Carbonation Resistance of Cement-Based Materials Improved by Nitrite

. Carbonation resistance ability is one of the most important durability-related proprieties of cement-based materials. Trough the carbonation depth experiment, isothermal conduction calorimetry, XRD, BET, and water vapor sorption, the efect of calcium nitrite (Ca(NO 3 ) 2 ) on the carbonation properties of cement-based materials is obtained. Te result indicates that the addition of Ca(NO 3 ) 2 improves the carbonation resistance property of cement-based materials if the hydration of cement pastes and microstructure is modifed earlier without afecting the late hydration process. In addition, the refned pores and higher tortuosity cut down the channels, thereby impeding the ingress of carbon dioxide gas into cementitious materials, as confrmed by BETand water vapor sorption. Te Ca(NO 3 ) 2 exhibits high performance in improving the carbonation resistance and extending the life of strengthened concrete.


Introduction
Concrete, being a primary construction material worldwide, has garnered signifcant academic attention regarding its structural durability [1].Various factors in diferent environments infuence the service life, thus afecting the durability of concrete [2].Carbon dioxide (CO 2 ) erosion is one such crucial factor.Within the realm of cementitious materials, the pivotal signifcance of carbonation encompasses its direct infuence on structural sustainability, given its potential to dismantle the passivating flm on reinforcement, thereby instigating corrosion [3][4][5].In the context of cementitious materials, external carbon dioxide intrudes and initiates a chemical reaction with calcium hydroxide and available water, leading to a discernible diminution of pH levels within the system, specifcally descending from approximately 12.5 to below 10.Te oxidation flm of steel can be destroyed because of the defciency of alkalinity, which increases the possibility of steel bar corrosion [6].Meanwhile, other hydration products, including C-S-H gel and AFt crystals, are also prone to react with carbon dioxide [6].Te cohesion of hydration products decreases after carbonation, which leads to macroscopically visible decreased mechanical strength and increased porosity [7].
Te negative impact of carbon dioxide erosion on cement is multifaceted, and the ability of carbonation resistance is closely associated with the internal pore structure of the cement matrix.Te ingress of carbon dioxide acts as a fundamental prerequisite for succeeding chemical reactions during the carbonation process.Tus, the pore structure emerges as a principal determinant that governs the gas permeability and carbonation attributes of cement-based materials.It is widely accepted that air permeability is related to the total porosity of the material [8].Te gas permeability coefcient correlates linearly with porosity coincidences for the same material [9,10].Bigger porosity is usually accompanied by a greater gas permeability coefcient.However, the infuences of the pore size, connectivity, and tortuosity cannot be ignored.Poor pore connectivity can cut of the transmission path of carbon dioxide.Te large pore tortuosity increases the resistance during the carbon dioxide transition [9].
It is evident that the carbonation resistance exhibited by cement-based materials is enhanced through the combined efects of reduced porosity, limited connectivity, and increased tortuosity.During the hydration process of cement, the aggregation of hydration products greatly afects the pore morphology and structure of concrete.Previous studies [11][12][13] investigating the infuence of various initial solidifcation conditions on the pore structure have demonstrated that inadequate early curing leads to autogenous shrinkage phenomena and afects pore structure formation.Accelerating the cement hydration process is a potential source of modifying the confguration of pore structures [14,15].Te relevant study indicates that the incorporation of a certain amount of Ca(NO 3 ) 2 into cement promotes the formation of ettringite and nitrate-containing AFm phases.Furthermore, Ca(NO 3 ) 2 accelerates the hydration of C3S [16].Ca(NO 3 ) 2 is an accessible chemical substance that can be extracted from pond sediments and industrial wastewater [17], employing Ca(NO 3 ) 2 to enhance the performance of cement proves advantageous for sustainable development.Nevertheless, the roles of Ca(NO 3 ) 2 , particularly on the carbonation resistance characteristics of concrete, have received limited attention in the literature.
Tis study mainly focuses on revealing how Ca(NO 3 ) 2 afects the carbonation resistance performance of cementbased materials.Te investigation incorporates a combination of experimental techniques, including BET analysis, isothermal conduction calorimetry, water vapor sorption, and XRD.Te fndings reveal that Ca(NO 3 ) 2 and the underlying modifcation mechanism impart an impact on the microstructure of cementitious materials.Tis study ofers a fresh perspective on the infuence of Ca(NO 3 ) 2 modifcation on the pore structure during the hydration process.It establishes a theoretical foundation for the utilization of Ca(NO 3 ) 2 in enhancing the carbonation resistance of the cement matrix.

