Chloride ingress and binding of coral waste filler-coral waste sand marine mortar incorporating metakaolin

Construction and Building Materials 190 (2018) 1069–1080

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Chloride ingress and binding of coral waste filler-coral waste sand marine mortar incorporating metakaolin Yunyao Wang a,b, Zhonghe Shui a, Rui Yu a, Yun Huang b,⇑ a b

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China

h i g h l i g h t s
Metakaolin can restrain chloride ingress and promote chloride binding of coral waste mortar.
Hemi- and mono-carboaluminate can converse to Friedel’s salt in chloride-rich environment.
Kuzel’s salt is not favored with the excessive amount of carbonate.
The outmost layers of mortars bind less chloride than the neighboring layers.

a r t i c l e

i n f o

Article history: Received 24 May 2018 Received in revised form 25 September 2018 Accepted 27 September 2018 Available online 4 October 2018 Keywords: Coral waste Metakaolin Chloride ingress Chloride binding Friedel’s salt Phase transformation

a b s t r a c t The present study investigates the chloride ingress and binding behaviors of coral waste filler-coral waste sand marine mortar and the influence imposed by metakaolin (MK) addition. The mortars were cured for 4 months and followed by exposing to 0.5 M sodium chloride solution for 45 days. Thereafter, the chloride profiles of mortars were determined by titration method and the phase assemblies of mortars at different depths were determined by X-ray diffraction and thermogravimetry. The results show that coral waste mortars demonstrate dramatically lower ability to resist chloride ingress than normal mortar due to the loose structure of coral waste mortar. Due to the combination of pozzolanic reaction, formation of additional carboaluminate and improved chloride binding capacity by MK addition, chloride penetration is significantly restrained. Carboaluminate can decompose to possible solid solution between hemicarboaluminate (C4 Ac0:5 H12 ) and Friedel’s salt (C4 ACl2 H10 ) in a low chloride concentration, while it directly converts to Friedel’s salt in a high chloride concentration. Since the large amount of coral waste in system restrains the formation of monosulfoaluminate (C4 AsH12 ), the Kuzel’s salt (C4 As0:5 ClH12 ), as an intermediate phase from monosulfoaluminate to Friedel’s salt, is not formed even in low chloride concentration. Owing to the synergistic effects of carbonation and the leaching of calcium ions, the outmost layers of the mortars bind less chloride compared to their neighboring layers. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Current marine engineering works on islands far from mainland are carried out more widely than ever to meet various demands. Since there are very few locally available construction materials for concrete production, millions of tons of raw materials like cement and aggregate are shipped from mainland to prepare concrete or mortar every year, resulting in huge fuel consumption and carbon emission. It is reported that 0.009 kg CO2 emission and 0.13 MJ of non-renewable energy consumption can be resulted for shipping one ton of raw materials per kilometer [1]. This ⇑ Corresponding author. E-mail address: [email protected] (Y. Huang). https://doi.org/10.1016/j.conbuildmat.2018.09.189 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

indicates the high cost as well as negative environmental impacts imposed by the transportation. Besides, since the transportation is relatively slow and dependent on the weather, the raw materials for some urgent civil works can hardly arrive in time. Therefore, it is of great meaning to find the alternative materials for concrete ingredients to reduce the raw materials transported from mainland, thus reducing the construction cost, fossil energy consumption and CO2 emission. In recent years, attentions have been paid to applying coral waste as concrete ingredient again, although the early studies focusing on the properties of coral reef aggregate concrete and mortar had been conducted in the middle of twentieth century [2,3]. Coral waste material is dug up as a waste during some engineering works like dock construction, waterway dredging and oil

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well construction. Coral waste is a type of porous carbonate material, which consists of aragonite and small part of calcite [4–6]. The mechanical properties of coral materials based concrete have been investigated [4,5,7–10]. A generally accepted conclusion is that concrete substituted by coral aggregate will reduce the compressive strength of concretes, but the reduction is modest (less than 15%) for coral material used as fine aggregate and remarkable for coral material used as both coarse and fine aggregate. For instance, it was reported that the 28-day compressive of coral fine aggregate-coral coarse aggregate concrete, prepared with 1000 kg binder per cubic meter of concrete and net water to binder ratio low to 0.2, was only 55.7 MPa [8]. These overviews mean that some measurements should be taken to further improve the mechanical properties of coral waste based concrete or mortar serving in harsh ocean environment. Since coral waste is exclusively used on isolated islands where chloride salt are abundant, the ability of coral waste concrete to resist chloride ingress should be studied because chloride corrosion is considered to be one of the major problems for the damaging of steel-reinforced concrete structure in marine environment [11–13]. Nowadays, the most widely used methods to evaluate the chloride resistance of concrete are Rapid Chloride Migration (RCM) according to NT build 492 and Rapid Chloride Penetration Test (RCPT) complying to ASTM C 1202. The two methods apply 0–60 voltage of electric field on the specimens and measure the rapid chloride diffusion coefficient or charge passed to evaluate the chloride resistance ability. Comparing to natural exposure, the durations of RCM and RCPT are too short, normally 6 h for RCPT and 24 h for RCM. This means the effect of chloride binding on chloride ingress is probably minimized. Besides, the applied electric field is also suspicious to affect the chloride binding, thus affecting the chloride ingress behaviors [14,15]. For the natural chloride exposure, the chloride profiles can be obtained to compare the chloride resistance ability directly, and the chemical reactions between penetrated chloride ions and hydrated matrixes at different depths can also be investigated. This means more and detailed information can be obtained to comprehensively evaluate the chloride ingress behaviors of concrete by chloride exposure method. In fact, RCM and RCPT have been used to evaluate the chloride resistance of coral aggregate concrete [6,9]. For instance, Cheng et al [9] reported the rapid chloride diffusion coefficient dropped from 9.5  1012 m2/s to 3.6  1012 m2/s when coral sand replaced river sand as fine aggregate. The similar results were also observed by Wang et al. [6] who used coral sand as partial fine aggregate to prepare Ultra-High Performance Concrete. However, as a more comprehensive method, chloride exposure method is rarely used to assess the chloride resistance of coral materials concrete. Da et al. [8] studied the chloride diffusion of coral aggregate concrete with net water to binder ratio (w/b) lower than 0.3 and found that the chloride penetrated as deep as 30 mm and 40 mm when respectively exposing to artificial seawater for 7 days and 28 days after 28-day pre-curing in lime-saturated water. This remarkably high ingress depth far exceeded that for the normal concrete, which was found that the chloride ingress depth of plain concrete with w/b of 0.5 after exposing to synthetic seawater for 2 months was just 10 mm [16]. However, Da et al did not study the interaction between penetrated chloride and hardened matrixes and also did not prepare a normal concrete as reference [8]. Thus, it is necessary to further clarify the chloride binding mechanisms and propose measurements to further improve the chloride resistance of coral waste concrete. When chloride ions ingress into concrete, the chloride ions will be bound either chemically or physically, and the rest of chloride ions will stay free. The chloride binding behaviors of cementitious materials are considered to reduce the free chloride concentration in pore solution, thus lowering the diffusion rate of chloride and

