Study on the Effects of Ultrasonic Agitation on CO 2 Adsorption Efﬁciency Improvement of Cement Paste

: To realize high-efﬁciency CO 2 absorption by fresh cement paste, ultrasonic vibration technology is introduced into the CO 2 absorption test device used in this study. Inﬂuences of ultrasonic frequency on the CO 2 absorption rate (CO 2 AR) and the ultimate absorption amount of fresh cement paste are analyzed. Furthermore, the inﬂuencing laws of the CO 2 absorption amount (CO 2 AA) on the ﬂuidity, pore distribution, and mechanical properties of cement paste under ultrasonic vibrating agitation are analyzed by measuring the variations of the CO 2 AA of cement paste. Results demonstrate that ultrasonic vibrating agitation not only can increase the CO 2 AR and ultimate absorption amount of fresh cement paste, but also can optimize the internal pore structure of materials and compressive strength of cement-based materials. 2 AA increases from 0% to 2.20% at a rate of 0.44%. Meanwhile, the percentages of slightly harmful pores are 70%, 72%, 74%, 77%, 77%, and 78%, respectively. The percentages of harmful pores are 21%, 19%, 15%, 12%, 11%, and 10%, respectively. The percentages of multi-harmful pores are 3%, 3%, 3%, 2%, 2%, and 2%, respectively.


Introduction
The greenhouse effect which is caused by excessive CO 2 emissions has brought many hazards. Global warming has become a primary issue of the top 10 global environmental problems. Nowadays, many methods exist to decrease global CO 2 emissions, such as changing the energy structure, chemical absorption, and blocked curing [1]. However, all of these methods have some technological defects. They all incur relatively high costs and cannot solve carbon emission problems within a short period of time. Portland cement is a traditional building material used most extensively around the world which has also attracted wide attention for reducing CO 2 emissions [2]. Cement production comes at the cost of great consumption of limestone and fuels, the pyrolysis and combustion of which release CO 2 . According to the statistics, approximately 1 ton of CO 2 emissions during the production of 1 ton of clinkers is produced [3]. From 2013 to 2016, the global cement output exceeded 4 billion tons, equating to about 16 million tons of global CO 2 emissions [4,5]. Decreasing CO 2 emissions in the Portland cement industry is therefore an important part of the global CO 2 emission reduction project [6].
Some studies have pointed out that cement in concrete can produce a significant amount of Ca(OH) 2 during the hydration process, accounting for about 20-30% of total hardened cement pastes, and largely exists in the pores of hardened cement pastes in their crystal form. In solution, Ca(OH) 2 can extremely easily react with CO 2 to produce CaCO 3 . As a result, concrete has considerable potential for CO 2 absorption [7,8]. Compared with mineral carbonisation and ocean storage, increasing the operability and application prospects of CO 2 absorption by concrete has recently become a hot research topic. However, realising high-efficiency CO 2 absorption by concrete is still a huge challenge [9].
Studies on CO 2 absorption of fresh cement pastes have been reported. It is found that the stirring rate, water-cement ratio and addition order of water reducing agents can significantly influence the CO 2 AR and ultimate absorption amount of fresh cement paste significantly. More importantly, the CO 2 AR and ultimate absorption amount of fresh cement paste can increase by appropriately setting these parameters [10]. Nevertheless, the stirring time of concrete is very short in practice, which is generally in the range of 30-90 s. Furthermore, reasonable settings of the stirring rate, water-cement ratio and addition order of water reducing agents alone cannot meet the fast CO 2 absorption rate required in practical concrete production [11,12]. Hence, the CO 2 absorption efficiency and ultimate absorption amount of fresh concrete still have room for development. Increasing the CO 2 absorption efficiency of fresh concrete more effectively is thus the key focus of this study.
In this study, an ultrasonic vibration device is introduced into the original CO 2 absorption device [13,14]. The CO 2 AR and absorption amount of fresh cement paste under ultrasonic vibrating agitation as well as the influences of ultrasonic vibration on the fluidity and porosity of CO 2 absorption are discussed through the unique ultrasonic performances. The collaborative action mechanism of fresh cement paste under ultrasonic vibrating agitation and CO 2 absorption was analysed through SEM observation. The variation laws of the internal microstructure after CO 2 absorption of fresh cement paste for different amounts are observed [15][16][17]. On this basis, the variation mechanism of basic performances of cement-based materials as a response to ultrasonic vibration is disclosed and the influences of cement flocculates on the crystal size of solution under ultrasonic waves were are analysed thoroughly [18][19][20].

Raw Materials
The P·O 42.5 cement, which is produced by the Xuzhou Zhonglian Cement Group, was chosen for this study. The mean grain size, density, standard-consistency water need, fineness (0.08 mm square hole sieve), and specific surface area were 14.813% Xav (µm), 3.14 g/cm 3 , 28.1%, 1.02%, and 3300 cm 2 /g, respectively. The specific chemical composition and mineral composition are listed in Tables 1 and 2. In this experiment, CO 2 is the highpurity CO 2 which is produced by a special gas plant in Xuzhou, the purity of which is ≥99.5%. The sand used in this experiment was the ISO cement test standard sand and the water used was tap water.

Reconstruction of the CO 2 Absorption Device
Two CO 2 absorption devices were manufactured to discuss the influences of ultrasonic agitation on the CO 2 AR and the ultimate absorption amount of fresh cement paste. One is a mechanical agitation device (Figure 1a) and the other is an ultrasonic vibrating agitation tank. The manufacturing process is introduced as follows. Firstly, the ultrasonic vibrating agitation tester with different ultrasonic vibrational frequencies is manufactured by connecting the ultrasonic vibrator into the original CO 2 absorption device. Next, a transducer, ultrasonic power supply, and qualified amplitude transformer are designed and selected according to the requirements of the vibration system on amplitude and vibration frequency. Finally, the ultrasonic power supply, transducer, amplitude transformer, and agitation tank are connected (Figure 1b).
The test was divided into Groups B and C. Group B used ultrasonic vibratin tion and Group C was moulded by mechanical agitation as a control group. Th sponding CO2 AA were 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively. pastes in Group B were numbered as B1, B2, B3, B4, B5, and B6. Cement pastes in C were C1, C2, C3, C4, C5, and C6. The fluidity, mechanical properties, pore str and microstructures of cement pastes in Groups B and C were tested to study th ences of ultrasonic vibrating agitation on the CO2 absorption of cement pastes.

Fluidity of Cement Paste after CO2 Absorption
The fluidity of Groups B and C was tested according to the Cement and Water ing Agent Compatibility Test Method (JC/T1083-2008) [25]. First, we poured the paste after CO2 absorption into the truncated cone and scraped flat. Then, we lifted t cated cone in the vertical direction and let the paste flow. Thirty seconds later, the ma Reprinted with permission from ref. [21]. Copyright 2021 Elsevier.

Fresh Cement Paste Preparation for CO 2 Absorption
Cement paste with a water-cement ratio of 0.5 was prepared. The stirring rate was set 210 ± 5 r/min. After the paste was stirred evenly, the ultrasonic vibrator and CO 2 flow valve were opened and CO 2 was supplied while stirring. The CO 2 AR and absorption amount of cement paste were measured by a CO 2 flowmeter. In this way, cement paste that meets the required CO 2 AA was prepared.

Determination of the Optimum Ultrasonic Frequency
Ultrasonic waves are acoustic waves with frequencies higher than 20 kHz, usually ranging between 20-100 kHz. The cavitation effect of an ultrasonic wave is related to its frequency. The ultrasonic frequency is negatively related to the cavitation effect of an ultrasonic wave. To reach a better ultrasonic cavitation effect, this research group developed three ultrasonic agitation devices with low ultrasonic frequencies of 20, 28, and 40 kHz [22][23][24].
The specimens were divided into four groups: A1, A2, A3, and A4. Groups A1, A2, and A3 were moulded using ultrasonic vibrating agitation. The ultrasonic frequencies of these three groups were 20, 28, and 40 kHz, respectively. Group A4 was used as a control group and moulded by mechanical agitation. The CO 2 AR and CO 2 AA were measured every 5 s to study the influences of ultrasonic frequency on real-time changes of the CO 2 AR and CO 2 AA. On this basis, the optimal ultrasonic frequency could be determined.

Experimental Procedure 2.5.1. Specimen Grouping
The test was divided into Groups B and C. Group B used ultrasonic vibrating agitation and Group C was moulded by mechanical agitation as a control group. The corresponding CO 2 AA were 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively. Cement pastes in Group B were numbered as B1, B2, B3, B4, B5, and B6. Cement pastes in Group C were C1, C2, C3, C4, C5, and C6. The fluidity, mechanical properties, pore structures, and microstructures of cement pastes in Groups B and C were tested to study the influences of ultrasonic vibrating agitation on the CO 2 absorption of cement pastes.

Fluidity of Cement Paste after CO 2 Absorption
The fluidity of Groups B and C was tested according to the Cement and Water Reducing Agent Compatibility Test Method (JC/T1083-2008) [25]. First, we poured the cement paste after CO 2 absorption into the truncated cone and scraped flat. Then, we lifted the truncated cone in the vertical direction and let the paste flow. Thirty seconds later, the maximum diameters of the paste were measured in two directions perpendicular to each other. Finally, the fluidity of the cement paste is the average of the two maximum diameters.

Pore structure of Hardened Cement Paste after CO 2 Absorption
The 40 × 40 × 160 mm standard cement mortar specimens were prepared using fresh cement pastes of Groups B and C according to Method of Testing Cements-Determination of Strength (GB/T17671-1999) [26][27][28]. All specimen moulds were removed after curing for 24 h, followed by standard curing to the regulated age under 20 ± 2 • C and humidity > 95%. At this moment, the compressive strength at 3, 7, and 29 d were determined by a LS80-65-160 hydraulic compression tester to discuss the influences of ultrasonic vibration on the mechanical properties of fresh cement paste after CO 2 absorption [29][30][31].

