Carbonation Resistance of Hybrid NanoSiO2 Modified Cementitious Surface Protection Materials

XIA Kailun, GU Yue＊, JIN Weizhun, JIANG Linhua＊, LÜ Kai, GUO Mingzhi

(1. College of Mechanics and Materials, Hohai University, Nanjing 211100, China; 2. College of Civil and Transportation Engineering, Hohai University, Nanjing 210000, China)

Abstract: Hybrid nanoSiO2 (HNS) modified cement pastes were explored as a kind of surface protection material (SPM). The carbonation resistance and mechanical properties of SPMs coated samples were tested.Thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP) were further employed to evaluate the chemical composition and microstructure characteristics of SPM. Besides, thermodynamic modeling was adopted to simulate the changes in the phase assemblages of SPM under the carbonation process. The results showed that SPM with 1 wt% HNS could effectively enhance the carbonation resistance. The incorporation of HNS could densify the microstructure and refine the pore structure. Moreover, the thaumasite can be stable at ambient temperature with the addition of HNS, which is beneficial to maintain alkalinity under the carbonation process.

Key words: hybrid nanoSiO2; carbonation resistance; surface protection materials; pore structure

1 Introduction

Currently, concrete is the most widely used material in engineering due to its excellent performance and low price[1]. Cement is the most important component of cementitious materials in concrete.From 1930 to 2013, the global carbon emissions from industrial cement production have reached nearly 40 billion tons[2]. Industrial cement production results in 5%-8% of global carbon emissions[3]. To a certain extent, it has caused the greenhouse effect of the earth[4]. The durability of concrete has a great impact on the stability and service life of concrete.Many concrete infrastructures have been damaged before reaching the expected service life. Repairments and reconstructions of concrete infrastructure are important aspects of cement consumption. Increasing the durability of concrete is an important way to extend the life of concrete structures and reduce global cement production, thus decreasing carbon emissions. In practical engineering problems, concrete is often exposed to many corrosive environments[5]and suffered from the corrosions of many physical and chemical factors[6], such as chloride, sulfate, water,and carbon dioxide. Among them, carbon dioxide diffusion will lower the pH of concrete, resulting in deterioration of the steel passivation film and making the steel rust easier when exposed to chloride ions[7-9].Hence, carbonation resistance is an important aspect of concrete durability. Due to the porous structure of the concrete surface, the impermeability is weak, and corrosions of concrete tend to proceed from the surface to the inside[10]. Therefore, the carbonation resistance of concrete is determined by the surface properties.

Nowadays, several methods to modify the concrete surface were investigated by researchers,such as surface coating, surface sealing treatment,hydrophobic impregnation, and so on[11]. NanoSiO2(NS) is a kind of lower price nanomaterial with pozzolanic effect[12], seeding effect[13,14], and filling effect that can compact the surface structure of concrete[15,16]. Therefore, many researchers have studied the application of NS in the modification of concrete surface. For example, Zhanget al[17]made a surface protection material (SPM) with NS (nanoSiO2) and SF(silica fume) and found that NS and SF can improve the pore structure. Meiet al[18,19]reported that in the NS and fly ash binary hybrid system, pore structure had been improved and nanoscale pores increased.Duet al[20]pointed out that NS can greatly reduce the capillary pores in the cement matrix, thereby enhancing impermeability. Sanchezet al[21]used colloidal NS for surface treatment of hardened mortar and found that it has the sealing effect and pores filling effect on the surface of hardened mortar. However, it is worth noting that NS tends to agglomerate due to its small particle size[22], especially in the pore solution of cement paste, which contains many kinds of positive ions. To ameliorate the dispersion of NS in pore solution, many researchers tried to find a kind of hybrid surface-modified nano-SiO2with well dispersion and functionality.

For NS, there are three types of hydroxyls,including isolated hydroxyl, vicinal hydroxyl, and germinal hydroxyl. These hydroxyls can provide the condition of surface modification of NS and serve as bridges for chemical or physical modification[23].Considering the possibility of surface modification of NS, various polymers can be added to the surface of NS to synthesize organic-inorganic hybrid nanoSiO2(HNS). The organic component can endow specific properties of HNS, such as enhanced dispersion and hydrophobic properties. Ernestoet al[24]functionalized silica particles with n-dodecyl groups(-C12H25) to synthesize hydrophobic particles and the water absorption of modified concrete decreased up to 45% relative to unmodified counterpart. Guet al[25-27]grafted shrinkage reducing admixture (SRA) on the NS to synthesize novel hybrid nanoparticles, which have capacity of reducing shrinkage and mitigating cracking of cement pastes at later ages. And a kind of nanoSiO2@graphene-oxide (NS@GO) nanoparticles consisting of graphene oxide and colloid nanoSiO2was reported to increase the flexural strength of cement composites by 49.2%. A kind of SPM modified with HNS used to enhance carbonation resistance was reported[28], and result showed the carbonation depth of modified SPM decreased by 79.0% compared to reference. Zhuet al[29]synthesized a modified hybrid core-shell nano-silica PMMA-SiO2and found the dispersion was improved a lot. Liet al[30,31]synthesized SiO2/PMHS hybrid nanocomposite and applied it to cementitious materials, which resulted in a significant improvement of permeability.