retarding the initiation of steel bar corrosion [11,12,17,18]. Active alumina in cementitious materials, not matter in the form of C3A or mineral admixtures rich in alumina, are well known to benefit the chloride binding of cement-based materials [14,19–21]. Metakaolin (MK) is a kind of artificial pozzolan calcined from kaolin soil. It has high pozzolanic activity and high alumina content compared to traditional mineral admixtures like fly ash and ground granulated blast-furnace slag. Previous studies have established that MK can densify the microstructure, improve the mechanical properties as well as durability [9,22–24]. Besides, when exposing to chloride environment, the chloride binding capacity of hardened matrix can be significantly increased by MK addition, due to the supplement of alumina from MK [19,25–27]. Additionally, the formation of additional carboaluminate in cement based materials in the presence of MK and carbonate is also well identified by many researchers, which is considered to fill the voids within matrix and densify the concrete structure [25,28–30]. As the chemical formula of coral waste can be generally expressed as calcium carbonate, there are probably chemical reactions between MK and coral waste. These literatures indicate that MK can be a promising material to restrain the chloride diffusion as well as improve the mechanical properties of coral waste based cement mixtures, due to these beneficial effects. However, the detailed investigation focusing on the concrete incorporating coral waste and MK is very few and the relevant mechanisms should be further clarified. The present study focuses on the impacts of MK on chloride ion ingress and binding of coral waste filler-coral waste sand marine mortar exposing to chloride solution and uncovers the influencing mechanisms. The chloride profiles of mortars were determined from titration method and the phase assemblies of mortars at different depths from exposing surface were identified from X-ray diffraction and thermogravimetry. Besides, the chloride binding behaviors of coral waste cement mixtures with and without MK were also studied. Based on results and discussion, the effects of MK on chloride ingress and binding of coral waste filler-coral waste sand marine mortars were identified and the relevant mechanisms were analyzed.

2. Materials and experimental 2.1. Materials Ordinary Portland cement of grade 42.5 containing 6.7 wt% calcite was used. Coral waste powder (CP) and metakaolin (MK) were used as supplementary cementitious materials to partly replace cement. The chemical composition and physical properties of cement, MK and CP are listed in Table 1. The chemical composition was measured by a PANalytical B. V. Axios advanced X-ray fluorescence spectrometer. The particle size analysis was performed on a Malvern Mastersizer 2000 laser particle size analyzer, with ethyl alcohol as dispersion medium. The Brunauer-Emmett-Teller specific surface area (BET SSA) was determined by a Micromeritics ASAP2020 automated volumetric apparatus. MK was provided by Maoming Kaolin Science and Technology Co. Ltd., China. CP, with mean particle size of 28.3 mm, was ground from coral waste sand (CS) that was collected from a pelagic island. Coral waste is very porous and the mineral composition of coral waste is aragonite, calcite rich in magnesium and a small amount of halite, which has been identified in the previous study of the lab [4–6,31]. The thermogravimetric analysis (TGA) results are illustrated in Fig. 1. Coral waste has been reported to subject to a phase conversion from aragonite to calcite at approximately 460 °C [32,33], but the phenomenon is not identified from TGA analysis here, probably due to the very little heat releasing or absorption during the phase conversion. Two obvious mass loss peaks at around 300 °C and

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Y. Wang et al. / Construction and Building Materials 190 (2018) 1069–1080 Table 1 Chemical composition and physical properties of cement, MK and CP. Species

CaO

SiO2

Al2O3

Na2O

K2O

Fe2O3

SO3

MgO

P2O5

Cl

L.O.I

Cement MK CP

59.82 0.03 49.02

21.5 54.06 0.31

5.86 41.98 0.12

0.2 0.33 1.34

0.67 0.56 0.56

2.78 0.76 0.08

2.01 0.13 0.31

2.18 0.09 2.97

0.09 0.57 –

– – 0.09

3.85 1.09 44.75

Species

Mean particle size by volume (lm)

BET SSA (m2/g)

Specific gravity (g/cm3)

Cement MK CP

13.0 3.4 28.3

0.437 33.48 Not measured

3.05 2.52 2.68

Fig. 1. TG-DTG-DSC curves of coral waste material.

770 °C are ascribed to the decomposition of organic components (Oc) in coral waste and calcite transformed from aragonite, respectively. The aggregates in this study were natural river sand (RS) and CS. CS was sieved to have the same particle size gradation with RS and their gradation is illustrated in Fig. 2. Due to the porous structure and rough surface of CS, it can significantly reduce the workability of mortars [4,34]. In this way, a polycarboxylate water-reducer (PCE) produced by Sobute New Materials Co., Ltd., China was used to ensure them comparable workability compared with 0-R sample.

2.2. Experimental details 2.2.1. Mix design and specimen preparation The mix proportions (binder content by mass is normalized to 100) and the spread diameter of mortar mixtures are listed in Table 2. Five mortars were prepared in this study. Two plain mortars were prepared using respective RS and CS as aggregate (samples named as respective 0-R and 0-C). The other three mortars were prepared with 30 wt% CP combined with 0 wt%, 5 wt% and 10 wt% MK (samples labeled as 30c-C, 5M30c-C and 10M30c-C). The mortars were mixed by a small planetary mixer and cast in 40 mm * 40 mm * 160 mm prism moulds and /100 mm * 200 mm cylinder moulds. Meanwhile, six pastes with different amount of MK and CP, water to binder ratio of 0.45, were mixed and cast in 40 mm * 40 mm * 40 mm moulds. All specimens were demoulded at 24 h and then cured in moist under 20 ± 2 °C and 98 ± 2% in relative humidity to certain age to carry out the experiments as Fig. 3 illustrated. 2.2.2. Evaporable water content The evaporable water content of the discs was examined to evaluate the water-filled porosity (P, wt%) in mortars [35]. After 4-month curing, the discs with 100 mm in diameter and 60 mm in height were cut from the cylinder specimens. The resulting discs were firstly saturated with distilled water in a vacuum tank conferring to the method for preparing water-saturated discs for Rapid Chloride Penetration Test (RCPT) according to ASTM C1202. Then, the water-saturated discs were dried at 105 °C in a vacuum oven to constant weight. The weight loss fraction with reference to ultimate weight of specimen is considered as the water-filled porosity.

