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Article

Evaluation of the Surface Performance of Mortar Matrix Subjected to Sodium Chloride Solution Modified with Hybrid Nanosilica Cement Paste

by
Kai Lyu
1,2,
Junjie Xu
3,
Yue Gu
3,*,
Kailun Xia
3,
Lei Wang
3,
Weiwei Liu
3 and
Xian Xie
3
1
College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, China
2
Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China
3
College of Mechanics and Materials Engineering, Hohai University, Nanjing 210098, China
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(16), 9876; https://doi.org/10.3390/su14169876
Submission received: 31 May 2022 / Revised: 10 July 2022 / Accepted: 28 July 2022 / Published: 10 August 2022
(This article belongs to the Special Issue Soil Dynamics and Earthquake Engineering in Sustainability)
Browse Figures
Review Reports Versions Notes

Abstract

:
In order to prolong the service life of cement-based materials subjected to external chloride ion attacks, two kinds of methods, a surface treatment and chloride immobilization, were combined by fabricating a nanosilica-modified cement paste and coating it on mortar samples as a surface treatment material (HSM). The protective effect of the HSM was evaluated according to its surface hardness, and an RCM test was carried out, which indicated that the attached layer could both increase the surface hardness and decrease the chloride diffusion coefficient. Then, the chloride immobilization mechanisms were illustrated in terms of chloride blocking resistance, chemical binding and physical binding X-ray diffraction (XRD), and thermogravimetric/derivative thermogravimetric (TG/DTG) and thermodynamic modeling. The results showed that the hybrid nanomaterials that modified the cementitious surface treatment materials may effectively improve the chloride-resistant property of a matrix with content of no more than 1%. This research outcome could provide evidence that hybrid nanosilica can be applied in surface treatment technology.

