Resistance of concrete and mortar against combined attack of chloride and sodium sulphate Mathias Maes, Nele De Belie ⇑ Magnel Laboratory for Concrete Research, Faculty of Engineering and Architecture, Department of Structural Engineering, Ghent University, Technologiepark – Zwijnaarde 904, B-9052 Ghent, Belgium article info Article history: Received 5 August 2013 Received in revised form 10 June 2014 Accepted 23 June 2014 Available online 1 July 2014 Keywords: Concrete Mortar Chlorides Sulphates Combined attack Blast-Furnace Slag abstract Marine environments are typically aggressive to concrete structures, since sea water contains high con- centrations of chlorides and sulphates. To improve predictions of concrete durability within such envi- ronments, it is important to understand the attack mechanisms of these ions in combination. In this research, the reciprocal influence of Cl and SO 4 2was investigated for four mixtures, namely with Ordinary Portland Cement, High Sulphate Resistant cement, and with Blast-Furnace Slag (50% and 70% cement replacement). Chloride penetration depths and diffusion coefficients were measured to investigate the influence of SO 4 2on Cl attack. Besides, length and mass change measurements were per- formed to examine the influence of Cl on SO 4 2attack. Since the formation of ettringite, gypsum and Fri- edel’s salt plays an important role, XRD-analyses were done additionally. It can be concluded that chloride penetration increases when the sulphate content increases at short immersion periods, except for HSR concrete. Concerning the sulphate attack, the presence of chlorides has a mitigating effect. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction A lot of damage is reported for constructions in marine environ- ments [1–8]. Marine environments are very aggressive, since sea water consists mainly of chlorides and sulphates. Both ions can be very harmful for the durability of concrete structures. However, almost no literature is found about the reciprocal influence. Chlo- rides affect durability by initiating corrosion of the reinforcement steel, and sulphates by deteriorating the concrete itself. Concrete structures in marine environments mostly have a high economic impact (e.g. bridges, wharfs, piers, tunnels, etc.), so it is important to know the attack mechanisms in detail in order to predict con- crete’s service life as exactly as possible. Concerning concrete deterioration due to chlorides, it is impor- tant to notice that corrosion will only be initiated by the free chlo- rides and not by the fraction that is chemically bound to the cement hydrates or physically adsorbed at the pore walls. So, chlo- ride binding is a significant factor related to reinforced concrete durability for three reasons [9]: reduction of the free chloride con- centration in the vicinity of the reinforcing steel will reduce the risk of corrosion; chloride binding will delay the chloride penetra- tion; formation of Friedel’s salt results in a less porous structure and slows down the transport of Cl -ions. Friedel’s salt (3CaOAl 2- O 3 CaCl 2 10H 2 O) is the result of chemical binding between chlo- rides and C 3 A. Chemical binding can also occur between chlorides and C 4 AF. Besides, physical binding occurs due to interac- tion with CSH. Factors influencing binding are [10–13]: chloride concentration, cement type, cement replacement, cation, alkalin- ity, temperature, water-binder factor, etc. Cement replacement by Blast-Furnace Slag (BFS) seems to have a positive influence on the resistance of concrete against chloride penetration. BFS concrete is already used in big marine structures because of the low hydration heat [14]. Besides, partial replace- ment of Ordinary Portland Cement (OPC) by BFS is considered as a promising way to improve concrete’s service life when the pre- diction is based on chloride penetration. In general, it is assumed that BFS concrete is able to bind more chlorides. This is attributed to increased Friedel’s salt formation. Replacement of 70% of the cement by BFS is the most suitable in view of chloride binding [15]. On the other hand, former research does not always show a higher chloride binding capacity of BFS concrete compared to OPC concrete [12,16]. Besides, when cured properly, BFS concrete possesses a finer pore structure resulting in a limited penetration depth of free chlorides and consequently a reduced risk for chloride initiated corrosion in comparison with http://dx.doi.org/10.1016/j.cemconcomp.2014.06.013 0958-9465/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +32 9 264 55 22; fax: +32 9 264 58 45. E-mail address: [email protected](N. De Belie). Cement & Concrete Composites 53 (2014) 59–72 Contents lists available at ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
Mathias Maes, Nele De Belie ⇑Magnel Laboratory for Concrete Research, Faculty of Engineering and Architecture, Department of Structural Engineering, Ghent University, Technologiepark – Zwijnaarde 904, B-9052Ghent, Belgium
a r t i c l e i n f o
Article history:Received 5 August 2013Received in revised form 10 June 2014Accepted 23 June 2014Available online 1 July 2014
Marine environments are typically aggressive to concrete structures, since sea water contains high con-centrations of chlorides and sulphates. To improve predictions of concrete durability within such envi-ronments, it is important to understand the attack mechanisms of these ions in combination.
In this research, the reciprocal influence of Cl� and SO42� was investigated for four mixtures, namely
with Ordinary Portland Cement, High Sulphate Resistant cement, and with Blast-Furnace Slag (50% and70% cement replacement). Chloride penetration depths and diffusion coefficients were measured toinvestigate the influence of SO4
2� on Cl� attack. Besides, length and mass change measurements were per-formed to examine the influence of Cl� on SO4
2� attack. Since the formation of ettringite, gypsum and Fri-edel’s salt plays an important role, XRD-analyses were done additionally.
It can be concluded that chloride penetration increases when the sulphate content increases at shortimmersion periods, except for HSR concrete. Concerning the sulphate attack, the presence of chlorideshas a mitigating effect.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction
A lot of damage is reported for constructions in marine environ-ments [1–8]. Marine environments are very aggressive, since seawater consists mainly of chlorides and sulphates. Both ions canbe very harmful for the durability of concrete structures. However,almost no literature is found about the reciprocal influence. Chlo-rides affect durability by initiating corrosion of the reinforcementsteel, and sulphates by deteriorating the concrete itself. Concretestructures in marine environments mostly have a high economicimpact (e.g. bridges, wharfs, piers, tunnels, etc.), so it is importantto know the attack mechanisms in detail in order to predict con-crete’s service life as exactly as possible.
Concerning concrete deterioration due to chlorides, it is impor-tant to notice that corrosion will only be initiated by the free chlo-rides and not by the fraction that is chemically bound to thecement hydrates or physically adsorbed at the pore walls. So, chlo-ride binding is a significant factor related to reinforced concretedurability for three reasons [9]: reduction of the free chloride con-centration in the vicinity of the reinforcing steel will reduce therisk of corrosion; chloride binding will delay the chloride penetra-
tion; formation of Friedel’s salt results in a less porous structureand slows down the transport of Cl�-ions. Friedel’s salt (3CaO�Al2-
O3�CaCl2�10H2O) is the result of chemical binding between chlo-rides and C3A. Chemical binding can also occur betweenchlorides and C4AF. Besides, physical binding occurs due to interac-tion with CSH. Factors influencing binding are [10–13]: chlorideconcentration, cement type, cement replacement, cation, alkalin-ity, temperature, water-binder factor, etc.
Cement replacement by Blast-Furnace Slag (BFS) seems to havea positive influence on the resistance of concrete against chloridepenetration. BFS concrete is already used in big marine structuresbecause of the low hydration heat [14]. Besides, partial replace-ment of Ordinary Portland Cement (OPC) by BFS is considered asa promising way to improve concrete’s service life when the pre-diction is based on chloride penetration.
