Durability study of AAMs: Sulfate attack resistance

Construction and Building Materials 229 (2019) 117100

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Durability study of AAMs: Sulfate attack resistance Josep Aliques-Granero a, Monique Tohoue Tognonvi a,b,⇑, Arezki Tagnit-Hamou a a b

Department of Civil Engineering, University of Sherbrooke, 2500, boul. de l’Université, Sherbrooke (Québec) J1K 2R1, Canada Department of Biology Sciences, University of Peleforo Gon Coulibaly, BP 1328 Korhogo, Cote d’Ivoire

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 High curing temperature of OPC

mortar foster expansion due to sulfate attack.  AAM systems better withstand sulfate attack than the ones of OPC.  Curing temperature required before sulfate attack depends on the material source.  Dimension and mass changes not suitable to assess degree of sulfate attack for AAM.  Microstructural characterization used to identify attack product.

a r t i c l e

i n f o

Article history: Received 4 May 2019 Received in revised form 23 September 2019 Accepted 26 September 2019

Keywords: Alkali activated materials Sulfate resistance Ordinary Portland cement Water absorption Cross-sectional dimension

a b s t r a c t This paper is a part of works intended to determine more appropriate durability test methods for alkali activated materials (AAMs). Indeed, the current methods used to characterize durability of AAMs are those initially developed for ordinary Portland cement (OPC) and blended OPC. As the chemistry that governs both AAMs and OPC systems is completely different, using OPC durability methods for AAMs could induce inaccurate results. This work aims to study sulfate resistance of alkali activated slag (AAS), alkali activated fly ash (AAFA) and OPC (used as reference) to propose appropriate sulfate resistance test for AAMs by using ASTM C1012 as a basis. Both temperature of curing and sodium sulfate concentration were studied. Expansion of samples was evaluated over time. Results showed that all the OPC bars exceeded the limit compliance with less expansion for those precured at 35 °C. The expansion was attributed in all cases to gypsum and ettringite formation. AAS samples underwent expansion that remains below the limit compliance. At high curing temperature, an initial high expansion attributed to water absorption by the formed porosity was observed. For AAFA systems, no expansion was detected whatever the curing regime and the sulfate concentration, showing a great resistance of such systems to sodium sulfate attack. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author at: Department of Biology Sciences, University of Peleforo Gon Coulibaly, BP 1328 Korhogo, Cote d’Ivoire. E-mail address: [email protected] (M.T. Tognonvi). https://doi.org/10.1016/j.conbuildmat.2019.117100 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Although AAMs appear to be the best alternative binder for OPC in construction field, there is still misinterpretation of their

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behavior in real service. The lack of long-term data is one of the facts that hinders the use of AAMs by the construction industry. As all the constructions are designed for a service life of decades, it would be valuable to have available long-term data of the performance of the materials. Most of the existing investigations have been done for OPC because it was traditionally the construction material par excellence. Thus, construction industry has a historical base in order to design an OPC-based concrete for future structures. On the contrary, AAMs data is relatively low compared to those of OPC. Consequently, there is a mistrust on how these materials will behave some decades later. To solve this problem, efforts in the development of new standards that could predict in a more realistic manner the long-term properties of the material, and consequently accelerate the industrial scale implementation of new materials need to be developed. There is currently RILEM technical committee, namely the TC 247-DTA ‘‘Durability testing of alkali activated materials”, dealing with regulation of such materials [1–3]. Due to the difference between OPC and AAMs chemistry, the use of durability standards of OPC-based concrete to characterize AAMs could induce incorrect results. In addition, owing to the large variety of raw materials susceptible to be activated, the properties of resulting AAMs vary. This makes difficult the standardization of the durability methods for such materials. Moreover, previous study on sulfuric acid resistance (5 wt%) of AAMs revealed different behavior as a function of the AAM type [4]. It has been demonstrated that gypsum was the main corrosion product in both AAS and OPC samples with different deterioration processes. Indeed, AAS specimens exhibited expansion with mass and dimension gains resulting in internal cracks and ultimately splitting of specimens into pyramid-like pieces. While OPC samples showed both mass and dimension loss resulting in their complete disintegration. The difference observed could be due to the nature of the hydration product, C-S-H in OPC system and C-(A)S-H for AAS system. AAFA, in the same conditions, showed no mass and no dimensional change. However, their compressive strength values were affected, as they decreased after acid exposure due to progressive dealumination and dealkalization of N–A–S–H gel during acid penetration [4]. Another important durability aspect in concretes of AAMs that needs to be studied is the external sulfate attack. Several service applications can subject concrete to external sulfate attack that may lead to its deterioration. Thus, it has been a classic point to study in OPC-based binders. The major part of the test methods used to evaluate chemical resistance to sulfate ions are conducted under accelerated conditions to reduce the testing time. Therefore, sulfate ions penetration in the samples is accelerated by immersing the samples in a high concentrated sulfate solution. The sulfate concentration in the solution is usually ten times higher than that accepted in practice [5]. ASTM C 452 and ASTM C1012 standards are often used for this test. ASTM C452 standard is very prescriptive and is only applicable to OPC. However, ASTM C1012 standard can also be applied to blends of OPC with pozzolans or slags, and blended hydraulic cements. None of them considers the possibility of using other materials than those above-mentioned. Several other methods can be found in literature [5]. Some works have been done to characterize AAMs depending on the aluminosilicate precursor used [6–9]. From the different findings, we can notice a great variability of procedures in terms of curing conditions, time and conditions of exposure, type of samples used (paste, mortar or concrete) and indicator used (expansion, compressive and flexural strength, mass change or analytical characterization). Therefore, there is an evident necessity to establish standardized procedures to better compare the results of different works. However, the new procedures should have some flexibility in