Experimental Materials and Preparation
Process.Te content of ordinary Portland cement (OPC) utilized during the investigation is presented in Table 1.Te composition analysis reveals that the cement comprises 56.7 wt.% C3S, 14.3 wt.% C2S, 3.2 wt.% C3A, and 12.0 wt.% C4AF, as determined by XRD measurement.Figure 1 depicts the distribution of particle sizes present in the cement.For the experimental setup, Ca(NO 3 ) 2 of chemical grade sourced from Sigma-Aldrich Co., Ltd., is employed.Te dosage of Ca(NO 3 ) 2 is varied at 1%, 2%, and 3% relative to the bulk of the cement.
Te experimental procedure involved the utilization of OPC to fabricate cement paste specimens with a prescribed 0.4 water-to-cement ratio.Te dimensions of the samples were set at 40 × 40 × 160 mm 3 .To prevent carbonation, a plastic flm was used to demold the cement paste samples after 1 day.Subsequently, the specimens were subjected to a curing process for 28 days in a dedicated curing room.Following the guidelines stated in Chinese standard GB/T 17671-2021, a concrete specimen measuring 100 mm × 100 mm × 400 mm in dimensions was fabricated as the subjected sample for the accelerated carbonation experiment.Te demolding process was executed 24 hours after casting, followed by a curing period that extended until 28 days.Keep the temperature at 20 °C, relative humidity at 95% during the curing process.Table 2 presents the mixture design of concrete samples, which will be used in this experiment.

Accelerated Carbonation and Mechanical Strength.
To reduce saturation before initiating accelerated carbonation, the samples were dried at 60 °C for 48 hours.Subsequently, the specimens were relocated to the carbonation chamber for specifc durations of 28, 56, 90, and 180 days.In compliance with the Chinese durability, criterion GB50082-2009 meticulously controlled to create specifc environmental conditions.Te carbonation chamber maintained a consistent temperature of 20 °C, while the relative humidity was carefully regulated at 70%.Additionally, the CO 2 concentration within the sample storage chamber was set at 20%.After subjecting the samples to carbonation experiments for the specifed duration, applying a 1% phenolphthalein alcohol solution using a spray method, then measure the carbonation depth.Use a hydraulic press to test the

X-Ray Difraction (XRD).
To determine the composition of the hydration phases in the respective samples, the samples were ground until they formed a fne powder.Subsequently, the powdered samples were sifted through an 200-mesh sieve to ensure uniformity in particle size.Te Xray difraction was performed in the angle range of 5-70 °with a time interval of 0.45 seconds each step utilizing the D8 Advance difractometer, manufactured by Bruker in Germany.Te Rietveld method, implemented through the utilization of TOPAS software, was employed to quantitatively determine the proportion of each phase present following the hydration process.

Specifc Surface Area and Porosimetry (BET).
To investigate the alterations in the pore structure resulting from the infuence of Ca(NO 3 ) 2 , a Micro TriStar II 3fex surface analyzer (model: 3 Flex, manufactured in the USA) was employed.Te samples were degassed in vacuum for 24 hours at a constant temperature of 40 °C to remove moisture and other impurities from the sample's surface and pores.Subsequently, the samples were placed in nitrogen for adsorption.Te relative pressure was maintained within the range of 1.3 × 10 −9 to 1 during the experimental procedure.
Once adsorption equilibrium was reached, the volume of adsorbed gas at diferent relative pressures was measured to obtain the isotherm adsorption curve.Te desorption isotherm curve was obtained by continuously decreasing the relative pressure to vacuum.After completion of the analysis, the Barrett-Joyner-Halenda (BJH) methods were implemented to elucidate the characteristics of the pore structure.

Dynamic Water Vapor Sorption (DVS).
To replicate the moisture dispersion occurring within the pore structure of the cement paste, a dynamic water vapor sorption experiment in which the samples weighed approximately 25 mg was carried out using a Vapor Sorption Analyzer (model: TGA Q5000SA).Te specimens were introduced into a controlled chamber where the RH was meticulously regulated.Te RH levels ranged from 8% to 98% with intervals of 10% during both the adsorption and desorption phases, while maintaining a constant temperature of 20 °C.
Te equilibration period for per step lasted for 8 hours.