In general, it is assumed that BFS concrete is able to bind morechlorides. This is attributed to increased Friedel’s salt formation.Replacement of 70% of the cement by BFS is the most suitable inview of chloride binding [15]. On the other hand, former researchdoes not always show a higher chloride binding capacity of BFSconcrete compared to OPC concrete [12,16]. Besides, when curedproperly, BFS concrete possesses a finer pore structure resultingin a limited penetration depth of free chlorides and consequentlya reduced risk for chloride initiated corrosion in comparison with
60 M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72
OPC concrete. The chloride diffusion coefficient of BFS concretereaches lower values than the coefficient of OPC concrete [16–20].
The other main attack mechanism in marine environments isexternal sulphate attack. This occurs when water contaminatedwith sulphates penetrates into the concrete by means of diffusionor capillary suction. Sulphates are mostly found in the form ofsodium sulphate (Na2SO4) or magnesium sulphate (MgSO4). Thecation associated with SO4
2� has an influence on the attack mecha-nism and the resulting deterioration [21]. Sodium sulphate attackwill result in expansive reaction products while magnesium sul-phate attack will result in reduction in strength. In current paper,the influence of Na2SO4 and the combined attack of NaCl and Na2-
SO4 is examined. Because of the different attack mechanism, theinfluence of MgSO4 and the combined attack of NaCl and MgSO4
will be discussed in following papers.Once penetrated into the concrete, sulphates react with the
lime formed during the hydration process of the Portland clinker.One of the reaction products is calcium sulphate or secondary gyp-sum. This calcium sulphate reacts with hydrated calcium alumi-nates, C3A, and forms ettringite (3CaO�Al2O3�3CaSO4�32H2O).Gypsum leads to reduction of stiffness and strength, expansionand cracking and eventually to transformation of the material intoa mushy and non-cohesive mass. Ettringite has the ability to swellstrongly, which results in a densification of the microstructure fol-lowed by internal stresses that lead to cracking and destruction ofthe concrete [22]. The factors influencing the rate of external attackare: the quantity of sulphate ions, the possibility of the sulphatesto penetrate into the concrete and the volume of C3A in the cementand the type of cement used to make the concrete.
Temperature changes can have an influence on the sulphateattack mechanism as well. At low temperatures and in presenceof soluble carbonate and reactive silicate, thaumasite (Ca3Si(CO3)(SO4)(OH)6�12H2O) can be formed. This is not expansive but lowersthe strength and has a negative influence on the microstructure. Itis generally assumed that thaumasite is only formed at tempera-tures below 15 �C. However, some researchers also found thauma-site at temperatures higher than 15 �C [23–25]. In the currentpaper, the influence of temperature fluctuations on the sulphateattack mechanism and on chloride penetration was not examined;the tests were performed at 20 �C. Nevertheless, in West-Europeanmarine environments sea water temperatures are often below10 �C while in the Middle East and South East Asia sea water tem-peratures can be higher than 30 �C. According to Aköz et al. [26]raising temperatures of sodium sulphate solutions in the range of20–40 �C have a beneficial influence on resistance of mortaragainst sulphate attack. However, they are not able to determinethe dominant factor affecting the performance of mortar in asodium sulphate solution at 40 �C. On the other hand theyobserved that raised temperatures until 40 �C could have negativeeffects on mortar resistance in magnesium sulphate solutions.They assume that this is due to decalcification of C–S–H to M–S–H, which leads to a porous structure. Concerning the influence offluctuating temperatures on chloride diffusion, some researchers[7,27,28] found that chloride diffusion rises with increases intemperature.
The use of cement replacement materials tends to improve theresistance against sulphate attack [29]. However, when BFS is usedand the samples are exposed to magnesium sulphate, deteriorationexceeds that observed in Portland cements [30]. In that case, thecalcium hydroxide is consumed by the pozzolanic reaction, sothe sulphates and the magnesium ions will react directly withthe C–S–H due to the absence of Ca(OH)2, resulting in a cohesion-less M–S–H [31]. Furthermore, BFS concrete which is partiallyimmersed in a sulphate solution will show severe deteriorationin the upper parts of the concrete in contact with air due to saltcrystallization [32,33]. This is in contrast with the situation with
complete immersion. When the concrete is completely immersedin the sulphate solution, BFS concrete will have a higher resistancethan OPC concrete.
Generally, the C3A-content of the cement plays a major role inthe attack mechanism of sulphates and in the binding behaviourof chlorides. When both ions penetrate the concrete together,C3A-binding will definitely influence this multi-ion transport. Inliterature a lot of papers are found on individual chloride and sul-phate attack. Nevertheless, only limited literature is available con-cerning combined environmental attack of chlorides and sulphates.Two groups of papers are found: firstly, the papers investigatingthe effect of chlorides on sulphate attack and secondly the papersinvestigating the effect of sulphates on chloride attack.
According to Al-Amoudi et al. [34] there are three possibleschools of thought concerning the influence of chlorides on sul-phate attack: (1) the sulphate attack mechanism is intensified,(2) sulphate attack is mitigated, and (3) the influence is insignifi-cant. In their research, they examined the role of chloride ions onexpansion and strength reduction due to sulphate attack by addinghigh volumes of sodium chloride, namely 15.7% Cl�, to mixedsodium and magnesium sulphate solutions. The sulphate concen-tration amounted to 2.1% SO4
2� in which the sodium and magne-sium sulphate were proportioned to provide 50% of the sulphateconcentration from each of them. They found that the deteriorationis more severe for specimens immersed in a pure sulphate solutionthan in a combined sulphate–chloride solution. Concerning theinfluence of the cement type, they concluded that the deteriorationdue to sulphate attack is not very different in cements with varyingC3A contents in the range of 3.5–8.5%. Besides, Al-Amoudi et al.also found that replacement of OPC by BFS has only a marginalbeneficial effect.
Also Abdalkader et al. [35] investigated the influence of chlorideon the performance of mortars subjected to sulphate exposure.Their tests were done at 5 �C with 6 g/l SO4
2� magnesium sulphatesolutions diluted with 5 g/l Cl� sodium chloride. They observedthat sulphate attack is more severe when the samples areimmersed in a combined solution compared to those stored in apure sulphate solution, since the presence of chlorides acceleratesdamage caused by thaumasite.
Based on the findings of Zuquan et al. [36] the influence of chlo-rides on the sulphate attack mechanism prolong the periods of theattack process, which results in a less severe deterioration. Theirtests were performed at 20 �C with a combined solution of 5% Na2
SO4 and 3.5% NaCl.Concerning the influence of sulphates on chloride attack, De
Weerdt and Geiker [37] concluded that chloride ions penetratemuch deeper into the concrete compared to other elements origi-nating from the seawater e.g. Mg2+, S2� and Na+ which have,approximately, a constant concentration from 20 mm inwards.This should mean that the binding competition between Cl� andSO4
2� only occurs in the outermost layers. Nevertheless, due tothe binding of chlorides, more free sulphate ions will be presentand they are able to penetrate deeper into the concrete. However,data found by Brown and Badger [22] indicate that Friedel’s saltconverts to ettringite in the presence of sodium sulphate solution.Thus, ettringite is the stable phase under these conditions. Thisshould mean that more free chlorides will be present in the con-crete. Based on the findings of Zuquan et al. [36] the influence ofsulphates (in the form of Na2SO4) on chloride diffusion is depen-dent on the exposure period, namely at early exposure periodsthe presence of sulphate decreases the concentration of chloridesand the chloride diffusion coefficient. But at later exposure periods,the presence of sulphates in combined solutions, cause anincreased ingress of chloride.