Mechanical Properties of Hardened Cement Paste after CO 2 Absorption
Cement pastes of Groups B and C were filled with cubic test moulds in a size of 40 × 40 × 40 mm. The cement paste was made compact by vibration. Then, 24 h later, the mould was removed and specimens were cured under standard conditions of 20 ± 2 • C and humidity > 95% to the regulated age, followed by 24 h of drying in an oven at a temperature of 120 • C. Specimens were crushed into blocks or cylinders. Pore structural distribution and porosity after 28 d were determined by mercury intrusion porosimeter, a PoreMaster33 with a testing range of 3.5 nm to 400 µm. The effects of ultrasonic vibration on the porosity of cement paste after CO 2 absorption are discussed [32][33][34].

Characterization of the Hydration Products in Cement Paste after CO 2 Absorption
The fresh cement paste of Groups B and C was put into cubic test moulds of 40 × 40 × 40 mm dimension. The cement paste was then compacted by vibration and cured under standard conditions for 12 h to prepare the cement paste samples. At this time, the strength of the cement paste sample is first established, which is conducive to sample preparation. Furthermore, the hydration reaction in the cement is in its initial stages and the internal structure is relatively loose, which makes it an ideal time to observe the products produced by the reaction of cement hydration products with CO 2 .
The cement paste samples were made into 1-mm-thick pieces, which were then immersed in absolute ethanol for 48 h to prevent the hydration of the cement. The pieces were dried in a constant temperature blast oven at 65 • C for 24 h and then placed in an ion sputtering apparatus for surface gold spraying. Then, scanning electron microscopy (SEM) and energy spectrum analysis were performed with a scanning electron microscope Quanta 250. The effects of ultrasonic vibration on the CO 2 ultimate absorption amount are shown in Figure 2. It can be seen that under mechanical agitation, the CO 2 AA in the cement paste increased gradually. When the CO 2 absorption reached 5, 10, 15, and 20 s, the CO 2 AA were 0.77%, 1.35%, 1.87%, 2.15%, and 2.47% of the cement mass, respectively. After an absorption time of 35 s, the cement paste could not further absorb CO 2 due to thickening and lost fluidity. At this point, the CO 2 AA of the cement paste reached 2.64% of the ultimate absorption amount. The 40 × 40 × 160 mm standard cement mortar specimens were prepared using fresh cement pastes of Groups B and C according to Method of Testing Cements-Determination of Strength (GB/T17671-1999) [26][27][28]. All specimen moulds were removed after curing for 24 h, followed by standard curing to the regulated age under 20 ± 2 °C and humidity >95%. At this moment, the compressive strength at 3, 7, and 29 d were determined by a LS80-65-160 hydraulic compression tester to discuss the influences of ultrasonic vibration on the mechanical properties of fresh cement paste after CO2 absorption [29][30][31].

Mechanical Properties of Hardened Cement Paste after CO2 Absorption
Cement pastes of Groups B and C were filled with cubic test moulds in a size of 40 × 40 × 40 mm. The cement paste was made compact by vibration. Then, 24 h later, the mould was removed and specimens were cured under standard conditions of 20 ± 2 °C and humidity >95% to the regulated age, followed by 24 h of drying in an oven at a temperature of 120 °C. Specimens were crushed into blocks or cylinders. Pore structural distribution and porosity after 28 d were determined by mercury intrusion porosimeter, a PoreMaster33 with a testing range of 3.5 nm to 400 μm. The effects of ultrasonic vibration on the porosity of cement paste after CO2 absorption are discussed [32][33][34].

Characterization of the Hydration Products in Cement Paste after CO2 Absorption
The fresh cement paste of Groups B and C was put into cubic test moulds of 40 × 40 × 40 mm dimension. The cement paste was then compacted by vibration and cured under standard conditions for 12 h to prepare the cement paste samples. At this time, the strength of the cement paste sample is first established, which is conducive to sample preparation. Furthermore, the hydration reaction in the cement is in its initial stages and the internal structure is relatively loose, which makes it an ideal time to observe the products produced by the reaction of cement hydration products with CO2.
The cement paste samples were made into 1-mm-thick pieces, which were then immersed in absolute ethanol for 48 h to prevent the hydration of the cement. The pieces were dried in a constant temperature blast oven at 65 °C for 24 h and then placed in an ion sputtering apparatus for surface gold spraying. Then, scanning electron microscopy (SEM) and energy spectrum analysis were performed with a scanning electron microscope Quanta 250.

Effects of Ultrasonic Frequency on the CO2 Ultimate Absorption Amount
The effects of ultrasonic vibration on the CO2 ultimate absorption amount are shown in Figure 2. It can be seen that under mechanical agitation, the CO2 AA in the cement paste increased gradually. When the CO2 absorption reached 5, 10, 15, and 20 s, the CO2 AA were 0.77%, 1.35%, 1.87%, 2.15%, and 2.47% of the cement mass, respectively. After an absorption time of 35 s, the cement paste could not further absorb CO2 due to thickening and lost fluidity. At this point, the CO2 AA of the cement paste reached 2.64% of the ultimate absorption amount.  Compared with mechanical agitation, the CO 2 AR of fresh cement paste is significantly increased under ultrasonic vibrating agitation. At 5, 10, 15, 20, and 25 s, the CO 2 AA of fresh cement pastes under the fixed ultrasonic frequency of 40 kHz were 0.85%, 1.41%, 2.07%, 2.42%, and 2.61% of cement mass, respectively. These are 10.4%, 4.4%, 10.7%, 12.6%, Appl. Sci. 2021, 11, 6877 5 of 18 and 5.7% higher than those under mechanical agitation at the same absorption times. The CO 2 AA reached a maximum at 30 s, approximately 2.66% of the ultimate absorption amount. Compared with mechanical agitation, the CO 2 ultimate absorption amount of fresh cement pastes under ultrasonic vibrating agitation is significantly increased with the shortening of absorption time.
With the reduction of ultrasonic frequency, the CO 2 ultimate absorption amount of cement paste is increased to some extent. At 5, 10, 15, and 20 s, the CO 2 AA of fresh cement pastes at the fixed ultrasonic frequency of 28 kHz were 0.94%, 1.65%, 2.2%, and 2.53% of the cement mass. These increased to 1.19%, 2.09%, 2.84%, and 3.12% by decreasing the ultrasonic frequency to 20 kHz. At an ultrasonic frequency of 28 kHz, the CO 2 AA of fresh cement pastes reached 2.73% of the ultimate absorption amount at 25 s. At an ultrasonic frequency of 20 kHz, the CO 2 AA of fresh cement pastes reached 3.17% of the ultimate absorption amount at 25 s.

Effects of Ultrasonic Vibration on CO 2 AR
The effects of ultrasonic vibration on CO 2 AR are shown in Figure 3. It can be seen that under mechanical agitation, the CO 2 AR was 0.063, 0.061, 0.058, 0.053, 0.046, and 0.035 ν(%/s) at the absorption times of 5, 10, 15, 20, 25, and 30 s, respectively. By increasing the absorption time, the cement paste gradually thickens causing the CO 2 AR to decrease accordingly. The cement paste changed from a fluid to pasty fluid gradually after 30 s. At 35 s, the CO 2 absorption of the paste reached saturation and the pastes could not absorb CO 2 any further.
cantly increased with the shortening of absorption time.
With the reduction of ultrasonic frequency, the CO2 ultimate absorption am cement paste is increased to some extent. At 5, 10, 15, and 20 s, the CO2 AA of f ment pastes at the fixed ultrasonic frequency of 28 kHz were 0.94%, 1.65%, 2.2 2.53% of the cement mass. These increased to 1.19%, 2.09%, 2.84%, and 3.12% creasing the ultrasonic frequency to 20 kHz. At an ultrasonic frequency of 28 k CO2 AA of fresh cement pastes reached 2.73% of the ultimate absorption amount At an ultrasonic frequency of 20 kHz, the CO2 AA of fresh cement pastes reached 3 the ultimate absorption amount at 25 s.

