration and pressure increased, owing to the decalcification of calcium-rich activation reaction products and the formation of less strength-contributing calcium carbonate polymorphs. Meanwhile, 2 h initial curing had the least impact on the compressive strength, whereas 0-, 4- and 20 h initial curing caused reductions in strength of up to 33%.
• Carbonation curing had a positive impact on the water absorption and volume of permeable voids of alkali-activated slag CMB mixes. Extending the initial and carbonation curing durations and increasing the carbonation pressure reduced the two properties, as the newly formed calcium carbonate, associated with higher CO₂ uptake, filled the pores and voids in the alkali-activated matrix.
• The pH values of alkali-activated slag CMB mixes were reduced with carbonation curing to reach a minimal value of 9.1. The drop in pH was evidenced by the formation of a brighter-colored surface and was related to the consumption of alkaline compounds and the formation of calcium carbonate.
• The masses and strengths of alkali-activated slag CMB mixes increased after 28 days of exposure to salt, except for the sample with the highest CO₂ uptake, where its strength was reduced by −4.09%. The mass and strength gains were associated with an increase in the crystalline phases of the CMB matrix. However, as the degree of carbonation increased, lesser activation reaction products were available to react with the NaCl, thereby reducing the mass gain.
• The X-ray diffractograms showed that carbonation curing resulted in the intensification of calcite peaks and reduction in C-S-H peaks. Such an observation explained the drop in strength of alkali-activated slag CMB mixes after carbonation curing. Under salt attack, new chloride-based components, such as Friedle’s salt and Halite, were developed, acting as filler materials, thereby densifying the concrete matrix and increasing its strength.
• The carbonation curing of alkali-activated slag CMB mixes reduced the global warming potential, i.e., carbon footprint, by up to 46%, from 85.2 to 46.1 kg CO₂ eq/m³. As all mixes satisfied the strength requirement for non-load-bearing CMBs, the mix subjected to 4 h initial curing followed by 20 h carbonation curing at a pressure of 5 bars (4a-20c-5b) was deemed most suitable among the alkali-activated slag CMB mixes developed herein, as it was associated with the lowest carbon footprint.