the mix design and curing conditions. Actually, AAMs performances highly depend on the formulation and sometimes requires a short special curing at elevated temperatures. Moreover, there are few works focusing on concretes, as no standard test method for evaluating the sulfate resistance of concrete is available. Gu et al. [10] studied the influence of sulfuric acid attack test methodologies including non-accelerated test, brushing, wetting and drying cycling and increased acid concentration on both conventional and AAFA concretes. Degradation of specimens were evaluated considering changes in physical, mechanical and microstructural properties. According to these authors, the rate of change of the simple measures was significantly mix dependent and unsuitable for the comparison of the degradation performance of the different materials. They also stated that owing to the continued hydration of the materials, compressive strength could not be an appropriate indicator of degradation. As solution, they proposed a heat curing of the material before testing. Moreover, the same authors demonstrated in another study [11] that despite the relatively high strength showed by alkali-activated concrete after sulfuric acid attack, the coarse aggregates appeared to be more subject degradation than those in OPC concrete. The concentration of the acid solution was found to significantly affect specimens as increasing the concentration accelerated the degradation process. This work aims to study the effect of the test conditions and the indicator of the degree of attack to analyze AAMs sulfate resistance. The parameters studied were sulfate concentration and curing regime before exposing the mortars to sulfate solution. Mass and cross-sectional dimension changes and microstructural evaluation such as SEM-EDX and XRD were used as indicators of the degree of attack. Finally, the applicability of these typical indicators of the degree of attack for AAMs according to the deterioration mechanism was highlighted. 2. Materials and methods 2.1. Materials Class F fly ash (FFA) and blast furnace slag (BFA) provided by Holcim (Montreal, QC, Canada) and Grancem Holcim (Everett, MA, USA) respectively were used as aluminosilicate sources. General-use Portland cement (OPC) from Holcim was used for comparative purposes in order to establish a direct relationship between test methods and results. Physical properties and chemical composition of studied materials are shown in Table 1 and their XRD patterns are displayed in Fig. 1. The cement is composed of crystal phases of alite (A), ferrite (F) and aluminate (C). The Bogue composition of this cement has been found to be: 65.9% C3S, 6.3% C2S, 7.4% C3A and 7.1% C4AF. Its density and fineness are 3.15 kg/ m3 and 3820 cm2/g respectively. The slag is mostly composed of CaO (42%) and SiO2 (35.1%), with an important content in Al2O3 (10.8%) and is amorphous with the presence of mullite as impurity (Fig. 1b). BFS presents the highest fineness of 6422 cm2/g with a density of 2.97 kg/m3 almost similar to the one of OPC. 2.1.1. LOI: lost on ignition The chemical composition of the fly ash with sum of SiO2 + Al2O3 + Fe2O3 of 85.9% higher than 70% according to ASTM C618, is effectively class F fly ash. Its mineralogical composition provided in Fig. 1c, shows the presence of a broad halo between 15 and 35° (2h) characteristic of an amorphous phase as well as the typical crystalline products of class F fly ashes, such as mullite (M), magnetite (N) and quartz (Q). Its density and fineness are 2.59 kg/m3 and 3723 cm2/g respectively. The mean diameter of particles of all studied materials is in the same range, as it is of 11.21, 10.3 and 12.63 lm for OPC, BFS and FFA respectively.

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J. Aliques-Granero et al. / Construction and Building Materials 229 (2019) 117100 Table 1 Physical properties and chemical composition of the raw materials. Physical properties OPC BFS FFA

Density (kg/m3) 3.15 ± 0.00 2.97 ± 0.01 2.59 ± 0.00

Particle size, D50 (lm) 11.21 10.03 12.63

Blaine surface (cm2/g) 3820 ± 15 6422 ± 56 3723 ± 39

Chemical composition (wt%) OPC BFS FFA

SiO2 19.5 35.1 45.9

Al2O3 4.6 10.8 23.2

Fe2O3 2.3 0.4 16.9

CaO 62.5 42.0 4.5

Na2O 0.2 0.2 1.3

K2O 0.8 0.3 1.9

MgO 2.0 7.9 1.0

SO3 3.0 1.1 0.1

LOI 5.2 1.7 4.2

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Fig. 1. Mineralogical composition of: (A) General use Portland cement. A: C3S, C: C3A, F: C4AF, (B) blast furnace slag and (C) fly ash. M: mullite, N: Magnetite, Q: quartz.