1. Introduction

As a type of civil engineering material, concrete based on Portland cement is used everywhere, including in roads, buildings, and bridges, and in the engineering of other infrastructure. It is practically the foundation of modern industrial society. However, alongside the enhancement of people’s awareness about the concept of environmental protection, the huge CO2 footprint caused by concrete is becoming a critical issue [1,2,3]. The prolongment of the service life of concrete would undoubtedly be a judicious and cost-effective strategy to address carbon emissions that may also fail to meet the substantial requirements of cement during the rebuilding process [1,2,4,5,6,7].
Regarding infrastructure in marine environments, the corrosion of chloride ions is of the utmost concern and must not be neglected [8]. The diffusion and migration of ions in concrete/mortar is likely to lead to the corrosion of the steel, which will ultimately result in the property degradation of the infrastructure [9,10]. This topic is of intense concern for practitioners in both the industrial and research fields, and enormous effort has been devoted to improving the chloride resistance of cementitious materials. According to the previous literature, chloride resistance can be improved via two kinds of methods: surface treatment with a protection layer and chloride immobilization [11,12]. Regarding the chloride immobilization method, chloride resistance is often achieved by refining the pore structure with a lower water–cement ratio and adding supplementary cementitious materials or nanoparticles [13,14]. However, this type of method has two main drawbacks: (1) the protection may be insufficient in highly aggressive environments, such as marine, saline, and alkaline environments, and (2) this method may lead to the entire material being overdesigned, which is less economically efficient [15,16]. Taking into account the entry order of the external erosional substance, the near-surface region is the most vulnerable zone in concrete. Thus, surface treatment technology plays an important role in improving the durability of concrete. Surface treatment tends to form a continuous layer that can act as a physical barrier that prevents the penetration of chloride ions into the cementitious matrix [17]. According to the chemical composition of the treatment agents, surface treatments can be subdivided into two categories: traditional polymer/nanocomposite coatings and cementitious coatings [18,19]. Traditional polymer coatings have a superior protective effect; however, they have the disadvantages of poor fire resistance, easy cracking and detachment, a limited service life, and difficult removal after their protective effect is lost [20]. Moreover, the bond between the cementitious matrix and the attached layer is strong enough because they are two different materials. Thus, in recent decades, cementitious coatings, including polymer-modified cementitious coatings and geopolymer coatings, have been comprehensively investigated in terms of their mix ratio design, micro- and macro-structure, and performance [21]. Compared with inorganic coatings, cementitious coatings are of superior durability and compatibility with the matrix; however, their protective effect is not as good as that of inorganic coatings. It is widely accepted that the protective effect of cementitious coatings is closely related to their microstructures. A denser and more homogeneous microstructure is more likely to achieve good resistance to chloride penetration.
As nanotechnology provides research into concrete with new concepts and vitality, various types of nanoparticles have emerged in the construction sector, meaning the enhanced performance of cementitious materials has been observed [22,23,24,25]. In recent decades, a great amount of research has been conducted to investigate the application of various nanomaterials in construction [26]. Ultimately, the introduction of nanoparticles into concrete/mortar can improve the mechanical properties and durability of composites by refining the pore structure, resulting in a denser microstructure, which is bound to increase the chloride resistance of composites [27,28]. Among these nanomaterials, nanosilica (NS) is a kind of nanoparticle that has been widely used in the surface treatment of cementitious materials. Hou [29], Javid [30], and Li [31] et al. proved that spraying NS on a mortar/concrete surface can significantly reduce the permeability, water absorption, and abrasive resistance of composites.
Most of the current literature focuses on the application of direct coating; other methods, i.e., applications in the form of integral nanomodified cementitious surface treatment materials, have attracted little attention. The latter method may maintain a similar deformation coefficient with the matrix, which decreases the risk of cracking. On the other hand, more recently, it has been reported that hybrid nanosilica seems to be more suitable for the modification of cement-based materials with better dispersibility and a certain degree of hydrophobicity [32,33]. Furthermore, when considering the effect of NS on reductions in the diffusion of chloride ions, in addition to migration resistance, the roles of physical binding and chemical binding should also be addressed [34].
Thus, in this research, two kinds of methods (surface treatment and chloride immobilization) were combined, which was achieved by producing a kind of protection layer with NS-modified cementitious material to achieve the best protection efficiency. The cement paste modified with hybrid nanosilica was prepared and coated on mortar samples to serve as a surface treatment material (HSM), and it was coated on the mortar surfaces as a protection layer. To determine the macro-properties of the HSM, a surface hardness test and a rapid chloride migration test were conducted. The effect of HN on chloride immobilization was illustrated in terms of physical binding, chemical binding, migration blocking with X-ray diffraction (XRD), and thermogravimetric/derivative thermo-gravimetric (TG/DTG) and thermodynamic modeling. The results showed that the cementitious surface treatment materials modified with hybrid nanomaterials could effectively improve the chloride-resistant property of the matrix with content of no more than 1%. This research outcome could provide evidence that hybrid nanosilica can be applied in surface treatment technology.

2. Materials and Methods

2.1. Materials

Ordinary Portland cement in accordance with Chinese standard GB175-2007 was used in all the mixes. Its chemical composition is given in Table 1. Natural quartz sand with a fineness modulus within 2.3~3.0 and silica content ≥96% was used as fine aggregate. Polycarboxylate superplasticizer produced by Jiangsu Subote New Materials Co., Ltd. (Nanjing, China) was introduced as a dispersion agent to ensure the workability of the cement composites.
The organic–inorganic hybrid nanoslica (HN) was a commercial product supplied by Jiangsu Sobute New Materials Co., Ltd. According to the supplier, the inorganic constituent of HN is colloidal nanosilica, and the organic component in HN is the aliphatic molecular group, which accounts for 13% by weight. The diameter of HN ranges between 30 and 60 nm. The schematic illustration of HN is shown in Figure 1.

2.2. Sample Preparation

The cement paste samples modified using hybrid nanosilica (HN) were prepared and served as surface treatment materials (HSMs). The water–cement ratio was 0.35 by weight. For comparison, three mixtures (BK, A, and B) with HN contents of 0%, 1%, and 2% (see Table 2), by the weight of cement, respectively, were fabricated. Mortar samples were prepared as matrix materials for the HSMs, abbreviated as REF. The cement–sand mass ratio of the mortar was 1:3, with the water–cement ratio being 0.53.
After the mortar was set for the initial setting time, the surface of the mortar was roughened by grinding it with abrasive paper to create a rough surface, which enhanced the bond behavior between the mortar matrix and the HSM layer. Then, the HSM was coated on the surface of mortar with a thickness of 20 mm. The mortar samples coated with HSM were demolded 1 day after being cast, and then, they were cured in a standard curing chamber (20 °C, 95% RH) until the tests were carried out. For the RCM (rapid chloride migration) test, the HSM–mortar samples were removed from the chamber after being cured for 28 days. For the rebound surface hardness test, the HSM–mortar samples were removed after 14 days. Thereafter, they were soaked in sodium chloride solution with three concentrations (0.1, 0.5, and 1 mol/L, respectively) until the measurements were carried out.

