Evaluation of CO2 sequestration efficiency by Pozzolime concrete

Reducing CO2 emissions from cement industry is vital. Portland cements manufacturing is responsible for 5 – 8 % of global greenhouse gases. Therefore, using alternative binders in concrete is necessary to reduce the environmental impact of cement. This work goals to investigate the efficiency of Pozzolime concrete in CO2 sequestration from the environment and then to convert it into calcium carbonate inside the concrete. The Pozzolime concrete was tested at the ages of 14, 28 and 56 days with two moist-curing ages; 14 and 28 days. The studied mixes were exposed to 15 and 25 % of CO2 concentration for a period of 24 hours. The efficiency was evaluated through compressive strength, carbonation depth, CO2-uptake and weight change. The results showed that higher concentration of CO2 for exposure of 24 hours caused a significantly higher carbonation depth. The maximum CO2 uptake was recorded at the age of 14 days for Pozzolime concrete, when exposed to 25% concentration of CO2.


Introduction
The Greeks knew the Lime and it was commonly used by the Romans. The Romans as well knew how to create a hydraulic binder, i.e. Lime-natural Pozzolanic cement by addition of materials, such as brick powder, volcanic ash or, pottery and tiles to lime [1]. In greenhouse gases (GHG) classification, the CO2 is the most important gas, which has a major influence on the climate change and international warming. Cement industry only produces approximately 5 -8 % of the global total CO2 emission [2]. Lime could be considered as an ecological binder due the needs of lower creation energy, lower CO2 emission during production and CO2 preoccupation by carbonation on setting [3]. For Portland cement, hydration causes the gain of strength and makes concrete to attain a reliable value on the 28 days. However, the Pozzolan reaction with lime is relatively slow, therefore, this mixture needs more than 28 days to develop such reliable strength [4]. Carbon oxide CO2 reacts with calcium oxide Ca(OH)2 inside the concrete to form calcium carbonate as shown in the following equation-1: This reaction speeds up to achieve strength. In the industries of bricks, blocks, and cement boards, it is a known procedure to cure these products by injecting CO2 gas through a sealed chamber at a favorite temperature from 25 -65 °C, and specified pressure, 0.83 MPa. This process needs moisture and porous structure to achieve its best results [5].  [7]. The chemical analysis and physical tests are shown in Table 1.

Mixes and curing systems
Based on the work of Kadum et al. [6], and trail mix. The mix proportions were selected for two Pozzolime mixes, P1 and P2, as shown in Table 3. The mixes were water-cured for two periods: 14 and 28 days. Then, the group that was cured for 14 days was tested for carbonation at the ages of 14, 28 and 56 days. Meanwhile, the group that was cured for 28 days was tested for carbonation at the ages of 28 and 56 days.

CO2 exposure chamber
The system in Figure 1 a [14]. Phenolphthalein is a liquid used to indicate the change in alkalinity. The change in treating concrete color from purple to white means that the pH drops beneath 8.5 in a water solution. The carbonation depth represents the distance from the top surface to the purple front and measurements were done by visual check [15].

Weight change.
When put the samples inside the CO2 sealed chamber, weight of specimens will change due to chemical reaction (because of exposure to CO2 concentrations at 15 and 25 % for 24 hours). The percentage of changes was taken by mathematical methods (the change in weight of the original weight) [16]. 2.4.4. CO2 uptake. The weight gain was meansured to calculate CO2 uptake for concrete specimens during the exposure period as showed in equation-2 [17]. Carbonation calculated by formula below and from water loss collected by spongy paper which, added to the final mass. Through in view of the system as a sealed system, it was imposing to include the vaporized water that was primarily inside the samples prior to carbonation.

