**1. Introduction**

Cement and concrete are used in building and construction work worldwide. The construction industry producing cement and concrete can be regarded as the world's largest industry [1–3]. Construction is responsible for a great amount of CO2 release [4,5]. One tonne of cement emits approximately 0.6 to 0.8 tonnes of CO2 [1,6–8]. The CO2 concentration in the atmosphere has increased from 280 to 420 ppm in 2020 [9]. The incorporation of waste and the proper use of resources are leading global challenges to control the negative environmental impact of cement and concrete and preserve the planet.

High-emission countries are actively exploring carbon capture and utilisation (CCU) or storage technologies. CCU is a novel method to reduce CO2 and turn CO2 into a commercially interest product [10–12]. The carbonation and chemical reactions in cement-based materials (CBMs) (Equations (1) to (5)) [13–18] occurs with CO2, affects cement hydration products, and increases CaCO3 production [5,19,20]. As early as 1970, the idea of CO2 capture through carbonation with CBM appeared [21]. Carbonation of CBM, as an alternate to CCU, reduces water absorption and curing time (useful in the precast industry), increases density, and improves fragmentation resistance and mechanical properties [22–24].

$$\text{Ca} \ (\text{OH})\_2 + \text{CO}\_2 \rightarrow \text{CaCO}\_3 + \text{H}\_2\text{O} \tag{1}$$

$$\text{C} - \text{S} - \text{H} + \text{CO}\_2 \rightarrow \text{CaCO}\_3 + \text{SiO}\_2 \cdot \text{nH}\_2\text{O} \tag{2}$$

**Citation:** Suescum-Morales, D.; Jiménez, J.R.; Fernández-Rodríguez, J.M. Use of Carbonated Water as Kneading in Mortars Made with Recycled Aggregates. *Materials* **2022**, *15*, 4876. https://doi.org/10.3390/ ma15144876

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 23 May 2022 Accepted: 8 July 2022 Published: 13 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

$$4\text{Cao} \cdot \text{Al}\_2\text{O}\_3 \cdot 13\text{H}\_2\text{O} + 4\text{CO}\_2 \rightarrow 4\text{CaCO}\_3 + 2\text{Al}(\text{OH})\_3 + 10\text{H}\_2\text{O} \tag{3}$$

$$\text{\textbullet\text{CaO}\cdot\text{SiO}\_2 + \text{\textbulletCO}\_2 + \text{nH}\_2\text{O} \rightarrow \text{SiO}\_2\cdot\text{nH}\_2\text{O} + \text{\textbulletCaCO}\_3\tag{4}$$

$$2\text{CaO}\cdot\text{SiO}\_2 + 2\text{CO}\_2 + \text{nH}\_2\text{O} \rightarrow \text{SiO}\_2\cdot\text{nH}\_2\text{O} + 2\text{CaCO}\_3\tag{5}$$

The concentration of CO2 in the environment must be increased for accelerated carbonation in different ways [14,15,18,23–30]. To increase this level of CO2, a carbonation chamber is usually necessary [13] with or without pressure [31,32], with different levels of CO2 or even submerging the samples in mixtures of different gases [33–35]. A review on the effect of CO2 in cement-based materials on the physico-mechanical properties was previously described in another study by Suescum-Morales et al. [36]. The carbonation rate is determined by the diffusion of CO2 gas in the samples [35]. Carbonation products make samples more dense, preventing the easy entry of CO2. The need for a high amount of CO2 in the curing environment implies the need for an accelerated carbonation chamber. An attractive alternative is to apply carbonation technology during cement kneading. To avoid the difficulty of CO2 diffusion and make CBM carbonation apply to in-situ products, the kneading water is replaced by carbonated water (water with high CO2 content). Thus, it is possible to start the carbonation simultaneously with the hydration process of the cement and increase carbonation [20,37]. Furthermore, the hydration reaction of the cement occurs much faster than that under normal curing conditions [5].

Construction and demolition waste (CDW) is produced during the demolition phases of several types of construction building or infrastructures (over 30 billion tonnes per year worldwide) [13,38,39]. CDW is composed of several types of waste, in addition to concrete and ceramics, such as glass, stone, bituminous material, and others. Recycled aggregates (RAs) are obtained from CDW with appropriate treatment (recycling plant treatment). A possible simple classification of RA may be made, in a simple way [25]: (i) if the waste is ceramic, the aggregate might be called recycled ceramic aggregates; (ii) if is concrete waste, may be called recycled concrete aggregates (RCA); and (iii) if it is a mixture of the two above, mixed recycled aggregates (MRA) [36,40].

Mixed recycled aggregate (MRA) is the most widely produced RA in the world. The non-existence of regulations and different sources is still limited the use of MRA [41]. Recycled masonry aggregate (RMA) differs from natural aggregates (NAs) mainly in terms water absorption, higher porosity, and lower density [42,43]. MRA has had different uses: as aggregates for masonry mortar, and as an aggregate for alkaline activated material or CBM [41–48]. RMA is a type of MRA obtained from screening and crushing walls waste [36,49–52].