In this paper, a new kind of surface protection material (SPM) was prepared by using cement paste with HNS incorporation. A core-shell hybrid nanoSiO2(HNS) was used, which has better dispersion compared to normal NS. To investigate the carbonation resistance of SPMs with different dosages of HNS incorporation,SPMs were prepared into three types with varied content of HNS. Each type of SPM was made into three different thicknesses to explore the impacts of thickness on the mechanical properties. The interface bonding strength between SPM and substrates was detected through the flexural strength test, and the compressive strength of SPM-covered samples was investigated. The carbonation depth was detected after 28 d carbonation. To further research the influence caused by HNS, thermogravimetric tests (TG), X-ray diffraction (XRD), and mercury intrusion porosimetry(MIP) were conducted.

2 Experimental

2.1 Materials

The ordinary Portland cement (OPC) is obtained from Anhui Conch Cement Co., Ltd. The mineral components of cement are listed in Table 1.

Table 1 Chemical components of OPC

Fig.1 Molecular structure of organic and inorganic constituent of HNS[28]

Commercially available organic-inorganic HNS was provided by the Jiangsu Sobute New Materials CO., Ltd. According to the supplier, the inorganic constituent of HNS is NS, and the organic component in HNS is aliphatic molecular group which accounts for 13% by weight. The diameter of HNS ranges between 30-60 nm. The schematic diagram of the molecular structure of HNS is presented in Fig.1.

2.2 Sample preparation

The cubic samples consist of different thicknesses of substrates and SPMs. The substrates were firstly cast into a 70.7 mm×70.7 mm×70.7 mm cubic mold. After the initial setting of substrates, the SPMs were molded onto it. The molding processes are presented in Fig.2.

The rectangular samples consist of substrates and SPMs in equivalent volume. The substrates were firstly cast into half of a 20 mm×20 mm×100 mm rectangular mold. After the initial setting of substrates, the SPMs were molded in the remaining part of the mold. The molding processes are presented in Fig.3.

Table 2 Contents of HNS in different SPMs

Fig.2 The molding process of cubic samples

Fig.3 The molding process of rectangular samples

Thew/cratio of substrates was 0.5 and thew/cratio of SPMs was 0.4, and the formulas of different SPMs and samples are listed in Table 2. All samples were cured in a relative humidity of more than 95%and (20±2) ℃ environment for 28 d and then being carbonated for 28 d. The concentration of CO2was about (20±2) %, and the relative humidity is about(70±5) % during carbonation.

2.3 Test Methods

2.3.1 Mechanical properties

The compressive strength and flexural strength were tested using the cubic samples and rectangular samples after cured for 28 d, respectively. The flexural strength test was conducted to measure the interfacial bonding strength between substrates and different SPMs. Three samples were tested in each formula.

2.3.2 Carbonation depth

The carbonation depth was detected by alcohol phenolphthalein solution after 28 d carbonation.

2.3.3 Chemical compound

XRD was carried out after 28 d carbonation using a Bruker D8 Advance X-ray diffractometer. The scanning range was from 5° to 70°, and the scanning rate was 10 °/min.

TGA was carried out by a thermal analyzer(NETZSCH STA449F3, Germany) in a nitrogen atmosphere. Specimens were heated from 30 to 1 000℃ at a rate of 20 ℃/min.

2.3.4 Microstructure

MIP test was carried out by a Micrometrics AutoPore IV 9510 (American Michael Instruments Corp., USA), with maximum pressure up to 415 MPa.Pore size ranging from 3 nm-360 μm was recorded.

2.3.5 Thermodynamic modeling

A Gibbs free energy minimization program was used to predict the changes of the phase composition during carbonation in a chemical system. The database of CEMDATA18 was used[32

3 Results and discussion

3.1 Carbonation

Fig.4 shows the carbonization depth of different SPMs. Compared with BK, the carbonation depth of group A decreases by 94.4%, 97.0%, and 96.7% in each thickness, respectively, indicating that HNS can enhance the carbonation resistance of hardened cement paste. The improvement of carbonation resistance may be due to the optimization of microstructure caused by HNS. The pozzolanic effect can induce the formation of dense C-S-H, and the filling effect can further refine the pore structure of cement paste.

Unexpectedly, group B, which contains more HNS than A, shows no significant improvement in carbonation resistance compared to BK. The reason may be ascribed to the agglomeration of HNS, and the microstructure may hardly be improved.