Both FFA and BFS were activated using an industrial sodium silicate solution (SS) and a sodium hydroxide pellets (SH) supplied by Brenntag Canada. SS chemical composition was: 29.3 wt% of SiO2, 9.1 wt% of Na2O and 61.6 wt% of H2O with the silica modulus in molar of 3.3. As previously described [4], a 50 wt% SH solution was prepared and used to modify the silica modulus of the SS. In the case of the BFS, as setting time was detected only 5 min after casting the mortar, a 2 wt% hydrated-lime slurry (HL) was added to the activator prior to addition to the mix. This action induces an increase in the setting time from 5 min to 62 min of mortars, measured according to the ASTM C807 standard. The silica sand used to produce mortars meets the specifications of the ASTM C778 standard. This sand was supplied by

‘‘GENEQ Inc” (Ottawa, Illinois (EE.UU)). The particle size of this sand ranges from 600 mm (sieve No. 30) to 150 mm (sieve No. 100). 2.2. Mortar preparation 25  25  285 mm mortar bars and 50  50 cm mortar cubes were prepared using the procedure previously described [4]. The mix design of the mortars is provided in Table 2. As, AAS mortar presented a fast setting (below 5 min), 2 wt% hydrated lime (HL) was required to delay the setting to 62 min according to ASTM C807. A modified ASTM C305 was used to produce AAFA mortars. The detail of the procedure was described in previous work [4]. OPC mortars were prepared in accordance with ASTM C305 without any

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Table 2 Mix proportions of OPC, AAS and AAFA mortars (g). Mortar (g)

OPC

AAS

AAFA

Binder w/s SS SH HL Sand

400 0.49 – – – 1100

400 0.45 93.8 29.6 8 1100

465 0.25 133.2 40.7 – 1279

modification. After mixing, the mortars were cast in molds. Then, samples were covered with a plastic film before subjecting to three different curing regimes: (1) 23 °C and 100% relative humidity; (2) 35 °C during the first 24 h followed by normal curing at 23 °C; (3) 24 h at 80 °C and storage at 23 °C. To prevent water evaporation for the curing temperature at 35 and 80 °C the first 24 h, mortars covered with a plastic film were sealed in plastic bags containing water [4]. After 24 h at the different studied temperatures, the specimens were removed from the molds and stored under normal conditions (23 °C and 100% RH) until the different tests. 2.3. Testing procedure Both the stability in water and the sulfate resistance of 25  25  285 mm mortar bars were studied. The goal of the water stability test was to assess the behavior of mortars in tap water. The water immersion test was performed following the procedure described in the ASTM C1038 standard. The expansion and mass of bars were measured as a function of time up to 1 week. Tests were performed on mortar bars cured at 35 °C (according to conditions applied for sulfate resistance test (ASTM C1012)) and 80 °C (in line with AAFA curing conditions) for 24 h. Sulfate resistance tests were carried out by using the ASTM C1012 standard as basis. The standard recommends to use sodium sulfate as a source of sulfates, at a concentration of 5 wt%. And test is performed on samples cured at 35 °C. However, it allows to use other sources of sulfates and other concentrations depending on the real service conditions that has to be reproduced. As the aim of this work is to evaluate the procedure rather than a real service conditions, sodium sulfate was used as the sulfate source with different concentrations of 2, 5 and 8 wt%. Also, the influence of curing regime was evaluated. Three different curing temperatures during the first 24 h have been studied: 23 °C (normal curing), 35 °C (according to ASTM C1012), and 80 °C (special curing temperature for AAFA) followed by normal curing (23 °C and 100% of RH) until the required strength. Indeed, according to ASTM C1012 standard, the compressive strength required for samples before submitting them to sulfate attack test must be equal to or higher than 20 MPa. Therefore, compressive strength test was performed on 50  50 cm mortar cubes according the ASTM C109 standard. And samples that did not achieve 20 MPa the first day, were stored in a controlled humidity and temperature chamber (23 °C and 100% of RH) until the required strength was reached. Specimens were then placed in the different sodium sulfate solution (2, 5 and 8 wt%). Once the test has started, the expansion of specimens was measured as a function of time up to 12 months for cross-sectional tests. After each measurement, the sulfate solution was renewed by a fresh sulfate solution. For mass measurement, only 12 month attacked samples were considered. Scanning electron microscopy, combined with energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD) were used to identify the resulting attack products after one year of exposure. XRD characterization were realized with an X’pert Pro MPD diffractometer in CuKa configuration. Data were