2.3. Test Methods

2.3.1. Rebound Surface Hardness

A rebound number is an indication of the surface hardness of a specimen. Here, the rebound numbers were recorded by using a rebound hammer to hit the surface of the specimens according to GBT50315. The nominal kinetic energy of the rebound hammer used was 0.196 J. For each surface, 10 points were randomly measured, and an averaged value was taken after the maximum and minimum values were eliminated.

2.3.2. Rapid Chloride Migration (RCM) Test

The rapid chloride migration test (RCM) was carried out to evaluate the chloride ion diffusion coefficients according to GBT50082-2009 [35]. The RCM method was first proposed by Tang and Nilsson [36], featuring a well-founded theoretical basis, short test cycle, and intuitive results. A cylindrical specimen which was 100 mm in diameter and 50 mm in height was measured in the RCM test. In the RCM testing tank, the catholyte was 10% NaCl solution, while the anolyte was 0.3 mol/L NaOH. The penetration depth of chloride ions was determined by spraying 0.1 mol/L AgNO3 onto the surface of the cut section that split from the specimen. Then, the chloride ion diffusion coefficient of the specimen was calculated according to Equation (1), and an averaged value over three samples was adopted to determine the representative value. Equation (1) is shown below, where D R C M is the non-steady-state migration coefficient of the sample, accurate at 0.1 × 10−12 m2/s; U is the test load voltage; T is the average value of the initial and final temperature of the anode solution; L is the specimen thickness; X d is the average value of the chloride ion penetration depth; and t is the electricity test time.
D R C M = 0.0239 × ( 273 + T ) L ( U − 2 ) t ( X d − 0.0238 ( 273 + T ) L X d U − 2 )

2.3.3. X-ray Diffraction (XRD)

X-ray diffraction analyses were carried out on powder paste mixtures after the isopropyl alcohol solvent exchange at each designed age using a Bruker D8 Advance diffractometer in a θ–θ configuration using Cu-Kα radiation. The scanning range was 5~70° (2θ) with a scanning rate of 5°/min.

2.3.4. Thermogravimetric and Differential Thermogravimetric (TG-DTG) Analyses

Thermogravimetric and differential thermogravimetric (TG-DTG) analyses were performed on a thermal analyzer (NETZSCH STA449F3, Selb, Germany) to measure the weight change of the samples when they were heated in a nitrogen atmosphere as a function of temperature. The temperature was set to increase from 30 to 1000 °C at a heating rate of 20 °C/min.

2.3.5. Thermodynamic Modeling

The equilibrium phase assemblages of the samples were calculated via thermodynamic modeling. The calculations were carried out using the Gibbs free energy minimization software (GEMS) [37]. The thermodynamic properties of aqueous species, complexes, and solids were sourced from the default GMES-PSI database. Additional data for cement hydrates were supplemented with CEMDATA18 [38]. During the modeling process, some parameters of the system were simplified as follows: (i) the CSHQ model from Lothen-bash was adopted for the calculation of the CSH phase [16]; (ii) the maximum degree of hydration for the binder was assumed to be 80% under a sodium chloride solution with different amounts; and (iii) the binding form of chloride ions in the model was chemically bound, while physical binding was not considered.