In this research, the influence of the multi-ion transport on theproper attack mechanisms was examined. Accelerated tests were
M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72 61
conducted in the laboratory by increasing the chloride and sul-phate concentration of the test solution. On the one hand, the influ-ence of sulphates on chloride attack was investigated by means ofdiffusion tests whereupon colour change boundaries and chloridediffusion coefficients were calculated. On the other hand, the influ-ence of chlorides on sulphate attack was examined by measuringthe length and mass change of specimens immersed in a combinedsolution. In addition, XRD-analyses supplemented with quantita-tive Rietveld analyses were performed.
2. Materials and methods
2.1. Materials
To examine the influence of sulphates on chloride penetration,four different concrete mixtures were used: two Portland cementmixtures, namely one with Ordinary Portland Cement (OPC) andone with High-Sulphate Resistant cement (HSR), as well as twoBlast-Furnace Slag (BFS) mixtures with Portland cement replace-ment levels of 50% (S50) and 70% (S70). Table 1 gives an overviewof the compositions. The total binder content (cement + slag = B)was maintained at 350 kg/m3 and the water-to-binder factor (W/B) at 0.45. These values are in accordance with EN 206–1 [38],when the concrete is applied in an XS2 environment. Superplasti-cizer (SP) based on polycarboxylic ethers was added to the mixtureto obtain a slump between 100 mm and 150 mm (consistency classS3).
Cubes with 150 mm side were cast and cured at 20 �C and a rel-ative humidity (R.H.) higher than 95%. They were demoulded after24 h and cured under the same conditions until the age of testing.Then a cylinder with diameter 100 mm was drilled from each cubeand this cylinder was cut in three specimens with a thickness of50 mm.
The characteristics of the fresh concrete are tabulated in Table 1as well. The slump was measured according to NBN EN 12350-2[39] with indication of the consistency class and air contentaccording to NBN EN 12350-7 [40].
To examine the influence of chlorides on the sulphate attackmechanism, four different mortar mixtures were used with thesame binder types as used in the concrete mixes. The water-to-bin-der factor (W/B) was also maintained at 0.45. The water-to-sandfactor was maintained at 0.15. Mortar cubes with a 20 mm sideand mortar prisms 20 � 20 � 160 mm were prepared and curedat 20 �C and 95% R.H. until the age of testing.
Besides, some cement paste samples were produced to performXRD-analysis. These samples also had a W/B-ratio of 0.45. Prismsof 40 � 40 � 160 mm were produced and cured at 20 �C and 95%R.H. After 28 days curing, slices of 40 � 40 � 7 mm were cut off
Table 1Concrete compositions and fresh concrete characteristics (slump according to NBN EN12350–2).
and these slices were vacuum saturated before immersing in thetest solutions.
In Table 2 the chemical composition of the Portland cementsand the slag, determined in accordance with NBN EN 196-2 [41]and using Wavelength Dispersive X-ray Spectroscopy (WD-XRF).Blaine’s fineness and density of the cement and slag are shownas well. The slag meets all the requirements mentioned in NBNEN 15167-2 [42] and in the Belgian guidelines for a technicalapproval for BFS.
Since HSR cement is used because of its low C3A-content, theC3A-content was calculated by using the Bogue equations. Accord-ing to EN 197-1 [13], the C3A-content for HSR cement is limited to3%. In current research, the C3A-content for HSR cement amountedto 2.50% and corresponds to the standard. For OPC the C3A-contentwas 7.92%.
2.2. Methods
To determine the resistance against chloride penetration andthe influence of sulphates hereon, a diffusion test was performed.Afterwards, Colour Change Boundaries (ccb), which give an indica-tion of the chloride penetration depths, and chloride profiles,which provide the data to calculate a chloride diffusion coefficient(Dnssd), were obtained. The cylindrical concrete specimens weretested at the age of 28 days.
To investigate sulphate attack and the effect of chlorides on thismechanism, length and mass change were measured. The mortarprisms and cubes were also tested at the age of 28 days.
2.2.1. Diffusion testThe resistance of concrete to chlorides was evaluated by the dif-
fusion test as described in NT Build 443 [43]. Using combined solu-tions next to the prescribed single chloride solution makes itpossible to examine the influence of sulphates on chloride penetra-tion. At the age of 28 days, the specimens were saturated in a 4 g/lCa(OH)2 solution. After 10 days of immersion in this solution, thespecimens were coated, except for the casting surface. The coatedspecimens were placed in the 4 g/l Ca(OH)2 solution for another10 days. Afterwards the specimens were placed in the test solution,an aqueous NaCl solution with a concentration of 165 g NaCl perliter solution, equal to the one described in NT Build 443. To deter-mine the influence of sulphates, two extra test solutions weremade. These test solutions (Table 3) contained 165 g/l NaCl as well,but the sulphate content amounted to 27.5 g/l Na2SO4 (18.6 g/lSO4
2�) and 55 g/l Na2SO4 (37.2 g/l SO42�). In the first combination,
the Cl�/SO42� ratio is equal to the ratio found in sea water from
the North Sea. The second combination has a higher Cl�/SO42� ratio
Table 2Chemical composition, Blaine fineness, water content and density of the cement typesand blast-furnace slag.
Influence of Cl� on SO42�-attack (Mass + Length change)
Ref. (SO42�) 50 g/l Na2SO4
Comb. 1 (SO42�) 50 g/l Na2SO4 + 50 g/l NaCl
62 M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72
in order to clarify the influence of the sulphates more in detail. Thetests took place at 20 �C and the period of immersion lasted for7 weeks (ccb + Dnssd) or for 14 weeks (ccb). After storage in the testsolutions, the ccb and chloride profiles were obtained.
The ccb was determined by means of the colorimetric method,more specifically by spraying a 0.1 M AgNO3 solution onto bothhalves of split specimens. This results in a visible white depositof AgCl2, where free chlorides have penetrated into the concrete.For analysis, photographs of the split specimens were taken andanalysed by using ImageJ Software. For each half specimen thepenetration depth was measured at 6–9 places with an intervalof 10 mm.
To obtain a chloride profile, chloride concentrations had to bemeasured at different depths. For practical reasons, chloride pro-files and the ccb could not be obtained from the same specimens.Both were determined on different specimens from the samebatch.
Powder was collected from the cylindrical specimens up to adepth of 20 mm, using a profile grinder. Layers of 2 mm thicknesswere ground. Acid-soluble as well as water-soluble chlorides wereextracted from the powder. The acid-soluble chloride content givesan indication of the total chloride content and the water-solublechloride content is used to estimate the free chloride content.The extraction and titration method is similar to the methoddescribed by Maes et al. [20]. Although, the titration solution wasslightly changed and had a total volume of 50 ml which consistsof 20 ml 0.3 mol/l HNO3 supplemented with 10 ml acid solublechloride extraction solution and 20 ml distilled water on the onehand or with 5 ml water soluble chloride extraction solution and25 ml distilled water on the other hand. The titration was executedwith 0.01 M AgNO3. For the analysis, namely a potentiometrictitration, a Metrohm MET 702 automatic titrator was used. Repeat-ability of the used extraction and titration method in this paperwas checked. Triplicate tests were conducted on four homogenizedsamples. The results indicate that the used water extractionmethod is quite repeatable, since the coefficient of variation rangedfrom 0.8% to 4.8%. This corresponds to the repeatability resultsobtained by Yuan [12].