obtained by scanning at 0.1° (2 h) with a divergence slit of 1/4°. A Hitachi S-3400 N scanning electron microscope in backscattering mode (BSE) was used to perform SEM observations on polished surfaces. 3. Results and discussion 3.1. Compressive strength results Fig. 2 displays the variation of the compressive strength of OPC, AAS, and AAFA mortar cubes according to curing temperature for 1, 7 and 28 days of curing. The mechanical behavior strongly depends on the nature of the mortar. In the case of OPC mortars (Fig. 2a), the best compressive strength after 28 days is obtained with a normal curing at 23 °C, with 40 MPa. Heating the specimens from 23 to 35 °C for the first day induces an increase in the compressive strength from 15 MPa to 21 MPa. However, the increase of the compressive strength over time of mortars cured at 35 °C is no significant as for the normal curing mortars. Since the compressive strength at 7 and 28 days is of 30 and 35 MPa for 35 °C while it is about 33 and 40 MPa respectively in the case of 23 °C. The same behavior is observed when the specimens are heated at 80 °C during the first day. Indeed, precuring the specimens at 80 °C results in a significant early strength development of 33 MPa which seems to decrease by 3 MPa (i.e. 30 MPa) at 7 days followed by a slight raise of 2 MPa (i.e. 32 MPa) after 28 days of curing. The low compressive strength of the OPC mortar cubes at later ages when they are cured at elevated temperatures is related to the total porosity which is higher [12–14]. According to Al-Dulaijan et al [15] and Elkhadiri et al. [16], when OPC mortars are subjected to elevated temperatures, specifically above 55 °C, the compressive strength continuously decreases because of an increase of the porosity. When elevated temperatures are stopped applying, the hydration is supposed to continue but with a low reaction rate. This could explain the slight increase of the compressive strength observed from 7 days to 28 days in this work. The variation of the compressive strength of AAS mortars as a function of the curing temperature is shown in Fig. 2b. One can observe that AAS mortars behave like OPC mortars. The best compressive strength of 59 MPa is obtained with a normal curing at 23 °C after 28 days. As for OPC, heating mortars during the first day from 23 °C to 35 or 80 °C leads to enhance the compressive strength from 25 MPa to 44 and 56 MPa respectively. Although, the compressive strength of AAS mortar cubes precured at 35 °C continue to increase over time, this increase is lower than the one observed with mortars of 23 °C-curing especially for 28 days of curing. Since for 35 °C of preheating, the 28-day compressive strength is of 52 MPa while it is about 59 MPa at 23 °C. In the case of 80 °C of preheating, the highest compressive strength of 56 MPa is obtained the first days. Afterward, it decreases by 11% (50 MPa) and 9% (51 MPa) for 7 days and 28 days respectively. Due to the low difference between the compressive strength at 7 days and the one obtained at 28 days, it can be considered as constant for these two ages. Similar results were reported by Bakharev et al. [17] and Chi et al. [18], where they obtained better final resistance for the AAS cured at 23 °C than those cured at elevated temperatures. The reaction mechanism that governs the formation of mortars in AAS system is comparable to that of OPC system. Actually, applying thermal curing to AAS system induces the increase of the porosity resulting in the decrease in the compressive strength as-above observed. These results are also in accordance with those obtained by Fernández-Jiménez et al. [19]. Indeed, these authors showed that when AAS were heated at 45 °C during the first 24 h before a normal curing, they first suffered a loss of compressive

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Fig. 2. Compressive strength for OPC (a), AAS (b) and AAFA (c) mortars at 1, 7 and 28 days for different curing temperatures during the first 24 h: 23, 35 and 80 °C, followed with a normal temperature of 23 °C until test.

strength at 7 days. However, the compressive strength slightly increased with a normal tendency afterwards. For AAFA system (Fig. 2c), as expected, the compressive strength for mortar cubes cured at normal temperature (23 °C) did not achieve a high compressive strength (less than 5 MPa). This is due to the particularity of these systems that need a thermal curing in order to accelerate the hardening process [20]. The highest strength obtained in this curing condition is 14 MPa at 28 days. Even preheating AAFA mortars at 35 °C during the first 24 h does not significantly improve the compressive strength. Nevertheless, this action induces a slight strength gain as the compressive which was less than 5 MPa the first day for normal curing, becomes 7 MPa when mortars were thermally treated at 35 °C and reaches 17 MPa at 28 days instead of 14 MPa. Thermal treatment of AAFA mortar cubes the first 24 h at 80 °C results in a significant improvement of the compressive strength. Unlike the progressive strength development in the case of 23 °C and 35 °C, mortar cubes precured at 80 °C develop their best performance the first day (34 MPa) followed by a slight decrease the 7 days before remaining constant up to 28 days. The mechanism processes occurred in the case FA is used as a precursor of AAM is of different nature from those that took place in both OPC and AAS systems. In AAFA systems, thermal curing at elevated temperatures is sometimes required (depending on composition and physical properties of FA) for a good dissolution of FFA particles. In fact, a bad dissolution of FFA lowers the reaction rate as well as the hydration degree. This leads to the presence of a significant anhydrous particles and more porous microstructure [21]. This is perfectly highlighted in obtained

results that show lesser strength development when mortar cubes are cured at 23 °C and 35 °C than those preheated at 80 °C. These results are in line with those obtained by Bakharev [22] even if there is a difference in the curing regime. Actually, in his study, thermal treatment was applied on fly ash-based geopolymer after 24 h of normal curing (at 23 °C). An increase in the compressive strength was only observed after thermal curing at elevated temperatures (75 and 95 °C) followed by either a constant evolution or a slight decrease of the strength. Such a behavior was also observed by other researchers in the case of metakaolin-based geopolymers [21,23]. In fact, both metakaolin and fly ash have been proved to undergo the same reaction process during their activation. According to these authors, the strength behavior of AAFA or metakaolin-based AAM significantly depended on curing conditions confirming results obtained in this study. 3.2. Dimensional stability in water Fig. 3 and Table 3 show respectively, the length and the mass change of the OPC, AAS and AAFA mortar bars stored in water during 7 days after subjecting them to thermal curing at 35 and 80 °C the first 24 h. As it can be observed, the dimensions of the OPC bars heated at 35 °C (Fig. 3a) remain practically constant until 7 days, showing a negligible shrinkage with a mass gain of 1.1% (Table 3) probability due to water absorption. The AAS bars exhibit an expansion up to 0.02% from 180 min to 3 days before keeping constant. These mortars show a very low mass gain of 0.32%. Contrary to the others, the AAFA bars undergo a rapid expansion of 0.05%

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Fig. 3. Dimensional stability of OPC, AAS and AAFA mortar bars submerged in water during time: a) Curing procedure 35 °C (24 h) + 23 °C, b) 80 °C (24 h) + 23 °C, c) Comparison of OPC and AAS cured at 35 °C with AAFA cured at 80 °C.