3. Results and Discussion

3.1. Surface Hardness Change

Surface rebound testing was employed to reflect the change in the surface hardness of the samples [39]. Figure 2 shows the surface rebound numbers of different samples under the 0.1 mol/L NaCl solution. In general, the rebound numbers of the samples were significantly increased after being coated with the HSM. For different types of HSM, in 28 d, their rebound numbers did not differ much. For instance, HSM-A was only slightly higher than HMS-B by 2%, which could be ignored considering the uncertainty in the testing process.
Figure 3 displays the surface rebound numbers of different samples under the 0.5 mol/L NaCl solution. It can be seen that the rebound numbers of samples with the HSM were all greater than that of the reference sample, similar to the pattern that was observed under the 0.1 mol/L NaCl solution. However, it could be seen that the three types of HSM exhibited different enhancement effects. In 28 d, the surface-hardness-enhancing effect of these HSMs followed an order: A > B > BK. Compared to HSM-BK, the rebound number of HSM-A and HSM-B increased by 21.9% and 9.8%, respectively. It was noted that in 28 d, the rebound number of HSM-B was lower than that of HSM-B in 14 d; however, this decreasing trend did not occur in HSM-A. This may be related to the magnification of the side effects caused by the agglomeration of hybrid nanomaterials at an extended age [40]. For nanomaterials, a higher dosage seemed to be more likely to trigger agglomeration.
The surface rebound numbers of different samples under the 1 mol/L NaCl solution are shown in Figure 4. As shown in Figure 4, when soaked in such a high concentration of NaCl solution, the surface rebound numbers of samples without HSMs were extremely low, which indicated that erosion inhibited the increase in strength caused by cement hydration. The specimens coated with HSMs all presented a pronounced enhancement effect both at early and later stages. At 28 d, in terms of the rebound number from high to low, the specimens were in the following order: A > B > BK. Compared to HSM-BK, the rebound number of HSM-A and HSM-B increased by19.8% and 9.9%, respectively. HSM-A exhibited the best hardness improvement effect under this concentration. It should be noted that the drop phenomenon appeared in both HSM-BK and HSM-B. This may have been associated with microstructure destruction due to chloride ion attacks [41]. Another interesting phenomenon is that the rebound numbers of HSM-A and HSM-B were both lower than that of HSM-BK in 7 d. This could have been attributed to the better pore-refining effect of hybrid nanoparticles in HSM-B at the early stage.
Overall, when the samples were soaked in the NaCl solution, chloride ions could penetrate the samples. Some of the chloride ions could be immobilized by hydration products, and other chloride ions could travel within the composites through connected pores. When considering the immobilization of chloride ions, three aspects should be addressed: migration resistance, chemical binding, and physical binding. For the BK samples, when subjected to the 0.5 mol/L NaCl solution, three factors worked together to immobilize the chloride ions. The AFm-like phase can chemically bind chloride ions and be transformed into F salt, which could enhance the microstructure. Therefore, increased surface hardness can be seen in Figure 3 for BK samples at 7 d. When the soaking time continued to increase, the chloride ions were continuously immobilized, the microstructure of the BK samples was constantly enhanced, and increased surface hardness could be observed. However, when subjected to the 1 mol/L solution, the BK samples did not have the capacity to continuously immobilize the penetrated ions, and the ions traveled within the composites, which weakened the microstructure of BK, and this is why a decrease could be observed in BK when the soaking time was extended from 7 d to 28 d.

3.2. RCM

The chloride diffusion coefficient has been considered to be a primary parameter which reflects the permeability of cementitious materials exposed to sodium chloride solutions.
In the RCM test, the thickness of the HSM was set to 20 mm. Figure 5 shows the chloride diffusion coefficients of different types of samples measured via RCM. The chloride diffusion coefficients of REF, HSM-A, HSM-B, and HSM-BK were 8.06, 3.16, 4.17, and 6.53 × 10−12 m2/s, respectively. In comparison, HSM-A showed the lowest coefficient. In particular, compared with REF, the chloride diffusion coefficients of HSM-A and HSM-B were reduced by 60.8%, and 48.3%, respectively. When compared with BK, a reduction of 51.6% could be seen in HSM-A. The chloride penetration resistance of cementitious materials is highly dependent on the pore structure of the composites. With the introduction of nanoparticles, the pore structure of the HSM can be greatly improved and refined via the pozzolanic effect, the nucleation effect, and the filling effect. In addition, HN particles can effectively block or cut off capillaries, forming tortuosity and more disconnected transport channels, which further improves the resistance to chloride permeability [42]. However, due to the large specific surface area of the HN particles, which may have triggered agglomeration, the coefficient of HSM-B was larger than that of HSM-A. The obtained results agreed well with findings in previous research [6]. In short, the results indicate that HSMs fabricated with a low dosage of hybrid nanomaterials may have denser microstructures.
The effect of hybrid NS on the diffusion of chloride ions should be addressed from three perspectives: chemical binding, physical binding, and migration resistance. The addition and dosage of HN particles may have both positive or negative effects on the chloride immobilization capacity of the three factors, and in the following section, this is illustrated in detail, coupled with XRD, TG/DTG, and thermodynamic modeling.