Fig. 2. Particle size gradation of aggregate.

2.2.3. Sodium chloride exposure The chloride exposure experiment was carried out on mortar discs as well as hardened paste powder.

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Table 2 Mix proportions of mortars and pastes. Mixture type

Mix

w/b

Cement

MK

CP

RS

CS

PCE

Spread diameter (mm)

Mortars

0-R 0-C 30c-C 5M30c-C 10M30c-C

0.5 0.5 0.5 0.5 0.5

100 100 70 65 60

0 0 0 5 10

0 0 30 30 30

250 0 0 0 0

0 250 250 250 250

0 2 2 2.5 3

165 145 150 145 140

Pastes

100% cement 30%CP 5%MK 10%MK 5%MK + 30%CP 10%MK + 30%CP

0.45 0.45 0.45 0.45 0.45 0.45

100 70 95 90 65 60

0 0 5 10 5 10

0 30 0 0 30 30

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

Not Not Not Not Not Not

4-month curing

Evaporable water

Mortar preparation

2-month curing

One normal mortar and four coral waste mortars;

o

Drying at 105 C to constant weight

measured measured measured measured measured measured

Compressive strength According to ISO 679

4-month curing

Chloride profile Chloride exposure

45-day exposure

0.5 M sodium chloride; Refresh weekly

Grinding powder

Titration

Layer by layer; 2mm per layer;

Free chloride-depths; Total chloride-depths; Bound chloride-depths

X-ray diffraction analysis; thermogravimetric analysis;

X-ray diffraction analysis;

Chloride exposure Paste preparation One plain paste and five blended pastes;

2-month curing

0.5 M sodium chloride; Paste powder to liquid ratio=1g : 4.5 ml

Bound chloride Titration

To evaluate the chloride binding capacity

Fig. 3. Sketch showing the procedure of experiment.

The mortar discs exposing to chloride solution were cut from the cylinder specimens cured for 4 months. The discs were 100 mm in diameter and 60 mm in height. Before exposing to chloride solution, the discs were coated with epoxy resin with the exception of one of the cutting surfaces. After the resin was hardened, the discs were saturated with water as mentioned in Section 2.2.2 and subsequently exposed to 0.5 M sodium chloride solution in a tank, with the uncoated cutting surface upwards. This concentration of chloride was selected because it was generally equivalent to the chloride concentration in sea water. The sodium chloride solution in the tank was refreshed weekly to maintain the constant concentration of sodium chloride. After exposing for 45 days, the discs were removed from the tank and the epoxy resin was scraped down. Then, the discs were ground layer by layer axially. Each layer was 2 mm in thickness except 3 mm for the outmost layer. The resulting powder was collected and dried at

50 °C in a vacuum oven for 24 h. These dried mortar powders would be used to test the free, total and bound chloride content. The chloride exposure experiment was also performed on hardened paste powder in order to compare the chloride binding capacity of different cement mixtures and the phase conversion of hardened pastes in chloride environment. The experiment procedures will be described in Section 2.2.6. 2.2.4. Water-soluble chloride The water-soluble chloride content was measured to evaluate the free chloride content after exposing to chloride solution [36]. 2 g of dried mortar powder and 20 ml of distilled water were placed in screw-capped vials at first. Thereafter, the vials were tightly capped and shaken for 2 min to homogenize and subsequently standed for 24 h. Then, the leaching solution was filtered out and 5 ml of filtrate was pipetted into an Erlenmeyer flask to

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be neutralized with 1 M H2SO4 solution. After that, the neutralized solution was titrated by 0.02 M AgNO3 solution with the indicator of KCrO4 solution. In this way, the free chloride content in 1 g of hydrated mortar can be computed as expressed in Eq. (1). Meanwhile, the pH value of leaching solution was also measured by a pH meter at 20 °C.

4  CAg  VAg  MCl m

Xf ¼

ð1Þ

where Xf free chloride content, mg/g-hydrated mortar; CAg standard concentration of AgNO3 solution, 0.01938 mol/L; VAg consumed volume of AgNO3 solution, ml; MCl molar mass of chloride, 35.45 g/mol; m mass of the hydrated powder, 2 g; 2.2.5. Acid-soluble chloride The acid-soluble chloride was measured to evaluate the total chloride content in hydrated mortar after exposing to chloride solution [20,36]. 2 g of mortar powder was dissolved by 20 ml of 2.2 M HNO3 solution in screw-capped vials. Once the gas was fully released, the vials were capped and aged for 24 h. After that, the solution was filtered out and 5 ml filtrate was pipetted into an Erlenmeyer flask. Then, 5 ml of 0.02 M AgNO3 solution was added to the flask to precipitate all the chloride. The residual silver ions could be obtained by titration with 0.02 M KSCN solution. In this way, the total chloride can be determined from Eq. (2).

  0 4  MCl  CAg  VAg  CCN  VCN

Xt ¼

m

ð2Þ

where Xf total chloride content, mg/g-hydrated mortar; CAg standard concentration of AgNO3 solution, 0.01938 mol/L; 0

VAg added volume of AgNO3 solution, 5 ml; CCN standard concentration of KSCN solution, 0.01956 mol/L; VCN comsumed volume of KSCN solution; ml; MCl molar mass of chloride, 35.45 g/mol; m mass of the hydrated powder, 2 g; Based on Eqs. (1) and (2), the bound chloride content, Xb , could be determined from Eq. (3).

Xb ¼ Xt  Xf

ð3Þ

2.2.6. Chloride binding capacity assessment The chloride binding capacity of the hardened mixtures were evaluated by the chloride binding isotherms determined from chloride profiles of mortars and chloride binding capacity of hardened cement paste powder. Based on the water-filled porosity (P, wt%) and free chloride content (Xf , mg/g-hydrated mortar) of mortars, the free chloride concentration in pore solution of mortars after exposing to chloride solution can be determined from Eq. (4), assuming that the pore structure of mortars before and after chloride exposure was not changed. In this way, the binding isotherms can be drawn by plotting bound chloride content against free chloride concentration.

