Influence of nano-SiO2 and carbonation on the performance of natural hydraulic lime mortars

Construction and Building Materials 235 (2020) 117411

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Influence of nano-SiO2 and carbonation on the performance of natural hydraulic lime mortars Kai Luo a, Jun Li a,⇑, Qing Han a, Zhongyuan Lu a,*, Xin Deng a, Li Hou a, Yunhui Niu a, Jun Jiang a, Xiaoying Xu b, Pan Cai b a State Key Laboratory of Environment-friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China b Sichuan Esheng Cement Group, Leshan 614222, China

h i g h l i g h t s
In NHL2 mortars carbonation percentage would increase with the increased NS.
NS doped NHL2 mortars can capture and utilize much more CO2.
The capture and solidification of CO2 could also improve the mechanical properties.
NS and accelerated carbonation curing can reduce the pore diameter of NHL2 mortars.

a r t i c l e

i n f o

Article history: Received 7 August 2019 Received in revised form 24 October 2019 Accepted 26 October 2019

Keywords: Natural hydraulic lime mortars Carbonation Nano-SiO2 Properties

a b s t r a c t As an ideal cementitious material for the maintenance of historic buildings and the replacement of interior decoration mortars, the widespread application of natural hydraulic lime (NHL) is expected to play an important role in CO2 capture and utilization. Structures and properties of NHL2 mortars with nano-SiO2 (NS) under carbonation were researched in this paper. NHL mortars could capture CO2 during hardening process of air-hardening contents, and the captured CO2 would be fixed in the matrix to form CaCO3. Carbonation percentage and extent of NHL2 mortars increased by the addition of NS whatever in atmospheric or accelerated carbonation condition. Much more compacted carbonation surfaces than that of the uncarbonated region for NHL2 mortars with NS was observed. The addition of NS will reduce the porosity and big pores (approximately 1 lm) while increase the mesopores (approximately 50 nm) of the mortars. Carbonation reaction of NHL2 mortars seemed to be improved by the addition of NS, which would be beneficial to the collection, solidification and further utilization of CO2. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Capture, collection, solidification and utilization of CO2 has attracted many more interests due to the urgent requirement of reducing greenhouse gas emissions [1–4]. Cementitious materials, including ordinary Portland cement (OPC) [5–7], lime [3], magnesia [8–10], fly ash [11,12], EAF slag [13], incinerator bottom ash [14] and steel slag [5,15], is all known to capture and immobilize CO2 through long-term carbonation in the air. However, durability of cement based materials (especially the concrete) would be degraded under carbonation [3,16]. The main hydration products of OPC (C-S-H gels) are believed to be decalcified during long-

⇑ Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (Z. Lu). https://doi.org/10.1016/j.conbuildmat.2019.117411 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

term exposure to atmospheric CO2 and gradually decomposed over time [3]. Moreover, carbonation could also neutralize the alkaline environment provided by OPC hydration (pH of the pore solution is >13) and increase the corrosion harm of the embedded steel. In recent years, researchers [17,18] found that carbonation curing could improve the mechanical properties of precast concrete (without reinforcing bars), CO2-curing concrete technology and high-carbonation cement clinker were thus developed [19,20]. However, carbonation curing conditions are relatively complicated for the preparation of precast concrete [21]. Lime based materials had better water and air permeability due to its porous and loose structures, which would bring much higher carbonization efficiency than that of conventional CO2 capture materials. It is much more interesting that the captured CO2 can be fixed in a matrix to form calcites and thus improve the properties of the lime based materials.

