New insights into how MgCl2 deteriorates Portland cement concrete

Cement and Concrete Research 120 (2019) 244–255

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Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres

New insights into how MgCl2 deteriorates Portland cement concrete Ning Xie

a,b

c

, Yudong Dang , Xianming Shi

d,⁎

T

a

Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, School of Materials Science and Engineering, University of Jinan, Jinan, Shandong 250022, China Western Transportation Institute and Civil Engineering Department, College of Engineering, 2327 University way #6, Montana State University, Bozeman, MT 597174250, USA c Yunnan Institute of Building Research, #150, Xuefu Road, Kunming 650223, China d Laboratory of Advanced & Sustainable Cementitious Materials, Dept. of Civil and Environmental Engineering, Washington State University, Sloan 101, P.O. Box 642910, Pullman, WA 99164-2910, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Magnesium chloride Portland cement concrete Degradation Microstructure Chemical mechanism

The consensus on mechanisms responsible for how chloride-based salts attack concrete has yet not been achieved. This work exposed laboratory-fabricated concrete samples to MgCl2 solutions of various concentrations along with freeze/thaw and wet/dry cycles, with NaCl solutions as the control. The laboratory investigation revealed a new chemical mechanism during the MgCl2-induced deterioration of concrete, i.e., the formation of multi-phase nano-sized crystals, including CaCl2, Mg(OH)2, and Mg3(OH)5Cl·(H2O)4. This mechanism was further corroborated by the testing of concrete samples cored from several selected concrete bridge decks, where cumulative exposure to MgCl2 deicer resulted in significantly compromised splitting tensile strength (as high as 50%) as well as reductions in microhardness (up to 60%, often at a depth of 25 to 50 mm). This study provides new insights into the risks of MgCl2 to the concrete-built environment. It also alerts that visual inspection could be misleading for condition assessment of concrete exposed to MgCl2.

1. Introduction Concrete is a ubiquitous building material used in modern construction and is known to be a cost-effective material for aggressive service environments [1,2]. Nonetheless, the mechanical properties of concrete (strength, modulus of elasticity, etc.) may deteriorate significantly over time as a result of its exposure to chemicals, such as the chloride-based salts commonly used for snow and ice control on roadways and parking lots. The mechanisms underlying the attack of chloride-based deicers (NaCl, MgCl2, etc.) to concrete remain debatable even though numerous studies have been conducted on the subject [3–8]. For concrete materials in service, their exposure to chloride-based deicers can induce both physical and chemical damages, which subsequently compromise the life-cycle integrity, reliability, and performance of buildings, structures, tunnels, pavements, etc. The physical mechanism of deicer attack, which mainly results from accelerated freeze/thaw (F/T) cycles, can lead to scaling, map cracking, or paste disintegration of Portland cement concrete (PCC) [4,5]. The chemical mechanism of deicer attacks is caused by the reactions of deicers with cement paste and/or aggregate phase, leading to reduced integrity and

⁎

strength [3–8]. For reinforced concrete structures, these physicochemical damages of the concrete matrix can exacerbate the ingress of moisture, oxygen and aggressive agents (e.g., chloride anions) onto the surface of reinforcing steel and increase the corrosion risk of steel rebar [7,9–11]. Numerous studies have been conducted in many laboratories, often in an accelerated manner, which reported the physicochemical deterioration of concrete as a function of deicer type and test protocol [3–8,12]. These studies illustrate the complexity of this concrete durability issue and suggest that there is more than one mechanism at work. Jain et al. [5] subjected PCC specimens to deicers solution in the laboratory, with NaCl, CaCl2 and MgCl2 at 10.5 M for wet/dry (W/D) cycles and 5.5 M for F/T cycles. They concluded that exposure to CaCl2 induced the most reduction in physical and mechanical properties of concrete. Shi et al. [6] reported that after approximately one year of continuous immersion to 8 wt% commercial deicers (NaCl, CaCl2, or MgCl2) at room temperature (without F/T cycles), detectable changes in the chemistry and morphology of cement hydrates inside the concrete specimens could be observed. Notably, with the PCC specimens exposed to the MgCl2-based deicer, the samples exhibited the most reduction in compressive strength. In yet another study [7], Dang et al.

Corresponding author. E-mail address: [email protected] (X. Shi).

https://doi.org/10.1016/j.cemconres.2019.03.026 Received 20 November 2017; Received in revised form 24 March 2019; Accepted 24 March 2019 0008-8846/ Published by Elsevier Ltd.

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MgCl2-H2O is located at −33.2 °C and the concentration of 21.6 wt%. The microstructure of concrete changes as a result of exposure to MgCl2 environment, in light of the formation of brucite, Friedel's salts, M-S-H, magnesium oxychloride, and/or secondary calcium oxychloride. These chemical reactions can result in severe deterioration of concrete materials. Furthermore, the formation of calcium oxychloride (or magnesium oxychloride) and brucite would make the cementitious material unsaturated, which leads to changes in its transport paths and damage behaviors [27]. Recent years have seen increased use of MgCl2 as the freezing point depressant in anti-icing or deicing products [11], it is thus necessary to shed light into the risks of MgCl2 to the concrete-built environment. Such mechanistic understanding of this concrete degradation risk is crucial for developing effective countermeasures of either prevention or mitigation. Although significant progress has been made in unraveling the impact of MgCl2 on concrete materials, the mechanisms of how MgCl2 reacts with cementitious materials remain relatively uncertain. Specifically, a research gap lies in the lack of linkage between the phenomena occurring at the micron or nanometer scales and the changes in engineering properties at the mesoscale or macroscopic scale. The objective of the present study is to unearth new insights into this research gap, by analyzing the complex reaction products of this chemical attack from multiple length scales. This work presents a study of laboratory-fabricated concrete samples to investigate the relative impact of MgCl2 vs. NaCl (coupled with F/T and W/D cycles) on Portland cement concrete and the associated mechanism(s). In addition, concrete cores taken from a selection of bridge decks in Oregon were investigated to further validate the impact of MgCl2 on concrete materials.