Fig.4 Carbonation depth of different SPMs

3.2 Mechanical properties

Fig.5 Mechanical properties of different samples cured for 28 d: (a) compressive strength; (b) flexural strength

Fig.5(a) presents the compressive strength of A,B, and BK with different thicknesses of SPMs. It can be observed that the compressive strength of A and BK are relatively close to each thickness. This suggests that the incorporation of 1 wt% HNS has little effect on the compressive strength of cement paste. Unexpectedly,the compressive strength of B decreases with the increase in thicknesses of SPM. What can be speculated that the SPM of B has degraded compressive strength resulting from the agglomeration of HNS. Also, the dense C-S-H formed by the pozzolanic effect and seeding effect can create a barrier around unhydrated cement grain and thus prevent continuous hydration[33].This adverse impact manifests that the threshold for the concentration of HNS must be taken into consideration.Redundant HNS may lead to a negative effect on mechanical properties.

From Fig.5(b), it can be found that the flexural strength of A increases by 48.0% compared to BK,indicating that 1 wt%HNS can effectively enhance the interfacial bonding strength between substrates and SPMs. The formation of C-S-H in the interfacial zone promoted by HNS may be the main reason for enhanced bonding strength. Conversely, compared to BK, the flexural strength of B decreases by 8%instead of increasing. This result may be due to the agglomeration and hindrance effect of HNS as well,manifesting that 1 wt% HNS is better dispersed than 2 wt% HNS.

3.3 X-ray diffraction analysis

Fig.6 shows the XRD patterns of samples after carbonation. It is worth noting that peaks at 18.1°associated with the portlandite are only evidently observed in A. The absence of this peak in B and BK may attribute to the consumption during carbonation reaction. Several peaks corresponding to calcite at 36.0°, 39.4° and 43.8° can be observed. The differences in intensity of these peaks manifest the lowest calcite content in A.

It is worth noting that most of the peaks corresponding to ettringite were accompanied by thaumasite. The higher intensity of these peaks reflects a lower carbonation degree in A. However, an ettringite peak at 24.8° is found to be missing in A, indicating that the intensity of those “E+T” peaks in A may partially come from thuamasite rather than ettringite.Thaumasite often exists stably in low-temperature systems at 0-5 ℃. However, the curing and carbonation process were proceeded in (20±2) ℃. At this circumstance, speculation can be put forward that the HNS may impact the phase composition of hardened cement paste, which makes thaumasite capable of formation in ambient temperatures.

Fig.6 XRD patterns of different samples after 28 d carbonation

3.4 Thermogravimetric analysis

Fig.7 shows the TG and DTG curves for different samples after 28 d carbonation. Two remarkable mass loss peaks at the range of 50-200 ℃ and 600-800 ℃ are observed. The peaks at the range of 50-200 ℃ correspond to the combined water in hydration products, such as C-S-H, AFt, and aluminate hydrates[33]. The peaks in the range of 680 to 800℃ correspond to the decompose of CaCO3at high temperature[34].

Fig.7 TG and DTG curves of different samples after 28 d carbonation: (a) TG; (b) DTG

It can be seen that in Fig.7(b) that the peaks at range of 50-200 ℃ in A and B is stronger than B and BK, indicating that more C-S-H remains in A and B than BK after 28 d carbonation. Additional C-S-H in A and B can be explained in two ways: (1) A and B have better carbonation resistance compared to BK, and less C-S-H was decalcified during the carbonation process;(2) Extra C-S-H was formed due to the pozzolanic reaction between HNS and portlandite.

The peaks at the range of 680-800 ℃ show that the amount of CaCO3in A is remarkably less than that in B and BK. This indicates that A has a lower degree of carbonation, which is consistent with previous results of carbonation depth. Content of CaCO3in different samples can be calculated through the weightless curves in Fig.7(a). The relative contents of CaCO3in A, B, and BK are 30.23%, 36.38%, and 38.45%. Comparing to BK, the contents of CaCO3decreased by 27.19% in A and 5.69% in B, respectively.Furthermore, the CaCO3peaks of B and BK shift to a higher temperature which demonstrates a higher degree of crystallization of CaCO3and confirms better carbonation resistance of A.