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K. Luo et al. / Construction and Building Materials 235 (2020) 117411

In fact, lime has been the most widely used inorganic binder in ancient times before the invention of Portland cement. Mortars or concretes constructed by limes in ancient Rome was found to have increased bonding strength after slowly reacting with atmospheric CO2. However, it can not meet the modern cultural relics restoration construction engineering requirement due to the lower water resistance and slower air-hardening reaction of lime [22]. Both hydraulic (dicalcium silicate, C2S) and air-hardening contents (Ca (OH)2) are existed in natural hydraulic lime (NHL), which has been used in masonry buildings up to the beginning of the 20th Century [23–25]. Mechanical properties and water resistance of the NHL could be improved due to the presence of C2S compared to that of air lime. According to European standard [26], there are high contents of Ca(OH)2 (>35 wt% for NHL2, >25 wt% for NHL3.5 and >15 wt% for NHL5) existed in NHL. And thus CO2 is the indispensable reactant for NHL hardening as depicting in Eq. (1),

CO2 þ CaðOHÞ2 ! CaCO3 þ H2 O

ð1Þ Fig. 1. X-ray diffraction pattern of NHL2.

Therefore, NHL is considered to be an ideal candidate for the CO2 capture, collection, solidification and utilization compare to that of OPC or lime [27]. However, capture and fixation efficiency of CO2 is also relatively lower in the early curing stage of NHL [28,29]. Our previous studies [30] revealed nano-SiO2 (NS) could improve the early mechanical properties increase the carbonation percentage of NHL mortars. Up to date, there is a little report refers to the effect of NS on the properties of NHL. In this study, structures and properties of NHL2 mortars with NS under carbonation were further researched. 2. Experimental program 2.1. Materials According to European standard [26], the chemical composition of NHL2 was shown in Table 1, mineral compositions shown in Fig. 1, the mainly mineral compositions of NHL2 are Ca(OH)2, SiO2, CaCO3, C4AF, Mayenite, Periclase and b-C2S, and it comes from Shanghai Desaibao Building Materials Co., Ltd.. The specific surface area and particle sizes of NS as 200 m2/g and 7–40 nm, and it Aladdin Industrial Corporation. The sand comes from Xiamen ISO standard sand Co., Ltd. The cumulative screen residue was (2000 lm, 0%), (1600 lm, 7%), (1000 lm, 33%), (500 lm, 67%), (160 lm, 87%) and (80 lm, 100%).

70 ± 5% RH. At the same time, the sample is blown by an electric fan to keep the air flowing.) 2 h and continued to cure to 3, 7 and 28 days. Accelerated carbonation curing, mortars were cured in carbonized curing box (which comes from JianYanHuaCe (Beijing) Instrument & Equipment Co., Ltd.) at 20 ± 3 °C, 70 ± 5% RH and CO2 concentration of 4 ± 1% for 1 day, then demoulded and continued to cure to 3, 7and 28 days in this condition. 2.3. Carbonation front test To observe the depth of carbonation, mortars after atmospheric curing and accelerated carbonation curing to a certain age (3 days, 7 days and 28 days) were divided into two parts by horizontal break, then 1% phenolphthalein alcohol solution was sprayed on the corresponding section. The carbonation percentage can be estimated by calculating the proportion of the colored area according to Eq. (2).

P

R¼

C Sc  100% St

ð2Þ

where R is carbonation percentage, Sc is carbonized area and St is the total area of fracture surface (1600 mm2). In order to reduce test errors in compositions and structures of different mortars, tested samples of different mortars were collected according to the schematic shown in Fig. 3.

2.2. Preparation 2.4. Mechanical properties test Table 2 shows the proportion of the NHL2 mortars with NS. Fig. 2 depicts the steps of NS dissolution and dispersion with mechanical agitation for 30 min and ultrasonically for 1 h. Then mix according to the order of adding water, NHL2 and sand, and molded in 40  40  160 mm3 steel molds. Inside, the waterbinder (NHL2 + NS) ratio (W/B) is 0.6 (determined by the fluidity (200 ± 5 mm)), and the ratio of NHL2 to sand is 1:3. Two curing methods, atmospheric curing (which has the same CO2 concentration compared to natural environmental carbonation.) and accelerated carbonation curing were adopted, respectively. Atmospheric curing (conditions: the temperature at 20 ± 3 °C, humidity at 70 ± 5% RH.), then demoulded (curing 1 day) to precuring [3] (conditions: the temperature at 20 ± 3 °C, humidity at