Cf ¼

Xf P  MCl

ground to less than 74 lm. After that, 8 g of powder and 36 ml of 0.5 M lime-saturated chloride solution (with solid to liquid ratio of 1 g:4.5 ml) were added in a 50 ml vial, and shaken to homogenize after tightly capped. The vial was stored in a glass desiccator with sodium hydroxide to eliminate CO2. One month later, the bound chloride content and the equilibrium chloride concentration were determined by titrating the filtrate with AgNO3 solution. 2.2.7. X-ray diffraction and thermogravimetric analysis X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were applied to examine the phase assemblies at different depths from exposing surface after chloride exposure. The instrument for XRD was a D8 Advance X-ray diffractometer with the parameters of Cu target, 40KV, 30 mA, 1° per minute. The instrument for TGA was Netzsch STA 449F3 thermal analyzer with the heat rate of 10 °C/min, with air flow. The mass loss of samples was monitored from room temperature to 1000 °C. 3. Results and discussion 3.1. Water-filled porosity and compressive strength The water-filled porosity at 4 months and compressive strength at 2 months of mortars are presented in Table 3. Due to the loose structure of coral waste, the mortars incorporating coral waste materials demonstrate obviously higher water-filled porosity and lower compressive strength compared to RS mortar. After MK is included, obviously less water-filled porosity and higher compressive strength are observed. This improvement can be ascribed to the combination of two effects. Firstly, MK addition will benefit the formation of carboaluminate [31]. Carboaluminate is a common phase in Portland cement containing calcite and its formation is more preferential in the system with high alumina no matter in the form of C3A or mineral admixture [37–39]. Carboaluminate possibly formed in the coral waste-bulk paste boundaries will fill the voids in transition area and improve the mechanical properties of coral waste aggregate mortars as well as reduce the water-filled porosity. Secondly, the pozzolanic activity of MK will form additional C-S-H gels to occupy the voids and densify the microstructure of matrix. The synergistic effects of carboaluminate formed and pozzolanic reaction of MK significantly increase compressive strength and obviously reduce the water-filled porosity when MK is incorporated. The mechanical properties of the mortars are not the emphasis of the present work and the detailed investigation will be conducted in another work. Since the densification of the coral waste-bulk paste boundaries and the pozzolanic reaction in bulk paste can block the channels for chloride ions to transport, the chloride ingress in the coral waste mortars can be restrained physically. This deduction is verified in the following sections. 3.2. Chloride profiles Fig. 4 shows the free chloride profile of mortars exposing to 0.5 M sodium chloride for 45 days. The use of coral waste promotes the chloride ingress, mainly due to the loose structure of coral

Table 3 2-month compressive strength and 4-month evaporable water of mortars. Mix

Compressive strength at 56 days (MPa)

Water-filled porosity (wt.%)

0-R 0-C 30c-C 5M30c-C 10M30c-C

49.1 34.8 28.2 41.7 56.9

8.43 19.61 26.35 17.74 11.06

ð4Þ

At the curing age of 2 months, the hardened pastes were crushed to less than 2.5 mm in diameter and then immersed in absolute alcohol for 3 days to stop hydration. Thereafter, the small grains were dried in a vacuum oven at 50 °C for 24 h followed by

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Fig. 4. Free chloride content at different depths from exposing surface of mortars.

waste-based mortar resulting sufficient ingress channels for chloride ions. When 5 wt% MK is used, the free chloride content referring to unit mass of dried mortar decreases remarkably. It can be partly attributed to the much denser microstructure (lower water-filled porosity) of mortars after MK is incorporated. However, the water-filled porosity cannot rationalize the different chloride ingress behaviors of 5M30c-C and 10M30c-C compared to 0-R. Although 5M30c-C and 10M30c-C demonstrate obviously higher water-filled porosity than 0-R as shown in Table 3, the chloride ingress depths and free chloride content are less or comparable for 5M30c-C and 10M30c-C compared to 0-R. This indicates that the chloride binding may play a very important role in chloride ingress behaviors of coral waste mortars. The distinguished ability of MK to improve the chloride binding capacity of normal cementbased materials has been frequently reported [19,25,27,31,40]. When MK is used in coral waste cement mixtures, it also improves the chloride binding capacity obviously. As reviewed in introduction, the chloride binding of hydrated concrete will retard the penetration of chlorides. Due to the joint effect of microstructure densification and chloride binding capacity improvement, the free chloride content in mortars sharply decreases. The chloride binding behaviors inhibiting free chloride diffusion can be reflected by comparing 10M30c-C, 5M30c-C with 0-R. Although 0-R demonstrates obviously lower porosity (as shown in Table 3) than 10M30c-C and 5M30c-C, 0-R still presents higher free chloride content and larger chloride penetration depth. Fig. 5 illustrates the total chloride profile of mortars exposing to sodium chloride solution. It reveals that the coral waste and MK affect the total chloride profile very similarly to the free chloride profile. Subtracting free chloride from total chloride, the bound chloride profile can be determined as illustrated in Fig. 6. It clearly shows that the outmost layers of the mortars do not tend to bind the most chlorides. It is expected to be mainly caused by the server leaching of calcium ions to exposure solution [41]. Another factor accounting for the less bound chlorides of the outer layers is the carbonation. As commonly known, calcium ions is necessary for the chloride binding of cement-based materials as the reactions shown in Eqs. (8)–(11) and calcium ions are verified to benefit the chloride binding process [25,42,43]. When large amount of calcium hydroxide (CH) leaches out or the mortars are carbonated, the calcium ions in the outer layers of the mortars are decreased and the formation of Fs is restrained. Besides, the solubility of Friedel’s salt is also hydroxyl and calcium ions dependent, and the solubility will be enlarged due to the leaching and carbonation

Fig. 5. Total chloride content at different depths from exposing surface of mortars.

Fig. 6. Bound chloride content at different depths from exposing surface of mortars.

induced calcium ions and hydroxyls concentration dropping [44,45]. An additional effect imposed by carbonation is that carbonation increases the carbonate content in mortars, which will restrain the formation of Fs by strengthening the ionic competition between Cl and CO2 3 to form AFm phases [43]. Since the formation of Friedel’s salt is restrained by leaching and carbonation, the chloride binding capacity of mortars can be reduced [14,46]. 