exposed PCC specimens to 15 F/T cycles in the presence of 3 wt% NaCl or 2.54 wt% MgCl2 solution. The PCC specimens exposed to diluted MgCl2 solution exhibited a considerable reduction in their splitting tensile strength or STS (up to 55%) but no apparent scaling, implying a predominantly chemical deterioration. In contrast, the PCC specimens exposed to diluted NaCl solution exhibited a limited loss in their STS but visible surface scaling and corresponding reductions in the modulus of elasticity, implying a predominantly physical deterioration. Sutter et al. [4] exposed mortar specimens to concentrated deicer solutions (18 wt% NaCl or 14 wt% MgCl2) and F/T cycles and demonstrated that MgCl2 has a more significant potential to damage cementitious composites, relative to NaCl. The same authors [3] reported that the field cores extracted from selected concrete bridge decks (Colorado, Idaho, Iowa, Montana, and South Dakota, exposed to either MgCl2 or both NaCl and MgCl2 deicers) exhibited some damage or distress, but those damages could not be conclusively attributed to the chemical attack by deicers. A certain level of consensus among existing laboratory studies [4,13–18] and several field cases [3,19] have demonstrated that MgCl2 can chemically degrade PCC. As shown in Eq. (1), MgCl2 can react with the cementitious calcium silicate hydrate (C-S-H) present in the cement paste and turn it into non-cementitious magnesium silicate hydrate (MS-H). As shown in Eq. (2), MgCl2 can also react with another type of cement hydration product, Portlandite, and produce a crystalline phase known as brucite (Mg(OH)2). The formation of brucite (and other crystalline phases mentioned later) in confined concrete pores induces significant crystallization pressure and expansive forces, which may result in localized cracking of the concrete [4,6,18,20]

MgCl 2 + C − S − H → M − S − H + CaCl2

(1)

MgCl 2 + Ca(OH)2 → Mg(OH)2 + CaCl2

(2)

2. Materials and methods

A few laboratory studies [4,21] reported the formation of yet another potentially detrimental phase, calcium oxychloride (3CaO·CaCl2·15H2O) as a result of exposing concrete to MgCl2. This crystalline phase formed in cement mortars exposed to 15% MgCl2 solutions for 84 days, as evidenced by optical microscopy, SEM, and microanalysis [4]. The proposed mechanism is based on Eqs. (2) and (3):

3Ca(OH)2 + CaCl2 + 12H2 O → 3CaO·CaCl2·15H2 O

2.1. Fabrication of laboratory concrete specimens Concrete specimens were fabricated in the laboratory under wellcontrolled conditions and then subjected to designed tests detailed in later sections. An ASTM specification C150-07 Type I/II low-alkali Portland cement from Diamond Mountain, MT was used, along with coarse aggregate (maximum size of 9.5 mm) and fine aggregate (clean, natural silica sand, maximum size of 4.75 mm, fineness modulus of 3.1) from the JTL Group (Belgrade, MT). The mix proportioning of concrete is summarized in Table 1, featuring a w/c ratio of 0.55 to allow accelerated degradation of the concrete. The chemical agent triethylamine (TEA) was used at a dosage of 0.05% by mass of cement, as a setaccelerating admixture; and this simulates a typical mix design used in cold-weather concreting. The dosage of the air-entraining agent (MicroAir™) was 0.006% by mass of cement, lower than the vendor-recommended dosage. This reduced dosage was used to entrain less air voids than those typically seen in concrete with well-designed F/T durability, aimed to simulate insufficiently air-entrained concrete and to accelerate the deterioration of concrete specimens in the laboratory investigation.

(3)

The petrographic evidence indicated that plate-shape calcium oxychloride crystals and their carbonate-substituted phase precipitated in air voids and cracks, along with consumption of Portlandite. In addition, Friedel's salt (3CaO·Al2O3·CaCl2·10H2O) was detected [21]. In another laboratory study [4], brucite was also observed in the outer layers of the PCC specimens exposed to concentrated MgCl2. Peterson et al. pointed out that the damage of concrete and mortar with exposure to chloride-rich environment mainly resulted from the expansion and deterioration associated with the formation of calcium oxychloride hydrate (3CaO·CaCl2·15H2O or 3CaO·CaCl2·12H2O) phases [22]. This point was further indicated by Weiss' group, who elucidated the deteriorating mechanisms of concrete materials exposed to chloride-based deicers. For instance, with exposure to MgCl2 environment, calcium leaching and the formation of calcium oxychloride phases are the critical causes of concrete deterioration [23]. Ghazy systematically investigated the impact of chloride-based salts on concrete materials and stated that the formation of complex oxychlorides, including magnesium oxychloride and calcium oxychloride, is the dominant factor resulting in the deterioration of concrete [24,25]. Apart from the formation of the oxychlorides [26], a laboratory study [8] reported that, with exposure to MgCl2 environment, the cementitious materials could chemically react with MgCl2 and form a variety of products in concrete. The authors plotted the phase diagram for MgCl2-H2O, and the comparison of freezing temperature for aqueous MgCl2 with NaCl, CaCl2, and potassium acetate (KAc), shown as Fig. 1 [8]. As demonstrated in the phase diagram, the eutectic point of