Table 3 Evolution of the mass change of specimens exposed to water as a function of the preheating temperature. Time (days)

Mass change (%) 35 °C (24 h) + 23 °C

0 0.01 (15 min) 0.125 (3 h) 2 4 6 7

80 °C(24 h) + 23 °C

OPC

AAS

AAFA

OPC

AAS

AAFA

0 0.40 0.52 0.52 0.92 1.06 1.06

0 0.15 0.17 0.25 0.31 0.32 0.32

0 0.23 0.65 1.24 1.12 1.05 1.02

0 2.00 3.93 4.05 4.05 4.17 4.26

0 1.01 1.20 1.28 1.36 1.39 1.74

0 1.29 2.89 2.86 2.83 2.67 2.81

from 15 min to about one hour where their size becomes progressively constant with a mass gain of 1% at the end of the 7-day experiment. Although the difference observed the first moments of the test, the dimensions of all the studied specimens are almost stabilized before the 7 days of testing. The result of mass change indicates that AAS samples at this temperature (35 °C) exhibit finer microstructure than the others (OPC and AAFA), as they show the less mass gain in water. At 80 °C (Fig. 3b), OPC bars show an expansion of 0.01% the first day before remaining practically constant until 7 days. A significant mass gain of 4.3% was observed suggesting the presence of high porosity (Table 3). The AAS bars experiment a significant expansion of 0.045% the first day followed by a slight increase

reaching 0.05% up to 7 days with a mass gain of 1.7%. The AAFA bars behave like OPC bars with a constant expansion of 0.01% from the first day up to the 7 days of testing. However, AAFA specimens present a mass gain of 2.8% lower than the one of OPC bars. As observed with the 35 °C heat curing, the dimensions of all the studied specimens are stabilized before the 7 days of testing suggesting that expansion occurs the first times of water exposition. OPC bars precured at 80 °C and AAS bars precured at 35 °C and 80 °C exhibit an expansion after 3 days of testing. This behavior can be related to the total porosity of the microstructure, as explained in SECTION 3.1. When the mortar bars are cured at elevated temperature, the total porosity is higher and thus the water uptake is higher, resulting in higher expansion. This is confirmed by the

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mass change of specimens (Table 3). Since increasing the preheating temperature induces a raise in the mass gain. This shows that the curing temperature has a significant effect on specimen porosity. Therefore, this expansion is attributed to the water absorption by the total porosity. And the stabilization of bar dimensions before the 7-day experiment could be due to the saturation of pores with water. In AAFA system, two behaviors can be observed depending on the preheating temperature. When the bars are heated at 35 °C, a strong expansion occurred early after only 15 min of exposition in water with a total expansion of 0.05%. While at 80 °C of preheating, the bars underwent slight expansion of 0.01% during the whole experiment. This result suggests that the optimum curing temperature of AAFA system is 80 °C. This explains the special thermal curing of the AAFA that requires energy in order to accelerate the hydration rate and thus improving microstructure properties of the studied material. As previously explained (Section 3.1), at 35 °C anhydrous FA content is higher than the one at 80 °C. This results in higher expansion due to water absorption. However, AAFA mortars precured at 80 °C showed higher mass gain of 2.8% while those preheated at 35 °C exhibit only 1% mass gain. This could indicate that, although at 80 °C, the total porosity was lower, the geopolymer product obtained would be more susceptible to adsorb water on its surface and then formed more hydrated product. When comparing each system with its best curing temperature (Fig. 3c) (35 °C for OPC and AAS and 80 °C for AAFA), one can note that all specimens have similar behavior despite minor differences. Indeed, in the OPC system, there is no expansion and for both AAS

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and AAFA systems, there is only 0.01% of expansion which can be neglected. Thus, it can be concluded that each system needs its own specific curing conditions in order to have comparable behaviors. 3.3. Sulfate resistance 3.3.1. Expansion results Sulfate expansion results for the OPC, AAS and AAFA mortar bars are shown in Figs. 4, 5 and 6 respectively. The influence of both the sodium sulfate concentration (2, 5 and 8 wt%) and the curing regime on the results was reported up to 1 year. It is important to note that after the special curing temperature during the first 24 h at 35 °C and 80 °C, the test was conducted at 23 °C until the end. Fig. 4a presents the expansion curves of all the OPC bars prepared and stored at 23 °C. As the 1-day compressive strength of 14.5 MPa was lower than the 20 MPa required by the ASTM C1012 standard, the bars were put in the sulfate solutions after 2 days with 22.2 MPa of strength. Whatever the sulfate concentration, all the bars exceeded the limit compliance specified in the ACI 201.2R (Guide to durable concrete) for Class 2 exposure, of 0.05% at 6 months and 0.10% at one year. The expansion was strongly related to the sulfate solution since it was higher and faster with increasing sulfate concentration. The values of the expansion are reported in the Table 4 according to the sulfate concentration and the time of exposure. Fig. 4b shows the OPC bars expansion when the curing temperature was 35 °C during the first 24 h in accordance with the ASTM

Fig. 4. Sulfate resistance for OPC bars (ASTM C1012) at 2, 5 and 8% of sulfate concentrations with curing temperature for first 24 h of: a) 23 °C, b) 35 °C, c) 80 °C, followed by a normal temperature conditioning at 23 °C.