The SANS CMT5105 testing machine (Shenzhen, China) was carried out to evaluate the mechanical properties of the NHL2 mortars with NS, with a loading rate of 2400 ± 200 N/s. 2.5. Micro performance test AutoPoreIV 9500 Mercury intrusion poremeasurement (MIP) was used to represent pore structure of mortars. X-ray diffraction (XRD) was used to determined the mineral composition of the NHL2 product, using a D/max 1400 diffractometer (Rigaku Corporation, Japan), with a Cu-Ka radiation, the diffractometer operates an integrated scanning step of 0.02° in continuous mode and

Table 1 Chemical composition of NHL2 (wt%). Elements

SiO2

CaO

Fe2O3

MgO

Al2O3

NHL2

9.38

78.65

1.96

4.49

3.15

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K. Luo et al. / Construction and Building Materials 235 (2020) 117411 Table 2 Mix design of the NHL2 mortars.

a

IDa

NS(g)

NHL2(g)

Sand(g)

Water(g)

w/b

Fluidity(mm)

A0 A1 A2 A3 C0 C1 C2 C3

0 4.5 9.0 13.5 0 4.5 9.0 13.5

450 450 450 450 450 450 450 450

1350

270 272 275 278 270 272 275 278

0.6

201 200 198 195 200 199 199 196

Note: A – atmospheric curing, C – accelerated carbonation curing.

Fig. 2. Schematic diagram of the NS solution preparation.

Fig. 4 are listed in Table 3. After 3 days accelerated carbonation curing, only a few carbonations occurred in sample C3 (Fig. 4(a)), and the corresponding carbonation percentage was 8.8% (Table 3). After 7 days accelerated carbonation curing, colored area of C0 remained unchanged. Colored areas in sample C1, sample C2 and sample C3 (Fig. 4(b)) were continuously reduced, and their carbonation percentage reached to 7.5%, 19.3% and 52.9%, respectively. As shown in Fig. 4(c), carbonation depths further increased with the prolonged accelerated cabonation curing ages, and the carbonation percentage for sample C0-C4 reached to 60.9%, 82.8%, 97.4% and 98.4%, respectively. As shown in the Fig. 4(d), no obvious carbonation traces were observed in the sample section after 28 days atmospheric curing. Based on above, carbonation depths and carbonation percentage would all increase with the increased contents of NS (0% to 3%) under carbonation condition, which indicated that the NHL2 mortars with NS were much more suitable for the collection and solidification of CO2. 3.2. Pore size distributions Fig. 3. Schematic of sampling positions.

1 sstep1 sweep from 10° to 80° 2h. Thermogravimetric (TG) was carried out, using Jupiter STA449C thermoanalyser (Netzsch, Germany) in alumina crucibles, at 10 °C/min heating rate inan flowing N2 (50 mL/min) atmosphere, from 25 °C to 1000 °C. Scanning electron microscope (SEM) was carried out to observe the microstructure of the sample cross-section (The samples were sprayed with gold before the test) and the corresponding EDS, using UItra55 digital scanning microscope (Carlzeiss NTS GmbH, Germany). 3. Results and discussions 3.1. Carbonation front observations Carbonation depths of different NHL2 mortars with NS are shown in Fig. 4, and the calculated carbonation percentage through

Table 4 and Fig. 5 shows that pore size distributions of NHL2 mortars with NS curing in atmospheric and accelerated carbonation condition for different ages (3, 7 and 28 days). Pore size distributions (Fig. 5) of the mortars significantly changed as the number of NS increases continuously. Pore size of the mortars curing in atmospheric environment is mainly around 1000 nm and its content would be reduced as the number of NS increases continuously to each curing age. Meanwhile, porosity of samples decreases to the increase in curing ages and NS, which is consistent with our previous research results [30]. As shown in Table 4, at the same curing age, with the increase of Ns content from 0% to 3%, the number of air voids (above 10000 nm) and macropores (50–10000 nm) decreased and the number of mesopores (2–50 nm) increased. In atmospheric, with the increase of Ns content from 0% to 3%, air voids decreased from 9.2% to 8.7%, 6.8% and 4.7%, macropores gradually decreased from 87.2% to 83.0%, 73.1% and 68.0%, while meso-