2.2. Cyclic F/T and W/D test in the presence of deicers The impacts of different deicers on concrete were assessed with the cyclic F/T and W/D test in the presence of deicer solutions. The concentrations of the deicer solutions were 1%, 3%, 5%, 7%, 10%, 15%, and 20% by weight of aqueous solution. Depending on the specific design of the experiments, 10 or 15 F/T along with W/D cycles were employed to simulate the temperature and moisture changes experienced by field concrete in an accelerated manner. Each cycle consisted of 48 h of full immersion of the concrete specimens in the diluted MgCl2 or NaCl solution at −30 ± 1.7 °C (−22 ± 3.1 °F) and RH 50 ± 5%, followed by 24 h of thawing and 12 h of exposure in dry air at room temperature 23 ± 1.7 °C (73.4 ± 3 °F, RH 25 ± 5%). The lower end 245

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Fig. 1. a) Phase diagram of MgCl2-H2O and b) the comparison of freezing temperature for aqueous MgCl2 with that of NaCl, CaCl2, and KAc [8].

2.4. Microhardness test

Table 1 Mix proportion of concrete, kg/m3. Cement

Water

TEA

407

223.9

0.2035

Fine aggregate 655

Coarse aggregate 1022

Microhardness test is an approach to evaluating the mechanical properties of brittle materials from a microstructural perspective. This technique entails applying a static load on a polished specimen surface for a certain period, followed by measuring the size of the indented marks to quantify the microhardness of the tested spot on the specimen. In this study, small specimens of 36 × 12 × 12 mm3 were cut from the original cores extracted from the bridge decks using a water-cooled diamond saw. Subsequently, the cut samples were polished sequentially using #320, #600, #800 and #1200 grit sandpapers to obtain an adequately flat mirror surface. Before the microhardness testing, the polished samples were cleaned in de-ionized water and then dried in a container with RH 59% for 12 h. A Digital Vicker's Microhardness Tester (HXS-1000AY, Shanghai Highwell Optoelectronic Technology Co., Ltd.) was employed for the measurements. During the loading process, 0.25 N (0.025 kgf) was applied and held for 10 s. The microhardness HV was calculated as follows:

Air-entraining agent 0.02442

temperature of F/T cycles was chosen to be sufficiently low to ensure that most of the concrete pore solution in the air voids and micro-pores would freeze despite the presence of the deicer solution. During the F/T cycles, the rate of temperature drop/rise of both solutions averaged at approximately 4.5 °C/h (8.1 °F/h); as such, the solution temperature reached its minimal or maximal value temperature in nearly 12 h. The rapid rate of freezing is an acceleration of freezing commonly seen in the field concrete. After the given number of F/T cycles, the test specimens were individually rinsed under running tap water and handcrumbled to remove any scaled-off material. The specimens were then air-dried for 24 h at 23 ± 1.7 °C (73.4 °F), RH 45–55%, before being weighed and subjected to mechanical testing and microscopic characterization.

HV =

STS is a critical property of concrete concerning its cracking resistance, shear capacity, anchorage capacity, and durability. In this study, the STS of laboratory-fabricated and field-cored specimens was determined following the standard test method ASTM C496/496M-17. The test method consisted of applying a diametrical compressive force along the longitudinal axis of the cylindrical concrete specimens at a rate of 25 to 50 pounds per second (11 to 23 kg/s). Two thin pieces of plywood were placed along the longitudinal axis of the concrete cylinder, such that the compressive load was evenly distributed. The maximum load at the failure was recorded and used to calculate the STS as follows.

P·C 145·π·L·D

(5)

where F = force, in kgf; d = mean diagonal length of the indentations, in mm. For each sample, four depth levels, 2–5 mm, 15–20 mm, 25–30 mm and 50–60 mm from the deck's driving surface were chosen for testing to examine the microhardness of paste and interfacial transition zone (ITZ) as a function of concrete depth, respectively. This aimed to examine the potential effect of deicer exposure and other factors on the microhardness of these two concrete phases. For each depth level, the microhardness was measured for at least 60 spots randomly selected from three different paste areas at least 100 μm away from aggregate. Similarly, a coarse aggregate with the diameter larger than 5 mm was chosen at each depth, and the microhardness was then measured for at least 60 spots randomly chosen from the ITZ around this aggregate (25 μm to 50 μm away from the edges of aggregate).

2.3. STS test

ST S (in MPa) =

1.8544F d2

2.5. Microscopic characterization (4) The fracture surface morphologies of selected field-cored or postexposure laboratory-fabricated PCC samples were examined using an FEI-Quanta 200F scanning electron microscope coupled with energydispersive X-ray spectroscopy (EDS) analyzer. For the SEM, a typical accelerating voltage of 20 kV was applied. The fracture surfaces of the samples were gold sputtered for 60 s before the microstructure observation. For EDS, a typical 15–20 kV accelerating voltage was applied with a scan time of 60 s per sampling area. The ratio of the elements of interest was determined based on EDS analyzer with face scanning

where: P = Load at failure (in pounds) C = Estimated length of contact on the top and bottom (for unscaled specimens, C = 2; but significantly scaled specimens would feature lower contact areas) L = Length of the specimen (in inches) D = Diameter of the specimen (in inches)

