Novel procedure of CO2 capture of the CaO sorbent activator on the reaction of one-part alkali-activated slag†

1. Introduction

Carbon dioxide (CO2) plays an important role in the Earth’s ecosystem, but high CO2 emissions exacerbate the greenhouse effect. The traditional building materials industry produces a large amount of CO2, and the use of alkali-activated cement can decrease the emission of this greenhouse gas.1 The precursors are derived from several Si- and Al-rich minerals and industrial wastes including metakaolin, ground blast furnace slags, and fly ash.2 Geopolymers have been considered as environmentally friendly substitutes that can replace Portland cement in construction materials.3 CO2 capture is generally based on the use of a variety of sorbents, such as inorganic chemisorbents, MOFs, and organoamine adsorbents, which exhibited high theoretical uptake capacity, excellent catalytic performances, and generated valuable byproducts respectively.4,5 In particular, CO2 mineral sequestration deserves special attention as it is obtained directly. Du et al.6 found CO2 has a strong affinity with montmorillonite, indicating a potential solution for CO2 sequestration into a deep shale reservoir. Calcium-based sorbents (CaO, Ca(OH)2) are an abundant material and can be obtained easily at low cost. Theoretically, 1 g of CaO can capture 786 mg of CO2. In addition, this sorbent could be applied at room temperature and atmospheric pressure.9,16 The reaction of calcium oxide (CaO) with CO2 at high temperatures has great significance.7,8 In the presence of CO2, CaO transforms into a stable carbonate, which can be a useful technology for stable CO2 storage.8,9 The reaction between CaO and CO2 proceeds in two stages.10-13 In the first stage, CO2 reacts rapidly with CaO to form dense calcium carbonate (CaCO3). In the next stage, CO2 in the gas phase can react with CaO only by diffusing through the initially formed layer of CaCO3 products. The diffusion coefficients for CaO and CaCO3 are 0.3 and 0.03 cm2 s−1, respectively.14 Thus, the initial fast stage and the subsequent slow stage are controlled by the kinetics of the reaction and diffusion of CO2 through the CaCO3 product layer, respectively. CaO requires temperatures in excess of 300 °C to react with CO2 at a reasonable rate.11 At low temperatures, the carbonation of CaO requires the assistance of water vapor, and the relative humidity of the gas phase should be more than 8% to carbonize CaO to obtain CaCO3 and temperature affects this reaction mildly, and relative humidity affects this reaction significantly.15 Very recently, Moreno et al.16 used hydrated lime samples exposed for six days to atmospheres of different relative humidity (RH) of 24, 58, 75, and 100%, respectively. They found that the CO2-capture capacity is related to the mechanism that governs the physisorption of water on calcium hydroxide (Ca(OH)2) at room temperature. Although CaO derived naturally from limestone or dolomite is inexpensive and widely available, its sorption capacity declines rapidly to only 10% of its theoretical capacity after several cycles of usage owing to severe particle sintering and attrition.17,18 However, the costs and profits of the process would not allow it to be implemented on a large scale, unless there is a way to extend applications of adsorption products.

Quicklime is an important component in cement production; it can also be used as an activator in alkali-activated cements. Alkali-activated slag (AAS) is less energy-intensive and emits less CO2 than the ordinary Portland cement.19 Ca(OH)2 and CaO are more mild, price competitive, and environmentally friendly activators than sodium hydroxide and sodium silicate.20,21 According to a previous report, CaO has better activation potential in alkali-activated materials (AAMs) than Ca(OH)2, but the early strengthening of the paste was slow due to the lower pH of the system.21 The setting time reflects the rate of the alkali-activated reactions. To satisfy the requirements of different applications, the setting time of the paste of Portland cement was adjusted by the addition of different additives. However, the setting time of an alkali-activated slag is very short to be applied conveniently, and the conventional retarder does not work in alkali-activated cement. Therefore, efforts have been directed at controlling the setting time. Pradip et al.22 reported that OPC accelerated the geopolymerization reaction and aided the fly-ash-based geopolymer to achieve a setting time comparable to that of the conventional cement concrete. Hubler et al.23 studied the addition of calcium silicate hydrate seeds to an AAS paste, and observed an earlier and larger hydration rate peak and a much higher compressive strength after 1 d of curing. This is because the accelerating properties of the calcium silicate hydrate seeds rely on the nucleation and growth being the rate-limiting steps in the hydration process. However, there are very few studies on decreasing the hydration rate of AAMs, and no study has been reported on controlling the setting time of alkali-activated cement by modifying the activator.

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Xem thêm : Mg + HNO3 → Mg(NO3)2 + N2O + H2O | Mg ra Mg(NO3)2

CaO has been investigated extensively because of its high theoretical CO2 uptake capacity, as well as the extensive availability of low-cost CaO precursors.24 However, according to the well-known cycle performance of CaO carbonation, the sorption capacity of CaO sorbents decreases dramatically with cyclic sorption and desorption processes. It is therefore important to find a direct application of the resultant adsorption products. This study investigated the mechanism of the conversion of CaO to Ca(OH)2 and CaCO3 at ambient temperature and constant humidity (∼75% RH), and the effect of partially carbonized CaO as an activator on the reaction rate of an alkali-activated slag. This process could mitigate CO2 emissions via rapid CO2 capture and promote the research on AAMs.

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