C3 A þ CaðOHÞ2 þ 2Cl þ 10H2 O ! C4 ACl2 H10 þ 2OH

ð8Þ



C4 AcH11 þ 2Cl þ CaðOHÞ2 ! C4 ACl2 H10 þ CaCO3 þ 2OH þ H2 O ð9Þ 

C4 Ac0:5 H12 þ 2Cl þ 0:5CaðOHÞ2 ! C4 ACl2 H10 þ 0:5CaCO3 þ 2OH þ 1:5H2 O

ð10Þ



C4 AsH12 þ 2Cl þ CaðOHÞ2 ! C4 ACl2 H10 þ 2OH þ CaSO4  2H2 O ð11Þ Fig. 7 illustrates the pH value of leaching solution of mortars at different distances from exposing surface. The lower pH value of

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As explained in Section 3.2, the chloride binding behaviors of coral waste mortar can be probably improved with MK addition, thus inhibiting chloride diffusion in mortars. However, whether the use of MK really improves the chloride binding behaviors of coral waste mortars as expected need further investigation. As aforementioned, the leaching of calcium ions and carbonation can obviously restrain the chloride binding behaviors, so that the data from the depths lower than LC depth is not used to determine chloride binding isotherms. Fig. 8 presents the chloride binding isotherms of mortars. Due to the fact that the data lower than LC depth is rejected, the isotherms of 30c-C, 5M30c-C and 10M30cC just cover narrow concentration range. Fig. 8 shows that 0-C

achieves higher chloride binding isotherm compared to 0-R. It can be explained by the small part of halite in coral waste, in which a certain proportion is trapped and cannot be extracted by immersing in water. In this way, this proportion of chloride is mistaken as bound chloride by hydration product of binder, leading to a higher background of the isotherm belonging to 0-C. After 30 wt% CP is incorporated, 30c-C shows comparable binding isotherms with 0C in the Cf range covered by 30c-C. It reveals that the dilution effect can be compensated by the chloride brought in by CP. When MK is added, the bound chloride content is significantly increased in the Cf covered range of 5M30c-C and 10M30c-C. This confirms the ability of MK to benefit the chloride binding of cement-based materials when the free chloride concentration is less than 0.1 M. However, the difference between 5M30c-C and 10M30c-C is not clear based on the present figure. The limited difference of chloride binding capacity between 5M30c-C and 10M30c-C can be ascribed to the denser structure of 10M30c-C, leading to a longer required time to reach equilibrium compared to 5M30c-C possessing less dense structure. Fig. 8 only verifies that MK improves the chloride binding capacity of coral waste mortar below the free chloride concentration of 0.1 M. However, for a higher free chloride concentration, no conclusion can be drawn for the figure. To identify the chloride binding capacity of mixtures at higher chloride concentration, the chloride binding behaviors of four paste powders exposing to 0.5 M sodium chloride are compared as shown in Fig. 9. The error bars present the standard deviation here. It clearly demonstrates that incorporation of 30 wt% CP can reduce the bound chloride content around 20%. It can be ascribed to the combination effect of C3A dilution and the strengthened ionic competition between chloride and carbonate ions to form AFm phases [31,47]. The further inclusion of MK gives a remarkable increase in bound chloride content, due to the increased amount of active alumina as mentioned in the introduction. Fig. 10 shows the XRD patterns of the pastes before and after chloride exposure. It can be drawn that the incorporation of MK can benefits the formation of carboaluminate, but the monocarboaluminate (Mc) is much more preferred by 5 wt% MK addition and hemicarboaluminate (Hc) is preferential by 10 wt% MK due to the limited amount of reactive carbonate available. However, the binary use of MK and CP results in remarkably stronger Hc and Mc diffraction peak compared to the single use of MK. This indicates that the inclusion of MK will probably densify the boundaries between coral materials (CP and CS) and matrix to improve the mechanical properties and prevent ions transport, due to the formation of carboaluminate. After chloride exposure, both Hc and

Fig. 8. Chloride binding isotherms of mortars.

Fig. 9. Bound chloride content of pastes at equilibrium time.

Fig. 7. pH of the leaching solution at different depths of mortars.

the outer layer leaching solution is clearly identified from the figure. This supports the previous explanation that the relatively less bound chlorides in outer layer can be ascribed to the leaching of calcium ions or that combined with carbonation. The depth affected by the leaching and carbonation (named as LC depth) can be roughly determined as where the pH tends to be stable and the LC depths are marked in Fig. 7. It can be found that the LC depths are in general consistent with the water-filled porosity of the various mortars. 3.3. Comparison of chloride binding capacity

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Aer Cl- exposure

Before Cl- exposure

Fig. 10. XRD patterns of hardened pastes at 2 months before and after chloride exposure (Et: ettringite; Fs: Friedel’s salt; Hc: hemicarboaluminate; Mc: monocarboaluminate; CH: calcium hydroxide; Ar: aragonite).

Mc decompose to Friedel’s salt [31,40,42]. The inclusion of 30 wt% CP presents negligible effect on the magnitude of Fs diffraction peak, which is beyond expectation. It is probable that the very small amount of halite in CP (0.09 wt% as shown in Table 1) dissolves into the exposure solution and results in a higher free chloride concentration, thus promoting the formation of Fs. Fig. 10 also reveals that the addition of 5 wt% MK significantly increase the intensity of Fs reflection and the further increase of MK replacement ratio does not continuously promote the Fs formation. It can be explained that the calcium ions are not sufficient to some extent caused by the pozzolanic reaction of excessive amount of MK, thus restraining the formation of Fs from other AFms (Eqs. (9)–(11)) and increasing the solubility of Fs. Although the Fs diffraction is not further increased by incorporating 10 wt% MK, the additional C-S-H formed by pozzolanic reaction probably still contributes to a higher chloride binding capacity of ‘‘10% MK + 30% CP” paste compared to ‘‘5% MK + 30% CP” paste sample. Besides, a roughly trend can be seen that the chloride exposure process increases the magnititude of the Et diffraction peak. It is ascribed to the released sulfates (Eq. (11)) derived from Fs formation to form delayed-Et with monosulfoaluminate in the presence of calcium ions [47,48]. 3.4. Phase assemblies of mortars at various depths from exposing surface 3.4.1. X-ray diffraction Fig. 11 presents the XRD patterns of mortars at different depths from exposing surface. A well consistent result between each mortar is that the outmost layers of the mortars do not demonstrate the strongest Fs diffraction, which is also supported by the bound chloride profile (Fig. 6). The diminished intensity of CH diffraction is also confirmed in the figure, complying with the results given in Fig. 7. Besides, a shift of ‘‘Fs” diffraction peak to lower 2h area is identified from XRD patterns with the increasing distance from exposing surface. The similar phenomenon was found by other