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K. Luo et al. / Construction and Building Materials 235 (2020) 117411

(a) Accelerated carbonation curing for 3 days

(b) Accelerated carbonation curing for 7 days

(c) Accelerated carbonation curing for 28 days

(d) Atmospheric curing 28 days Fig. 4. Carbonation fronts of the mortars.

Table 3 Carbonation percentage of NHL2 mortars with NS. Curing age

C0(%)

C1(%)

C2(%)

C3(%)

3 days 7 days 28 days

0 0 60.9

0 7.5 82.8

0 19.3 97.4

8.8 52.9 98.4

pores gradually increased from 3.6% to 8.3%, 20.1% and 27.3% in 28 days curing, respectively. The same trend were obtained for mortars curing in accelerated carbonation condition. Macropores around 1000 nm decreased as the number of NS increases continuously, but macropores further decreased compared to that of mortars curing in atmospheric environment at the same age. At 28 days, Air voids of C0-C3 gradually decreased from 8.7% to

5.9%, 5.6% and 4.5%, macropores gradually decreased from 86.4% to 83.3%, 71.1% and 67.9% while mesopores increased from 4.9% to 10.9%, 23.3% and 27.6%, respectively. Preliminary, the change of pores is mainly due to the filling of NS. Subsequently, the distribution of pores was further changed with the formation of C-S-H through C2S hydration and pozzolanic reaction and the product of carbonation. Which is the main reason for the increase in mesopores, with the prolongation of curing time and the increase of NS [31,32]. The reduction of air voids and macropores makes the mortars much more compact. On the one hand, the increased mesopores could increase the specific surface areas. Thus, CO2 adsorption efficiency also increases [33]. On the other hand, large amount of gases containing CO2 was dissolved in the pore solution to the mortars under carbonation, the as-

5

K. Luo et al. / Construction and Building Materials 235 (2020) 117411 Table 4 Detailed pores size distribution data collected through Fig. 5. Sample

NS (%)

Curing age (Days)

Porosity (%)

Air voids (%)

Macropores (%)

Mesopores (%)

A0

0

A1

1

A2

2

A3

3

C0

0

C1

1

C2

2

C3

3

3 7 28 3 7 28 3 7 28 3 7 28 3 7 28 3 7 28 3 7 28 3 7 28

28.7 28.0 27.1 27.8 27.2 25.9 26.9 26.3 25.1 26.4 25.5 24.7 27.5 25.6 22.4 26.1 24.8 21.2 25.8 23.6 20.1 24.1 22.1 19.2

11.6 10.8 9.2 9.2 8.7 8.7 8.1 7.2 6.8 6.9 5.8 4.7 11.2 10.3 8.7 8.0 6.8 5.9 6.6 6.3 5.6 6.0 5.5 4.5

86.0 86.6 87.2 83.1 83.0 83.0 74.2 73.9 73.1 69.3 69.3 68.0 84.5 85.2 86.4 82.9 83.1 83.3 74.4 72.0 71.1 69.4 68.2 67.9