Compressive strength
As illustrated in Table 4, the compressive strength of Pozzolime concrete was significantly affected by CO2 concentration, exposure time, and wet curing period. Results showed that exposing to CO2 for 24 hours causes a higher increase in strength.  For mix P1, the increase in compressive strength at the age of 14 days of water curing was 3.4 and 13.7 % for 15 and 25 % of CO2 gas concentration, respectively. The effect of water curing is obvious at the age of 28 days. For curing at 14 days only and testing at 28 days the increase was 2.3 and 5.5 % for 15 and 25 % of CO2 gas concentration respectively. Meanwhile, for 28 days curing the increase in strength is very clear and it was 2.3 and 7.0 % for 15 and 25% of concentration respectively. At the age of 56 days the increase at 14 days curing only was 7.4 and 14.3 % for 15 and 25 % of CO2 concentration, respectively. Same trends were diagnosed for mix P2. This gain in compressive strength for two mixes P1 andP2, is due to the formation of calcium carbonate, which densified the microstructure. The increase in compressive strength also starts from the exothermic nature of carbonation reaction that will increase the heat and cause quick solidification [18]. Results also indicated that the curing time with exposing concrete to CO2 % at an earlier age, provides higher gain in compressive strength [16]. This due to the development of more hydration products that block the pores and reduce the next entrance of CO2 to the matrix [19]. The strength increasing slightly for mixing P1 than the P2 at an early age, while at age 28 days approximately the same, but at later age 56 days the mix P2 increasing than mix P1.

Carbonation penetration depth
The results of carbonation depth are shown in Table 5. These measures were done by using digital caliper. The results displayed that the greatest recorded depth was 9.12 mm at the age of 56 days with 14 days of wet-curing only and with 25 % of CO2 concentration for mix P1. However, for mix P2 the maximum penetration depth was 20.1 mm at age of 56 days for 14 days of wet-curing and 25 % of CO2 concentration. On the other hand, the minimum values were 2.1 and 4.21 mm at the age of 28 days with 14 days of wet-curing for zero CO2 concentration for mixes P1 and P2 respectively. This could be due to the consequent reaction of lime with the CO2. Generally, the reduction in depth from the age of 14 to 28 days could be due to the continuous hydration that filled the pores and reduced the entrance of more CO2 to the matrix [19,20].  Table 6 declares that the specimens of Pozzolime concrete have increased in weight with different CO2 exposures when compared to the reference mixes [16]. The maximum increase in weight was at early age for 14 days wet curing and 25 % CO2 concentration. The recorded values were 2.83 and 3.11 % for mixes P1 and P2, respectively. This could be attributed to the carbonation of lime which densifies the microstructure of concrete [21]. There was a detection of water loss in lime under pressure. It is well known that the carbonation of lime would produce water. Therefore, calculation of water loss should take into account both original mixing water and water released from lime carbonation [22].  Table 7 lists the CO2 uptake that was determined via the increase in weight from CO2 exposure with initial mass and with the drying binder of Pozzolime concrete as explained previously, the mass of carbonated Pozzolime finally is including the water loss, and the weight of original dry binder [16]. The maximum CO2 uptake value was 4.278 % at the age of 14 days of wet-curing and 25 % concentration for mix P2. Meanwhile, the corresponding value for mix P1 was 3.578 %, i. e. About 20 % reduction in uptake. This trend was observed for all calculated values and that could be due to the less available lime in mix P1. Another concluding remark was that the reduction in uptake with the increase in wet-curing period and testing age and that could be explained by the blocking of pores through early carbonation and reduction of permeability due to continuous hydration [19].

Conclusions
The following conclusions can be drawn from the current study: 1-Results showed that exposing specimens to CO2 gas for 24 hours causes an increase in compressive strength due to carbonation process.  7 2-Exposing concrete to CO2 at earlier ages, yields higher increase in compressive strength due to more pours and calcium oxide Ca(OH)2 able to react with CO2 gas. 3-The maximum measured carbonation depth values were 9.12 and 20.1 mm for Pozzolime mixes P1 and P2, respectively, at the age of 56 days with 14 days of wet-curing and 25 % of CO2 concentration due to P1 more finest materials and densify than P2. 4-The increase in CO2 concentration at the early age of exposure will increase the weight of concrete for all tested Pozzolime mixtures. 5-The maximum CO2 uptake by Pozzolime concrete happens when it is exposed to higher CO2 concentration and for longer duration of exposure at an early age. 6-The increase of lime in the mix will increase the CO2 uptake at early age more than the later age.