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samples exposed to 20 wt% NaCl (or 20 wt% MgCl2) solution exhibited nearly intact external dimensions, implying that the concrete underwent less number of F/T cycles as a result of the concentrated NaCl significantly reducing the freezing point of the concrete pore solution. A similar test was implemented by Sutter et al. to investigate the impacts of various deicers on the durability of concrete [4]. Different from the results presented here, their study found that the external morphology of the concrete samples with exposure to MgCl2 solution was worse than the samples with exposure to NaCl solution. In their study, the impacts of CaCl2, NaCl, and MgCl2 solutions were evaluated by immersing the concrete samples into the solutions with a concentration of 15% for 84 days at a constant temperature (without F/T and W/D cycles). As a result, the concrete deterioration mainly resulted from the chemical reactions between the cement hydrates and the ions from the deicer solution. In the present study, however, the impacts of the NaCl and MgCl2 on concrete were tested along with F/T and W/D cycles. The F/T and W/D cycles greatly affected the solubility of various reactants (e.g., Portlandite) and reaction products (e.g., brucite) as well as the ingress rate of water and other detrimental species (e.g., Cl−, Mg2+, and Na+) into the concrete. Furthermore, the concrete deterioration resulted from a combination of chemical attack and physical attack. The physical attack may be a result of conventional frost damage (e.g., freezing of evaporable water and subsequent volume expansion, hydraulic pressure, and osmotic pressure) coupled with salt weathering (i.e., the buildup of internal stress due to salt precipitation during the W/D cycles). Fig. 4 presents the data of mass loss of the concrete samples exposed to MgCl2 and NaCl solutions, respectively. Fig. 4a reveals mass gains (shown as negative mass losses) as a result of concrete exposure to MgCl2 solutions, which is mainly ascribed to the accumulation of magnesium chloride salt inside the concrete as a result of W/D cycling. The highest mass gain was no more than 1.0%. In contrast, Fig. 4b indicates that the mass loss of concrete samples exposed to the NaCl solutions clearly increased with increasing number of F/T and W/D cycles. The highest and lowest mass loss for the samples after 10 cycles were approximately 17% and 8% in the NaCl solution with concentration of 5 wt% and 15 wt%, respectively. Fig. 5 shows the STS of laboratory-fabricated concrete cylinders after exposure to 0, 3, 8 and 15 F/T and W/D cycles, in the presence of 3 wt% and 15 wt% of MgCl2 and NaCl solution, respectively. As demonstrated in Fig. 5a, relative to the control group (0 cycles), the concrete cylinders exposed to 3 wt% MgCl2 solution exhibited a more sizeable reduction (from 6.4 to 2.8 MPa) in their STS than their counterparts (from 6.4 to 5.3 MPa) exposed to 3 wt% NaCl solution after 15 F/T and W/D cycles. In the solutions with a concentration of 15 wt %, as shown in Fig. 5b, similar regularity can be observed. The STS of the samples with exposure to 15 wt% of MgCl2 solution distinctively

mode corresponding to the low-magnification SEM images. The thermal properties of the paste powder samples were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), using a Mettler Toledo-TGA/SDTA851e. The measurements were performed in air with a temperature range from 25 °C to 1000 °C and a heating rate of 15 °C/min. The precipitate formed after exposing laboratory-fabricated PCC samples to 20 wt% MgCl2 (coupled with 15 F/T and W/D cycles) was examined using both the X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). For the XRD examination, the precipitate was filtered and then dried in air at room temperature for 24 h. After being dried, the remained white powder was used as the sample for the XRD examination. Care was taken to minimize any carbonation risk of this power sample. The XRD pattern was obtained on a Rigaku D/max-rA X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 45 kV and 20 mA. The diffraction scans were performed from 5° to 90° at a rate of 1°/min, with a step size of 0.02°. For the TEM examination, the dried powder was dispersed in ethanol via ultrasonication. Then a small drop of the dispersion was transferred to a copper grid for TEM observation. The transmission electron microscopy (TEM) images and corresponding selected area electron diffraction (SAED) patterns were collected by a Hitachi-7700 microscope with an accelerating voltage of 100 kV.

3. Results and discussion 3.1. Laboratory investigation 3.1.1. Deicer deterioration of laboratory-fabricated samples: MgCl2 vs. NaCl Fig. 2 presents the original states of the concrete samples in the 1 wt % and 20 wt% deicer solutions of MgCl2 or NaCl, respectively, after exposure to 15 F/T and W/D cycles. It was evident that the exposure of concrete to 20 wt% MgCl2 (along with F/T and W/D cycles) resulted in the formation of white precipitates (shown as Fig. 2a). Note that only laboratory-fabricated specimens were subjected to such cyclic F/T and W/D test in the laboratory. The field-cored specimens were directly subjected to mechanical testing and microscopic characterization without further exposure. Fig. 3 illustrates the external dimensions of the laboratory-fabricated concrete cylinders exposed to MgCl2 and NaCl solutions of various concentrations (0 wt%, 10 wt%, and 20 wt%) after 10 F/T and W/D cycles. All the concrete samples exposed to MgCl2 solutions exhibited no visible surface distress, even after 10 F/T and W/D cycles. In contrast to the samples exposed to 10 wt% MgCl2, the concrete samples with exposure to 10 wt% NaCl solution exhibited significant surface scaling after 10 F/T and W/D cycles. It is interesting to note that the concrete

Fig. 2. Concrete samples exposed to 15 F/T and W/D cycles and a) 20 wt% or b) 1 wt% chloride deicer solutions. 247

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Fig. 3. Laboratory-fabricated PCC samples after exposure to 10 F/T and W/D cycles and a) 0% MgCl2; b) 10 wt% MgCl2; c) 20% MgCl2; d) 0% NaCl; e) 10 wt% NaCl; f) 20 wt% NaCl solutions.