There are two ways to produce accelerated carbonation in RCA [14]: in the aggregate itself [7,42,53–55] or in the mixture of RCA with Portland cement [24,27,28,56]. However, these studies do not investigate the effect of CO2 on CBMs made with RMA or RCA using carbonated water as kneading water. This research would fill this information gap. Nor has any literature been found that studies the simultaneous use of carbonated water and CO2 curing.

This study mainly investigates the physico-mechanical properties of a porous CBM with RMA, and carbonated water as kneading water and for subsequent curing in CO2. To observe the effect of carbonated water on the microstructure of the hardened samples, with NA and RMA, and cured under both regimes, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and backscattered electron (BSE) were performed. Thermogravimetric analysis and differential thermal analysis (TGA/DTA) was also performed to determine the amount of CaCO3 in all cases. No studies have been found that simultaneously use carbonated water as kneading water, under accelerated curing, and using RMA as aggregate. The production of precast or in-situ non-reinforced CBM products could be possible with the following approach: incorporation of waste (RMA), with the added value to CO2, and inclusion of carbonated water as kneading water, which can be very revolutionary.

#### **2. Materials and Methods**

#### *2.1. Materials*

NA was used as a reference. NA and RMA were used in previous research [25,36,49,50]. The components of RMA according to UNE EN 933-11:2009 were: red ceramic bricks (53.9%), masonry mortar (39.8%), unbound aggregates (5.7%), concrete (0.4%), and gypsum particles (0.2%). Water absorption and dry bulk density (DBD) were measured according to UNE-EN 1097-6:2013 [57]. DBD for NA and RMA was 2.63 g·cm−<sup>3</sup> and 2.14 g·cm−3, respectively. The amount of CaCO3 for NA and RMA (197 and 239 kg/m<sup>3</sup> respectively) was calculated using TGA/DTA. Water absorption was 0.79% for NA and 9% for RMA. CEM II/A-V 42.5 R was used as cement [58] with a DBD of 2.89 g·cm−<sup>3</sup> according to the characteristics provided by the manufacturer.

As kneading water, two types of commercial water were used: normal water (H2O) and carbonated water (CO2·H2O), both from the manufacturer Fuente Primavera, Spain. For H2O, the pH value was 7.7 and the initial concentration of CO2 was between 0.2–0.5 mg·L<sup>−</sup>1. For CO2·H2O, these values were 4.8 (for pH) and 14.1–14.4 mg CO2·L<sup>−</sup>1.

## *2.2. Experimental Design and Curing Conditions*

NA and RMA were sieved: 2/4, 1/2, 0.5 /1, 0.25/0.5, 0.125/0.25, and 0/0.125 fractions to reconstitute the lower limit indicated by ASTM C 144-04 [59]. Two gaps were achieved by deleting the following fractions: 0.25/0.5 and <0.125 mm. Its mineral skeleton (with two gaps) can facilitate the input of CO2 and total carbonation of the mix (more porous). Table 1 shows the reconstituted particle size distribution of NA.


**Table 1.** Spindle-shaped particle size limits.

Equations (6) and (7) calculate the dry mass of each component:

$$\text{Dry mass of element} = \frac{V \cdot 1 \cdot \text{p}\_{rd\text{ element}}}{6} \tag{6}$$

$$\text{Dry mass of NA} = \frac{V \cdot 5 \cdot \text{\text{\textdegree{}}\_{\text{rel}} \text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedbl}}}{6} \tag{7}$$

where ρ*rd cement* = 2.89 g/cm3 and ρ*rd natural aggregate*= 2.63 g/cm3, which are the DBD of cement and NA, respectively. A volume (V) of 1600 cm<sup>3</sup> was manufactured in each mix. According to Equation (7), the mass of NA was 3507 g. The mass of each fraction for NA (Table 2) were obtained according to the 2 gap realized in Table 1. A total substitution of NA by RMA in volume fraction by fraction was realized. To replace NA by RMA, the bulk density was used [60].


**Table 2.** Aggregate weights used by different fractions.

**\* BD** = **Bulk density**.

The samples were subjected to two curing environments (both with 21 ± 2 ◦C and 65 ± 10% of relative humidity): (i) normal climatic chamber (CC) and (ii) accelerated carbonation chamber (CO2·C). For the CC, the CO2 concentration was 0.04%, and for the CO2·C it was 5%. The CO2 used for this condition was provided by Linde (99.99995% purity). Figure 1 shows the experimental design carried out.

**Figure 1.** Experimental design carried out.

#### *2.3. Kneading Process*

Table 3 shows the composition of the mixes studied. The aggregates were presaturated, according to the water absorption of each one of them (NA or RMA). Therefore the *w*/*c* ratio used can be considered as effective. The kneading process was in accordance with previous research [25,36].


**Table 3.** Weights used for the different mixes (g).

(\*) CC or CO2-C.

Prismatic 40 × 40 × 160 mm casts were used [61]. The samples were keep in the mould for 3 h. The samples were covered to prevent CO2 input/output during this time. After this time (3 h), the samples were demoulded because the aim was to demould the samples very quickly, similar to what happens in a precast plant. According to Pan et al. [35], this pre-curing time is crucial to avoid water-saturated capillary pores resulting in a low penetration rate of CO2 for the samples cured in CO2·C. The samples were then cured in two chambers: CC and CO2·C for 1, 3, and 7 days of curing.