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Fig. 5. Sulfate resistance for AAS bars (ASTM C1012) at 2, 5 and 8% of sulfate concentrations with curing temperature for the first 24 h of: a) 23 °C, b) 35 °C, c) 80 °C.

C1012 standard. As previously observed for normal curing samples, all the bars exceeded the limit compliance at 6 months and 1 year, except the bars exposed in the lowest sulfate concentration (2 wt%), that passed the test at 6 months, but failed it at 1 year. However, even if the test failed too, results were better compared to the bars exposed all the time at 23 °C (Fig. 4a). In addition, there was a correlation between the sulfate concentration and the expansion. Actually, expansion increased with increasing the sulfate concentration. Fig. 4c shows expansion results for the OPC bars precured at 80 °C during the first 24 h. In this case, with an unusual curing conditions for this system, the bars experimented a fast and high expansion after only 50 days. This expansion was almost the same for all the sulfate concentrations up to 150 days. From these days, expansion became sulfate concentration dependent as it increased with the concentration. All the bars exceeded the limit compliance of 0.01% (at 1 year) after 50 days. All these results were in accordance with the mass gain of OPC specimens after 12 months of exposure reported in Table 5. Indeed, the mass gain directly related to the expansion increases with increasing both sulfate concentration and curing temperature. If samples cured at normal temperature (23 °C) or at 35 °C showed moderate mass gain (2–3%), those precured at the highest temperature (80 °C) exhibits the greatest mass gain that varied from 6 to 8% according to the solution concentration. Since the mass gain in the case of sulfate attack was higher than the one observed with water exposition, the excess in mass would result from the formation of more expansive products due to sulfate attack. Fig. 5a shows the sulfate expansion results for the AAS mortar bars cured at 23 °C. As it can be observed, there were no expansion

whatever the sulfate concentration. Therefore, the sulfate concentration had no influence on the results. Fig. 5b shows the sulfate expansion results for the AAS mortar bars precured at 35 °C during the first 24 h. A rapid expansion of 0.05% was observed after the first week of immersion and became constant afterwards with the same tendency as the bars exposed all the time at 23 °C (Fig. 5a). This behavior correlates well with the results of dimensional stability test. The bars exposed only to tap water gained a similar expansion at 7 days like those exposed to sulfates. Therefore, the expansion observed in this case is attributed to the water absorption of the bars but not to sulfate attack. This was confirmed by the constancy of the mass gain of about 0.3% in both water and sulfate solution (Tables 3 and 5). Fig. 5c shows the sulfate expansion results for the AAS mortar bars heated at 80 °C during the first 24 h. As observed with the bars cured at 35 °C, a rapid expansion occurred after the first week of immersion followed thereafter by a constant value. In this case, a slight gain in the mass (0.5–0.6%) that apparently did not affect the integrity of samples was observed (Table 5). This expansion correlates well also with the results of dimensional stability test, and it is attributed as well to the water absorption. According to these results, AAS appears to well withstand the sulfate attack. Fig. 6 gathers the sulfate expansion results for the AAFA mortar bars cured at 23 °C (Fig. 6a), 35 °C (Fig. 6b) and 80 °C (Fig. 6c) respectively. For the bars cured at 23 and 35 °C, the test was started after 28 days of curing, even if the compressive strength did not achieve the 20 MPa required by ASTM C1012 standard. The compressive strength was 14 MPa and 18 MPa at 28 days for curing temperature of 23 and 35 °C respectively (Fig. 2c). And, due to the slow hardening reactions of AAFA at low curing

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Fig. 6. Sulfate resistance for AAFA bars (ASTM C1012) at 2, 5 and 8% of sulfate concentrations according to curing temperature for first 24 h of: a) 23 °C. b) 35 °C. c) 80 °C.

Table 4 Expansion of OPC mortar bars at 6 and 12 months 6 months Na2SO4 wt% 23 °C 35 °C (24 h) + 23 °C 80 °C (24 h) + 23 °C Limit compliance* *

2 0.07 0.04 1.14 <0.05%

12 months 5 0.09 0.06 1.13

8 0.16 0.07 1.22

2 0.20 0.12 1.68 <0.10%

5 0.32 0.16 2.04

8 0.42 0.20 2.36

According ACI 201.2R for Class 2.