Effects of Ultrasonic Vibration on CO2 AR
The effects of ultrasonic vibration on CO2 AR are shown in Figure 3. It can that under mechanical agitation, the CO2 AR was 0.063, 0.061, 0.058, 0.053, 0.0 0.035 ν(%/s) at the absorption times of 5, 10, 15, 20, 25, and 30 s, respectively. By ing the absorption time, the cement paste gradually thickens causing the CO2 AR crease accordingly. The cement paste changed from a fluid to pasty fluid gradua 30 s. At 35 s, the CO2 absorption of the paste reached saturation and the pastes co absorb CO2 any further.
The CO2 AR of fresh cement paste increased significantly under ultrasonic v agitation compared to that under mechanical agitation. At an ultrasonic frequenc kHz, the CO2 AR of the cement paste was 0.08, 0.075, 0.067, and 0.055 ν(%/s) at the tion times of 5, 10, 15, and 20 s, respectively, which are 27%, 23%, 15.5%, and 3.8% compared to those under mechanical agitation. After 20 s, the cement paste chang fluid to pasty fluid and the CO2 absorption of paste reaches saturation at 27 s. At an ultrasonic frequency of 28 KHz, the CO2 AR of cement paste is 0.08 0.072, and 0.054 ν(%/s) at the absorption times of 5, 10, 15, and 20 s, respectively. T AR of cement paste increases to some extent with the decrease of ultrasonic freque an ultrasonic frequency of 20 kHz, the CO2 AR of cement paste is 0.108, 0.1, an ν(%/s) at 5, 10, and 15 s respectively. The CO2 AR of cement paste increases signif According to the results, it can be concluded that the CO2 absorption time of paste gradually shortens, while the CO2 AR and CO2 AA further increase when trasonic frequency gradually decreases from 40 kHz to 28 and 20 kHz. This is du fact that when the ultrasonic frequency increases, the ultrasonic intensity will i accordingly. When the increased ultrasonic intensity is excessive, there are ex bubbles produced, which conversely increase attenuation of scattering, forming The CO 2 AR of fresh cement paste increased significantly under ultrasonic vibrating agitation compared to that under mechanical agitation. At an ultrasonic frequency of 40 kHz, the CO 2 AR of the cement paste was 0.08, 0.075, 0.067, and 0.055 ν(%/s) at the absorption times of 5, 10, 15, and 20 s, respectively, which are 27%, 23%, 15.5%, and 3.8% higher compared to those under mechanical agitation. After 20 s, the cement paste changes from fluid to pasty fluid and the CO 2 absorption of paste reaches saturation at 27 s.
At an ultrasonic frequency of 28 KHz, the CO 2 AR of cement paste is 0.085, 0.08, 0.072, and 0.054 ν(%/s) at the absorption times of 5, 10, 15, and 20 s, respectively. The CO 2 AR of cement paste increases to some extent with the decrease of ultrasonic frequency. At an ultrasonic frequency of 20 kHz, the CO 2 AR of cement paste is 0.108, 0.1, and 0.077 ν(%/s) at 5, 10, and 15 s respectively. The CO 2 AR of cement paste increases significantly.
According to the results, it can be concluded that the CO 2 absorption time of cement paste gradually shortens, while the CO 2 AR and CO 2 AA further increase when the ultrasonic frequency gradually decreases from 40 kHz to 28 and 20 kHz. This is due to the fact that when the ultrasonic frequency increases, the ultrasonic intensity will increase accordingly. When the increased ultrasonic intensity is excessive, there are excessive bubbles produced, which conversely increase attenuation of scattering, forming barriers of the ultrasonic wave. On the other hand, increasing of ultrasonic intensity will also lead to increasing of nonlinear attenuation, which is disadvantageous for uniform agitation. Consequently, particles of the cement paste are not well distributed. Therefore, when the ultrasonic frequency is 20 kHz, the cavitation effect of ultrasonic agitation is best, and the corresponding CO 2 AR and CO 2 AA is highest. The changes of divergence of cement paste with CO 2 AA are shown in Figure 4. From Figure 4a, it can be seen that under mechanical agitation, the divergence values of cement paste are 159, 140, 131, 122, 106, and 98 mm when the CO 2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively. Figure 4b shows that the cement paste surface looks finer and more watery after ultrasonic vibrating agitation. Under ultrasonic vibrating agitation, divergence values of the cement paste are 174, 154, 143, 133, 115, and 105 mm, respectively. These are 9.4%, 10% 9.2%, 8.5%, and 7.1% higher than those under mechanical agitation. Ultrasonic vibration can increase the divergence of cement paste effectively. The divergence of cement paste decreases gradually with the increase of CO 2 AA and the paste loses fluidity slowly to gradually become a pasty fluid.

Effects of Ultrasonic
Appl. Sci. 2021, 11, 6877 6 of 18 of the ultrasonic wave. On the other hand, increasing of ultrasonic intensity will also lead to increasing of nonlinear attenuation, which is disadvantageous for uniform agitation. Consequently, particles of the cement paste are not well distributed. Therefore, when the ultrasonic frequency is 20 kHz, the cavitation effect of ultrasonic agitation is best, and the corresponding CO2 AR and CO2 AA is highest.

Divergence of Cement Paste after CO2 Absorption
The changes of divergence of cement paste with CO2 AA are shown in Figure 4. From Figure 4a, it can be seen that under mechanical agitation, the divergence values of cement paste are 159, 140, 131, 122, 106, and 98 mm when the CO2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively. Figure 4b shows that the cement paste surface looks finer and more watery after ultrasonic vibrating agitation. Under ultrasonic vibrating agitation, divergence values of the cement paste are 174, 154, 143, 133, 115, and 105 mm, respectively. These are 9.4%, 10% 9.2%, 8.5%, and 7.1% higher than those under mechanical agitation. Ultrasonic vibration can increase the divergence of cement paste effectively. The divergence of cement paste decreases gradually with the increase of CO2 AA and the paste loses fluidity slowly to gradually become a pasty fluid.

Effects of Ultrasonic Vibration on the Fluidity of Cement Paste after CO2 Absorption
The effects of ultrasonic vibration on the divergence of cement paste after CO2 absorption were drawn according to test results of divergence ( Figure 5). Clearly, under the same CO2 AA, the divergence of fresh cement paste under the ultrasonic vibrating agitation is higher than that under mechanical agitation. Moreover, the divergence of cement paste values were 174, 154, 143, 133, 115, and 105 mm when the CO2 AA were 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively, which decreased by 13.0%, 7.7%, 7.5%, 15.7%, and 9.5%, respectively.

Effects of Ultrasonic Vibration on the Fluidity of Cement Paste after CO 2 Absorption
The effects of ultrasonic vibration on the divergence of cement paste after CO 2 absorption were drawn according to test results of divergence ( Figure 5). Clearly, under the same CO 2 AA, the divergence of fresh cement paste under the ultrasonic vibrating agitation is higher than that under mechanical agitation. Moreover, the divergence of cement paste values were 174, 154, 143, 133, 115, and 105 mm when the CO 2 AA were 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively, which decreased by 13.0%, 7.7%, 7.5%, 15.7%, and 9.5%, respectively. Appl. Sci. 2021, 11, 6877 7 of 18 The above results indicate that the ultrasonically agitated fluidity of cement paste is increased compared with that mechanically agitated. Cement particles in unit volume of cement paste is significantly increased when ultrasonic agitation is adopted, which means that ultrasonic agitation is better to break the flocculation structures formed by cement particles compared with mechanical agitation. The break of flocculation structures means that there is more free water in the paste to ensure fluidity of cement paste. However, both fluidities of cement paste agitated ultrasonically and mechanically decreases with an increase in CO2 AA and the decrement is significant. This is due to the fact that the structure of the cement paste gradually changes when CO2 AA gradually increases, which is disadvantageous for fluidity of cement paste.

The Most Available Geometric Diameter of Pore Distribution
The differential curve of the pore diameter of hardened cement paste under CO2 AA was tested by the mercury intrusion method ( Figure 6). For CO2 AA of 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, the most available geometric diameters in the pore distribution differential curve of hardened cement paste are 112.5, 118.5, 92.5, 83.1, 80.7, and 72.7 nm under mechanical agitation. Under ultrasonic vibrating agitation, the most available geometric diameters in the pore distribution differential curve of hardened cement paste are 78.7, 83.3, 70.5, 60.6, 49.4, and 48.2 nm, respectively, which are decreased by 30%, 29.7%, 23.8%, 27.1%, 38.8%, and 33.7% compared to those under mechanical agitation.
It can be obtained according to Figure 6 that the most available geometric diameter of pore distribution of the hardened carbonized cement paste with ultrasonic agitation is lower than that with mechanical agitation. When the CO2 AA is respectively 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, the most available geometric diameter of pore distribution of the hardened carbonized cement paste with ultrasonic agitation gradually decreases by 15.4%, 14%, 18.5%, and 2.4% compared to that with mechanical agitation, which indicates that the most available geometric diameter of pore distribution of the hardened carbonized cement paste with ultrasonic agitation gradually decreases with an increase in CO2 AA. Therefore, the most available geometric diameter of pore distribution of the hardened carbonized cement paste can effectively be reduced with ultrasonic agitation. The above results indicate that the ultrasonically agitated fluidity of cement paste is increased compared with that mechanically agitated. Cement particles in unit volume of cement paste is significantly increased when ultrasonic agitation is adopted, which means that ultrasonic agitation is better to break the flocculation structures formed by cement particles compared with mechanical agitation. The break of flocculation structures means that there is more free water in the paste to ensure fluidity of cement paste. However, both fluidities of cement paste agitated ultrasonically and mechanically decreases with an increase in CO 2 AA and the decrement is significant. This is due to the fact that the structure of the cement paste gradually changes when CO 2 AA gradually increases, which is disadvantageous for fluidity of cement paste.

The Most Available Geometric Diameter of Pore Distribution
The differential curve of the pore diameter of hardened cement paste under CO 2 AA was tested by the mercury intrusion method ( Figure 6). For CO 2 AA of 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, the most available geometric diameters in the pore distribution differential curve of hardened cement paste are 112.5, 118.5, 92.5, 83.1, 80.7, and 72.7 nm under mechanical agitation. Under ultrasonic vibrating agitation, the most available geometric diameters in the pore distribution differential curve of hardened cement paste are 78.7, 83.3, 70.5, 60.6, 49.4, and 48.2 nm, respectively, which are decreased by 30%, 29.7%, 23.8%, 27.1%, 38.8%, and 33.7% compared to those under mechanical agitation. Wu Zhongwei divided the concrete pore into four types according to influences of pore size on the durability of concretes: Harmless pores (<20 nm), slightly harmful pores It can be obtained according to Figure 6 that the most available geometric diameter of pore distribution of the hardened carbonized cement paste with ultrasonic agitation is lower than that with mechanical agitation. When the CO 2 AA is respectively 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, the most available geometric diameter of pore distribution of the hardened carbonized cement paste with ultrasonic agitation gradually decreases by 15.4%, 14%, 18.5%, and 2.4% compared to that with mechanical agitation, which indicates that the most available geometric diameter of pore distribution of the hardened carbonized cement paste with ultrasonic agitation gradually decreases with an increase in CO 2 AA. Therefore, the most available geometric diameter of pore distribution of the hardened carbonized cement paste can effectively be reduced with ultrasonic agitation.
It can be obtained from Figure 7, with an increase in CO 2 AA, that harmless pores and slightly harmful pores of the hardened carbonized cement paste will increase, while harmful pores and multi-harmful pores will decrease. However, the increment of harmless pores and slightly harmful pores and the decrement of harmful pores and multi-harmful pores are more significant when ultrasonic agitation is adopted.