2.4 2.6 3.6 7.7 8.3 8.3 17.7 19.0 20.1 23.8 24.9 27.3 4.3 4.5 4.9 9.1 10.1 10.9 19.0 21.7 23.3 24.6 26.3 27.6

formed CaCO3 could fill the pores and increased the compactness of the mortars [34,35]. 3.3. Mineral compositions Mineral compositions of the NHL2 mortars with NS curing at 28 days are shown in Fig. 6. The mainly mineral compositions of A0-A3 are Ca(OH)2, SiO2, CaCO3 and C2S. Characteristic diffraction peaks at Ca(OH)2 and C2S simultaneously decreased as the number of NS increases continuously. It seems that NS could accelerate both hydration and carbonation reaction. Mortars curing in accelerating carbonation are mainly composed of CaCO3, Ca(OH)2, SiO2 and C2S. Little characteristic diffraction peaks of Ca(OH)2 are observed in C3, CaCO3 would be the main products in this case. XRD tests can only qualitatively describe mineral composition and simply observe relative content changes of different phases. TG can detect and calculate the mineral content based on the weight loss characteristics of reaction products of cement-based materials [36,37]. TG curves of NHL2 mortars with NS after 28 days curing are shown in Fig. 7, four mass loss stages including evaporation of adsorbed water below 100 °C, C-S-H gel is dehydrated at the temperature of 100–400 °C, Ca(OH)2 dehydroxylation at the temperature of 400–570 °C, and the CaCO3 is decomposed into CO2 and CaO at the temperature of between 570 and 800 °C [30,36,37]. In NHL2, the mineral compositions of Ca(OH)2 is 41.75%, CaCO3 is 8.07%. The content of main reaction products of NHL mortars calculated through TG tests and the relative mineral compositions change of Ca(OH)2 and CaCO3 compared to that of NHL2 are listed in Tables 5 and 6, respectively. C-S-H and CaCO3 in A0-A3 increases while Ca(OH)2 decreases with the increase of NS, which indicated that NS can react with Ca(OH)2 to produce additional C-S-H [30] and promote the cabonation reaction when curing in atmospheic condition. C-S-H and CaCO3 would also increase while Ca(OH)2 decrease with the increase of NS under accelerated carbonation conditions. However, C-S-H contents in sample A0-A3 (A0 4.52%, A1 4.74%, A2 4.87% and A3 5.35%) are slightly higher than that of in sample C0-C3 (C0 3.48%, C1 4.59%, C2 4.25% and C3 4.37%) as shown in Table.5.

According to Eq.(3), the proportion of CaCO3 produced by Ca (OH)2 with mass ratio of 1% is 1.35%. Therefore, the increment of CaCO3 should be C0 37.22%, C1 41.97%, C2 42.17%, C3 44.20%. Compared with Table 6 (C1 44.59%, C2 46.59%, C3 50.41%), additional CaCO3 was found. As reported [38], C2S and C-S-H can all react with CO2 according to Eqs. (4) and (5), respectively, in accelerated carbonation condition.

CaðOHÞ2 þ CO2 ! CaCO3 þ H2 O

ð3Þ

2ð2CaO  SiO2 Þ þ ð2  xÞCO2 þ yH2 O ! xCaO  SiO2  yH2 O þ ð2  xÞCaCO3 C  S  H þ CO2 ! CaCO3 þ SiO2 þ H2 O

ð4Þ ð5Þ

As shown in Table 6, consumption of Ca(OH)2 in accelerated carbonation condition (C0 27.54%, C1 31.06%, C2 31.21% and C3 32.71%) is significantly higher than that of in atmospheric enviroment (A0 5.26%, A1 6.80%, A2 9.16% and A3 11.42%). Correspondingly, CaCO3 contents in sample C0-C3 are also significantly higher than that of in sample A0-A3. In combination with Tables 5 and 6, it is further demonstrated that the addition of NS contributes to the improvement of the carbonation reaction [33], which is attributed to the improved CO2 adsorption by the increased mesopores (Fig. 5 and Table 4). And accelerated carbonation helps in much higher carbonation than addition of NS (1% to 3%). As shown in Table 7, in atmospheric conditions, the effect of NS on hydration and carbonation of HHL mortars is almost the same, when NS content is 1%. However, the promotion of carbonation by NS is significantly higher than that of hydration with the increased contents of NS (2% to 3%). IN accelerated carbonation conditions, the extent influence of NS on carbonation is much higher than hydration, and the competition is more significant with the increased contents of NS (1% to 3%). It is known that the mass loss occurred at 570–800 °C is mainly the release of CO2 [39]. The content of CO2 utilization in NS mortars after 28 days curing can be estimated by TG curves (Fig. 7) according to Eq.(6).