Carbonation Depth.
Te measurement of the carbonation depth provides a means to assess the degree of erosion inficted upon the cement matrix by CO 2 .Te impact of various Ca(NO 3 ) 2 concentrations on the degree of carbonation depth is shown in Figure 2. Evidently, the Ca(NO 3 ) 2 -treated concrete specimens exhibit reduced carbonation depth compared to the reference group at each designated carbonation duration.Specifcally, in the sample containing 1% Ca(NO 3 ) 2 , the carbonation depth at 28 days reduced from 15.2 mm to 12.2 mm; at 56 days, it reduced from 16.5 mm to 12.9 mm; at 90 days, it reduced from 17.8 mm to 13.4 mm; and at 180 days, it reduced from 19.2 mm to 16.0 mm.With the incorporation of 1% and 2% of Ca(NO 3 ) 2 , a signifcant reduction in the initial stage (28 days) carbonation depth is observed.Subsequently, the rate of carbonation depth growth signifcantly decreases, and it is highly likely that even after 195 days, the carbonation depth will not exceed that of the control group.However, it is imperative to acknowledge that increasing the proportion of Ca(NO 3 ) 2 has not yielded the desired outcomes.In the case of the specimen infused with 3% Ca(NO 3 ) 2 following a storage of 180 days, the carbonation depth surpasses even that of the reference group.

Mechanical Strength and Cement Hydration.
Te evaluation of compressive strength at a hydration duration of 28 days is a widely applied method to characterize the earlystage hardening of cement pastes.Figure 3 illustrates the compressive strength behavior of samples with diferent amounts of Ca(NO 3 ) 2 .Te results showed that after Ca(NO 3 ) 2 was mixed into the concrete mixture, the strength of the matrix was improved to a certain extent, but the efect was not signifcant.Specifcally, the compressive performance of the samples rises from 38.1 MPa to 40.9 MPa and 42.2 MPa when 1.0% and 2.0% Ca(NO 3 ) 2 are incorporated into the plain concrete, respectively.Te compressive strength of samples containing 2% Ca(NO 3 ) 2 is higher than that of samples containing 1% Ca(NO 3 ) 2 , which aligns with the experimental fndings of Dorn et al. [16].Te incorporation of 1% and 2% Ca(NO 3 ) 2 resulted in a greater increase in compressive strength, and this is attributed to the involvement of Ca(NO 3 ) 2 in the hydration reactions within the cement matrix.Te addition of Ca(NO 3 ) 2 facilitates the  [18].However, when the dosage reaches 3.0%, the compressive performance decreases to 40.2 MPa.Te excessive participation of Ca(NO 3 ) 2 in reactions promotes the abundant formation of AFm, which occupies the space intended for the hydration of C3S.Tis inhibits the hydration of C3S and reduces the generation of C-S-H, thereby partially ofsetting the increase in compressive strength resulting from the signifcant formation of ettringite and AFm.Tese fndings highlight the impact of diferent dosages on the compressive strength performance of the specimens.Te measurement of hydration heat serves as a crucial means to investigate the hydration process of cement.In Figure 4(a), the heat release rate of cement paste with different amounts of Ca(NO 3 ) 2 is depicted.Te results demonstrate that the presence of Ca(NO 3 ) 2 reduces the induction period of cement hydration, as shown by the earlier appearance of the initial peak.Moreover, the maximum value of the groups containing Ca(NO 3 ) 2 surpasses that of the reference group, indicating an enhancement in compressive strength.Figure 4(b) displays the cumulative heat release profles of samples with varying concentration.Te cumulative exothermic heat liberation is closely linked to the degree of cement hydration, displaying a positive correlation that strengthens as the hydration process progresses.Prior to 40 hours, the cumulative heat release of the reference group lags that of the other samples, suggesting that Ca(NO 3 ) 2 promotes early hydration.After 150 hours, the total heat release of the sample incorporating various quantities of nitrate reaches 5000 J, indicating the progress of the hydration during the early stage.