and MgCl2, respectively. For the sample exposed to water, the C-S-H phase was well maintained, and little crystal precipitate could be detected (Fig. 6a). For the sample exposed to 3 wt% NaCl, however, the fracture surface of the NaCl sample was non-flat and showed many needle-shaped precipitates, which are nanometers in diameter and randomly distributed in the hydration products without any orientation. For the concrete samples exposed to 3 wt% MgCl2 solutions, however, featured different microstructures than those exposed to tap water and or 3 wt% NaCl solution. One typical microstructure observed in the MgCl2 exposed samples was the large quantity of homogeneous precipitate phase, which was randomly distributed in the cement paste matrix, as shown in Fig. 6c). Another typical one was the highly porous microstructure with plate-like crystalline precipitates, as shown in Fig. 6d). Figs. 7 to 9 demonstrate the SEM/EDS results of the concrete samples after 15 F/T and W/D cycles in tap water, 3 wt% NaCl, and 3 wt% MgCl2 solutions, respectively. For the concrete sample exposed to tap water (Fig. 7), the Ca content was 15.57% (regarding atomic concentration), and no Mg signal was detected (Fig. 7b). For the concrete

dwindled (from 6.4 to 4.8 MPa), while for those with exposure to 15 wt % of NaCl solution, the variation is relatively stable (from 6.4 to 6.3 MPa) after 15 times of F/T and W/D cycles. 3.1.2. Microscopic analyses of selected laboratory-fabricated concrete samples Although it is difficult to accurately characterize the element information through the SEM/EDS analysis of the fracture surfaces, it is still the most widely accepted tool to semi-quantitatively characterize the morphologies and structures of the phases and chemical information of the cement paste [24]. Compared with the BSE, which gives the chemical and phase structure information by analyzing a polished surface, the SEM/EDS of a fracture surface will sacrifice the accuracy of the element quantity but provide original morphology information to elucidate the potential synergistic deteriorate mechanisms of concrete with exposure to chemicals, via the direct morphology observation along with quasi-quantitative analysis. Fig. 6 presents the SEM images of the fracture surfaces of the concrete samples after 15 F/T and W/D cycles in tap water, 3 wt% NaCl,

Fig. 4. Mass loss of laboratory-fabricated PCC samples as a function of F/T cycles with various concentrations of a) MgCl2 and b) NaCl solutions. 248

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Fig. 5. STS of laboratory-fabricated PCC samples after exposure to F/T and W/D cycles in a) 3 wt% NaCl and 3 wt% MgCl2 solution, and b) 15 wt% NaCl and 15 wt% MgCl2 solution.

3.1.3. Phase characterizations of the precipitated product Fig. 10 shows the XRD pattern of the white precipitates (see Fig. 2) collected from 20 wt% MgCl2 solution after 15 F/T and W/D cycles of the concrete samples. The XRD results reveal crystalline peaks of CaCl2, Mg(OH)2, and Mg3(OH)5Cl·4(H2O). The main characteristic peaks of CaCl2, Mg(OH)2, and Mg3(OH)5Cl·4(H2O) were clearly observed, which suggested the presence of these crystals in the white precipitates. The main peaks of Mg(OH)2 were also observed, but the signals were much lower than those of Mg3(OH)5Cl·4(H2O). Similar to our previous study [7], the exposure to NaCl deicer and F/T cycles typically leads to visible scaling and significant mass change of concrete, whereas the exposure to MgCl2 deicer and F/T cycles did not result in any of these visible symptoms (Fig. 3). Instead, the MgCl2induced deterioration of concrete featured decalcification of the binder phase, formation of new crystalline phase, and degradation of the overall microstructure, which were responsible for the observed loss in

sample exposed to NaCl (Fig. 8), the Ca contents were 11.96 at.%, suggesting the Ca leaching of the concrete. As demonstrated in the highmagnification SEM image (Fig. 8c), the diameter of nano-sized needleshaped precipitates were about 50 nm with Ca, Si, Al, Cl, and Na contents of 12.08 at.%, 2.49 at.%, 0.77 at.%, 0.31 at.%, and 0.93 at.%, respectively (Fig. 8d). Fig. 9 presents the SEM/EDS results of the concrete samples after 15 F/T and W/D cycles in 3 wt% MgCl2 solution, at low, medium, and high magnification levels, respectively. In this sample, the Mg content was as high as 12 at.% corresponding to the two typical fracture surface images. In addition, as can be seen from the high magnification SEM/ EDS results, the plate-like precipitates were composed of high Mg and Cl, about 22 at.% and 0.7 at.%, respectively. The Ca and Si content was as low as 1 at.% and 0.1 at.%, respectively.

Fig. 6. SEM micrographs of the concrete samples after 15 F/T and W/D cycles in a) tap water, b) 3 wt% NaCl solution, c) homogeneous precipitate phase and d) crystalline precipitate phase in 3 wt% MgCl2 solution. 249

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Fig. 7. SEM/EDS results of the concrete samples after 15 F/T and W/D cycles in tap water a) high-magnification SEM image and b) corresponding EDS results.

reactions (1) to (3), depending on the reaction conditions. One additional chemical reaction that might have occurred is presented as follows.

its mechanical properties. The STS reduction seen in the concrete samples averaged 56% and 18%, after 15 F/T and W/D cycles in 3 wt% MgCl2 and 3 wt% NaCl, respectively (Fig. 5). Fig. 11 gives the TEM image and the corresponding SAED pattern of the white precipitated product. As can be seen from the TEM image (Fig. 11a), the precipitated products are nano-sized rod shape crystals with diameters ranging from 100 to 200 nm. The lengths of the nanocrystals are ranged from 500 nm to 2 μm. As such, it was very hard for it to be directly observed and characterized by SEM in concrete samples. Corresponding to the XRD result, the SAED pattern (Fig. 11b) shows three adjacent circles of the first layer diffraction spot, which means that the white precipitate is not a single phase product. It is noteworthy that CaCl2, brucite, oxychlorides, and Mg3(OH)5Cl·4(H2O) peaks were observed in the XRD pattern of the reaction product between 20 wt% MgCl2 and laboratory-fabricated concrete samples. The TEM and the corresponding SAED pattern further illustrated the morphology and structures of these reaction products. This reveals that the chemical reactions by which MgCl2 deteriorates Portland cement concrete are more complicated than the well-known

a MgCl 2 + b C − S − H + c H2 O → d M − S − H + e Mg(OH)2 + f CaCl2 + Mg3 (OH)5 Cl·4H2 O