Porosity
The total porosity of hardened cement pastes after different CO2 AA und sonic vibrating agitation is shown in Figure 8. Clearly, the porosities of the h cement paste under mechanical agitation are 17.5%, 17.1%, 16%, 15.1%, 14.3%, an when the CO2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively ultrasonic vibrating agitation, the porosities of the hardened cement paste ar 15.1%, 14.6%, 13.4%, 12.1%, and 11.5%, which are 10.9%, 11.7%, 8.8%, 1.3%, 15. 16.7% lower compared to those under mechanical agitation, respectively. Moreo porosities of hardened cement paste decrease by 3.2%, 3.3%, 8.2%, 9.7%, and 5% w CO2 AA increases at the rate of 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20% conti under ultrasonic vibrating agitation. According to the above analyses, conclusions can be drawn that the porosi hardened carbonized cement paste whether mechanically agitated or ultrasonic tated decreases when CO2 AA increases. However, the porosity decrement is m vious when ultrasonic agitation is applied. Reasons for the significant porosity de when ultrasonic agitation is applied lies in two aspects. On the one hand, ceme cles can be minimized by ultrasonic waves, which is advantageous for the hydra gree of cement particles. On the other hand, the flocculation structures formed by particles can be broken by ultrasonic agitation, which is advantageous for ceme cles to get accesses to water to improve the hydration degree. The improved hy degree of cement particles indicates that more hydration products of C-S-H and formed, which improves the density of the hardened cement paste, thus reducin ity of the hardened cement paste.

Mean Pore Size
The mean pore sizes of hardened cement paste after different CO2 AA und sonic vibrating agitation are shown in Figure 9. It can be seen that under mechan itation, the mean pore sizes of hardened cement paste are 88, 79, 75, 68, 65, and when CO2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, respectively. Mea the mean pore sizes of hardened cement paste are 80, 73, 65, 56, 50, and 45 nm u trasonic vibrating agitation, which are decreased by 9.1%, 7.6%, 13.3%, 17.6%, 23. 25% compared to those under mechanical agitation. Furthermore, the mean pore According to the above analyses, conclusions can be drawn that the porosity of the hardened carbonized cement paste whether mechanically agitated or ultrasonically agitated decreases when CO 2 AA increases. However, the porosity decrement is more obvious when ultrasonic agitation is applied. Reasons for the significant porosity decrement when ultrasonic agitation is applied lies in two aspects. On the one hand, cement particles can be minimized by ultrasonic waves, which is advantageous for the hydration degree of cement particles. On the other hand, the flocculation structures formed by cement particles can be broken by ultrasonic agitation, which is advantageous for cement particles to get accesses to water to improve the hydration degree. The improved hydration degree of cement particles indicates that more hydration products of C-S-H and CH are formed, which improves the density of the hardened cement paste, thus reducing porosity of the hardened cement paste.
According to the above analyses, it can be obtained that both mechanical trasonic agitation is helpful to minimize the mean pore size and that the mean p gradually decreases with an increase in CO2 AA. However, the decrement in me size when ultrasonic agitation is applied is more obvious. This is due to the fact products of CaCO3 crystals are helpful to minimize pore sizes and optimize p distribution of the hardened carbonized cement paste.

Compressive Strength
The effects of ultrasonic vibration on the compressive strength of ceme materials after CO2 absorption are shown in Figure 10. At the age of 3 d, the CO 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20% with a corresponding compressive str the hardened cement paste of 5.9, 5.4, 5.7, 5.2, 5.6, and 5.3 MPa under mechanic tion. According to the data, the compressive strength may increase and decrease increase of CO2 AA, but the fluctuation amplitude is not very large. At the age of d, the compressive strength of hardened cement paste is increased to some exten the compressive strengths are 6.9, 6.7, 6, 6.1, 6.5, and 6.5 MPa, respectively. The v law of the compressive strength firstly decreases slowly but decreases significant the CO2 AA is 0.88%. Subsequently, it increases with the increase of CO2 AA.
At the age of 28 d, the compressive strengths of the hardened cement paste 7.4, 7.6, 7.6, 7.2, and 7.1 MPa, respectively. In view of the data, the compressive changes stably and even presents a slowly increasing trend in the early stages absorption as the absorption amount increases. However, the compressive stre creases to a small extent with the increase of absorption amount, though the v amplitude is not evident. At different ages, the compressive strength of fresh paste does not increase significantly with an increase of CO2 AA. It does change extent in the middle, however, it does not change greatly with CO2 AA.
When the CO2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, under u vibrating agitation, the compressive strengths of the hardened cement paste at 3 5.5, 5.9, 5.3, 5.8, and 5.4 MPa, respectively. In view of the data, the variation law pressive strength is very close to that under mechanical agitation. At 7 d, the com strengths of hardened cement paste are 6.9, 6.7, 6, 6.1, 6.5, and 6.5 MPa, resp which then increases to 7.4, 7.6, 7.5, 7.9, 7.5, and 7.4 MPa at 28 d, respectively. Me the compressive strength at 7 and 28 d under ultrasonic vibrating agitation app the results under mechanical agitation as the CO2 AA increases. According to the above analyses, it can be obtained that both mechanical and ultrasonic agitation is helpful to minimize the mean pore size and that the mean pore size gradually decreases with an increase in CO 2 AA. However, the decrement in mean pore size when ultrasonic agitation is applied is more obvious. This is due to the fact that the products of CaCO 3 crystals are helpful to minimize pore sizes and optimize pore size distribution of the hardened carbonized cement paste.

Compressive Strength
The effects of ultrasonic vibration on the compressive strength of cement-based materials after CO 2 absorption are shown in Figure 10. At the age of 3 d, the CO 2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20% with a corresponding compressive strength of the hardened cement paste of 5.9, 5.4, 5.7, 5.2, 5.6, and 5.3 MPa under mechanical agitation. According to the data, the compressive strength may increase and decrease with the increase of CO 2 AA, but the fluctuation amplitude is not very large. At the age of 7 and 28 d, the compressive strength of hardened cement paste is increased to some extent. At 7 d, the compressive strengths are 6.9, 6.7, 6, 6.1, 6.5, and 6.5 MPa, respectively. The variation law of the compressive strength firstly decreases slowly but decreases significantly when the CO 2 AA is 0.88%. Subsequently, it increases with the increase of CO 2 AA. The variation laws of compressive strength under mechanical agitation and ultrasonic vibrating agitation are essentially identical. The compressive strength of hardened cement paste under ultrasonic vibrating agitation is increased to some extent compared with those after equal CO2 AA at the same age under mechanical agitation. With the increase of CO2 AA, the compressive strength changes slightly, indicating that ultrasonic vibration influences the compressive strength of cement paste after CO2 absorption slightly.

Compressive Strength
The effects of ultrasonic vibration on the comprehensive strength of cement-based materials after CO2 absorption are shown in Figure 11. It can be seen that when the CO2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, under mechanical agitation, the compressive strength of hardened cement paste at 3 d are 24.9, 24.4, 24.9, 24, 25, and 24.9 MPa, respectively. In view of the data, the compressive strength of cement paste after CO2 absorption within the curing stage changes slightly. Even though it fluctuates to some extent, the fluctuation range is very small. At   At the age of 28 d, the compressive strengths of the hardened cement paste are 7.4, 7.4, 7.6, 7.6, 7.2, and 7.1 MPa, respectively. In view of the data, the compressive strength changes stably and even presents a slowly increasing trend in the early stages of CO 2 absorption as the absorption amount increases. However, the compressive strength decreases to a small extent with the increase of absorption amount, though the variation amplitude is not evident. At different ages, the compressive strength of fresh cement paste does not increase significantly with an increase of CO 2 AA. It does change to some extent in the middle, however, it does not change greatly with CO 2 AA.
When the CO 2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, under ultrasonic vibrating agitation, the compressive strengths of the hardened cement paste at 3 d are 6, 5.5, 5.9, 5.3, 5.8, and 5.4 MPa, respectively. In view of the data, the variation law of compressive strength is very close to that under mechanical agitation. At 7 d, the compressive strengths of hardened cement paste are 6.9, 6.7, 6, 6.1, 6.5, and 6.5 MPa, respectively, which then increases to 7.4, 7.6, 7.5, 7.9, 7.5, and 7.4 MPa at 28 d, respectively. Meanwhile, the compressive strength at 7 and 28 d under ultrasonic vibrating agitation approaches the results under mechanical agitation as the CO 2 AA increases.
The variation laws of compressive strength under mechanical agitation and ultrasonic vibrating agitation are essentially identical. The compressive strength of hardened cement paste under ultrasonic vibrating agitation is increased to some extent compared with those after equal CO 2 AA at the same age under mechanical agitation. With the increase of CO 2 AA, the compressive strength changes slightly, indicating that ultrasonic vibration influences the compressive strength of cement paste after CO 2 absorption slightly.