Chloride contents, in wt% of concrete, were calculated usingEqs. (1) and (2) (ct = total chloride content, cw = water-soluble chlo-ride content).
ct ð%Þ ¼10� 100� 35:45� 0:01� Vol: AgNO3 ðmlÞ
1000� 2ð1Þ
cw ð%Þ ¼10� 100� 35:45� 0:01� Vol: AgNO3 ðmlÞ
1000� 2:5ð2Þ
where 10 represents the dilution factor; 35.45 is the atomic mass ofchlorides (g/mol); 0.01 is the concentration of the titration solution(mol/l); and 2 (or 2.5 for water-soluble chloride content) is the massof the concrete powder in the extraction solution. The factor0.01 can be replaced by the exact concentration of the AgNO3-solu-tion, resulting from the calibration and with unit mol/l.
The water-soluble chloride concentration gives an indication ofthe free chloride content in the concrete. In this research, the free
chloride content cf is assumed to be 80% of the water-soluble chlo-ride content, in accordance with the findings of Yuan [12]. He com-pared the water-soluble chloride content and the free chloridecontent which was measured by means of a pore solution extrac-tion. Furthermore, the bound chloride content is the differencebetween total and free chloride content.
The non-steady state diffusion coefficient Dnssd was calculatedin accordance to the method described in NT Build 443 [43].Non-steady-state diffusion coefficients and chloride surface con-centrations were obtained by fitting Eq. (3) to the measured chlo-ride profiles, using a non-linear regression analysis in accordancewith the least squares method.
where c(x, t) is the chloride concentration at depth x and time t(mass% concrete), ci the initial chloride concentration (mass% ofconcrete), cs the chloride concentration at the surface (mass% ofconcrete), Dnssd the non-steady-state diffusion coefficient (m2/s), xthe distance from the surface until the middle of the consideredlayer (m) and t the exposure time (s).
Compared to the mass of concrete, it was reasonable to assumethat the initial chloride concentration ci in Eq. (3) equalled 0% (inreality, this value can differ from 0%). The first layer was excludedfrom the regression analysis, since the measured chloride concen-tration in the first layer is generally considered not representative.
The performed method exhibits a minor adjustment comparedto the method prescribed in NT Build 443 [43]. The straightforwardmethod prescribed in there is not suitable for detecting smallchanges in diffusion coefficients [44]. Thus, first an average surfacechloride concentration was calculated per concrete mixture,regardless the used test solution. This surface concentration wasused to fit Eq. (3) to the measured chloride profiles again. This timeonly the non-steady state diffusion coefficient was estimated. Thisway, by using a constant chloride surface concentration per mix-ture, the small differences between the diffusion coefficientsbecome more visible.
2.2.2. Mass changeTo examine the influence of chlorides on the sulphate attack
mechanism, mass change measurements were done. At the ageof 28 days, mortar cubes with a 20 mm side were immersed inthe test solutions, namely a 50 g/l Na2SO4 solution and a 50 g/l Na2-
SO4 + 50 g/l NaCl solution (Table 3). The Na2SO4 concentration isconform the ASTM C 1012-4 standard [45], the concentration ofthe added NaCl is equal to make a clear distinction between thesingle-ion solution and the multi-ion solution. Firstly, the speci-mens were vacuum saturated with a 4 g/l Ca(OH)2 solution. Themass of the specimen (surface dry) was measured just beforeimmersion in the test solution, after saturation, mref and at differ-ent time intervals mx: every 2 weeks until 8 weeks immersion,then every month until 6 months immersion and then every 4–5 months until 20 months immersion. Consequently, the masschange was calculated as follows:
Mass change ð%Þ ¼ mx �mref
mref� 100 ð4Þ
2.2.3. Length changeAnother method to examine the influence of chlorides on the
sulphate attack mechanism is to measure length change. Mortarprisms 20 � 20 � 160 mm were prepared with metal studs embed-ded in the mortar, in order to measure more accurately, see Fig. 1.The end of the stud is used as measuring point during the test. Toanalyze the results, the total length of the studs was subtractedfrom the total measured length.
Fig. 1. Metal studs placed in the moulds to be embedded in the mortar specimens.The end of the stud is used as fixed measuring point.
Table 4Free chloride surface concentrations for specimen immersed at the age of 28 days andat the age of 84 days, respectively.
M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72 63
At 28 days, the prisms were vacuum saturated and immersed inthe test solutions, cf. mass change measurements (Table 3). Thelength of the specimen (‘length between the ends of the metalstuds’ – ‘length of the metal studs’) was measured just beforeimmersion lref and at different time intervals lx: every 2 weeks until10 weeks immersion, then every 11 weeks until 10 months immer-sion. So, the length change was calculated as follows:
Length change ð%Þ ¼ lx � lref
lref� 100 ð5Þ
2.2.4. XRD-analysisTo do the XRD-analysis OPC, S50 and S70 cement paste samples
were immersed in different test solutions. After a certain immer-sion time, the samples were crushed to pass a 500 lm sieve. Sincethe internal standard approach was selected for absolute phasequantification and estimation of the amorphous or non-identifiedphase content by XRD analysis [46–48], a 10 wt% ZnO internalstandard was added to the obtained powder. Finally, the powderswere side-loaded into sample holders to reduce preferred orienta-tion effects. The XRD data were collected on a Thermo ScientificARL X’tra diffractometer equipped with a Peltier cooled detector.Samples were measured in h/2h geometry over an angular rangeof 5–70� 2h (Cu Ka radiation) using a 0.02� 2h step size and 1 s/stepcounting time.
Afterwards, every XRD-measurement was quantitatively ana-lysed by means of the Rietveld method for whole-powder patternfitting to investigate the reaction product formation. Topas Aca-demic V4.1 software was used for Rietveld refinement [48,49].
Table 5Non-steady state diffusion coefficients for specimen immersed at the age of 28 daysand at the age of 84 days respectively.
The free chloride concentrations at the surface are calculatedusing Eq. (3) and the measured free chloride profiles. Next to thesurface concentrations, also diffusion coefficients are estimatedduring the first regression analysis. However, these coefficientsare not used for further calculations considering the reasons givenin Section 2.2. Table 4 gives the mean surface concentrations andstandard deviations on the individual values, based on five mea-surements per mixture, regardless the test solution.
These results are used to recalculate the diffusion coefficients. Itshould be noted that these experimental surface concentrationsare not comparable to realistic surface concentrations in marine
environments, since the degradation is accelerated by increasingthe chloride content.
Table 5 shows the non-steady state diffusion coefficients Dnssd
(10�12 m2/s), for OPC and HSR concrete as well as for BFS concreteat different sulphate contents at 28 days and at 84 days.