XRD Analysis Results and Phase Composition.
Te promotion of Ca(NO 3 ) 2 on the hydration degree is presented in Section 3.2.Tere seems to be no positive infuence on the extent of the late hydration process.To investigate such an efect, the pastes underwent X-ray difraction (XRD) analysis and were characterized using the Rietveld method with TOPAS software.Figure 5 presents the XRD patterns of cement pastes with varying quantities of Ca(NO 3 ) 2 after 28d of hydration.It is obvious that no new phase appears when Ca(NO 3 ) 2 is incorporated.Te addition of Ca(NO 3 ) 2 did not result in a signifcant change in the C3A content, which aligns with the conclusion drawn by Ye et al. [19].Furthermore, each phase number was identifed.As the main component generated during the whole hydration process, C-S-H constitutes a signifcant portion, approximately 60%, of the overall phase composition.Moreover, the amount of C-S-H refects the extent of hydration.Compared with the reference paste, pastes with Ca(NO 3 ) 2 have less C-S-H.When the dosages of Ca(NO 3 ) 2 were 1%, 2%, and 3%, the C-S-H amount decreased from approximately 62% to 60%, 58%, and 58%, respectively.Tis phenomenon may be attributed to the extensive formation of AFm phases, which occupy a signifcant portion of the available space for C-S-H formation.Ca(NO 3 ) 2 exhibited a negative efect on the extent of the late hydration process, that is why the cumulative heat release was less than the reference paste in Section 3.2.In addition, more Ca(OH) 2 can be detected in pastes with Ca(NO 3 ) 2 , and the reaction between Ca(OH) 2 and infltrating CO 2 leads to a deceleration in the difusion rate of CO 2 within the matrix, thereby explaining the reduction in carbonation depth.