Compressive Strength
The effects of ultrasonic vibration on the comprehensive strength of cement-based materials after CO 2 absorption are shown in Figure 11. It can be seen that when the CO 2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, under mechanical agitation, the compressive strength of hardened cement paste at 3 d are 24.9, 24.4, 24.9, 24, 25, and 24.9 MPa, respectively. In view of the data, the compressive strength of cement paste after CO 2 absorption within the curing stage changes slightly. Even though it fluctuates to some extent, the fluctuation range is very small.  The variation laws of compressive strength under mechanical agitation and ultrasonic vibrating agitation are essentially identical. The compressive strength of hardened cement paste under ultrasonic vibrating agitation is increased to some extent compared with those after equal CO2 AA at the same age under mechanical agitation. With the increase of CO2 AA, the compressive strength changes slightly, indicating that ultrasonic vibration influences the compressive strength of cement paste after CO2 absorption slightly.

Compressive Strength
The effects of ultrasonic vibration on the comprehensive strength of cement-based materials after CO2 absorption are shown in Figure 11. It can be seen that when the CO2 AA are 0%, 0.44%, 0.88%, 1.32%, 1.76%, and 2.20%, under mechanical agitation, the compressive strength of hardened cement paste at 3 d are 24.9, 24.4, 24.9, 24, 25, and 24.9 MPa, respectively. In view of the data, the compressive strength of cement paste after CO2 absorption within the curing stage changes slightly. Even though it fluctuates to some extent, the fluctuation range is very small.  According to the above experimental results, it can be obtained that the increment in compressive strength of the hardened carbonized cement paste is more significant when ultrasonic agitation is applied compared with that when mechanical agitation is applied. Under ultrasonic agitation, cement particles are broken to smaller ones and flocculation structures formed by cement particles are broken by ultrasonic waves, which can promote cement hydration to produce more C-S-H. Therefore, the compressive strength is significantly increased by applying ultrasonic. According to the above experimental results, it can be obtained that the increment in compressive strength of the hardened carbonized cement paste is more significant when ultrasonic agitation is applied compared with that when mechanical agitation is applied. Under ultrasonic agitation, cement particles are broken to smaller ones and flocculation structures formed by cement particles are broken by ultrasonic waves, which can promote cement hydration to produce more C-S-H. Therefore, the compressive strength is significantly increased by applying ultrasonic.

Mechanical Agitation Molding
SEM images of the mechanical agitation moulded specimens of cement paste after 10,000 amplifications are shown in Figure 12. Obviously, the microstructure of cement paste without CO 2 absorption in the early hardening stage shows relatively sparse cement particle distribution and gelatinization structures in the paste compared with the microstructure when CO 2 AA is 0.44%. Moreover, a significant number of pores in the paste and hydration products are scattered around, revealing poor structural integrity (Figure 12a). In contrast, cement paste with a CO 2 AA of 0.44% shows few CaCO 3 crystals under SEM. After amplification, there are few CaCO 3 needle-like crystal whiskers in the microstructure which penetrate in gel substances (Figure 12b) [35]. However, no crystal whiskers are found in cement paste that has not absorbed CO 2 .
paste without CO2 absorption in the early hardening stage shows relatively sparse cement particle distribution and gelatinization structures in the paste compared with the microstructure when CO2 AA is 0.44%. Moreover, a significant number of pores in the paste and hydration products are scattered around, revealing poor structural integrity (Figure 12a). In contrast, cement paste with a CO2 AA of 0.44% shows few CaCO3 crystals under SEM. After amplification, there are few CaCO3 needle-like crystal whiskers in the microstructure which penetrate in gel substances (Figure 12b) [35]. However, no crystal whiskers are found in cement paste that has not absorbed CO2.
When the CO2 AA is increased to 0.88%, hydration products are distributed more extensively in the paste. The CaCO3 crystals of hydration products and CaCO3 needle-like crystal whiskers are increased, while the number of pores is decreased ( Figure  13c). When the CO2 AA increases to 1.32%, the internal structure becomes more compact in the early stage of CO2 absorption of the paste. Additionally, CaCO3 needle-like crystal whiskers of hydration products are increased, which interweave both vertically and horizontally (Figure 12d).

Ultrasonic Vibrating Agitation Moulding
SEM images of ultrasonic vibrating agitation moulded specimens of cement paste after 10,000 amplifications are shown in Figure 13. It can be seen that under ultrasonic vibrating agitation, the microstructure of cement pastes without CO2 absorption in the early hardening stage shows relatively uniform distributions of cement particles and gel structures compared with the microstructure when the CO2 AA is 0.44%. Moreover, there are no big pores, and the pores are in a relatively uniform distribution accompanied by a reduction of distances among flocculating constituents (Figure 13a). In contrast, the cement paste with a CO2 AA of 0.44% shows some CaCO3 crystals and the existence of some CaCO3 needle-like crystal whiskers penetrating in the gel substances, which are similar to those of mechanical agitation moulded specimens (Figure 13b).
When the CO2 AA is 0.88%, CO2 and Ca 2+ react continuously as CO2 is supplied continuously, accelerating the hydration process of cement and producing hydration products continuously. As a result, pores in the paste are filled continuously, effectively decreasing the quantity and diameter of pores at the same time as increasing the volume of flocculating constituents and quantity of CaCO3 needle-like crystal whiskers ( Figure  13c). When the CO2 AA is 1.32%, the flocculating constituents increase in uniform dis- When the CO 2 AA is increased to 0.88%, hydration products are distributed more extensively in the paste. The CaCO 3 crystals of hydration products and CaCO 3 needle-like crystal whiskers are increased, while the number of pores is decreased (Figure 13c). When the CO 2 AA increases to 1.32%, the internal structure becomes more compact in the early stage of CO 2 absorption of the paste. Additionally, CaCO 3 needle-like crystal whiskers of hydration products are increased, which interweave both vertically and horizontally (Figure 12d).

Energy Dispersive Spectrum (EDS) Analysis
In order to better understand the content changes of the elements of the paste after CO2 absorption by adopting ultrasonic vibration technology, EDS analysis was conducted with the needle-like production in cement paste without CO2 absorption, respectively and the cement paste with 1.32% CO2 absorption by mechanical agitation, as well as that with 1.32% CO2 absorption by ultrasonic agitation, as illustrated in Figure 14a-c. The contents of each element are shown in Table 3.

Ultrasonic Vibrating Agitation Moulding
SEM images of ultrasonic vibrating agitation moulded specimens of cement paste after 10,000 amplifications are shown in Figure 13. It can be seen that under ultrasonic vibrating agitation, the microstructure of cement pastes without CO 2 absorption in the early hardening stage shows relatively uniform distributions of cement particles and gel structures compared with the microstructure when the CO 2 AA is 0.44%. Moreover, there are no big pores, and the pores are in a relatively uniform distribution accompanied by a reduction of distances among flocculating constituents (Figure 13a). In contrast, the cement paste with a CO 2 AA of 0.44% shows some CaCO 3 crystals and the existence of some CaCO 3 needle-like crystal whiskers penetrating in the gel substances, which are similar to those of mechanical agitation moulded specimens (Figure 13b).
When the CO 2 AA is 0.88%, CO 2 and Ca 2+ react continuously as CO 2 is supplied continuously, accelerating the hydration process of cement and producing hydration products continuously. As a result, pores in the paste are filled continuously, effectively decreasing the quantity and diameter of pores at the same time as increasing the volume of flocculating constituents and quantity of CaCO 3 needle-like crystal whiskers (Figure 13c). When the CO 2 AA is 1.32%, the flocculating constituents increase in uniform distribution and the number of pores is decreased significantly in view of the microstructure. Furthermore, the CaCO 3 whiskers continue to increase and interact to form cubic network structures (Figure 13d). Compared with mechanical agitation, the distributions of cement particles and hydration products under ultrasonic vibration are more uniform, and the number of pores decreases, whilst the production of CaCO 3 needle-like crystal whiskers increases. Hence, cement becomes more compact in the early hardening stage [36].

Energy Dispersive Spectrum (EDS) Analysis
In order to better understand the content changes of the elements of the paste after CO 2 absorption by adopting ultrasonic vibration technology, EDS analysis was conducted with the needle-like production in cement paste without CO 2 absorption, respectively and the cement paste with 1.32% CO 2 absorption by mechanical agitation, as well as that with 1.32% CO 2 absorption by ultrasonic agitation, as illustrated in Figure 14a-c. The contents of each element are shown in Table 3.
of cement particles and hydration products under ultrasonic vibration are more unifo the number of pores decreases, whilst the production of CaCO3 needle-like crystal w increases. Hence, cement becomes more compact in the early hardening stage [36].

Energy Dispersive Spectrum (EDS) Analysis
In order to better understand the content changes of the elements of the pa CO2 absorption by adopting ultrasonic vibration technology, EDS analysis w ducted with the needle-like production in cement paste without CO2 absorption, tively and the cement paste with 1.32% CO2 absorption by mechanical agitation, as that with 1.32% CO2 absorption by ultrasonic agitation, as illustrated in Figur The contents of each element are shown in Table 3. According to Table 3, the main elements of the measure points in Figure 14a-O, and Ca, respectively. Meanwhile, there are also some Si and Al elements. It obtained from Table 3 that the contents of C, Ca, and Si elements are increased, w of Al element is obviously reduced when CO2 is absorbed by the cement past    According to Table 3, the main elements of the measure points in Figure 14a-c are C, O, and Ca, respectively. Meanwhile, there are also some Si and Al elements. It can be obtained from Table 3 that the contents of C, Ca, and Si elements are increased, while that of Al element is obviously reduced when CO 2 is absorbed by the cement paste under mechanical agitation. However, under ultrasonic agitation, the contents of C and Ca elements are increased more obviously and that of Al element continues to decrease.
According to the above results, it can be concluded that with the absorption of CO 2 , the content of C element of the cement paste is increased and there are CaCO 3 crystals in the cement paste. Moreover, due to the "cavitation effect" of ultrasonic agitation, CO 2 is effectively dispersed in cement paste, leading to more CaCO 3 crystals being produced by adopting ultrasonic agitation. Therefore, cement paste is able to absorb more CO 2 , thus forming more CaCO 3 , when ultrasonic agitation is applied compared with that when mechanical agitation is applied.