3.3. XRD

To obtain information regarding the phase assemblages of the samples, X-ray diffraction analysis was carried out. Figure 6 represents the XRD patterns of samples after being soaked in the 1 mol/L NaCl solution for 28 d. The diffraction peaks of ettringite, Friedel salt, portlandite, calcite, and CSH are clearly identified and labeled in the figure. For all of the samples, the main peaks seemed to be similar, which implies the incorporation of hybrid nanomaterials would not change the mineral composition of hydrated products under a NaCl solution. The peak at around 9° is related to ettringite. Due to the utilization of hybrid nanosilica, the microstructure of the HSM could be refined and densified, and not enough space existed for the formation of ettringite [7]. Additionally, the HN particles could consume the released aluminum phase to form the CSH phase. Thus, according to the peak height, the ettringite content followed the order of BK > A > B, which indicates the strong ability of HN to inhibit the formation of ettringite. The peaks at around 18° and 34° were attributed to portlandite. These peaks were likely due to the pozzolanic reaction of hybrid nanomaterials that consume a certain calcium hydroxide and transform it into CSH, and this reaction resulted in the peak height of CH, which followed the order of BK > B > A. However, the physical chloride-binding capacity of the CSH was related to both the amount and Ca/Si of the CSH. When HN was added, the amount of CSH increased, while the Ca/Si of the CSH gel decreased. Increases in the amount of CSH strengthened the capacity of the HSM to absorb chloride ions, while the decreased Ca/Si of C-S-H weakened its capacity to immobilize chloride ions. This is why in Figure 5, the chloride diffusion coefficient of HSM-B is even higher than that of HSM-A. This agreed with the findings in previous research [30].
Friedel salt is an important mineral phase in cementitious materials formed after chloride-induced corrosion. The effect of NS on chloride-induced corrosion can be illustrated by chemical binding, which is involved in reactions between chloride ions and the AFm-like phase ([Ca2Al(OH)6·2H2O]+) to form Friedel salt (FS, [Ca2Al(OH)6·2H2O]2·Cl2·4H2O) [5]. To observe the XRD patterns of samples near the peak of Friedel salt more clearly, in Figure 7, the peaks at around 11°are magnified. It can be seen that the peak intensity of HSM-A was similar to that of HMS-B but lower than that of HMS-BK. In terms of chemical binding, the addition of HN particles hindered the formation of FS and weakened the capacity of the HSM to immobilize chloride ions.