researchers [49] and it was attributed to the formation of solid solution between Cl-AFm and OH-AFm [47]. However, in a system with the presence of sulfate, excessive chloride and carbonate ions, OH-AFm is not stable and the hydroxyls can be exchanged. This means the solid solution between Cl-AFm and OH-AFm is probably not preferred here. It has been proven that Fs can form solid solution with Mc and OH-AFm [47,50]. Since Hc can be considered as an intermediate phase from OH-AFm to CO3-AFm, the solid solution between Fs and Hc (labeled as SSFs-Hc) is probably existed. This hypothesis is also supported by the XRD patterns, where the peak of solid solution disappears as approaching to the peak position of Hc. With the decreasing intensity of SSFs-Hc, the reflection corresponding to Mc becomes stronger. It means that SSFs-Hc should be partly formed from Mc, but indirectly. The chloride ions probably firstly react with Mc to form Fs and then Fs reacts with Hc to form SSFs-Hc. The formation of SSFs-Hc here is actually contradictory to the published results that the formation of Kuzel’s salt is favored in low chloride concentration [19,25,42,47]. The contradictory can be ascribed to the large amount of carbonate incorporated in this study, which can convert the monosulfoaluminate (Ms) to Mc [37,51,52]. As De Weerdt et al [42] addressed, the Kuzel’s salt was formed from the decomposition of Ms in low chloride concentration. Since Ms is almost exhausted by the carbonate incorporated, the transformation of Ms to Kuzel’salt is restrained, so that the Kuzel’s salt is absent here. Actually, in a high chloride concentration, both Hc and Mc can directly decompose to Fs, which are also verified by Fig. 10 [31,40,42]. This highlights the different reaction mechanisms of carboaluminate and chloride ions between high and low chloride concentration. With regard to the layers not affected by external chlorides (19– 21 mm for 0-R, 5M30c-C and 10M30c-C), the binary use of MK and coral waste material does result in more intense reflections of Mc and Hc. The increasing of MK incorporation from 5 to 10 wt% does not result in stronger reflection of Mc but stronger reflection of Hc. All these phenomena are consistent with the results from XRD patterns of pastes (Fig. 10).

Y. Wang et al. / Construction and Building Materials 190 (2018) 1069–1080

(a) 0-R

(b) 0-C

(c) 30c-C

(d) 5M30c-C

(e) 10M30c-C

Fig. 11. XRD patterns of mortars at different depths from exposing surface.

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3.4.2. Thermogravimetric analysis Fig. 12 presents the DTG curves of mortars at different depths from exposing surface. The decomposition temperatures of specific species are referred to the reports [41,53]. However, due to the higher heating rate of those studies (20 °C/min), all species demon-

strate approximately 15 °C higher decomposition temperature compared to the present study. The decomposition of Fs is divided into two temperature regions, 100–130 °C for the loss of four interlayer water molecules and 260–380 °C for the release of six main layer water molecules. The second weight loss region of Fs is also

(a) 0-R

(b) 0-C

(c) 30c-C

(d) 5M30c-C

(e) 10M30c-C

Fig. 12. DTG curves of mortars at different depths from exposing surface.

Y. Wang et al. / Construction and Building Materials 190 (2018) 1069–1080

interfaced by the decomposition of organic components (Oc), so that the differential area of the weight loss peak cannot be used to quantify Fs content in mortar as the ref. did [41]. For all samples, the weight loss peak of CH is significantly diminished for the outmost layers of mortars comparing to its neighboring layers, which is consistent with the XRD patterns (Fig. 11) and the pH of leaching solution (Fig. 7). The DTG curves of 0-R sample (Fig. 12(a)) demonstrate the occurrence of carbonation by comparing the weight loss peak of calcite of outmost layer to that of neighboring layer. It confirms that the outer layers of the mortars are carbonated and it is one of the reasons accounting for the low bound chloride content of the outermost layers. The carbonation may be partly attributed to the long-time moist curing. Nevertheless, the pre-carbonated process was also confirmed to harm the chloride binding capacity of concrete [43]. As for the weight loss peak corresponding to carboaluminate (Mc and Hc), it demonstrates gradually increasing magnititude from outer layers to inner layers, this trend is shown in all mortars. It indicates that carboaluminate is not stable in chloride environment, which is consistent with XRD results shown in Figs. 10 and 11. As aforementioned, in a carbonate-containing system, Mc can indirectly transform to SSFs-Hc in a low chloride concentration and directly decompose to Fs at a higher chloride concentration. When comparing 5M30c-C, 10M30c-C to 0-R and 30c-C, the binary use of MK and coral waste result in obvious higher weight loss corresponding to carboaluminate, indicating the increasing amount of carboaluminate in mortar incorporating MK and coral waste. This result is also supported by XRD patterns in Fig. 10. However, a contradictory phenomenon observed in Fig. 12 compared to Fig. 11 is that the weight loss peak of carboaluminate generated in outer layers in DTG curves, whereas the diffraction signal of carboaluminate cannot be observed in XRD patterns. It may result from the lowly crystallized carboaluminate decomposition and the interface from SSFs-Hc decomposition. 4. Conclusions The present study investigated the chloride ingress and binding of marine mortar incorporating metakaolin and coral waste (used as both filler and aggregate). The total chloride, free chloride and bound chloride profiles were determined and the phase assemblies of mortar layers at the different distance from exposing surface were analyzed. Based on the results, the following conclusions can be drawn. 1) MK or its hydration products can react with coral waste to form monocarboaluminate (Mc) in a lower MK content and to form hemicarboaluminate (Mc) in a higher MK content. The formation of Mc and Hc can densify the coral wastebulk paste boundaries, thus improving the mechanical properties as well as the ability to resist mass transport. 2) Using of coral waste in marine mortars will remarkably promote chloride ingress due to the loose structure of coral waste. Metakaolin addition can remarkably reduce the chloride ingress depth due to the synergistic effects of pozzolanic reaction (densify the bulk paste), increased amount of carboaluminate formed (densify the hardened paste-coral waste boundaries) and the enhanced chloride binding capacity of coral waste marine mortar. 3) The low bound chloride content of the outmost layer is attributed to the synergistic effects of calcium hydroxide leaching out during exposure and carbonation during moist-curing or exposure. These two effects result in the reduction of calcium ions available for the formation of Friedel’s salt.