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K. Luo et al. / Construction and Building Materials 235 (2020) 117411

Fig. 5. Pore sizes distributions of the mortars: (a) A0-A3 for 3 days, (b) C0-C3 for 3 days, (c) A0-A3 for 7 days, (d) C0-C3 for 7 days, (e) A0-A3 for 28 days, (f) C0-C3 for 28 days.

  M ¼ P sample  P NHL2  1000

ð6Þ

where M is the mass of captured CO2 (g/(kg cementitious)), Psample is the percentage change of the sample between 570 °C and 800 °C, PNHL2 is the percentage change of the NHL2 between 570 °C and 800 °C, 1000 is the mass of cementitious (NHL + NS) unit is g. As shown in Table 8, the mass of captured CO2 in NHL mortars all increases with the increase of NS (0% to 3%) whatever in atmospheric (5.5, 6.5, 12.6, 15.6 g/(kg cementitious)) or accelerated carbonation (159.6, 196.5, 205.3, 222.1 g/(kg cementitious)) condition

after 28 days curing, which indicates the carbonation percentage and extent could be increased by the addition of NS. In addition, the mass of captured CO2 in NHL mortars curing in accelerated carbonation condition is much higher that of curing in atmospheric environment, compared A0 with C0, A1 with C1, A2 with C2, and A3 with C3. And accelerated carbonation helps in much higher carbonation than addition of NS (0% to 3%). If 3%-NS-NHL2 used for historical building restoration and interior decoration is 1 million tons/year in the world, 222,000 tons of CO2 will be captured and

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K. Luo et al. / Construction and Building Materials 235 (2020) 117411

Table 7 The extent of influence of NS on hydration and carbonation compared with A0 and C0.

Fig. 6. Mineral compositions of NHL2 mortars curing for 28 days.

Sample

C-S-H (%)

CaCO3 (%)

A1 A2 A3 C1 C2 C3

0.22 0.35 0.83 1.11 0.77 0.89

0.23 1.62 2.30 8.37 10.37 14.19

increase in NS (0%-3%) are observed, which are consistent with the MIP results. Morphology of the samples curing in accelerated carbonation condition for 28 days is shown in Fig. 9. There are small amount of flakes (2), gelatinous and granular products (1) in sample C0. EDS pattern of the marked aeration Fig. 9 and the corresponding datas are shown in Fig. 10 and Table 9, respectively. Flake-like (Ca/O molar ratio is 1/2) and granular products (Ca/O molar ratio is 1/3 could be identified as Ca(OH)2 and CaCO3, respectively. Gelatinous products containing Ca, Si and Al could be identified as C-S-H and C-A-S-H. After 28 days of curing, the flakes of C1, C2 and C3 decreased, while the products of gel (3), granular, strip (4) and petal (5) increased gradually (Fig. 9). As shown in Fig. 10 and Table 9, a strip and petal shape products are also identified as CaCO3. Obviously, CaCO3 increases while Ca(OH)2 decreases from the increase in NS under accelerated carbonation conditions. EDS data were consistent with TG results. 3.5. Mechanical properties

Fig. 7. TG curves of the samples after 28 days curing.

Table 5 Mineral compositions of NHL mortars calculated through TG tests. Sample

C-S-H (%)

Ca(OH)2 (%)

CaCO3 (%)

A0 A1 A2 A3 C0 C1 C2 C3

4.52 4.74 4.87 5.35 3.48 4.59 4.25 4.37

36.49 34.95 32.59 30.33 14.21 10.69 10.54 9.04

9.30 9.53 10.92 11.60 44.29 52.66 54.66 58.48

utilized every year. NHL2 mortars with NS can capture and utilize much more CO2, compared with other building materials [3,39].