(6)

This is indirectly supported by a previous study [28], in which the strong CaCl2·2MgCl2·(H2O)12 peaks were detected with the mixture of CaCl2 and MgCl2. The needed CaCl2 could stem from the reaction (1) discussed earlier, before being consumed by the reaction with MgCl2 and water to form tachyhydrite. The DSC thermogram of the white reaction product is presented in Fig. 12a). This figure reveals the presence of three distinctive peaks during the heating process. The first one, attributed to the loss of the confined water (or crystallization water), started at about 150 °C. The second major peak started at about 250 °C, corresponding to the decomposition of M-S-H [23]. The third peak started at about 350 °C, corresponding to the decomposition of magnesium hydroxide (i.e.,

Fig. 8. SEM/EDS results of the concrete samples after 15 F/T and W/D cycles in 3 wt% NaCl a) and b) low-magnification SEM image and the corresponding EDS results, and c) and d) high-magnification SEM image and the corresponding EDS results [12]. 250

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Fig. 9. SEM/EDS results of the concrete samples after 15 F/T and W/D cycles in 3 wt% MgCl2 a) and b) low-magnification, c) and d) medium magnification, and e) and f) high-magnification.

brucite) [23]. The phase compositions were further supported by the marked difference in the thermogravimetric pattern (TGA), shown in Fig. 12b). The TGA plot also exhibited three distinctive plateaus at about 90 °C, 250 °C, and 350 °C, respectively, which corroborates the three DSC peaks mentioned above. The SEM/EDS results of post-exposure laboratory-fabricated samples shed light on the mechanisms by which the NaCl and MgCl2 deicers deteriorate the concrete in the laboratory setting. As reported previously [12], salt scaling is more than a physical mechanism, because the exposure of concrete to NaCl and F/T and W/D cycles resulted in the formation of nano-sized, needle-shaped precipitates and reduced Ca contents in cement hydrates. Frost damage of concrete in the presence of MgCl2 is also more than a physical mechanism, resulting in microstructures with a large quantity of homogeneous precipitate phase and highly porous microstructure with plate-like crystalline precipitates rich in Mg and Cl (Fig. 9). The results strongly support the assumption of chemical reactions between MgCl2 and the C-S-H phase, and the characterization of reaction products by XRD and TEM was able to shed additional light on such reactions. The conversion of C-S-H to M-S-H is known to affect the properties of cement paste significantly. The silica in C-S-H is organized in single

Fig. 10. XRD pattern of the white precipitate from the concrete samples exposed to 20 wt% MgCl2 solution and 15 F/T and W/D cycles.

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Fig. 11. HRTEM image and its corresponding SAED pattern of the white precipitate from the concrete samples exposed to 20 wt% MgCl2 solution and 15 F/T and W/ D cycles a) HRTEM image, and b) corresponding SAED pattern.

be misleading for assessing the condition of concrete with exposure to MgCl2 environment, as the chemical attack by MgCl2 did not exhibit apparent signs of distress (until severe disintegration of the concrete occurred). As a result, to further investigate the real impact of MgCl2 in the field environment, a few representative bridge decks in the State of Oregon were examined for possible concrete coring to support this investigation. The random sampling of the Oregon Department of Transportation (ODOT) bridge deck population considered several factors such as deck age, traffic volume, geolocation and winter severity, and data availability, as detailed elsewhere [33]. Note that the deck selection avoided the coastal districts where chlorides from the marine environment may reach the deck and compromise the role of deicer-sourced chlorides in the investigation. To examine the likely contribution of MgCl2 deicer to degradation of ODOT bridge decks, seven PCC decks were selected for in-depth research in this work, and the relevant data are provided in Table 2. Note that these decks featured a variety of climatic and traffic conditions, diversity in cumulative MgCl2 deicer usage, and no exposure to other chlorides (even though there may be additives such as corrosion inhibitor in the commercial deicers). For the vast majority of the deck cores, no significant deterioration was apparently visible other than a limited level of surface scaling. In other words, there were no signs of significant longitudinal, transverse or diagonal cracking and no evidence of visible efflorescence. The STS test results of the concrete cores taken from the selected bridge decks are provided in Table 2. As shown in Table 2, the concrete cored from bridge deck #5 exhibited the lowest average STS of 2.9 MPa, which is only 50% of the concrete cored from bridge deck #1 (5.8 MPa). All the field cored samples exhibited an STS much lower

chains whereas that in M-S-H is in the form of silica sheets [4,8]. In addition to a different molar ratio of Si/Ca, M-S-H contains more chemically bound water than C-S-H [29,30]. As the introduction of MgCl2 into the concrete pore solution is anticipated to reduce its pH, this facilitates the formation of M-S-H, because a lower pH promotes the reaction between the Mg(OH)2 and the silica [31,32]. As demonstrated in Eqs. (1) and (2), CaCl2 can form as a result of reactions between MgCl2 and the main components of the cement paste, namely the C-S-H gel and Ca(OH)2. As such, chemical and physical interactions between MgCl2 and CaCl2 should be considered for the concrete MgCl2–rich environment. In the present study, with exposure to F/T and W/D cycles, there were significant variations in water vapor pressure and temperature both temporally and spatially across the concrete matrix. With the freezing process, the water vapor pressure in the pores could be high, which was conducive to the formation of xCaCl2·yMgCl2·zH2O phases, such as tachyhydrite. The specific ratios between x, y, and z depend on the internal reaction conditions (e.g., water vapor pressure, temperature, and species concentrations) that define the thermodynamics and kinetics of the relevant reactions. In addition to decomposition of the C-S-H gel, the crystallization process (as evidenced by Fig. 9e) of nano-crystals likely induced crystallization pressure inside the concrete. These physicochemical mechanisms help explain the considerable STS loss of the concrete samples after F/T and W/D cycles in MgCl2 solution, despite the absence of visible scaling of the concrete.