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4) Calcium carboaluminate is not stable in chloride solution. At a low chloride concentration, carboaluminate can transform to the solid solution between hemicarboaluminate and Friedel’s salt in the presence of reasonable amount of carbonate, rather than the formation of Kuzel’ salt. In a carbonatecontaining system, the monosulfoaluminate formed in later age will react with carbonate to form monocarboaluminate, leading to an obvious reduction of monosulfoaluminate content. In this way, the reaction between monosulfoaluminate and chloride ions to form Kuzel’s salt is restrained. Conflict of interest None. Acknowledgements The authors acknowledge the financial supports of National Key Research and Development Program of People’s Republic of China (No. 2017YFB0310905), ‘‘Nature Science Foundation Project of China (Nos. 51679179 and 51608409)” and ‘‘Yang Fan Innovative & Entrepreneurial Research Team Project (No. 201312C12)”. References [1] E. Bundesverband Der Deutschen Ziegelindustrie E. V, Green building challenge handbuch, 2017. (Accessed October 12th 2017). [2] Howdyshell, The use of coral as an aggregate for portland cement concrete structures, Army Construction Engineering Research Laboratory, 1974. [3] W.R. Lorman, Characteristics of Coral Mortars, Armed Services Technical Information Agency, 1960. [4] Q.K. Wang, P. Li, Y.P. Tian, W. Chen, C.Y. Su, Mechanical properties and microstructure of portland cement concrete prepared with coral reef sand, J. Wuhan Univ. Technol. 31 (5) (2016) 996–1001. [5] X.P. Wang, Z.H. Shui, R. Yu, M. Bao, G.M. Wang, Effect of coral filler on the hydration and properties of calcium sulfoaluminate cement based materials, Constr. Build. Mater. 150 (2017) 459–466. [6] X.P. Wang, R. Yu, Z.H. Shui, Q.L. Song, Z.H. Zhang, Mix design and characteristics evaluation of an eco-friendly Ultra-High Performance Concrete incorporating recycled coral based materials, J. Clean. Prod. 165 (2017) 70–80. [7] L. Lin, Z. Yanlin, L. Haibo, H. Chao, Coral aggregate pre-wet on the mechanical properties of concrete, Concrete 1 (2011) 85–86 (in Chinese). [8] B. Da, H.F. Yu, H.Y. Ma, Y.S. Tan, R.J. Mi, X.M. Dou, Chloride diffusion study of coral concrete in a marine environment, Constr. Build. Mater. 123 (2016) 47– 58. [9] S.K. Cheng, Z.H. Shui, T. Sun, R. Yu, G.Z. Zhang, S. Ding, Effects of fly ash, blast furnace slag and metakaolin on mechanical properties and durability of coral sand concrete, Appl. Clay Sci. 141 (2017) 111–117. [10] J.W. Tang, H. Cheng, Q.B. Zhang, W. Chen, Q. Li, Development of properties and microstructure of concrete with coral reef sand under sulphate attack and drying-wetting cycles, Constr. Build. Mater. 165 (2018) 647–654. [11] A.K. Suryavanshi, J.D. Scantlebury, S.B. Lyon, Corrosion of reinforcement steel embedded in high water-cement ratio concrete contaminated with chloride, Cem. Concr. Compos. 20 (4) (1998) 263–281. [12] O.A. Kayali, M.N. Haque, The Cl-/OH- ratio in chloride-contaminated concrete-a most important criterion, Mag. Concr. Res. 172 (47) (1995) 235–242. [13] M. Manera, O. Vennesland, L. Bertolini, Chloride threshold for rebar corrosion in concrete with addition of silica fume, Corros. Sci. 50 (2) (2008) 554–560. [14] Q. Yuan, C. Shi, G. De Schutter, K. Audenaert, D. Deng, Chloride binding of cement-based materials subjected to external chloride environment-A review, Constr. Build. Mater. 23 (1) (2009) 1–13. [15] M. Castellote, C. Andrade, C. Alonso, Chloride-binding isotherms in concrete submitted to non-steady-state migration experiments, Cem. Concr. Res. 29 (11) (1999) 1799–1806. [16] V.G. Papadakis, Effect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress, Cem. Concr. Res. 30 (2) (2000) 291–299. [17] J.B. Newman, C. Arya, Problem of predicting risk of corrosion of steel in chloride contaminated concrete, in: Proceedings of the Institution of Civil Engineers Part 1-Design and Construction, 1990, pp. 875–888. [18] G. Lin, Y.H. Liu, Z.H. Xiang, Numerical modeling for predicting service life of reinforced concrete structures exposed to chloride environments, Cem. Concr. Compos. 32 (8) (2010) 571–579. [19] M.D.A. Thomas, R.D. Hooton, A. Scott, H. Zibara, The effect of supplementary cementitious materials on chloride binding in hardened cement paste, Cem. Concr. Res. 42 (1) (2012) 1–7.