Table 5 Mass change of specimens as a function of the preheating temperature after 12 months of exposure to sulfate solution. Mass change (wt%) OPC Na2SO4 wt% 23 °C 35 °C (24 h) + 23 °C 80 °C (24 h) + 23 °C

2 1.58 2.46 5.81

AAS 5 2.27 2.71 7.10

8 2.90 3.05 8.08

temperatures, it would take too long time to achieve the 20 MPa required before staring the test. Accordingly, it was decided to start the test after 28 days. Since the goal of this test was rather to study the influence of the test parameters, it was thought interesting to evaluate the influence of the compressive strength. As it can be observed, there was not any expansion for all the studied conditions. It was found that the sulfate concentration had no influence on the results. Instead, a little shrinkage during time was observed

2 0.29 0.28 0.54

AAFA 5 0.31 0.31 0.50

8 0.32 0.39 0.56

2 1.68 1.77 2.84

5 2.06 2.06 2.78

8 2.42 2.11 2.75

for all the bars in all the conditions. In addition, the mass gain of 2.8% (for T = 80 °C) is similar to the one observed in the water stability test indicating that this gain is only due to water absorption. Therefore, AAFA shows a good resistance to sulfate attack. According to results, unlike OPC mortars which do not well resist to sulfate attack, AAS and AAFA mortars better withstand a sulfate attack whatever the solution concentration. This difference is probably due to the chemical nature of the reaction products

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formed during each mortar. Mineralogical and microstructural characterizations were therefore required to identify the different reaction products. 4. Mineralogical characterization by means of X-ray diffraction XRD experiments were performed on attacked samples after 1 year of sulfate exposition (2, 5, and 8 wt%). For each system, a piece of sample collected from a bar according to the sulfate concentration and the curing regime, was analyzed. XRD patterns for the OPC specimens are provided in Fig. 7 according to the three studied curing temperatures, 23 °C (Fig. 7a), 35 °C (Fig. 7b) and 80 °C (Fig. 7c). In all cases, similar peaks, characteristics of quartz (Q), C-S-H gel (X), portlandite (P), ettringite (E) and gypsum (G), were observed with different intensity depending on the curing temperature. For curing temperature of 23 °C (Fig. 7a), the intensity of the main peak of portlandite (2h = 18.0°) slightly decreased when the sulfate concentration in the solution was higher, meanwhile the intensity of other portlandite peaks remained almost constant. At the same time, the intensity of the main peak of the ettringite (2h = 9.1°) slightly increased with the sulfate concentration. The intensity of the main peak of gypsum observed at 2h = 11.6° also increased when the sulfate concentration raised, while remaining very similar for both 5 and 8 wt%. These results are in line with the fact that when more sulfates are available in the solution, most of them react with the portlandite present in the OPC mortar,

evidenced by a decrease in the intensity of the portlandite peaks. Therefore, the portlandite reacts with the sulfates to form in a first step gypsum and thereafter converts to ettringite [24]. Both gypsum and ettringite were found to increase with the sulfate concentration resulting in the decrease in the portlandite. The ettringite and gypsum were thus responsible for the expansion observed in Fig. 4a. The region attributed to the C-S-H gel (2h = 29.0°) was almost the same in the 3 studied cases, indicating the stability of the gel in these conditions. The huge peaks of quartz observed in all the conditions were due to the sand used for the confection of the mortar specimens. These peaks of quartz were not illustrated completely because of rescaling the figure to better differentiate the important peaks. This rescaling is applied in all the XRD patterns. For curing regime of 35 °C (during the first 24 h) – 23 °C (Fig. 7b), there was not a clear evolution of the peak intensity with the sulfate concentration. Regarding the peaks of gypsum and ettringite, they seemed to keep constant with the sulfate concentration. For the portlandite, the main peak intensity increased with 5% of sulfates, but decreased at 8% to the minimum value. With a normal behavior, when more sulfates were present in the solution, they would be able to react with the portlandite. For this reason, the peak intensity of portlandite expected at 5% should be lower than that of 2%. The ettringite and gypsum were therefore responsible for the expansion observed in Fig. 4b. In the case of the C-S-H gel (X), the intensity of the peak was constant in all cases. When comparing these results with those of the bars cured at 23 °C all

Fig. 7. XRD patterns for OPC mortar bars exposed at different sodium sulfate solutions (2, 5, and 8 wt%) after 1 year of exposition. Curing temperature during the first 24 h of: a) 23 °C; b) 35 °C; c) 80 °C. Peaks: E, ettringite; G, gypsum; P, portlandite; Q, quartz; X, C-S-H gel.

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Fig. 8. XRD patterns for AAS mortar bars exposed at different sulfates dissolutions (2, 5, and 8 wt%) after 1 year of exposition. a) curing temperature during the first 24 h: 23 °C; b) 35 °C; c) 80 °C. Peaks: Q, quartz; X, C-(A)-S-H gel; HT, hydrotalcite; C, calcite.