On the one hand, the influence of increased sulphate concentra-tion is not very clear. Generally a small increase in diffusion coef-ficient is noticed at 28 days, especially in case of a high sulphateconcentration of 55 g/l Na2SO4. For the specimen tested at theage of 84 days, it rather seems that Dnssd decreases when sulphatesare added to the 165 g/l NaCl reference solution. Although, the dif-fusion coefficient decrease due to the addition of 55 g/l Na2SO4 forconcrete with an age of 84 days is smaller (range 13–19%) than theincrease for concrete with an age of 28 days (range 21–28%). Onthe other hand, the diffusion coefficients clearly decrease whenthe OPC is replaced by BFS and increase when the OPC is replacedby HSR cement, regardless the age and the composition of the testsolution.
Fig. 2 shows the chloride penetration depths (ccb) for OPC, HSR,S50 and S70 concrete immersed in the chloride solutions as men-tioned in Table 3. Fig. 2a and b shows the penetration depth forspecimens immersed at the age of 28 days for a period of 7 weeksand 14 weeks, respectively. Next, Fig. 2c and d shows the penetra-tion depth for specimens immersed at the age of 84 days, also for aperiod of 7 weeks and 14 week, respectively.
From Fig. 2a and b it can be seen that the penetration depth inHSR concrete immersed at 28 days decreases significantly (One-way ANOVA and Dunnett’s T3 Post Hoc test, level of signifi-cance = 0.05) when the Na2SO4 content in the 165 g/l NaCl solutionincreases from 0 g/l (Ref. (Cl�)) to 55 g/l (Comb. 2 (Cl�)). The sul-phate increase from 0 g/l to 27.5 g/l (Comb. 1 (Cl�)) has no statisti-cally significant influence on the chloride penetration depth.Besides, the penetration depth increases for OPC and BFS concreteimmersed at 28 days when the sulphates are added to the chloridesolution. This increase is significant for both mixtures, regardless
Fig. 2. Chloride penetration depth after 7 and 14 weeks immersion starting at an age of 28 days (a and b) or at an age of 84 days (c and d), in three different solutions (cf.Table 3). The error bars represent standard errors on the average.
64 M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72
the immersion time (One-way ANOVA and Dunnett’s T3 Post Hoctest, level of significance = 0.05). After an immersion period of7 weeks it seems that OPC concrete has the worst resistanceagainst chlorides when 55 g/l Na2SO4 is present in the combinedsolution. Notwithstanding, after an immersion period of 14 weeksit is clear that HSR concrete has the lowest resistance against chlo-rides, regardless the sulphate content.
Also from Fig. 2c and d, it can be seen that the chloride penetra-tion depth in HSR concrete decreases significantly after 7 weeksimmersion when the Na2SO4 content in the 165 g/l NaCl solutionincreases from 0 g/l to 27.5 g/l and 55 g/l (One-way ANOVA andDunnett’s T3 Post Hoc test, level of significance = 0.05). After14 weeks immersion, no significant differences in chloride pene-tration were measured for HSR. Oppositely, the penetration depthincreases for OPC and BFS concrete when the sulphates are addedto the chloride solution. However, after 7 weeks immersion, thechloride penetration increase for both BFS mixtures was not statis-tically significant. Similar to the concrete immersed at an age of28 days, OPC concrete has the worst resistance against chloridesafter an immersion period of 7 weeks when 55 g/l Na2SO4 is pres-ent in the combined solution. However, after an immersion periodof 14 weeks HSR concrete has the lowest resistance against chlo-rides, regardless the sulphate content.
Overall, the penetration depth for concrete immersed at an ageof 84 days is comparable or even slightly higher than the penetra-tion depth for concrete immersed at an age of 28 days. Neverthe-less, the penetration depth is expected to decrease in time.
Fig. 3. Mass change as a function of the immersion time in a 50 g/l Na2SO4 solution.
3.2. Influence of chlorides on sulphate attack
The influence of chlorides on the attack mechanism of sodiumsulphate is shown by means of mass and length change of mortarspecimen. For the mass results, the values shown in the graphs arethe average of at least five specimens. The length results are theaverage of at least three specimens.
3.2.1. Mass changeThe results of the mass change measurements are shown in
Figs. 3–5. In the graphs, the mass change is shown in percentchange compared to the initial mass, cf. Eq. (4). The first graph,Fig. 3, shows the results obtained for all four mortar mixtures afterimmersion in a 50 g/l Na2SO4 solution without addition of chlo-rides. Standard errors on the average are shown for the last mea-surement. The standard errors on the average values of S50 andS70 are too small to be visible in the graph, 0.11% and 0.07%respectively.
From Fig. 3, it is clear that OPC mortar specimens lose a largerpart of their mass compared to HSR and BFS mortar specimenswhen permanently immersed in a sodium sulphate solution. Onlythe mass loss for OPC after 616 days in the solution is significant(t-test, level of significance = 0.05). Until 320 days, the differencesbetween the four mixtures are rather small. It is clear that the massof the OPC specimens starts decreasing slowly around 100 days in
Fig. 4. Mass change as a function of the immersion time in a combined 50 g/lNa2SO4 + 50 g/l NaCl solution.
M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72 65
the solution, while the mass of the specimens of the other mixturesdoes not change obviously during this immersion period. After thefirst 300 days of the immersion period, 2% mass decrease was mea-sured for OPC. Until 300 days of immersion, no statistically signif-icant mass losses were measured. At the end of the testing period,after 616 days in the solution, the mass loss for OPC amounted to26.5% compared to the initial mass. For the HSR mixture, the massloss was around 2% and for the BFS mixtures around 0%.
It should be noted that the mass of the HSR mortar cubes wasincreased with 1.2% after 450 days immersion in the 50 g/l Na2SO4
solution. Nevertheless, from that point on the mass of the HSRcubes started decreasing until �2.0% after 616 days storage inthe test solution, which is a significant decrease (t-test, level of sig-nificance = 0.05). In contrast, the mass of the specimens made withBFS did not change significantly during the whole test period.
Fig. 4 shows the results of the mass change measurement forthe specimens stored in the combined test solution, namely 50 g/l Na2SO4 + 50 g/l NaCl. Standard errors on the average are shownfor the last measurement.
Fig. 5. Mass change per mixture as a function of the immersion time in a 50 g/l Na2SO4 anchange’-axis is different for the OPC-graph.)
Looking at Fig. 4, it seems that all the specimens undergo a massincrease during the immersion period of 616 days. However, after450 days, the mass of the OPC samples is no longer increasing.After 616 days in the combined solution, the mass of the HSRspecimens has increased with 2.5%, which was significantly more(One-way ANOVA and Dunnett’s T3 Post Hoc test, level of signifi-cance = 0.05) than the mass change of the OPC, S50 and S70 mix-tures, which amounted to 1.4%, 1.0% and 0.5% respectively.
Fig. 5 gives an overview of the mass change for the individualmortar mixtures after immersion in the Ref. (SO4
2�) and Comb. 1(SO4
2�) solutions, with indication of the standard errors on the aver-age for the last measurement.
The mass of the OPC mortar cubes is clearly influenced by thecomposition of the test solution. A decrease in mass of 26.5% wasmeasured after 616 days immersion in a single sodium sulphatesolution, while an increase of 1% is measured after immersion inthe sodium sulphate solution with addition of sodium chloride.For the other mixtures, the differences are not that distinct.