3.4. TG-DTG

To quantitatively evaluate the content of CH and Friedel salt further, TG-DTG was conducted. Figure 8 shows the DTG patterns of the samples after being soaked in the 1 mol/L NaCl solution for 28 d. It can be seen that several endothermic peaks appeared on the DTG patterns. The first sharp mass loss at 60~120 °C was associated with the evaporation of free water in the samples. The weak mass loss at 60~120 °C corresponded to the dehydration of CSH gels, ettringite, and gypsum. It is noted that at this temperature, it was difficult to distinguish these minor phases from each other. The broad peak at 310~385 °C was involved in the decomposition of Friedel salt [43], and the DTG results indicated that the amount of FS followed an order of A > B > BK, which agreed with the XRD analysis. The sharp mass loss that appeared at 400~500 °C was related to the dehydration of calcium hydroxide [44], and it could be seen that the amount of CH in HSM-A and HSM-B was lower than that in HSM-BK, which was in agreement with the XRD analysis. The mass loss at 600~780 °C was due to the decomposition of calcium carbonate with different polymorphs [45]. The positions of the peaks in all the samples were almost identical, indicating the samples contained similar mineral types. This was consistent with the results of the XRD.
Figure 9 shows the TG patterns of samples after being soaked in the 1 mol/L NaCl solution for 28 d. According to the temperature ranges of mass loss mentioned above, the mass rates of calcium hydroxide (CH) and Friedel salt in the samples could be further quantitatively calculated.
The mass rates of CH content in the samples (after being soaked in the 1 mol/L NaCl solution for 28 d) via calculation are listed in Table 3 (second row). It is can be seen that the mass rates of CH in the samples followed an order: BK > HSM-A > HSM-B. Compared with BK, the mass rate of CH in HSM-A and HSM-B decreased by 3.0% and 8.5%, respectively, and this could have been due to the fact that the HN could help to consume the CH and transform it into CSH; the higher the quantity of HN, the less CH was observed.
The mass rates of the calculated Friedel salt content in the samples (after being soaked in the 1 mol/L NaCl solution for 28 d) are listed and compared in Table 2 (third row). As indicated in the table, the mass rates of Friedel salt in HSM-A, HSM-B, and BK were 6.427%, 6.124%, and 6.557%, respectively. Specifically, in comparison with BK, the mass rate in HSM-A and HSM-B decreased by 2.0% and 6.6%, respectively. HSM-B had the weakest ability to bind chloride ions due to the fact that the least ettringite formed within HSM-B. This observation was consistent with the previous XRD experiment. The results further confirm that the introduction of hybrid nanomaterials reduced the ability of cementitious materials to chemically bind chloride ions. The underlying reasons for this may include: (i) the addition of hybrid nanomaterials lowered the calcium–silicon ratio of CSH gels; normally, CSH gels with low Ca/Si ratios have weaker chloride-binding capacities [46]; (ii) HSMs with hybrid nanomaterials have denser microstructures, which hinder the diffusion and adsorption of chloride ions into the interior space.

3.5. Thermodynamic Modeling

In order to investigate the chemical effect of NS on HMS in contact with the NaCl solution, the thermodynamic modeling of cement pastes with different SiO2 contents was carried out. Considering the difficulty in achieving the ideal uniform dispersion state, the presence of local enrichments of NS in some areas of cement pastes could be inferred. Therefore, the SiO2 contents used in thermodynamic modeling were set at 0 wt%, 1 wt%, and 5 wt% to correspond with the concentrations of NS in real-life situations.
Figure 10a–c show the predicted phase assemblages of cement pastes with different NS contents in contact with increasing amounts of NaCl solution. In Figure 10a, it is shown that hydrotalcite, monocarbonate, ettringite, Si-Hg, and C-S-H were the main hydrates in the cement pastes in the non-exposed period. As the contact with NaCl solution increased, the monocarbonate level firstly decreased, leading to the generation of Friedel salt. At high NaCl levels, a reduction in the total volume of solid phases and a formation of MSH were predicted to occur.
As shown in Figure 10b, the content of hydrates, including monocarbonate and Si-Hg, decreased in the non-exposed period with the incorporation of SiO2, while the C-S-H and calcite contents increased to a certain extent. This result can be seen in Figure 10c more obviously, in which monocarbonate disappeared and the Si-Hg level was remarkably reduced. This mechanism can be explained by the calcium occupation induced by silicon derived from SiO2. The spontaneous transformation process suggests that calcite and C-S-H are more stable than monocarbonate and Si-Hg, which indicates that the generation of calcite and C-S-H is prioritized in the hydration process. Monocarbonate (3CaO·Al2O3·CaCO3·11H2O) and Si-Hg (3CaO·x Al2O3·(1 − x) Fe2O3·0.84SiO2·4.32H2O) possessed higher calcium-to-carbon ratios and calcium-to-silicon ratios than those of calcite and C-S-H, which means that these two metastable phases were formed in a calcium-enriched or silicon-insufficient condition. Therefore, it was shown that the introduction of silicon has the ability to combine excessive calcium in monocarbonate and release CaCO3, leading to the generation of C-S-H and calcite. A similar mechanism can be inferred for the reaction between Si-Hg and SiO2. In fact, this transformation also occurred in other hydrates upon introducing SiO2. Figure 10d shows the phase volumes of hydrates with different SiO2 contents. Increasing trends in C-S-H and calcite could be observed with the consumption of other hydrates including monocarbonate, Si-Hg, and ettringite. It can be speculated that SiO2 has a strong calcium-fixing ability and can guarantee the prioritization of C-S-H generation and the delay of C-S-H decomposition. For NS, this effect may be more pronounced due its enhanced chemical activity in the nanoscale. Considering the absorption of C-S-H to chloride, this can explain the enhanced chloride resistivity of HSM-A.
Furthermore, it was shown that SiO2 also has an effect on the generation of Friedel salt (3CaO·Al2O3·CaCl2·10H2O). As shown in Figure 10e, the maximum volume of Friedel salt decreased along with the increase in SiO2 content, which could be attributed to the calcium occupation of SiO2. This result was consistent with a previous observation in XRD and TG examinations and indicated that the incorporation of NS may lower the chemical combination capacity to chloride of cementitious materials. However, due to the refined microstructure and the generation of additional C-S-H, NS can still enhance the chloride resistance of cement pastes at the early stage when exposed to a chloride-enriched environment.

