*Article* **CO2 Curing E**ffi**ciency for Cement Paste and Mortars Produced by a Low Water-to-Cement Ratio**

## **Seong Ho Han, Yubin Jun, Tae Yong Shin and Jae Hong Kim \***

Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea; ha8890@kaist.ac.kr (S.H.H.); ssjun97@gmail.com (Y.J.); tyshin@kaist.ac.kr (T.Y.S.)

**\*** Correspondence: jae.kim@kaist.ac.kr

Received: 3 July 2020; Accepted: 28 August 2020; Published: 2 September 2020

**Abstract:** Curing by CO2 is a way to utilize CO2 to reduce greenhouse gas emissions. Placing early-age cement paste in a CO2 chamber or pressure vessel accelerates its strength development. Cement carbonation is attributed to the quickened strength development, and CO2 uptake can be quantitatively evaluated by measuring CO2 gas pressure loss in the pressure vessel. A decrease in CO2 gas pressure is observed with all cement pastes and mortar samples regardless of the mix proportion and the casting method; one method involves compacting a low water-to-cement ratio mix, and the other method comprises a normal mix consolidated in a mold. The efficiency of the CO2 curing is superior when a 20% concentration of CO2 gas is supplied at a relative humidity of 75%. CO2 uptake in specimens with the same CO2 curing condition is different for each specimen size. As the specimen scale is larger, the depth of carbonation is smaller. Incorporating colloidal silica enhances the carbonation as well as the hydration of cement, which results in contributing to the increase in the 28-day strength.

**Keywords:** CO2 curing; size effect; colloidal silica; cement-based material; casting method

### **1. Introduction**

Greenhouse gas emissions in the industrial sector are of serious concern. In the construction industry, a large amount of CO2 is emitted during the production of cement by the calcination process. Various studies have been conducted to reduce the amount of CO2 emitted in the manufacturing process or to utilize emitted or captured CO2 for sustainable development [1,2]. CO2 curing for cement-based materials has been demonstrated as one possible way of utilizing CO2 [3–7].

The carbonation of calcium silicates such as C3S, β-C2S, and γ-C2S in Portland cement generally occurs more quickly than their hydration. Therefore, the CO2 curing of early-age concrete facilitates faster development of its strength [8]. The carbonation of anhydrous alite (C3S) and belite (C2S) is expressed as [9]:

$$\rm CaCO\_3S + (3-x)CO\_2 + nH \rightarrow C\_xSH\_n + (3-x)CaCO\_3 \tag{1}$$

and

$$2\text{β-C}\_2\text{S} + (2-x)\text{CO}\_2 + n\text{H} \rightarrow \text{C}\_x\text{SH}\_n + (2-x)\text{CaCO}\_3\tag{2}$$

where CxSHn refers to the calcium silicate hydrates of (CaO)*x*(SiO2)(H2O)*n*, simply denoted by C-S-H. The carbonation products are the calcium silicate hydrates and calcium carbonate (CaCO3). In addition, the calcium hydroxide, a product by the calcium silicate hydration, is also carbonated:

$$\text{Ca(OH)}\_{2} + \text{CO}\_{2} \rightarrow \text{CaCO}\_{3} + \text{H}\_{2}\text{O} \tag{3}$$

The calcium carbonate, produced at an early age, precipitates in pores of the cement paste. Consequently, cement-based materials obtain pore refinement, leading to enhanced durability and strength [10].

The degree of carbonation was usually estimated by the mass-curve or mass-gain method [11–13]. Equation (4) calculates CO2 uptake by measuring the increase in the mass of intact samples. The increase in the mass corresponds to the mass of reacted CO2. The mass-curve method [11] then evaluates

$$\text{CO}\_2 \text{ update } (\%) = \frac{\text{Mass of specimen subjected to CO}\_2}{\text{Initial mass of specimen}} \times 100\tag{4}$$