Pore Structure (BET).
As discussed, Ca(NO 3 ) 2 shows a slight infuence on the cement hydration process.Specifcally, Ca(NO 3 ) 2 slightly promotes the early 4 Advances in Materials Science and Engineering hydration stage.But it exhibited a slight negative efect on the extent of the late hydration process.Terefore, it seems that the hydration degree is not the main factor infuencing the performance of concrete, including compressive strength and carbonation properties.Te relationship of Ca(NO 3 ) 2 and pore structure was evaluated using the Brunauer-Emmett-Teller (BET) apparatus.
Te BET technique enables the determination of specifc surface area and distribution of pore sizes, providing valuable insights into the pore structure properties of the cement paste samples.Te nitrogen sorption-desorption curves of cement paste samples with varying dosages of Ca(NO 3 ) 2 are depicted in Figure 6.At low to moderate relative pressures (P/P 0 � 0∼0.8), with an increase in the content of Ca(NO 3 ) 2 , the isotherm adsorption curves shift closer to the x-axis, indicating a weakening interaction between Ca(NO 3 ) 2 and N 2 .At high relative pressures (P/P 0 � 0.8∼1.0), a signifcant increase in N 2 adsorption is observed for all samples, indicating the presence of slit-like pores predominantly formed by the accumulation of lamellar particles (e.g., lamellar calcium hydroxide crystals).Te adsorption capacity of N 2 and the area enclosed by the hysteresis loop decrease with an increase in the content of Ca(NO 3 ) 2 .Te inclusion of Ca(NO 3 ) 2 in the samples results in the formation of fner pores during the hydration process, accompanied by a reduction in the total number of pores.Te changes in the shape of the hysteresis loop refect an improvement in the pore structure, likely attributable to the Te application of the BJH method allowed for the quantifcation of pore characteristics, containing its cumulative volume and distribution, as illustrated in Figure 7. Te results suggest that the samples incorporating Ca(NO 3 ) 2 exhibited reduced nitrogen gas absorption in comparison with the reference group.Tese fndings suggest that the inclusion of Ca(NO 3 ) 2 infuences the pore structure of the sample, resulting in altered nitrogen sorption behavior.Conventionally, reduced nitrogen adsorption is associated with diminished porosity and fewer interconnected pores.Moreover, specimens with higher Ca(NO 3 2 concentrations demonstrate a propensity toward a further reduction in nitrogen adsorption, and this phenomenon can be ascribed to a diminishment in porosity and a decrease of connected pores.Furthermore, Figure 7(a) demonstrates that the cumulative pore volume of the Ca(NO 3 ) 2 -treated sample exhibits a decreasing trend as the dosage increases.As the dosage of Ca(NO 3 ) 2 increases, the pore volume of small pore sizes (20-38 Å) decreases continuously.Within the intermediate pore size range (38-100 Å), the sample with 1% Ca(NO 3 ) 2 exhibits a slightly larger pore volume compared to the reference sample, while the sample with 3% Ca(NO 3 ) 2 has a slightly smaller pore volume than the reference sample.In contrast, the sample with 2% Ca(NO 3 ) 2 has a signifcantly smaller pore volume than the reference sample.Within the large pore size range (100-150 Å), the pore volume of the samples with 1% and 3% Ca(NO 3 ) 2 remains the same as the reference sample.However, the sample with 2% Ca(NO 3 ) 2 has a smaller pore volume than the reference sample.By calculating the total area underneath the pore volume curve, it can be observed that the cumulative pore volume decreases with an increasing dosage of Ca(NO 3 ) 2 .Te reduction in cumulative pore volume signifes an enhancement in the compactness of the cement matrix.Moreover, in the pore distribution curves as shown by Figure 7(b), there is a degradation observed in the peaks at approximately 3.5 nm for the samples incorporating Ca(NO 3 ) 2 .Tis observation suggests that the presence of Ca(NO 3 ) 2 contributes to the densifcation of the pore characteristics in the cement paste.
Previous studies have reported that the correlation between relative pressure and adsorbed quantity can be applied to calculate the fractal factor of the pore structure in porous materials.Tis fractal factor provides valuable insights into the complexity and interconnectedness of the pore network within the cement paste.Table 3 shows that the Ca(NO 3 ) 2 addition declines the fractal dimension factor Ds, which decreases from 7.58958 to 7.16103, 6.97029, and 5.30373, respectively, when the Ca(NO 3 ) 2 amount changes from 0% to 3.0%.A higher Ds value represents the rougher and more irregular pore surface.Terefore, Ca(NO 3 ) 2 addition clearly decreases Ds, indicating a reduction in the complexity of the pore structure and a decrease in pore connectivity.
It has been confrmed that there are two types of calcium silicate hydrates with diferent density, which are produced during diferent stages of hydration process.Te thermal analysis results of paste samples containing various amounts of nitrate are presented in Figure 8. Tese results can be utilized to determine the hydration level by quantifying the nonevaporable water content in comparison with the fully hydrated samples.Based on the fndings by Taylor [20], it is established that completely hydrated cement pastes generally comprise around 23% nonevaporable water.By applying this information, the hydration degree can be computed, and the results are summarized in Table 4. Te incorporation of Ca(NO 3 ) 2 reduces the degree of hydration in the cement matrix, and the higher the dosage of Ca(NO 3 ) 2 , the greater the reduction in the degree of hydration.In accordance with Jennings' calcium silicate hydrate theory and previous calculations [21,22], the ratio and surface area of LD C-S-H formed during the early stages of the hydration process were determined.Te results presented in Table 4 indicate that the addition of Ca(NO 3 ) 2 leads to an increase in the content of LD C-S-H generated.However, the surface area of LD C-S-H exhibits an opposite trend, with a decrease of 12.26%, 18.14%, and 59.96% observed when 1.0%, 2.0%, and 3.0% Ca(NO 3 ) 2 were added, respectively.Tis is associated with the morphology of LD C-S-H, wherein if the formed LD C-S-H aggregates and forms clusters, it results in a reduction in specifc surface area.

Moisture Difusion (DVS).
Te transportation of gas molecules within porous materials is a complex process, wherein molecules of the medium can be adsorbed onto the pore surfaces of varying sizes under diferent applied relative pressures.Te larger volume pores gradually undergo capillary condensation and multiple-layer adsorption, while the smaller holes gradually get flled with media molecules.Te utilization of dynamic vapor sorption (DVS) is an effcient method for evaluating water vapor transport in porous materials.In the DVS measurement technique, the moisture content of the sample is considered as a function of water activity and adsorption kinetics, using adsorption 6 Advances in Materials Science and Engineering isotherms to investigate water adsorption properties.Tese water absorption characteristics are determined by the changes in sample weight during humidity absorption or difusion processes.Figure 9 presents the weight variation of cement pastes containing Ca(NO 3 ) 2 under varying humidity conditions during vapor sorption measurements.Based on the weight response data, the hysteresis loop is calculated and depicted in Figure 10.It is observed that the highest hysteresis content is attained at a humidity level of 75% for all samples.Furthermore, the hysteresis values of the samples incorporating Ca(NO 3 ) 2 are signifcantly lower than those of the reference.Notably, the hysteresis content of the samples with 1.0% and 2.0% Ca(NO 3 ) 2 is lower than that of the reference, while the content of the paste with 3.0% Ca(NO 3 ) 2 is higher than that of the reference within the range of 55% to 85% relative humidity.Tis phenomenon aligns with the carbonation depth process and will be further discussed in Section 4.    Advances in Materials Science and Engineering