3.2. Field investigation The laboratory testing results suggest that visual inspection might

Fig. 12. DSC and TGA curves of the precipitates from the concrete samples exposed to 20 wt% MgCl2 solution and 15 F/T and W/D cycles a) DSC and b) TGA. 252

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Table 2 Summary of the cored concrete samples from bridge decks. Bridge deck no. 1 2 3 4 5 6 7

Splitting tensile strength (MPa) 5.8 ± 1.4 3.2 4.6 ± 1.8 4.2 ± 0.3 2.9 ± 0.2 5.6 3.9 ± 0.4

ODOT conventional rating Fair Satisfactory Good Poor Good Satisfactory Good

Average daily traffic (ADT) 56,700 9793 5454 14,200 20,600 29,440 8332

Truck %

Year built

10 16 42 15 10 7 33

1972 1985 2003 1962 2002 1990 1985

Annual no. of F/T cyclesa 25 26 248 119 41 90 174

Annual MgCl2 deicer usage, (L/ ln-km/year)b 255 9082 7214 6389 5018 398 4939

a Note that the number of annual F/T cycles was estimated from the historical (2005 to 2011) hourly and sub-hourly historical air temperature data from adjacent weather station [27]. b Data were averaged from the winter maintenance records of fiscal year 2005 to 2011.

ratio or supplementary cementitious materials [36,37]. While detailed elsewhere [34], those cored ODOT bridge decks exposed to over 5000 L/ln-km (1370 mL/m2) annually and substantial F/T cycling exhibited significantly lower microhardness values across the concrete matrix, with a reduction of up to 60% in the mean microhardness value. The peak of microhardness reduction in a certain concrete depth (often 25 to 50 mm, instead of its surface) is consistent with the report that chloride concentration profile can feature a peak occurring at a certain depth beneath the concrete surface, likely due to wetting and drying cycles [38]. Compared with the interior of the concrete, the concrete surface is also more prone to alternations by carbonation, drying shrinkage, etc. This peak of degradation some depth inside the concrete may be responsible for the absence of significant surface distress (abrasion, performant deformation or cracking) of the concrete decks, making it difficult for the inspector to capture this unique distress caused by MgCl2.

than 6.9 MPa, the typical value of ODOT bridge deck concrete that was not chemically compromised [34]. 3.2.1. Microhardness analysis of field cored samples Out of the seven field bridge decks, two representative cores were chosen to examine the microhardness of concrete specimen as a function of exposure history, testing depth and area (paste vs. ITZ). Fig. 13 compares the microhardness gradients for two concrete bridge decks, i.e., deck #1 exposed to MgCl2 with 255 L/ln-km (69.8 mL/m2) and 25 F/T cycles per year and 56,700 ADT in 2008; and deck #2 exposed to MgCl2 with 9082 L/ln-km (2483 mL/m2) and 26 F/T cycles per year and 9793 ADT in 2008. These two decks were selected because they present the two extreme ends of the annual deicer usage spectrum while experiencing a similarly low number of F/T cycles annually. As illustrated in this figure, the bridge deck exposed to less MgCl2 (despite more traffic) generally exhibited higher microhardness values than the one exposed to more deicer. Furthermore, the statistically significant differences were at the interior (25–30 mm) for cement paste phase (mean value of 100 MPa vs. 70 MPs) and at the very top surface (2–5 mm) for ITZ phase (mean value of 80 MPa vs. 62 MPa). This suggests that higher exposure to MgCl2 alone (in the absence of substantial F/T cycling) may not induce significant degradation across the entire matrix of the field concrete. Note that it is typical for the microhardness values from hardened cementitious materials to show significant scatter and to follow a lognormal statistical distribution [35]. The observed reductions in the microhardness values may be attributed to the formation of new crystalline phases inside the concrete exposed to MgCl2, which reduced the adhesion between cement hydration particles and the cohesion of the ITZ phase. The microhardness measurements helped explain the observed strength reduction in the concrete cores. Previous studies have suggested that the microhardness can be used to monitor the performance changes of hardened cement pastes and ITZ due to the variation of w/c

3.2.2. Microscopic analyses of selected field cored samples This section is devoted to the use of SEM/EDS for microscopic characterization of selected field concrete cores and helps to interpret the observed macroscopic engineering property differences among them. Out of the seven field bridge decks, a few representative cores were chosen for such characterization to enable meaningful comparisons. Two bridge decks, which were rated as 6-SATISFACTORY by the ODOT using conventional visual inspection approach, i.e., decks #6 and #2, were examined. The two decks showed significantly different STS value of 5.6 MPa and 3.2 MPa, respectively. Fig. 14 gives the typical SEM images and corresponding EDS results represent the satisfactory, poor, and good conditions of the bridges by conventional visual inspection. As illustrated in Fig. 14a, the sample cored from deck #6, which was evaluated as satisfactory by ODOT, featured nearly intact cement hydrates and very few visible pores in the paste. In

Fig. 13. Microhardness distributions of concrete specimens from two ODOT bridge decks exposed to MgCl2 deicer at: a) 255 L/ln-km/year (deck #1) and b) 9082 L/ ln-km/year (deck #2). 253