4. Conclusions

In this study, hybrid-nanosilica-modified cement pastes were prepared as surface treatment materials (HSMs). The influence of HSMs on the chloride-resistant property of the matrix was evaluated using several experiments. From this study, the main conclusions can be drawn as follows:
(1) HSMs with 1 wt% hybrid nanosilica (HSM-A) could be effective in the inhibition of the degradation of surface hardness in a matrix exposed to sodium chloride solution. The enhancement effect of HMS-A was more remarkable in the high-concentration erosion solution. HSMs with higher dosages did not seem to be as effective as HSM-A.
(2) The use of HSM-A could reduce the chloride diffusion coefficient by 51.6% compared with the normal surface treatment materials. In comparison with the blank sample, lower contents of Friedel salt and calcium hydroxide were observed in the microstructure of HSM-A after the erosion of the sodium chloride solution. This indicates that the introduction of hybrid nanosilica would weaken the capacity of cementitious materials to immobilize chloride ions.
(3) The results of thermodynamic modeling showed an increasing trend of C-S-H and calcite with the consumption of other hydrates, including monocarbonate, Si-Hg, and ettringite, when the SiO2 was intruded into the cement paste, which indicated that SiO2 has a strong calcium-fixing ability which guarantees the prioritization of C-S-H generation and the delay of C-S-H decomposition. This further verified the enhanced resistivity of HSM-A to chloride erosion.
Overall, considering the findings of this research, the effect of hybrid nanosilica on the anti-corrosion resistance of HSMs should be comprehensively considered. On the one hand, the introduction of hybrid nanosilica can densify microstructures via the pozzolanic effect, the nucleation effect, and the micro-filling effect, which help to block chloride migration. Moreover, it was shown that the hybrid nanosilica consumed CH and transformed it into CSH, and more chloride ions could be absorbed. However, on the other hand, the hybrid nanosilica hindered the formation of FS and decreased the Ca/Si of CSH, which weakened the chloride-binding capacity of CSH. Thus, a proper hybrid nanosilica dosage is recommended, and in our current research, it was shown that the dosage should not exceed 1%.
When considering the limitations of our research, the bonding behavior between the matrix and HSM was not considered, as well as the surface texture, moisture, etc. Moreover, the water to cement ratios of the matrix and HSM were different, which led to varying mechanical properties. In the future, the overall mechanical performance of the mortar matrix and HSM should be evaluated. Thus, in further research, we will try to develop an HSM with the same or a similar water to cement ratio to increase the bonding behavior and compatibility between the matrix and the coated layer.