Discussion
4.1.Ca(NO 3 ) 2 Improves Carbonation Resistance.It should be noted that the addition of Ca(NO 3 ) 2 in an amount smaller than 3.0% can improve the carbonation durability of cementitious materials.Te carbonation process of cementitious material mainly relates to the microstructure, including the hydration products, pore structure, and the pore solution.Ca(NO 3 ) 2 exhibits little positive efect on the hydration of cement.However, there is no C-S-H growth because of the addition of Ca(NO 3 ) 2 , and the formation amount of calcium hydroxide improves, thereby sustaining the alkaline environment and retarding the neutralization of cementitious materials.As detected by BET, the pore structure reveals a linear negative correlation between pore volume and average pore size with the dosage of Ca(NO 3 ) 2 , which contradicts the relationship between carbonation depth of cement and the amount of Ca(NO 3 ) 2 used.When diferent dosages of Ca(NO 3 ) 2 admixture are added to cementitious materials, the pore structure undergoes refnement, leading to a decrease in pore density.Consequently, the fow of carbon dioxide gas into the cementitious material is restricted.Furthermore, the interconnectivity of pores is also a crucial factor that infuences the carbonation resistance of cementitious materials.Te nitrogen sorption and desorption curve indicate that the pore connectivity in cementitious materials is declined by the incorporation of Ca(NO 3 ) 2 , which further leads to a reduction in the difusion of carbon dioxide.Regarding the nitrogen sorption and desorption curves, the curve corresponding to the reference group displays the biggest total volume adsorbed in all samples (around 29 cm 3 /g).Moreover, the adsorbed volume in the reference group, which is around 11 cm 3 /g, is higher than that of samples with diferent dosages of Ca(NO 3 ) 2 at a relative pressure value of 0.5.However, the addition of 3.0% Ca(NO 3 ) 2 can reduce the carbon dioxide resistance of cementitious materials in 90 days, which is also related to the pore connectivity.Justnes's experimental studies [23] have

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Advances in Materials Science and Engineering shown that the steady-state migration coefcient D m when 2.0% Ca(NO 3 ) 2 is added into the cementitious materials doubles than those of the reference sample.Te D m is tightly related to the porosity and pore connectivity.More specifcally, the higher porosity and connectivity result in a higher value of D m .Due to the fact that the addition of diferent amounts of Ca(NO 3 ) 2 decreases the porosity, it can be inferred that the D m can be reduced by adding 3.0% Ca(NO 3 ) 2 , so the pore connectivity of cementitious materials with 3.0% Ca(NO 3 ) 2 increases.Accordingly, the gas migration coefcient of cementitious material with a higher amount of Ca(NO 3 ) 2 can be reduced.According to the fractal scaling law, a bundle of tortuous pores is assumed in cement-based material, and the efective permeability k can be derived according to the Hagen-Poiseuille equation and Darcy's law, as shown in the following: where D T is the tortuosity fractal dimension, ϕ represents the porosity, and d max represents the maximum pore size.Tortuosity fractal dimension is related to connectivity as shown in the previous study [24].Based on the reference value, that of the plain cementitious material sample is estimated at 1.0 and shows more connected pore structure.In addition, the pore system with micrometer scale shows disconnected characteristics and the gas difusion process is determined by the connected capillary pores based on the common value 500 nm [25].Te results of the average pore diameter tested by BET in Table 5 show that the lower tortuosity fractal dimension and the porosity have great efects on the permeability of concrete.Te moisture diffusion experiments using DVS measurement show that the addition of 3.0% Ca(NO 3 ) 2 increases the pore system connection when the hysteretic curve presents higher content.On the other hand, when the relative humidity is more than 55%, a water layer forms on the surface of the pore system and the amount of moisture in and out of the sample with 3.0% Ca(NO 3 ) 2 is smaller than that of other samples, which confrms the fact that the higher amount of Ca(NO 3 ) 2 improves the connectivity of cementitious materials.