After 616 days, HSR mortar underwent a small mass decreasewhen immersed in the single sulphate solution and a small, butstatistically significant, mass increase when immersed in the com-bined sulphate + chloride solution.
3.2.2. Length changeThe results of the length change measurements are shown in
Figs. 6–8. In these graphs, the length change is shown in percentcompared to the initial length, cf. Eq. (5).
Fig. 6 shows the results obtained for all four mortar mixturesafter 497 days immersion in a 50 g/l Na2SO4 solution without addi-tion of chlorides, with indication of the standard errors on theaverage for the last measurement.
As can be seen from the graph in Fig. 6, the length of the OPCspecimen was influenced the most. The significant length increase(t-test, level of significance = 0.05) amounted to 0.59% after497 days in the test solution. Until 140 days of immersion, thedifferences between the mixtures were negligible. From that
d in a combined 50 g/l Na2SO4 + 50 g/l NaCl solution. (Remark: the scale of the ‘Mass
Fig. 6. Length change as a function of the immersion time in a 50 g/l Na2SO4
solution.
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immersion period on, a clear length increase was found for OPCand HSR mortar, with a big increase for OPC between 300 and497 days. After 497 days, the length change for S50 and S70 stayedstable, 0.04% and 0.02%, while it slightly increased for HSR, 0.13%.
The graph in Fig. 7 shows the length change results afterimmersion in a combined solution with 50 g/l Na2SO4 and 50 g/lNaCl, with indication of the standard errors on the average forthe last measurement.
The results for the length change measurements after immer-sion in the combined sulphate and chloride solution are compara-ble to the results found after immersion in a single sulphatesolution. Here also the OPC mortar underwent the highest lengthincrease, namely 0.42%, followed by the HSR mortar and the BFSmixtures. Also after immersion in a sulphate solution with additionof chlorides, the BFS samples underwent almost no length change.
Fig. 8 gives an overview of the mass change for the individualmortar mixtures after immersion in the Ref. (SO4
2�) and Comb. 1(SO4
2�) solutions, with indication of the standard errors on the aver-age for the last measurement.
The length increases for the OPC prisms after immersion in thecombined solution were in the same order of magnitude as in thesingle sulphate solution until 300 days. After 497 days of immer-sion, the length increase in the combined solution was clearlysmaller than in the single sulphate solution. Besides, it seemed thatthe HSR concrete is less resistant to expansion when it is immersedin a 50 g/l Na2SO4 + 50 g/l NaCl solution. The length increase afterimmersion in the combined solution amounted to 0.15% after300 days of immersion compared to 0.10% in the single sulphatesolution. Notwithstanding, between 300 days and 497 days in the
Fig. 7. Length change as a function of the immersion time in a combined 50 g/lNa2SO4 + 50 g/l NaCl solution.
combined solution, the length change is stabilised for HSR whileit still increases in the single sulphate solution, which means thatthe length change was exactly the same for the HSR specimen inboth solutions.
4. Discussion
4.1. Influence of sulphates on chloride attack
In general, BFS concrete has the highest resistance against chlo-ride penetration. The chloride penetration fronts as well as the dif-fusion coefficients are the smallest for these mixtures, regardlessthe sulphate concentration in the combined solutions. This is inaccordance with the general assumption, namely that the replace-ment of Ordinary Portland Cement by BFS results in an increasingresistance against chloride penetration [20].
Concerning the influence of sulphates on chloride penetrationin concrete, the main trends obtained by measuring the chloridepenetration depth are in accordance with the main trends fromthe diffusion coefficients. Based on the diffusion coefficientsobtained at the age of 28 days (start of the immersion period), itseems that the free chloride diffusion increases when sulphatesare added to the 165 g/l NaCl solution, especially when the sul-phate content amounts to 55 g/l Na2SO4. Nevertheless, when thespecimens were immersed at the age of 84 days, the diffusion coef-ficients remained constant or decreased slightly when the sulphatecontent increased. A possible explanation for this phenomenoncould be the on-going hydration and the formation of expansivereaction products (e.g. ettringite) leading to a densification of thematrix which makes it more difficult for the chloride and sulphateions to penetrate. Because of this, the influence of sulphates onchloride penetration becomes inferior compared to the densifica-tion of the matrix.
On the other hand, the trends found for the chloride penetrationdepths, measured by means of the colorimetric method, are quitesimilar at the different testing ages. At the age of 28 days as wellas at the age of 84 days, after 7 and 14 weeks of immersion, thechloride penetration depth increased significantly for OPC andBFS concrete (except at 84 days after 7 weeks immersion) whenNa2SO4 was added to a NaCl solution. On the other hand, for HSRconcrete, the chloride penetration depth slightly decreased whenthe sulphate content increased. So, it can be determined that a sul-phate content of 27.5 g/l Na2SO4 added to a 165 g/l NaCl solutionhas almost no influence on the chloride penetration depth. Mean-while a sulphate content of 55 g/l Na2SO4 influences the chloridepenetration depth significantly when considering the effect of thebinder type. At low sulphate concentrations it is clear that HSRshows the highest chloride penetration depth. However, after a7 weeks immersion period in the combined solution with 55 g/lNa2SO4, it is remarkable that the chloride penetration in the OPCconcrete is slightly higher than in HSR concrete. Since the C3A con-tent of HSR is already very low from the beginning, there is nocompetition between Cl� and SO4
2� to bind the C3A. So, chloridescan penetrate the concrete unhindered. The same phenomenon isfound for OPC when high amounts of sulphates are added to thechloride solution. A part of the C3A binds with the sulphates andthe amount of free chlorides increases. This means that theadverse/harmful effect of HSR compared to OPC, in chloride con-taining environments can disappear when high sulphate contentsare present. Nevertheless, this phenomenon is not observed whenthe concrete is immersed for 14 weeks.
Overall, the diffusion coefficients found at the age of 28 daysand the chloride penetration depths at 28 days as well as at84 days confirm the findings of Al-Amoudi et al. [50]. They foundthat the concentration of free chlorides in the pore solution
Fig. 8. Length change per mixture as a function of the immersion time in a 50 g/l Na2SO4 solution and a combined 50 g/l Na2SO4 + 50 g/l NaCl solution.
M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72 67
increases significantly when sulphates were present in the chloridesolutions. This finding can be easily explained by the fact that apart of the C3A will preferentially bind with sulphates. Also the factthat Friedel’s salt will convert to ettringite when sodium sulphateis present [22] leads to an increase in free chlorides (and a higherchloride diffusion coefficient). Another factor enhancing the freechloride diffusion, and as a consequence the chloride penetrationdepth, is the increase in alkalinity of the pore solution due to theaddition of sodium sulphate to the chloride solution, which inhib-its chloride binding [35 cited in 34]. However, the obtained resultsare opposite to the conclusions of Zuquan et al. [36]. Although, itshould be noted that the test results of Zuquan et al. [36] areobtained after 90 days, 250 days and 400 days immersion, whichis longer than in current paper (49 days to 98 days). They concludethat the presence of sulphates reduces the chloride diffusion coef-ficient and the chloride concentration by 30–60% at early exposureperiods. They also attribute this to gradual formation of ettringitewhich leads to a compacted microstructure and decreases theingress of chlorides. Overall, according to the diffusion coefficientsobtained in this paper, the conclusions of Zuquan et al. [36] shouldbe nuanced since after very short exposure periods, namely49 days (7 weeks) to 98 days (14 weeks), the presence of sulphateslead to an increase in chloride penetration.