Author Contributions

Conceptualization, K.L. and J.X.; methodology, Y.G.; validation, K.X., X.X. and L.W.; formal analysis, K.L.; investigation, W.L.; resources, J.X.; data curation, Y.G.; writing—original draft preparation, W.L.; writing—review and editing, X.X.; visualization, L.W.; supervision, K.L.; funding acquisition, K.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Fundamental Research Funds for the Central Universities, grant number B210201041, and the National Natural Science Foundation of China, grant number 52178202, 52108206, and 51808188. Additionally, the authors thank the Jiangsu Research Institute of Building Science Co., Ltd. and the state key laboratory of high-performance civil engineering materials for funding this research project.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to acknowledge the financial support from the Fundamental Research Funds for the Central Universities (B210201041) and the National Natural Science Foundation of China (Grant Nos. 52178202, 52108206, and 51808188). Additionally, the authors thank the Jiangsu Research Institute of Building Science Co., Ltd. and the state key laboratory of high-performance civil engineering materials for funding this research project.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic illustration of HN particle.
Figure 1. Schematic illustration of HN particle.
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Figure 2. Surface rebound numbers of different samples under 0.1 mol/L NaCl solution.
Figure 2. Surface rebound numbers of different samples under 0.1 mol/L NaCl solution.
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Figure 3. Surface rebound numbers of different samples under 0.5 mol/L NaCl solution.
Figure 3. Surface rebound numbers of different samples under 0.5 mol/L NaCl solution.
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Figure 4. Surface rebound numbers of different samples under 1 mol/L NaCl solution.
Figure 4. Surface rebound numbers of different samples under 1 mol/L NaCl solution.
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Figure 5. Chloride diffusion coefficients of different types of samples.
Figure 5. Chloride diffusion coefficients of different types of samples.
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Figure 6. XRD patterns of samples after being soaked in 1 mol/L NaCl solution for 28 d.
Figure 6. XRD patterns of samples after being soaked in 1 mol/L NaCl solution for 28 d.
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Figure 7. XRD patterns of samples near the peak of Friedel salt (soaked in 1 mol/L NaCl solution).
Figure 7. XRD patterns of samples near the peak of Friedel salt (soaked in 1 mol/L NaCl solution).
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Figure 8. DTG patterns of samples after being soaked in 1 mol/L NaCl solution for 28 d.
Figure 8. DTG patterns of samples after being soaked in 1 mol/L NaCl solution for 28 d.
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Figure 9. TG patterns of samples after being soaked in 1 mol/L NaCl solution for 28 d.
Figure 9. TG patterns of samples after being soaked in 1 mol/L NaCl solution for 28 d.
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Figure 10. Thermodynamic modeling results: (a–c) phase assemblages of blends in contact with NaCl solution; (d) volume changes in hydrates with different SiO2 contents; (e) volume of Friedel salt in contact with NaCl solution.
Figure 10. Thermodynamic modeling results: (a–c) phase assemblages of blends in contact with NaCl solution; (d) volume changes in hydrates with different SiO2 contents; (e) volume of Friedel salt in contact with NaCl solution.
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Table
Table 1. Chemical composition of Portland cement (wt.%).
Table 1. Chemical composition of Portland cement (wt.%).
CaOSiO2Al2O3MgOFe2O3SO3K2ONa2O
62.8320.55.611.73.843.071.310.21
Table
Table 2. Mix proportions of the prepared cement paste samples.
Table 2. Mix proportions of the prepared cement paste samples.
Sample Water to Cement Ratio HN Content (%)
BK0.350
A0.351
B0.352
Table
Table 3. Mass rates of CH and Friedel salt content in samples after being soaked in 1 mol/L NaCl solution for 28 d.
Table 3. Mass rates of CH and Friedel salt content in samples after being soaked in 1 mol/L NaCl solution for 28 d.
SampleABBK
CH content (%)12.1711.4912.56
Friedel salt content (%)6.436.126.56
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Lyu, K.; Xu, J.; Gu, Y.; Xia, K.; Wang, L.; Liu, W.; Xie, X. Evaluation of the Surface Performance of Mortar Matrix Subjected to Sodium Chloride Solution Modified with Hybrid Nanosilica Cement Paste. Sustainability 2022, 14, 9876. https://doi.org/10.3390/su14169876

AMA Style

Lyu K, Xu J, Gu Y, Xia K, Wang L, Liu W, Xie X. Evaluation of the Surface Performance of Mortar Matrix Subjected to Sodium Chloride Solution Modified with Hybrid Nanosilica Cement Paste. Sustainability. 2022; 14(16):9876. https://doi.org/10.3390/su14169876

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Lyu, Kai, Junjie Xu, Yue Gu, Kailun Xia, Lei Wang, Weiwei Liu, and Xian Xie. 2022. "Evaluation of the Surface Performance of Mortar Matrix Subjected to Sodium Chloride Solution Modified with Hybrid Nanosilica Cement Paste" Sustainability 14, no. 16: 9876. https://doi.org/10.3390/su14169876

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