Mechanical properties of NHL2 mortars with NS curing in atmospheric and accelerated carbonation conditions are shown in Fig. 11. The addition of NS could increase the compressive strength of NHL2 mortars whatever in accelerated carbonation condition or in atmospheric environment, which was consistent with our previous research results [30]. NS with high pozzolanic activity could consume Ca(OH)2 and form enough C-S-H, thus increase the compressive strength of mortars curing in atmospheric environment. On the other hand, NS can fill the gaps between particles, which would enhance CO2 adsorption efficiency through the increased content of mesopores. Thus compressive strength growth rates of mortars curing in accelerating carbonation with curing ages and NS content (0%–3%) are also higher than that of curing in atmospheric environment, such as A0 (2.3 MPa) and C0 (8.2 MPa), A1 (2.5 MPa) and C1 (8.9 MPa), A2 (3.7 MPa) and C2 (14.3 MPa), A3 (4.6 MPa) and C3 (19.2 Mpa) after 28 days curing. In addition, large amount of CO2 was dissolved in the pore solution to the mortars under accelerated carbonation condition, which would also to accelerate the carbonation reaction. The consumption of Ca(OH)2 and formation of CaCO3 filled the pores again and increased the compactness of the mortars. Consequently, compressive strength of mortars curing in accelerating carbonation is higher than that

Table 8 Mass of captured CO2 (g/(kg cementitious)).

3.4. Microstructures Morphology of the samples curing in atmospheric condition for 28 days is shown in Fig. 8. Much more compact structures of the

Phase

A0

A1

A2

A3

C0

C1

C2

C3

CO2

5.5

6.5

12.6

15.6

159.6

196.5

205.3

222.1

Table 6 Changes of Ca(OH)2 and CaCO3 content. Phase

NHL2(%)

A0(%)

A1(%)

A2(%)

A3(%)

C0(%)

C1(%)

C2(%)

C3(%)

Ca(OH)2 CaCO3

41.75 8.07

5.26 +1.23

6.80 +1.46

9.16 +2.85

11.42 +3.53

27.54 +36.22

31.06 +44.59

31.21 +46.59

–32.71 +50.41

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K. Luo et al. / Construction and Building Materials 235 (2020) 117411

Fig. 8. SEM images of mortars curing in atmospheric environment for 28 days.

Fig. 9. SEM images of mortars curing in accelerated carbonation condition for 28 days.

K. Luo et al. / Construction and Building Materials 235 (2020) 117411

9

O

Si Ca C Mg

Al Ca

5 4 3 2 1

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Fig. 11. Compressive strength of NHL2 mortars with NS.

Energy (keV) Fig. 10. EDS pattern of red areas in SEM images. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 9 EDS datas of red areas in SEM images. Area

1 2 3 4 5

Element, Atomic % C

O

Mg

Al

Si

Ca

10.45 0.57 0.58 10.91 10.84

65.96 65.00 54.04 65.40 65.72

0.00 0.30 0.40 0.13 0.50

0.00 0.35 5.27 0.17 0.41

0.98 1.30 14.34 1.86 1.40

22.62 32.48 25.36 21.53 21.13

of mortars curing in atmospheric environment at the same NS dosage. 4. Conclusion (1) Carbonation depths and rate of NHL mortars would all increase with the increased contents of NS (0% to 3%) whatever in atmospheric or accelerated carbonation conditions. (2) The addition of NS will reduce the porosity and big pores while increase the proportion of mesopores and increase the compactness of NHL mortars for young ages. (3) NHL2 mortars with NS can capture and utilize much more CO2, specially in accelerated carbonation condition. And accelerated carbonation helps in much higher carbonation than addition of NS (1% to 3%). (4) The capture and solidification of CO2 could also improve the mechanical properties of NHL2 mortars with NS after 3, 7 and 28 days curing.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Sichuan Science and Technology Program (No. 19ZDZX0096, No. 2018GZ0152), and the National Natural Science Fund [grant number 51402246]

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