The mass-curve method needs caution when monitoring the increase in the mass of samples in a chamber. The water presented in a sample partially vaporizes due to the heat production from the carbonation, and excess water vapor condenses on the chamber wall. The mass-gain method compensates the water amount lost by vaporization to reduce experimental error [12–14]. Equation (5) then expresses a calculation of CO2 uptake:

$$\text{CO}\_2 \text{ update } (\%) = \frac{(\text{The mass increase of a cast sample by CO}\_2 \text{ curing}) + (\text{water loss})}{\text{Mass of raw materials for a sample}} \times 100 \quad (5)$$

Another way to consider the CO2 uptake evaluation is by measuring the decrease in CO2 gas pressure in a sealed reactor. The decrease in the CO2 pressure monitored for the whole carbonation process can be converted into the CO2 mole consumed for the carbonation. Such a pressure monitoring method is less prone to error. This paper, therefore, proposes the pressure monitoring method.

The efficiency of CO2 curing is related to water in the pore system of a sample. Water invariably allows for the mixing and subsequent workability of cement-based materials. However, if there is much free water in the sample, water also fills the pore of a sample fully, resulting in hindering CO2 gas from entering the sample interior. CO2 diffusion into a sample is limited if its pores are fully saturated. On the other hand, CO2 in gaseous form does not react, and so its dissolution in liquid water precedes for the carbonation. Sufficient water, that is, more than the reacting amount accounted for in Equations (1) and (2), is required for the CO2 dissolution. Previous studies [15,16] suggested the use of a relatively low water-to-cement ratio (W/C less than 0.25) in CO2 curing, and made samples by compaction molding. The cement compacts produced by the low water-to-cement ratio had a large amount of air-filled pores, which resulted in a higher CO2 diffusion and CO2 uptake.

Controlling the pore system, by the use of nano-sized particles, also affects the efficiency of CO2 curing. The nucleation effect of nano-sized limestone powder on CO2 curing was reported [17,18]. In addition to the nano-sized limestone powder, this study further investigates the effect of incorporating colloidal silica. The nano-sized silica particles reportedly nucleate the hydration of cementitious materials in accompany with their pozzolanic reaction [19]. As a result, it reduces the degree of chloride ion penetration [20,21] even though the increase in compressive strength is not substantial [22]. Lastly, the optimization for CO2 curing conditions, together with the effect of specimen size, is also investigated for the purpose of controlling the pore system.

#### **2. Experimental Details**

#### *2.1. Materials*

Ordinary Type I Portland cement and ISO standard sand (ISO 679) [23] were used to produce samples in this study. Table 1 shows the chemical composition of the cement determined by X-ray fluorescence spectrometry. The specific density of the sand was 2620 kg/m3. Its grain size ranged from 0.08 to 1.60 mm. The colloidal silica (commercial grade) used in this study mainly contained particles from 10 to 20 nm, and the SiO2 content in the aqueous solution was 40%. The pH of the colloidal silica ranged from 9.5 to 10.5.


**Table 1.** Chemical composition of ordinary Portland cement (wt.%).

#### *2.2. Sample Preparation*

Table 2 lists the mix proportions of the samples. A planetary mixer was used for a total of 5 min mixing. The mixes were then fabricated by two methods: (1) Compacting or (2) conventional consolidating-in-mold procedure following ASTM C109 [24]. The compacting method was applied to the samples with a relatively low W/C: Paste (W/C = 0.15) and Mortar (W/C = 0.35), which considered the condition of brick production in practice. Each mix was filled in a confined mold, and then it was compacted by 5 kN compression for 30 min. The dimensions of the paste and mortar compacts were as a 40-mm cube. In contrast, the dimensions for the samples, Paste (W/C = 0.4) and Mortar (W/C = 0.5), produced by the conventional procedure [24], were various as 25-, 40-, and 50-mm cubes so as to analyze the specimen size effect on CO2 curing. The sealed curing for the premature sample in a mold proceeded for 24 h at approximately 25 ◦C.

**Table 2.** Mix proportion of samples.


† Each stiff sample in a confined mold was compressed by 5 kN (within 30 min). ‡ The mortar flow of each sample was within 150 to 250 mm.

An additional two mortar samples incorporating colloidal silica were produced to analyze the effect of colloidal silica. Their mix proportions and fabrication methods were the same as the samples in Table 2, with 4% colloidal silica per cement mass.