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Fig. 14. The SEM images and their corresponding EDS results of selected coring samples with conditions of a) and b) “satisfactory”, c) and d) “poor”, and e) and f) “good”.

structure was not very smooth compared with that seen in the core from deck #6. The EDS results confirm the findings from SEM observations. Fig. 14b), d) and f) present the EDS results of cores from three bridge decks evaluated by visual inspection as csatisfactory”, “poor”, and “good”, respectively. The Mg content (in the studied area) was significantly higher in the bridge deck rated as “poor” than in the deck rated as “satisfactory” (0.65 at.% vs. 0.38 at.%), while the Ca content was slightly lower in the former deck (10.89 at.% vs. 11.16 at.%). The higher Mg/Ca ratio in the deck rated as “poor” implies the leaching of Ca2+ out of concrete caused by the chemical reaction of MgCl2 with Carich cementitious phases, as reported previously [5]. Fig. 14f) presents the EDS results of the bridge deck rated as “good” by the ODOT (using conventional visual inspection approach) but featured a relatively low average STS of 4.6 MPa. In this sample, the Mg content was as high as 0.67 at.% and the Ca content was as low as 9.28 at.%, which suggested more severe Ca2+ leaching than the deck rated as “poor”. This highlights the alarming fact that visual inspection failed to detect the premature deterioration inside the concrete exposed to MgCl2. The SEM/EDS results of cored samples shed light on the mechanisms by which the MgCl2 deicer deteriorates the concrete in the field environments. The microscopic evidence further suggested that the

addition, the typical lamellar shape C-S-H phase was well maintained. The surfaces of the C-S-H phase were smooth, and little other precipitates or crystals were observed. This favorable microstructure corresponded very well with the macroscopic engineering property of this concrete core. However, the bridge deck #4, which was evaluated as POOR by the ODOT, showed a relatively low average STS value of 4.2 MPa. Compared with deck #6, the lower STS of deck #4 can be ascribed to the significantly higher exposure to MgCl2 deicer (6389 vs. 398 L/ln-km/year). As illustrated in this figure, the microstructure featured a highly porous microstructure and absence of high-density cement hydrates. The SEM images reveal that the microstructure of the C-S-H phase was not a dense lamellar structure but a penetrable porous network. This poor microstructure agreed very well with the very low ranking by ODOT for its macroscopic appearance. Finally, the bridge deck #7, which was rated as GOOD by the ODOT using conventional visual inspection approach, showed average STS values of 3.9 MPa. These relatively low STS values suggest that the visual inspection approach failed to capture the poor condition inside these concrete decks. As illustrated in the SEM image, a small number of crystalline precipitates were observed. The microstructure of this sample exhibited a typical porous structure and the presence of some precipitates on the surfaces of the C-S-H phase. The surface of the lamellar shaped C-S-H 254

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concrete in the field environment had been affected by both physical and chemical damages. In addition to chemical degradation of the C-SH gel, the crystallization process likely induced crystallization pressure inside the concrete, and these physicochemical mechanisms resulted from excessive MgCl2 exposure and help explain the reduced microhardness values and low STS values observed in field concrete decks.

[6]

[7]

[8]

4. Conclusions [9]

1. The exposure of laboratory-fabricated concrete to 20 wt% MgCl2 (along with F/T and W/D cycles) resulted in the formation of white precipitates, which were characterized by XRD to be mainly CaCl2, brucite, oxychlorides, and Mg3(OH)5Cl·4(H2O) phases. The present study reveals that the chemical reactions by which MgCl2 deteriorates PCC are complicated and the reaction products may include multiple phases. 2. None of the concrete cylinders exposed to MgCl2 solutions of various concentrations (up to 20 wt%) exhibited any visible surface distress even after 10 F/T and W/D cycles. The concrete exposed to 3 wt% MgCl2 solution showed a much higher reduction in their STS than their counterparts exposed to 3 wt% NaCl solution. 3. For the fracture surface of laboratory-fabricated concrete, those exposed to 3 wt% NaCl showed many needle-shaped precipitates, which are nanometers in diameter. In contrast, those exposed to 3 wt% MgCl2 solutions featured a large quantity of homogeneous precipitate phase and highly porous microstructure with plate-like nano-sized crystalline precipitates (with rich Mg and Cl contents and limited Ca and trace Si contents). 4. Based on testing of concrete samples cored from seven selected ODOT concrete bridge decks, cumulative exposure to MgCl2 deicer can significantly compromise the STS of concrete (as high as 50%), notably when the annual deicer usage exceeded 5000 L/ln-km (1370 mL/m2). The microscopic evidence further suggested that the concrete in the field environment had been affected by both physical and chemical damages, including chemical degradation of the C-S-H gel and formation of new crystalline phases. These mechanisms resulted from excessive MgCl2 exposure and help account for the reductions in microhardness (up to 60%, often at a depth of 25 to 50 mm) and STS values observed in the field concrete decks.

[10] [11] [12] [13]

[14]

[15]

[16] [17]

[18]

[19] [20] [21] [22]

[23] [24] [25] [26] [27]

Acknowledgments [28]

The authors acknowledge the financial support provided by the National Natural Science Foundation of China (Project No. 51772128, 51672107, and 51761145023), Natural Science Foundation of Shandong Province (SZR1643), the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001), and the Oregon Department of Transportation (ODOT) and the USDOT Research & Innovative Technology Administration (RITA) through Alaska University Transportation Center and Western Transportation Institute. The authors would like to thank Prof. Jueshi Qian, Mr. Maowei Niu and Mr. Zhou Wu at Chongqing University for their assistance in the microhardness testing, and Prof. Wenshou Wang from Shandong University and Prof. Zhihua Yang for their support on XRD and HRTEM analysis.

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