Cement Paste without CO 2 Absorption
The hydration process of cement paste without CO 2 absorption is shown in Figure 15. Clearly, a gel film that is composed of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) crystals is formed on the surface of cement particles in the early hydration stage. With the continuous increase of hydration time, the gel film thickens day by day due to the increasing gels. Meanwhile, the cement begins to harden slowly until finishing the hydration of the cement.
Appl. Sci. 2021, 11, 6877 14 of 18 mechanical agitation. However, under ultrasonic agitation, the contents of C and Ca elements are increased more obviously and that of Al element continues to decrease. According to the above results, it can be concluded that with the absorption of CO2, the content of C element of the cement paste is increased and there are CaCO3 crystals in the cement paste. Moreover, due to the "cavitation effect" of ultrasonic agitation, CO2 is effectively dispersed in cement paste, leading to more CaCO3 crystals being produced by adopting ultrasonic agitation. Therefore, cement paste is able to absorb more CO2, thus forming more CaCO3, when ultrasonic agitation is applied compared with that when mechanical agitation is applied.

Cement Paste without CO2 Absorption
The hydration process of cement paste without CO2 absorption is shown in Figure  15. Clearly, a gel film that is composed of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) crystals is formed on the surface of cement particles in the early hydration stage. With the continuous increase of hydration time, the gel film thickens day by day due to the increasing gels. Meanwhile, the cement begins to harden slowly until finishing the hydration of the cement.

Cement Paste after CO2 Absorption under Mechanical Agitation
The hydration process of cement paste after CO2 absorption under mechanical agitation is shown in Figure 16. It can be seen that under mechanical agitation, CO2 gases scatter uniformly in the cement paste and firstly dissolve into H2CO3. H2CO3 reacts with Ca(OH)2 which is precipitated from the initial hydration of cement to produce flocculent CaCO3 crystals which adhere to the surface of cement particles (Figure 16a). With the in- mechanical agitation. However, under ultrasonic agitation, the contents of C and Ca elements are increased more obviously and that of Al element continues to decrease. According to the above results, it can be concluded that with the absorption of CO2, the content of C element of the cement paste is increased and there are CaCO3 crystals in the cement paste. Moreover, due to the "cavitation effect" of ultrasonic agitation, CO2 is effectively dispersed in cement paste, leading to more CaCO3 crystals being produced by adopting ultrasonic agitation. Therefore, cement paste is able to absorb more CO2, thus forming more CaCO3, when ultrasonic agitation is applied compared with that when mechanical agitation is applied.

Cement Paste without CO2 Absorption
The hydration process of cement paste without CO2 absorption is shown in Figure  15. Clearly, a gel film that is composed of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) crystals is formed on the surface of cement particles in the early hydration stage. With the continuous increase of hydration time, the gel film thickens day by day due to the increasing gels. Meanwhile, the cement begins to harden slowly until finishing the hydration of the cement.

Cement Paste after CO2 Absorption under Mechanical Agitation
The hydration process of cement paste after CO2 absorption under mechanical agitation is shown in Figure 16. It can be seen that under mechanical agitation, CO2 gases scatter uniformly in the cement paste and firstly dissolve into H2CO3. H2CO3 reacts with Ca(OH)2 which is precipitated from the initial hydration of cement to produce flocculent CaCO3 crystals which adhere to the surface of cement particles (Figure 16a). With the in-cement particles; Appl. Sci. 2021, 11, 6877 14 of 18 mechanical agitation. However, under ultrasonic agitation, the contents of C and Ca elements are increased more obviously and that of Al element continues to decrease. According to the above results, it can be concluded that with the absorption of CO2, the content of C element of the cement paste is increased and there are CaCO3 crystals in the cement paste. Moreover, due to the "cavitation effect" of ultrasonic agitation, CO2 is effectively dispersed in cement paste, leading to more CaCO3 crystals being produced by adopting ultrasonic agitation. Therefore, cement paste is able to absorb more CO2, thus forming more CaCO3, when ultrasonic agitation is applied compared with that when mechanical agitation is applied.

Cement Paste without CO2 Absorption
The hydration process of cement paste without CO2 absorption is shown in Figure  15. Clearly, a gel film that is composed of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) crystals is formed on the surface of cement particles in the early hydration stage. With the continuous increase of hydration time, the gel film thickens day by day due to the increasing gels. Meanwhile, the cement begins to harden slowly until finishing the hydration of the cement.

Cement Paste after CO2 Absorption under Mechanical Agitation
The hydration process of cement paste after CO2 absorption under mechanical agitation is shown in Figure 16. It can be seen that under mechanical agitation, CO2 gases scatter uniformly in the cement paste and firstly dissolve into H2CO3. H2CO3 reacts with Ca(OH)2 which is precipitated from the initial hydration of cement to produce flocculent CaCO3 crystals which adhere to the surface of cement particles (Figure 16a). With the in-hydration products of flocculating gel).

Cement Paste after CO 2 Absorption under Mechanical Agitation
The hydration process of cement paste after CO 2 absorption under mechanical agitation is shown in Figure 16. It can be seen that under mechanical agitation, CO 2 gases scatter uniformly in the cement paste and firstly dissolve into H 2 CO 3 . H 2 CO 3 reacts with Ca(OH) 2 which is precipitated from the initial hydration of cement to produce flocculent CaCO 3 crystals which adhere to the surface of cement particles (Figure 16a). With the increase of CO 2 AA, the CaCO 3 gel layer on the cement particle surface thickens and the cement paste viscosifies gradually, decreasing the fluidity accordingly [37]. cement paste viscosifies gradually, decreasing the fluidity accordingly [37].
The CaCO3 gel layer on the cement particle surface has a loosened structure and then the cement particles begin to hydrate gradually. Calcium metasilicate (C-S-H) gel and CaCO3 gel which are the hydration products of cement combine on the cement particle surface into a gel layer. Some flocculent CaCO3 gel crystals penetrate the C-S-H and flocculent CaCO3 gels as needle-like whiskers, forming a skeletal network (Figure 16b).
In the late hydration stage of cement, hydration products on the cement particle surface increase significantly and the wrapper thickens accordingly. Moreover, CaCO3 crystals are wrapped in the hydration products. Meanwhile, the CaCO3 needle-like crystal whiskers are still in the paste to form an effective network structure (Figure 16c), which improves the gelling performance among cement particles.

Hydration Mechanism of Cement Paste under Ultrasonic Agitation
The hydration mechanism of cement paste under ultrasonic agitation is shown in Figure 17. When CO2 gases are supplied under ultrasonic vibration, they firstly react with Ca(OH)2 which is precipitated from the initial hydration to produce flocculent CaCO3 crystals. The solid surfaces suspended in the liquid are damaged dramatically due to the "cavitation effect" of the ultrasonic wave [38]. When an ultrasonic wave radiates and spreads throughout the paste, it will produce numerous small bubbles in the paste, which will break continuously. More than 1000 instant high pressure regions can be produced at the rupture of bubbles [39][40][41]. The rupture explosions in the series will release a considerable amount of energy to cause great impacts on the surrounding areas. On the one hand, this causes continuous impacts on the cement particle surfaces which have not hydrated completely in the cement paste to make the gel hydration products on the surface peel off quickly, thus getting new cement particles. On the other hand, CaCO3 flocculating constituents are cut effectively under the ultrasonic wave effect and the abundant "nano" crystals are produced scattered among cement particles (Figure 17a).
In the early reaction stage of CO2, the CaCO3 gel layer on the cement particle surface has a relatively loose structure. When CO2 gases are consumed completely, the cement particles begin to hydrate gradually, producing hydration products continuously including "nano" CaCO3 crystals and a certain quantity of CaCO3 whiskers. Due to the "cavitation effect" of ultrasonic vibration, the "nano" CaCO3 crystals cannot adhere to the cement particle surface. Since the surfaces of hydration products from the reaction of cement particles and water are positively or negatively charged and the crystal nucleus has some adsorption capacity, some hydration products can adhere onto "nano" CaCO3 crystal surfaces. These "nano" CaCO3 flocculating constituents are effectively filled in the spaces among the cement particles. Moreover, CaCO3 whiskers depend on flocculating constituents and form a dense networked structure (Figure 17b). The hydration process of c 15. Clearly, a gel film that is co cium hydroxide (CH) crystals i hydration stage. With the conti day by day due to the increasin until finishing the hydration of t The hydration process of c tation is shown in Figure 16. It scatter uniformly in the cement Ca(OH)2 which is precipitated fr CaCO3 crystals which adhere to crease of CO2 AA, the CaCO3 gel layer on the cement particle surface thickens and the cement paste viscosifies gradually, decreasing the fluidity accordingly [37].
The CaCO3 gel layer on the cement particle surface has a loosened structure and then the cement particles begin to hydrate gradually. Calcium metasilicate (C-S-H) gel and CaCO3 gel which are the hydration products of cement combine on the cement particle surface into a gel layer. Some flocculent CaCO3 gel crystals penetrate the C-S-H and flocculent CaCO3 gels as needle-like whiskers, forming a skeletal network (Figure 16b).
In the late hydration stage of cement, hydration products on the cement particle surface increase significantly and the wrapper thickens accordingly. Moreover, CaCO3 crystals are wrapped in the hydration products. Meanwhile, the CaCO3 needle-like crystal whiskers are still in the paste to form an effective network structure (Figure 16c), which improves the gelling performance among cement particles.