In addition, a XRD analysis was performed on cement pastesamples immersed in a single 165 g/l NaCl solution to identifythe reaction products. The XRD profile, as can be seen in Fig. 9,shows peaks referring to the presence of ettringite, as well as Fri-edel’s salt.
The observed ettringite formation is not caused by sulphateattack since there was no external sulphate source present. Theettringite is formed during the hydration. Notwithstanding the factthat ettringite is an expansive reaction product, it will not causedeterioration during hydration. The ettringite fraction is also muchsmaller than when sulphate attack is considered. More importantin this case, immersion in a single chloride solution is the presence
of Friedel’s salt. This indicates the binding of penetrated chlorides.Since only free chlorides will initiate corrosion, chloride binding isan important issue. From Fig. 9 it is clear that an increase in BFScontent results in a (small) increase in Friedel’s salt formation/chloride binding. However, this is not in accordance with formerresults described in Maes et al. [20], where it was found that chlo-ride binding in BFS concrete is lower than in OPC concrete.Although, in that paper, chloride binding was investigated by mea-suring chloride profiles only.
XRD- and Rietveld results concerning combined attack areshown in Section 4.2 of this paper.
4.2. Influence of chlorides on sulphate attack
Overall, OPC mortar shows the lowest resistance against sodiumsulphate attack. The mass loss as well as the length increase wasthe highest for OPC. This is in accordance with previous research[29]. Next, from the obtained results it seems that the performanceof BFS mortar with 50% and 70% cement replacement in sodiumsulphate rich environments is better than that of HSR mortar. Nev-ertheless, the resistance of HSR mortar in sulphate containing envi-ronments is much better than that of OPC mixtures.
The influence of chlorides on sulphate attack is the most clearfor the mass changes of the OPC mortar. When OPC mortar isimmersed for 616 days in a single sodium sulphate solution, morethan a quarter of the initial mass of the specimen is lost due tocracking and spalling of the outermost layers. When OPC samplesare immersed in a combined solution of sodium sulphate andsodium chloride for the same period, no mass is lost. Notwith-standing, the mass of OPC mortar stored in the combined solutiontends to decrease again after 450 days of immersion. However, notin the same order of magnitude as the decrease in the single sul-phate solution. In general, it can be concluded that the presenceof chloride ions in a sulphate rich environment has a mitigatingeffect on the mass loss of OPC mortar due to sodium sulphate
Reaction product
Amount [%]OPC S50 S70
Gypsum 1.22 2.9 -Ettringite 15.63 10.32 8.85
Friedel’s salt 14.17 17.65 18.94
5 10 15 20 25 30 352θ
OPC S50 S70Ettringite Friedel's salt Gypsum
°
Fig. 9. XRD-profiles measured for OPC, S50 and S70 after 6 months immersion in a 165 g/l NaCl solution + Quantitative Rietveld analysis after XRD-measurements. Theamount of reaction products is compared to the internal standard (%).
68 M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72
attack. This finding is in accordance with the findings of Al-Amoudi[34]. Nevertheless, the mitigating effect is rather a delaying effectsince it seems that the deterioration will occur at a later time. Thisstatement is in accordance with the conclusions of Zuquan et al.[36].
After 616 days in the solution the specimens were also visuallyanalysed, as tabulated in Table 6.
The visual inspection of the specimens confirms the measure-ments described in Section 3.2 of this paper. The OPC samplesimmersed in the 50 g/l Na2SO4 solution are clearly deteriorated.
Table 6Deterioration at the end surfaces of mortar cubes, used for mass change measurements, e
Mix 50 g/l Na2SO4
OPC
HSR
S50
S70
The visual deterioration of the OPC specimens stored in the com-bined solutions as well as HSR mortar stored in the single sulphatesolutions is rather negligible, however some small cracks wereobserved. Besides, neither the HSR samples stored in the combinedsolution nor the BFS samples show deterioration.
Concerning the length change, it seems that there is no obviouseffect of the chlorides on the sulphate attack mechanism until300 days of immersion, regardless the mortar type. However, aftermore than 300 days of immersion, namely 497 days, it becomesclear that the length of the OPC specimens in the single sulphate
xposed to sulphate and sulphate + chloride solutions for 616 days.
50 g/l Na2SO4 + 50 g/l NaCl
Table 7Deterioration at the end surfaces of mortar prisms, used for length change measurements, exposed to sulphate and sulphate + chloride solutions for 300 days and 497 days.
Fig. 10. Crack types observed in OPC specimen after 497 days of immersion in a 50 g/l Na2SO4 solution or in a 50 g/l Na2SO4 + 50 g/l NaCl solution.
M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72 69
solution increases due to sulphate attack and that chlorides have amitigating effect on this. These findings are also translated intomore cracks in the specimen, as can be seen in Table 7.
The OPC samples are already clearly cracked at the edges after astorage period of 300 days in the single sulphate solution as well asin the combined solution. However, no big differences were mea-sured by length change nor observed by visual inspection. After497 days of immersion, the amount of cracks is increased andtransverse and longitudinal cracks are observed along the wholelength, as can be seen in Fig. 10. Furthermore, the edges start tocrumble. The samples stored in the single sodium sulphate solu-tions show more cracks compared to the samples stored in thecombined solution. Besides, also the HSR samples show some smalldeterioration at the edges. Nevertheless, the observed
deterioration does not extend after 300 days of immersion, it staysquite stable. This observation is in accordance with the lengthchange measurements. In contrast, the BFS samples do not showany deterioration after any immersion time.
Based on these findings the length change measurements are inaccordance with the mass change measurements. In both casesOPC specimens show a bad resistance to sulphate attack whenstored for more than 300 days. Besides, the influence of the chlo-rides on the mass loss and length change due to sodium sulphateattack becomes visible for OPC after more than 300 days immer-sion. The chlorides have no influence on sulphate attack in BFSmortar since no deterioration was observed after immersion inthe single sulphate solution nor after immersion in the combinedsolution.
Fig. 11. XRD-profiles measured for OPC, S50 and S70 after 6 months immersion in a 50 g/l Na2SO4 – solution. + Quantitative Rietveld analysis after XRD-measurements. Theamount of reaction products is compared to the internal standard (%).
Reaction product
Amount [%]OPC S50 S70
Gypsum 1.8 4.6 5.9Ettringite 17.9 22.7 26.2
Friedel’s salt 15.2 19.2 22.8
5 10 15 20 25 30 352θ
OPC S50 S70Ettringite Friedel's salt Gypsum
°
Fig. 12. XRD-profiles measured for OPC, S50 and S70 after 6 months immersion in a combined solution. + Quantitative Rietveld analysis after XRD-measurements for OPC,S50 and S70 specimen immersed in a combined Na2SO4 and NaCl solution for 6 months. The amount of reaction products is compared to the internal standard (%).
70 M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72
In order to explain these findings by means of formation ofreaction products, some XRD-measurements were performed onOPC, S50 and S70 cement paste samples. The cement paste sampleswere stored for 6 months in solutions similar to the test solutions,namely a 50 g/l Na2SO4 solution and a combined solution of 50 g/lNa2SO4 and 50 g/l NaCl. The XRD-profiles are shown in Figs. 11 and
12. The peaks that indicate ettringite, gypsum and Friedel’s salt arehighlighted in the profiles since these phases are assumed to be themain reaction products in sulphate and chloride rich environ-ments, together with gypsum. The amounts, quantified by Rietveldanalysis for three phases in particular, namely ettringite, Friedel’ssalt and gypsum are shown in a table underneath the graphs.