1080

Y. Wang et al. / Construction and Building Materials 190 (2018) 1069–1080

[20] O.R. Ogirigbo, L. Black, Chloride binding and diffusion in slag blends: influence of slag composition and temperature, Constr. Build. Mater. 149 (2017) 816– 825. [21] M.J. Kim, K.B. Kim, K.Y. Ann, The influence of C3A content in cement on the chloride transport, Adv. Mater. Sci. Eng. 2016 (2016) 1–8. [22] L. Courard, A. Darimont, M. Schouterden, F. Ferauche, X. Willem, R. Degeimbre, Durability of mortars modified with metakaolin, Cem. Concr. Res. 33 (9) (2003) 1473–1479. [23] B.B. Sabir, S. Wild, J. Bai, Metakaolin and calcined clays as pozzolans for concrete: a review, Cem. Concr. Compos. 23 (6) (2001) 441–454. [24] C.S. Poon, S.C. Kou, L. Lam, Compressive strength, chloride diffusivity and pore structure of high performance metakaolin and silica fume concrete, Constr. Build. Mater. 20 (10) (2006) 858–865. [25] Z.G. Shi, M.R. Geiker, K. De Weerdt, T.A. Ostnor, B. Lothenbach, F. Winnefeld, J. Skibsted, Role of calcium on chloride binding in hydrated Portland cementmetakaolin-limestone blends, Cem. Concr. Res. 95 (2017) 205–216. [26] N. Saikia, S. Kato, T. Kojima, Thermogravimetric investigation on the chloride binding behaviour of MK-lime paste, Thermochim. Acta 444 (1) (2006) 16–25. [27] G.M. Wang, Y. Kong, Z.H. Shui, Q. Li, J.L. Han, Experimental investigation on chloride diffusion and binding in concrete containing metakaolin, Corros. Eng. Sci. Technol. 49 (4) (2014) 282–286. [28] M. Antoni, J. Rossen, F. Martirena, K. Scrivener, Cement substitution by a combination of metakaolin and limestone, Cem. Concr. Res. 42 (12) (2012) 1579–1589. [29] K. Vance, M. Aguayo, T. Oey, G. Sant, N. Neithalath, Hydration and strength development in ternary portland cement blends containing limestone and fly ash or metakaolin, Cem. Concr. Compos. 39 (2013) 93–103. [30] J. Cabrera, M.F. Rojas, Mechanism of hydration of the metakaolin–lime–water system, Cem. Concr. Res. 31 (2) (2001) 177–182. [31] Y. Wang, Z. Shui, Y. Huang, T. Sun, P. Duan, Properties of coral waste-based mortar incorporating metakaolin: Part II. Chloride migration and binding behaviors, Constr. Build. Mater. 174 (2018) 433–442. [32] N. Yang, W. Yue, The Handbook of Inorganic Matalloid Materials Atlas, Wuhan University of Technology Press, Wuhan, China, 2000. [33] J. Lech, A. Zak, Aragonite?calcite transformation studied by EPR of Mn2+ ions, Chem. Phys. Lett. 157 (3) (1989) 175–179. [34] F. Chen, G. Zhang, S. Ding, M. Qing, Half-dry coral sand concrete preparation technology under natural conditions, China Harbour Eng. 37 (5) (2017) 68–72 (in Chinese). [35] G. Medjigbodo, E. Rozière, K. Charrier, L. Izoret, A. Loukili, Hydration, shrinkage, and durability of ternary binders containing Portland cement, limestone filler and metakaolin, Constr. Build. Mater. 183 (2018) 114–126. [36] T.U. Mohammed, H. Hamada, Relationship between free chloride and total chloride contents in concrete, Cem. Concr. Res. 33 (9) (2003) 1487–1490. [37] V.L. Bonavetti, V.F. Rahhal, E.F. Irasser, Studies on the carboaluminate formation in limestone filler-blended cements, Cem. Concr. Res. 31 (6) (2001) 853–859.

[38] T. Matschei, B. Lothenbach, F.P. Glasser, The role of calcium carbonate in cement hydration, Cem. Concr. Res. 37 (4) (2007) 551–558. [39] K. De Weerdt, K.O. Kjellsen, E. Sellevold, H. Justnes, Synergy between fly ash and limestone powder in ternary cements, Cem. Concr. Compos. 33 (1) (2011) 30–38. [40] H. Zibara, R.D. Hooton, M.D.A. Thomas, K. Stanish, Influence of the C/S and C/A ratios of hydration products on the chloride ion binding capacity of lime-SF and lime-MK mixtures, Cem. Concr. Res. 38 (3) (2008) 422–426. [41] Z.G. Shi, M.R. Geiker, B. Lothenbach, K. De Weerdt, S.F. Garzon, K. EnemarkRasmussen, J. Skibsted, Friedel’s salt profiles from thermogravimetric analysis and thermodynamic modelling of Portland cement-based mortars exposed to sodium chloride solution, Cem. Concr. Compos. 78 (2017) 73–83. [42] K. De Weerdt, A. Colombo, L. Coppola, H. Justnes, M.R. Geiker, Impact of the associated cation on chloride binding of Portland cement paste, Cem. Concr. Res. 68 (2015) 196–202. [43] M. Saillio, V. Baroghel-Bouny, F. Barberon, Chloride binding in sound and carbonated cementitious materials with various types of binder, Constr. Build. Mater. 68 (2014) 82–91. [44] C.L. Page, Vennesland, Pore solution composition and chloride binding capacity of silica-fume cement pastes, Mater. Struct. 16 (1) (1983) 19–25. [45] J. Ma, Z. Li, Chemical equilibrium modeling and experimental measurement of solubility for Friedel’s salt in the NaOHClNO3H2O systems up to 200 °C, Ind. Eng. Chem. Res. 49 (19) (2010) 8949–8958. [46] B. Martin-Perez, H. Zibara, R.D. Hooton, M.D.A. Thomas, A study of the effect of chloride binding on service life predictions, Cem. Concr. Res. 30 (2000) 1215– 1223. [47] M. Balonis, B. Lothenbach, G. Le Saout, F.P. Glasser, Impact of chloride on the mineralogy of hydrated Portland cement systems, Cem. Concr. Res. 40 (7) (2010) 1009–1022. [48] S.O. Ekolu, M.D.A. Thomas, R.D. Hooton, Pessimum effect of externally applied chlorides on expansion due to delayed ettringite formation: proposed mechanism, Cem. Concr. Res. 36 (4) (2006) 688–696. [49] A. Ipavec, T. Vuk, R. Gabrovsek, V. Kaucic, Chloride binding into hydrated blended cements: the influence of limestone and alkalinity, Cem. Concr. Res. 48 (2013) 74–85. [50] U.A. Birnin-Yauri, F.P. Glasser, Friedel’s salt, Ca2Al(OH)6(Cl, OH)2H2O: its solid solutions and their role in chloride binding, Cem. Concr. Res. 28 (12) (1998) 1713–1723. [51] B. Lothenbach, G. Le Saout, E. Gallucci, K. Scrivener, Influence of limestone on the hydration of Portland cements, Cem. Concr. Res. 38 (6) (2008) 848–860. [52] J. Bizzozero, K.L. Scrivener, Limestone reaction in calcium aluminate cement– calcium sulfate systems, Cem. Concr. Res. 76 (2015) 159–169. [53] M. Zajac, A. Rossberg, G. Le Saout, B. Lothenbach, Influence of limestone and anhydrite on the hydration of Portland cements, Cem. Concr. Compos. 46 (2014) 99–108.