Hydration Mechanism of Cement Paste under Ultrasonic Agitation
The hydration mechanism of cement paste under ultrasonic agitation is shown in Figure 17. When CO2 gases are supplied under ultrasonic vibration, they firstly react with Ca(OH)2 which is precipitated from the initial hydration to produce flocculent CaCO3 crystals. The solid surfaces suspended in the liquid are damaged dramatically due to the "cavitation effect" of the ultrasonic wave [38]. When an ultrasonic wave radiates and spreads throughout the paste, it will produce numerous small bubbles in the paste, which will break continuously. More than 1000 instant high pressure regions can be produced at the rupture of bubbles [39][40][41]. The rupture explosions in the series will release a considerable amount of energy to cause great impacts on the surrounding areas. On the one hand, this causes continuous impacts on the cement particle surfaces which have not hydrated completely in the cement paste to make the gel hydration products on the surface peel off quickly, thus getting new cement particles. On the other hand, CaCO3 flocculating constituents are cut effectively under the ultrasonic wave effect and the abundant "nano" crystals are produced scattered among cement particles (Figure 17a).
In the early reaction stage of CO2, the CaCO3 gel layer on the cement particle surface has a relatively loose structure. When CO2 gases are consumed completely, the cement particles begin to hydrate gradually, producing hydration products continuously including "nano" CaCO3 crystals and a certain quantity of CaCO3 whiskers. Due to the "cavitation effect" of ultrasonic vibration, the "nano" CaCO3 crystals cannot adhere to the cement particle surface. Since the surfaces of hydration products from the reaction of cement particles and water are positively or negatively charged and the crystal nucleus has some adsorption capacity, some hydration products can adhere onto "nano" CaCO3 crystal surfaces. These "nano" CaCO3 flocculating constituents are effectively filled in the spaces among the cement particles. Moreover, CaCO3 whiskers depend on flocculating constituents and form a dense networked structure (Figure 17b). the supplied CO 2 gases; forming more CaCO3, when ultrasonic agitation is applied compared with that when mechanical agitation is applied.

Cement Paste without CO2 Absorption
The hydration process of cement paste without CO2 absorption is shown in Figure  15. Clearly, a gel film that is composed of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) crystals is formed on the surface of cement particles in the early hydration stage. With the continuous increase of hydration time, the gel film thickens day by day due to the increasing gels. Meanwhile, the cement begins to harden slowly until finishing the hydration of the cement.

Cement Paste after CO2 Absorption under Mechanical Agitation
The hydration process of cement paste after CO2 absorption under mechanical agitation is shown in Figure 16. It can be seen that under mechanical agitation, CO2 gases scatter uniformly in the cement paste and firstly dissolve into H2CO3. H2CO3 reacts with Ca(OH)2 which is precipitated from the initial hydration of cement to produce flocculent CaCO3 crystals which adhere to the surface of cement particles (Figure 16a). With the in-flocculent gel hydration products; The CaCO3 gel layer then the cement particles b and CaCO3 gel which are t ticle surface into a gel layer flocculent CaCO3 gels as ne In the late hydration surface increase significant crystals are wrapped in t crystal whiskers are still in which improves the gelling

Hydration Mechanism of
The hydration mechan Figure 17. When CO2 gases Ca(OH)2 which is precipita crystals. The solid surfaces "cavitation effect" of the u spreads throughout the pa which will break continuo produced at the rupture of lease a considerable amoun On the one hand, this caus have not hydrated complet the surface peel off quickly flocculating constituents a abundant "nano" crystals a In the early reaction st has a relatively loose struc particles begin to hydrate cluding "nano" CaCO3 cry "cavitation effect" of ultraso cement particle surface. Si cement particles and water has some adsorption capac crystal surfaces. These "nan spaces among the cement constituents and form a den CaCO 3 crystals; cement paste viscosifies gradually, decreasing the fluidity accordingly [37].
The CaCO3 gel layer on the cement particle surface has a loosened structure and then the cement particles begin to hydrate gradually. Calcium metasilicate (C-S-H) gel and CaCO3 gel which are the hydration products of cement combine on the cement particle surface into a gel layer. Some flocculent CaCO3 gel crystals penetrate the C-S-H and flocculent CaCO3 gels as needle-like whiskers, forming a skeletal network (Figure 16b).
In the late hydration stage of cement, hydration products on the cement particle surface increase significantly and the wrapper thickens accordingly. Moreover, CaCO3 crystals are wrapped in the hydration products. Meanwhile, the CaCO3 needle-like crystal whiskers are still in the paste to form an effective network structure (Figure 16c), which improves the gelling performance among cement particles.

Hydration Mechanism of Cement Paste under Ultrasonic Agitation
The hydration mechanism of cement paste under ultrasonic agitation is shown in Figure 17. When CO2 gases are supplied under ultrasonic vibration, they firstly react with Ca(OH)2 which is precipitated from the initial hydration to produce flocculent CaCO3 crystals. The solid surfaces suspended in the liquid are damaged dramatically due to the "cavitation effect" of the ultrasonic wave [38]. When an ultrasonic wave radiates and spreads throughout the paste, it will produce numerous small bubbles in the paste, which will break continuously. More than 1000 instant high pressure regions can be produced at the rupture of bubbles [39][40][41]. The rupture explosions in the series will release a considerable amount of energy to cause great impacts on the surrounding areas. On the one hand, this causes continuous impacts on the cement particle surfaces which have not hydrated completely in the cement paste to make the gel hydration products on the surface peel off quickly, thus getting new cement particles. On the other hand, CaCO3 flocculating constituents are cut effectively under the ultrasonic wave effect and the abundant "nano" crystals are produced scattered among cement particles (Figure 17a).
In the early reaction stage of CO2, the CaCO3 gel layer on the cement particle surface has a relatively loose structure. When CO2 gases are consumed completely, the cement particles begin to hydrate gradually, producing hydration products continuously including "nano" CaCO3 crystals and a certain quantity of CaCO3 whiskers. Due to the "cavitation effect" of ultrasonic vibration, the "nano" CaCO3 crystals cannot adhere to the cement particle surface. Since the surfaces of hydration products from the reaction of cement particles and water are positively or negatively charged and the crystal nucleus has some adsorption capacity, some hydration products can adhere onto "nano" CaCO3 crystal surfaces. These "nano" CaCO3 flocculating constituents are effectively filled in the spaces among the cement particles. Moreover, CaCO3 whiskers depend on flocculating constituents and form a dense networked structure (Figure 17b).

CaCO 3 needle-like crystal whiskers).
The CaCO 3 gel layer on the cement particle surface has a loosened structure and then the cement particles begin to hydrate gradually. Calcium metasilicate (C-S-H) gel and CaCO 3 gel which are the hydration products of cement combine on the cement particle surface into a gel layer. Some flocculent CaCO 3 gel crystals penetrate the C-S-H and flocculent CaCO 3 gels as needle-like whiskers, forming a skeletal network (Figure 16b).
In the late hydration stage of cement, hydration products on the cement particle surface increase significantly and the wrapper thickens accordingly. Moreover, CaCO 3 crystals are wrapped in the hydration products. Meanwhile, the CaCO 3 needle-like crystal whiskers are still in the paste to form an effective network structure (Figure 16c), which improves the gelling performance among cement particles.

Hydration Mechanism of Cement Paste under Ultrasonic Agitation
The hydration mechanism of cement paste under ultrasonic agitation is shown in Figure 17. When CO 2 gases are supplied under ultrasonic vibration, they firstly react with Ca(OH) 2 which is precipitated from the initial hydration to produce flocculent CaCO 3 crystals. The solid surfaces suspended in the liquid are damaged dramatically due to the "cavitation effect" of the ultrasonic wave [38]. When an ultrasonic wave radiates and spreads throughout the paste, it will produce numerous small bubbles in the paste, which will break continuously. More than 1000 instant high pressure regions can be produced at the rupture of bubbles [39][40][41]. The rupture explosions in the series will release a considerable amount of energy to cause great impacts on the surrounding areas. On the one hand, this causes continuous impacts on the cement particle surfaces which have not hydrated completely in the cement paste to make the gel hydration products on the surface peel off quickly, thus getting new cement particles. On the other hand, CaCO 3 flocculating constituents are cut effectively under the ultrasonic wave effect and the abundant "nano" crystals are produced scattered among cement particles (Figure 17a). surfaces increase, thus thickening the wrapper on the cement particle surface. The "nano" CaCO3 crystals become wrapped by hydration products of the cement which are produced continuously. Consequently, flocculating constituents in the spaces of cement particles become tighter. Moreover, the microstructure of the cement after hydration seems to have better gelling performances and a more compact structure than that under mechanical agitation since the CaCO3 needle-like crystal whiskers interweave between the cement particles ( Figure 17c).