M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72 71
From the profiles obtained by analysing the cement paste froma single sulphate solution, as shown in Fig. 11, it is clear thatettringite is present.
The peaks referring to ettringite in the OPC samples are moredistinct than in the BFS samples. Quantitative Rietveld analysisshows that, next to ettringite, also a large fraction of gypsum isexplicitly present in the OPC paste. Besides, in the S50 sample amajor fraction of ettringite is found while almost no gypsum ispresent. In the S70 sample (almost) none of both phases are found.Significantly more gypsum and ettringite have formed in the OPCmixture compared to the BFS mixtures. According to the smallpeaks in the profile, small fractions of Friedel’s salt seem to bepresent as well. However, after the quantitative Rietveld analysis,no Friedel’s salt was found.
The total fraction of expansive reaction products amounts to52.37% for OPC, 29.92% for S50 and 5.85% for S70. The large fractionof expansive reaction products for OPC can cause the mass andlength increase. After formation of these reaction products cracksappear, which results in spalling and crumbling leading to massloss (see Tables 6 and 7). No Friedel’s salt is formed in these cementpastes, which is obvious since they were not exposed to chlorides.These findings provide an explanation for the mass and lengthchange results and for the observed deterioration after immersionin a 50 g/l Na2SO4.
In the end, some cement paste samples were subjected to aXRD-analysis after immersion in combined test solutions. In theprofiles, clear peaks belonging to ettringite as well as to Friedel’ssalt were found, as shown in Fig. 12.
Ettringite as well as Friedel’s salt represent a large fraction ofthe crystalline phases. The amount of gypsum of the samplesimmersed in the combined solution is rather small compared tothe amount of gypsum found in the OPC samples after immersionin a single sulphate solution. In general, the amount of Friedel’s saltincreases when the slag content increases. This was also found forthe samples immersed in a single chloride solution, even theamounts are in the same order of magnitude. According to theseresults, it can be stated that sulphates have only limited influenceon chloride binding. However, based on the diffusion test and thechloride profiles it was clear that chloride penetration slightlyincreases when sulphates are present in the solution. Based onthe results described in Section 3.2 it can be concluded that chlo-rides will bind less when sulphates are present since the free chlo-ride concentrations increases in combined solutions (see resultsccb).
On the other hand, the ettringite fraction in OPC samples isclearly influenced by the presence of chlorides in the solution com-pared to a pure sulphate solution. The formation of ettringite andgypsum in OPC samples decreases, namely from 33.99% and18.38% in the 50 g/l Na2SO4 solution to 17.9% and 1.8% in the50 g/l Na2SO4 + 50 g/l NaCl solution.
The amount of ettringite in the S50 samples immersed in thesingle sulphate and the combined sulphate chloride solution arecomparable. Overall, the reciprocal influence of sulphate andchloride ions on the sulphate binding behaviour in BFS concreteis negligible. Nevertheless, for the S70 samples a large increaseis measured in the ettringite content compared to the contentmeasured in the samples immersed in the single sulphate. It isnot clear how this increase can be explained. Probably, the valuefound after immersion in the single sulphate solution should beneglected.
The link between the amount of reaction products and thedeterioration, which was not observed, in BFS concrete is notimmediately clear since it is generally assumed that a higheramount of ettringite results in a higher degree of deterioration.However, based on the results in this paper the opposite is found,the high ettringite fraction does not lead to deterioration. As
described by Kunther et al. [51], deterioration due to expansionof the reaction products will only occur when there is a coexis-tence of ettringite and gypsum. Next to the volume increase,which is the prerequisite for expansion, also supersaturation inthe solution and force exerted by the formed minerals are deci-sive. In this paper, it is clear that most deterioration is observedfor the samples where ettringite and gypsum coexist, namely theOPC samples in the single sulphate solution. In all the samplesimmersed in the combined solution the amounts of ettringiteare rather high, although, the gypsum content is small. This couldexplain why almost no deterioration is found for samplesimmersed in the combined sulphate and chloride solution.
5. Conclusions
� Free chloride penetration in Ordinary Portland Cement concreteincreases when the sodium sulphate content in the chloridecontaining environment increases, regardless the age of theconcrete. So, the presence of sodium sulphate in a chloride solu-tion aggravates the penetration of free chlorides. Nevertheless,this conclusion is only valid for immersion periods between7 weeks and 14 weeks.In high sulphate resistant concrete the chloride penetrationdepth stays stable or decreases slightly when sodium sulphateand sodium chloride are both present in a solution. After ahort immersion period (7 weeks) in a chloride solution withhigh sodium sulphate content, the chloride penetration depthis even lower than in Ordinary Portland Cement concrete. Thisbeneficial effect of high sulphate resistant concrete disappearswhen the concrete is immersed for a longer period (14 weeks).Concerning the influence of sodium sulphate on the chloridediffusion coefficient, it can be concluded that the diffusion coef-ficient increases when the sodium sulphate content of thechloride solution increases. Nevertheless, when immersionstarts at 84 days, the diffusion coefficient does not changesignificantly.� Chlorides have a mitigating effect on sodium sulphate attack.
Sulphate deterioration is delayed by the presence of chlorides.Both, sulphates and chlorides will bind with the C3A hydrationproducts to form ettringite and gypsum on the one hand andFriedel’s salt on the other hand. However, chlorides’ reactionproduct, Friedel’s salt, is not stable in the presence of sodiumsulphate solution [22]. So, it is assumed that over time Friedel’ssalt will disappear, more ettringite and gypsum will form anddeterioration will occur.� When mortar is exposed to a sodium sulphate and chloride con-
taining environment, a large fraction of ettringite is present,however, almost no deterioration is observed. This can be attrib-uted to the small amount of gypsum. Deterioration due to sodiumsulphate attack only occurs when ettringite and gypsum arefound together. So, the finding of Kunther et al. [51] is confirmedby the results in this paper. In specimens exposed to a singlesodium sulphate environment, where deterioration is observed,both ettringite and gypsum are present in large amounts.� In general, replacement of Ordinary Portland Cement by Blast-
Furnace Slag improves the resistance of concrete/mortar againstchloride penetration and sodium sulphate attack. Concerning theinfluence of sulphates on chloride attack in BFS concrete, thetrends are similar to the trends found for ordinary Portlandcement concrete, namely an increase in chloride penetrationwhen the sodium sulphate content in the combined solutionincreases.Besides, chlorides have no influence (neither positive nor nega-tive) on sodium sulphate attack in BFS mortar. BFS mortaralready possesses a high resistance against sodium sulphate
72 M. Maes, N. De Belie / Cement & Concrete Composites 53 (2014) 59–72
attack, since no significant mass change or expansion isobserved when immersed in a pure sodium sulphate solution.
Acknowledgements
Research funded by a Ph.D. grant of the Agency for Innovationby Science and Technology (IWT). The authors would like to thankPhilip Van den Heede and Hugo Eguez Alava for critically readingthe manuscript.
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