Te Modifcation Mechanism on Microstructure by
Ca(NO 3 ) 2 .Te microstructure change of cementitious materials is tightly related to the cement hydration process.It has been observed in calorimetric measurements that the addition of Ca(NO 3 ) 2 will increase the hydration peak and the hydration fow of the acceleration period, but the hydration degree after the deceleration period is not extended.Based on the calculation of LD C-S-H, the ratio of LD C-S-H is observably increased with the additional amount of calcium nitrate, while the surface area of LD C-S-H decreases.It should be noted that the low-density C-S-H generated in the period of early hydration includes the induction and acceleration period.Te increased amount of LD C-S-H permits greater ion difusion into the pore solution, so the reaction of hydration products gets enhanced.Te research fndings by Dorn et al. [16] demonstrate that the addition of Ca(NO 3 ) 2 signifcantly increases the concentration of Ca 2+ in the pore solution of cement paste within the frst hour of hydration.In a previous study [21], the hydration rate of the acceleration period is linearly dependent on the addition of a temperature-rising inhibitor, which proves that the rate of acceleration period is primarily governed by the growth of C-S-H, which serves as the limiting step.However, the study also seems to show that the cement hydration accelerated by alkali salts results from the ion motility and the changes of the difusion characteristics.
Taking the development of calcium silicate hydrates and the infuence of alkali into consideration [22], the incorporation of calcium ions plays an important role in the growing hydrate, which determines the hydration rate [26], but the presence of anion shows the charge shielding, which means that a single calcium ion incorporated into the hydrates causes the two plus surface charge.It can be inferred that mobile anions in the close vicinity are needed to minimize the free energy of a hypothetical positively charged transition state.Terefore, the addition of more Ca(NO 3 ) 2 , ion motility, and the changes in the difusion characteristics result in the change of the surface area and the charge state of hydrates.Te mechanism is consistent with the results of pore structure and the density of low-density calcium silicate hydrates, which can also be used to analyze STXM and NMR results of previous studies [27][28][29].

Conclusions
In this study, the infuence of Ca(NO 3 ) 2 on the carbonation properties of cement-based materials was comprehensively examined utilizing isothermal conduction calorimetry, XRD, BET analysis, and water vapor sorption.From the conducted analyses, the following pivotal fndings can be deduced: (1) Te incorporation of Ca(NO 3 ) 2 enhances the carbonation resistance of cementitious materials, provided that the dosage is lower than 3.0% (2) Te carbonation resistance improvement of Ca(NO 3 ) 2 addition results from the increase of small pores and the change of tortuosity, as the lower tortuosity fractal dimension and porosity have obvious efects on the permeability of concrete (3) Te moisture difusion experiments have also shown that the addition of 3.0% Ca(NO 3 ) 2 shows a more connected pore system when the hysteretic curve presents higher content, which confrms the fact that the higher amount of Ca(NO 3 ) 2 improves the connectivity of cementitious materials (4) Te modifcation mechanism on microstructure by Ca(NO 3 ) 2 is mainly attributed to the fact that ion motility and the changes of the difusion characteristics result in the change of the surface area and the charge state of hydrates

Figure 6 :
Figure 6: Nitrogen sorption and desorption curve for samples with varying amounts of Ca(NO 3 ) 2 .

Figure 8 :
Figure 8: Termal analysis of cement pastes incorporated with varying proportions of nitrate at the stage of 28-d hydration.

Figure 9 :
Figure 9: Dynamic vapor isothermal curve of cement paste containing (a) 0%, (b) 1.0%, (c) 2.0%, and (d) 3.0% chemical admixture.Te black and blue lines represent relative humidity and moisture content shown on the primary and secondary y-axis.

Figure 10 :
Figure 10: (a) Efect of diferent amounts of Ca(NO 3 ) 2 on moisture content adsorbed by cement paste; (b) the hysteretic curve recalculated by moisture content at the same humidity.Te adsorption and desorption are ftted by the dashed lines.

Table 1 :
Chemical and phase composition of Portland cement.

Table 2 :
Mixture design of concrete used in the study.

Table 3 :
Pore structure and fractal parameters calculated from nitrogen sorption isotherm.

Table 4 :
Chemical bound water and C-S-H gel characteristics parameter in cement pastes.