3.5 Pore structure analysis

Pore size distributions of different samples before and after 28 d carbonation are shown in Fig.8.Samples before and after carbonation are marked with“-b” and “-a” for short,e g, A-b represents for group A before carbonation. The pores are classified into four categories including gel pores(≤10 nm), transition pores(10-100 nm), capillary pores(100-1 000 nm) and big pores[35,36]. The specific values of each category are listed in Table 3. It is easy to be found that the total porosity of A-b and B-b have decreased by 39.30%and 1.83% compared to BK-b. This result indicates that the incorporation of 1 wt% HNS can effectively reduce the total porosity of cement paste. Furthermore,the big pores, transition pores, and capillary pores,which are often considered harmfully, are partly diminished in A-b according to Fig.8, compared to B-b and BK-b. This suggests the HNS is not only capable of reducing total porosity but also able to refine pore structure. The remarkable porosity reduction can be recognized both in B-a and BK-a compared to B-b and BK-b. Volume difference between portlandite (32.3 cm3/mol) and calcite (36.9 cm3/mol) may account for this variation[37]. The infinitesimal distinction in total porosity between A-b and A-a can be taken as evidence of low carbonation degree.

Fig.8 Pore size distribution of different samples before and after 28 d carbonation

Table 3 Pore volume(mL/g) in different diameter ranges/(mL/g)

As shown in Fig.9, the most probable apertures of A-b and B-b are smaller than that of BK-b. These results suggest the aperture is distributed over ranges in smaller diameters due to the refining affection of HNS.After carbonation, the most probable apertures enlarged in both A-a, B-a, and BK-a. This enlargement may originate from the coarsening of calcite grains[38]. The most probable aperture of A-a is smaller than B-a and BK-a, indicating that A-a has a dense microstructure to restrain the grains coarsening.

Fig.9 Pore size distribution of different samples before and after 28 d carbonation: (a) A-b and A-a; (b) B-b and B-a; (c)BK-b and BK-a

3.6 SEM

Fig.10(a) presents the microstructure of the compact C-S-H in group A. As can be seen, the dense C-S-H gel in A are gathered into the spherical cluster and the strong cross-link between C-S-H clusters is observed. Compared to A, Fig.10(b) presents a looser structure of C-S-H in BK, indicating that without HNS incorporation, the density and interweaving of C-S-H are remarkably decreased. This result confirms the two effects of HNS attached on the surface of spherical cement particles. The first is the pozzolanic effect resulting in formation of C-S-H. The second is that the seeding effect leads to acceleration on C-S-H formation. The combination of the two enhancements promotes the refining effect on microstructure.

Fig.10(c) shows the microstructure of B. More Ca(OH)2and less C-S-H can be observed compared to A, indicating that the HNS in B does not produce the same effect as that in A. The main reason may be considered to be agglomeration. Fig.10(d) displays the agglomerated HNS in B. Clumps formed by HNS can be observed with sizes ranging from tens to hundreds of nanometers.

Fig.10 SEM images of non-carbonated samples: (a) A; (b) BK; (c)B; (d) High resolution image of B

3.7 Thermodynamic modeling

According to the XRD result, thaumasite was confirmed to exist in group A. To investigate the impact of thaumasite on carbonation resistance of cement paste, a Gibbs-free energy minimization simulation program (GEMs) was used to simulate the changes of phase assemblage during the carbonation process in two situations. In T situation, thaumasite was allowed to generate in 20 ℃ to imitate A samples. In N situation, the generation of thaumasite was disabled in 20 ℃ to imitate B and BK samples.

Fig. 11 Changes of phase assemblage of cement pastes in contact with CO2: (T) with the formation of thaumasite; (N) Normal situation without thaumasite

Fig.12 Changes of pH value and Ca/Si ratio of cement pastes in contact with CO2: (a) pH value; (b) Ca/Si ratio

Comparing to B and BK, A has a more similar phase composition to T situation. This result indicates that the phase composition may be one reason for the enhanced carbonation resistance of HNS modified samples.

4 Conclusions

In this study, three kinds of SPMs with 0 wt%,1 wt%, 2 wt% HNS were prepared and applied to substrates with different thicknesses. The carbonation resistance of SPMs was evaluated through a series of tests. The main conclusions can be summarized as follows:

a) Incorporation of 1 wt% HNS can effectively enhance the carbonation resistance. Meanwhile, it can also enhance the interface bonding between substrates and SPMs but has limited effect on compressive strength. The mechanism of these enhancements can be explained as the densification of microstructure.

b) Incorporation of 2 wt% HNS has little enhancement on carbonation resistance. Similarly, it has no positive effect on compressive strength and interface bonding between substrates and SPMs. An adverse impact on compressive strength is observed when the dosage of HNS is too high. This may be related to the hindrance effect of HNS on cement continuous hydration.

c) Incorporation of 1 wt% HNS can reduce total porosity of cement paste and refine pore structure by reducing harmful pores.

d) Incorporation of 1 wt% HNS may impact the phase composition of hardened cement paste, which makes thaumasite stable at ambient temperature. The existence of thaumasite may be beneficial to maintain the alkalinity of pore solution under the carbonation process. This contributes to improve the corrosion resistance of steel.

Acknowledgments

The authors thank Jiangsu Research Institute of Building Science Co., Ltd and the State Key Laboratory of High-performance Civil Engineering Materials for funding this research project.