Conclusions
The results of our previous investigation of CO2 absorption performance of fresh cement paste indicate that the CO2 absorption rate and the ultimate absorption amount of fresh cement paste can effectively be increased. However, the mixing process of concrete is relatively very short in practice. Therefore, the rapid and significant CO2 absorption by concrete cannot be realized only by reasonably setting the mechanical mixing rate and water-cement ratio.
As a consequence, a new method of ultrasonic agitation is applied to facilitate the CO2 absorption performance of cement paste in this investigation. Meanwhile, the CO2 absorption rate and the ultimate absorption amount of cement paste under ultrasonic agitation is thoroughly studied. The results indicate that by applying ultrasonic agitation, the CO2 absorption rate and the ultimate absorption amount of cement paste can be effectively increased. Once the carbonized cement paste is hardened, the hardened cement paste increases the compressive strength and decreases the porosity and pore sizes, indicating the refined pore structure of the hardened cement paste.
Due to the limited investigation on CO2 absorption of cement paste, there are few literatures reporting about it. Moreover, due to the limitation of the experimental device, only some critical problems are studied in this investigation regarding the CO2 absorption of cement paste. However, there are still many problems that require thorough investigations.   The hydration process of cement paste without CO2 abs 15. Clearly, a gel film that is composed of calcium silicate h cium hydroxide (CH) crystals is formed on the surface of ce hydration stage. With the continuous increase of hydration day by day due to the increasing gels. Meanwhile, the ceme until finishing the hydration of the cement. The hydration process of cement paste after CO2 absorp tation is shown in Figure 16. It can be seen that under mech scatter uniformly in the cement paste and firstly dissolve into Ca(OH)2 which is precipitated from the initial hydration of ce CaCO3 crystals which adhere to the surface of cement particle Cement particle; ticle surface into a gel layer. Some flocculent CaCO flocculent CaCO3 gels as needle-like whiskers, form In the late hydration stage of cement, hydrat surface increase significantly and the wrapper thi crystals are wrapped in the hydration products. crystal whiskers are still in the paste to form an eff which improves the gelling performance among cem Figure 16. Hydration mechanism of fresh cement paste tion: (a) Carbonization reaction stage of cement paste afte cement paste after CO2 supply; (c) late hydration stage Cement particle; the supplied CO2 gases; CaCO3 crystals; CaCO3 needle-like crystal whi

Hydration Mechanism of Cement Paste under Ultra
The hydration mechanism of cement paste un Figure 17. When CO2 gases are supplied under ultra Ca(OH)2 which is precipitated from the initial hy crystals. The solid surfaces suspended in the liquid "cavitation effect" of the ultrasonic wave [38]. W spreads throughout the paste, it will produce nu which will break continuously. More than 1000 i produced at the rupture of bubbles [39][40][41]. The ru lease a considerable amount of energy to cause gre On the one hand, this causes continuous impacts o have not hydrated completely in the cement paste t the surface peel off quickly, thus getting new cemen flocculating constituents are cut effectively under abundant "nano" crystals are produced scattered am In the early reaction stage of CO2, the CaCO3 g has a relatively loose structure. When CO2 gases a particles begin to hydrate gradually, producing cluding "nano" CaCO3 crystals and a certain qua "cavitation effect" of ultrasonic vibration, the "nano cement particle surface. Since the surfaces of hyd cement particles and water are positively or negati has some adsorption capacity, some hydration pro crystal surfaces. These "nano" CaCO3 flocculating c spaces among the cement particles. Moreover, CaC constituents and form a dense networked structure the supplied CO 2 gases;

Cement Paste without CO2 Absorption
The hydration process of cement paste without CO2 absorption is shown in Figure  15. Clearly, a gel film that is composed of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) crystals is formed on the surface of cement particles in the early hydration stage. With the continuous increase of hydration time, the gel film thickens day by day due to the increasing gels. Meanwhile, the cement begins to harden slowly until finishing the hydration of the cement.

Cement Paste after CO2 Absorption under Mechanical Agitation
The hydration process of cement paste after CO2 absorption under mechanical agitation is shown in Figure 16. It can be seen that under mechanical agitation, CO2 gases scatter uniformly in the cement paste and firstly dissolve into H2CO3. H2CO3 reacts with Ca(OH)2 which is precipitated from the initial hydration of cement to produce flocculent CaCO3 crystals which adhere to the surface of cement particles (Figure 16a). With the in-flocculent gel hydration products; ticle surface into a gel layer. Some flocculent CaCO3 gel flocculent CaCO3 gels as needle-like whiskers, forming In the late hydration stage of cement, hydration surface increase significantly and the wrapper thicken crystals are wrapped in the hydration products. Me crystal whiskers are still in the paste to form an effectiv which improves the gelling performance among cemen Figure 16. Hydration mechanism of fresh cement paste after tion: (a) Carbonization reaction stage of cement paste after CO cement paste after CO2 supply; (c) late hydration stage of ce Cement particle; the supplied CO2 gases; CaCO3 crystals; CaCO3 needle-like crystal whisker

Hydration Mechanism of Cement Paste under Ultrasoni
The hydration mechanism of cement paste under Figure 17. When CO2 gases are supplied under ultrason Ca(OH)2 which is precipitated from the initial hydrat crystals. The solid surfaces suspended in the liquid are "cavitation effect" of the ultrasonic wave [38]. When spreads throughout the paste, it will produce numer which will break continuously. More than 1000 insta produced at the rupture of bubbles [39][40][41]. The ruptur lease a considerable amount of energy to cause great im On the one hand, this causes continuous impacts on th have not hydrated completely in the cement paste to ma the surface peel off quickly, thus getting new cement pa flocculating constituents are cut effectively under the abundant "nano" crystals are produced scattered amon In the early reaction stage of CO2, the CaCO3 gel la has a relatively loose structure. When CO2 gases are c particles begin to hydrate gradually, producing hydr cluding "nano" CaCO3 crystals and a certain quantity "cavitation effect" of ultrasonic vibration, the "nano" Ca cement particle surface. Since the surfaces of hydratio cement particles and water are positively or negatively has some adsorption capacity, some hydration product crystal surfaces. These "nano" CaCO3 flocculating const spaces among the cement particles. Moreover, CaCO3 constituents and form a dense networked structure (Fig ticle surface into a gel layer. Some flocculent CaCO3 gel flocculent CaCO3 gels as needle-like whiskers, forming a In the late hydration stage of cement, hydration surface increase significantly and the wrapper thicken crystals are wrapped in the hydration products. Me crystal whiskers are still in the paste to form an effectiv which improves the gelling performance among cement Figure 16. Hydration mechanism of fresh cement paste after tion: (a) Carbonization reaction stage of cement paste after CO cement paste after CO2 supply; (c) late hydration stage of ce Cement particle; the supplied CO2 gases; CaCO3 crystals; CaCO3 needle-like crystal whiskers

Hydration Mechanism of Cement Paste under Ultrasonic
The hydration mechanism of cement paste under Figure 17. When CO2 gases are supplied under ultrasoni Ca(OH)2 which is precipitated from the initial hydrati crystals. The solid surfaces suspended in the liquid are "cavitation effect" of the ultrasonic wave [38]. When spreads throughout the paste, it will produce numer which will break continuously. More than 1000 insta produced at the rupture of bubbles [39][40][41]. The ruptur lease a considerable amount of energy to cause great im On the one hand, this causes continuous impacts on th have not hydrated completely in the cement paste to ma the surface peel off quickly, thus getting new cement pa flocculating constituents are cut effectively under the abundant "nano" crystals are produced scattered among In the early reaction stage of CO2, the CaCO3 gel lay has a relatively loose structure. When CO2 gases are co particles begin to hydrate gradually, producing hydr cluding "nano" CaCO3 crystals and a certain quantity "cavitation effect" of ultrasonic vibration, the "nano" Ca cement particle surface. Since the surfaces of hydratio cement particles and water are positively or negatively has some adsorption capacity, some hydration product crystal surfaces. These "nano" CaCO3 flocculating const spaces among the cement particles. Moreover, CaCO3 constituents and form a dense networked structure ( Fig   CaCO 3 needle-like crystal whiskers).
In the early reaction stage of CO 2 , the CaCO 3 gel layer on the cement particle surface has a relatively loose structure. When CO 2 gases are consumed completely, the cement particles begin to hydrate gradually, producing hydration products continuously including "nano" CaCO 3 crystals and a certain quantity of CaCO 3 whiskers. Due to the "cavitation effect" of ultrasonic vibration, the "nano" CaCO 3 crystals cannot adhere to the cement particle surface. Since the surfaces of hydration products from the reaction of cement particles and water are positively or negatively charged and the crystal nucleus has some adsorption capacity, some hydration products can adhere onto "nano" CaCO 3 crystal surfaces. These "nano" CaCO 3 flocculating constituents are effectively filled in the spaces among the cement particles. Moreover, CaCO 3 whiskers depend on flocculating constituents and form a dense networked structure (Figure 17b).
In the late hydration stage of the cement, hydration products on the cement particle surfaces increase, thus thickening the wrapper on the cement particle surface. The "nano" CaCO 3 crystals become wrapped by hydration products of the cement which are produced continuously. Consequently, flocculating constituents in the spaces of cement particles become tighter. Moreover, the microstructure of the cement after hydration seems to have better gelling performances and a more compact structure than that under mechanical agitation since the CaCO 3 needle-like crystal whiskers interweave between the cement particles ( Figure 17c).

Conclusions
The results of our previous investigation of CO 2 absorption performance of fresh cement paste indicate that the CO 2 absorption rate and the ultimate absorption amount of fresh cement paste can effectively be increased. However, the mixing process of concrete is relatively very short in practice. Therefore, the rapid and significant CO 2 absorption by concrete cannot be realized only by reasonably setting the mechanical mixing rate and water-cement ratio.
As a consequence, a new method of ultrasonic agitation is applied to facilitate the CO 2 absorption performance of cement paste in this investigation. Meanwhile, the CO 2 absorption rate and the ultimate absorption amount of cement paste under ultrasonic agitation is thoroughly studied. The results indicate that by applying ultrasonic agitation, the CO 2 absorption rate and the ultimate absorption amount of cement paste can be effectively increased. Once the carbonized cement paste is hardened, the hardened cement paste increases the compressive strength and decreases the porosity and pore sizes, indicating the refined pore structure of the hardened cement paste.
Due to the limited investigation on CO 2 absorption of cement paste, there are few literatures reporting about it. Moreover, due to the limitation of the experimental device, only some critical problems are studied in this investigation regarding the CO 2 absorption of cement paste. However, there are still many problems that require thorough investigations.