the time, the peaks of the portlandite, gypsum and ettringite, in this case of 35 °C, appeared to have lower intensity. Lower quantity of the gypsum and ettringite will result in lower expansion. This is corroborated by the expansion results shown in Fig. 4a and b, where the bars cured at 35 °C the first 24 h exhibited lower expansion than those cured the whole time at 23 °C in each sulfate concentration. At 80 °C (during the first 24 h) – 23 °C regime curing (Fig. 7c), as for 35 °C (Fig. 7b), there was not a clear evolution of the peak intensity with the sulfate concentration. The intensities of the main peak of portlandite, gypsum and ettringite were almost similar for 2% and 8% of sulfate concentration, while for 5% the intensity was lower. Concerning the expansion (Fig. 4c), there was a clear difference according to the sulfate concentration, with more expansion for the highest sulfate concentration. Compared to the bars cured at 23 °C and 35 °C, the main peak of the portlandite showed lower intensity in this case, while the main peak of gypsum and ettringite were higher. This is in line with the expansion results (Fig. 4a, b and c), where the bars cured at 80 °C during the first 24 h suffered a much higher expansion and mass gain than those cured at 23 °C and 35 °C, attributed to a significant formation of gypsum and delayed ettringite. Fig. 8 provides the XRD spectra for the AAS specimens when curing temperature was 23 °C (Fig. 8a), 35 °C (Fig. 8b) and 80 °C (Fig. 8c). The peaks found were quartz (Q), C-(A)-S-H gel (X) and hydrotalcite (HT). These compounds are in accordance with the findings of other studies for AAS at normal curing conditions [25,26]. No other peaks of crystalline products due to sulfate attack were found, in accordance with the expansion results shown in

Fig. 5a, b and c. The intensities of the main peak of C-(A)-S-H gel (2h = 29.3°) and hydrotalcite (2h = 11.3°) were practically the same in all the cases, indicating the stability of AAS to sodium sulfate attack. The peaks of quartz varied depending on the sampling process, and it was due to the sand used in the confection of the mortars. Similar results were obtained by other authors in similar attack conditions [27]. Fig. 9 provides the XRD patterns in the case of the AAFA specimens for curing temperature of 23 °C (Fig. 9a), 35 °C (Fig. 9b) and 80 °C (Fig. 9c). In this case, as observed with the AAS specimens, no peaks of the presence of new crystalline products due to the sulfate attack was found. The peaks found were mostly the crystalline phases of the unhydrated FA (M, mullite; N, magnetite) displayed on Fig. 1 plus the quartz due to the sand used for the confection of the mortars. The absence of expansion products such as ettringite and gypsum highlighted by XRD experiments in the case of alkali activated materials (AAS and AAFA) explains well the ability of such a material to withstand sulfate attack. 5. Microstructural analysis by means of SEM-BSE coupled with EDX SEM-BSE experiments were performed on attacked samples after 1 year of exposition to sulfate solutions. For each system, a piece of one bar exposed only to 5% of sulfate concentration for each curing regime was collected to analysis. Results of OPC specimens are displayed in Fig. 10 for the curing temperature of 23 °C (Fig. 10A), 35 °C (Fig. 10B) and 80 °C (Fig. 10C). SEM images show the presence of ettringite crystals

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Fig. 9. XRD patterns for AAFA mortar bars exposed at different sulfates dissolutions (2, 5, and 8 wt%) after 1 year of exposition. Curing temperature during the first 24 h: a) 23 °C; b) 35 °C; c) 80 °C. Peaks: Q, quartz; M, mullite; N, magnetite.

responsible for the expansion. For the bars cured at 23 °C and 35 °C (Fig. 10A and B), the presence of fine crystals and deposits of ettringite in the regions near the aggregates, are observed. At curing temperature of 80 °C (Fig. 10C), significant deposits of ettringite can be observed. In this case, all the region is practically covered by delayed ettringite evidenced by EDX analysis. Therefore, these bars exhibit higher expansion than those cured at 23 °C and 35 °C. The formation of significant amount of delayed ettringite is related to the elevated temperature used in this case, 80 °C. It is well known that the external sulfates react with some components of the hydrated OPC paste [24,28]. XRD analysis performed after 12 months of exposure to sulfates confirmed the presence of gypsum and ettringite. Precipitation of ettringite near the aggregates evidenced in SEM observations, could be the consequence of a higher concentration of portlandite in these regions [29]. Portlandite may react with sulfates to form gypsum, and then with calcium aluminate compounds to form ettringite. Gypsum crystals were revealed by XRD analysis but difficult to detect by SEM. Moreover, samples precured at 80 °C present similar fast expansion regardless of the sulfate concentration until 150 days. However, at later ages (above 150 days), the expansion increases with the sulfate concentration, showing a direct influence of the sulfate concentration. Accordingly, there seems to be two different effects. In a first stage, at early ages, an expansion due to an internal sulfate attack (ISA) could occur. This type of attack is favored in OPC systems when they have been subjected to high temperatures over 60–70 °C [30,31]. At these conditions, primary ettringite normally produced during hydration and beneficial for the concrete is not formed or, if it is already formed, it decomposes

to monosulfoaluminate. As a consequence, more of the sulfates, no stabilized by the primary ettringite, may be either in the pore solution or incorporated in the C-S-H gel. In presence of high humidity and microcracks (also produced by the thermal curing), the sulfates normally adsorbed in the C-S-H gel can be released and produce again ettringite (thus called delayed-ettringite: DEF) with a subsequent expansion [30–32]. This stage which would be less affected by the external sulfate concentration, is therefore responsible for the fast expansion experienced by OPC bars. In a second stage, at later ages, the influence of an external sulfate source is manifested (external sulfate attack: ESA) and thus the reaction of external sulfates with the calcium and aluminum compounds of the cement paste will produce again ettringite, causing further expansion. In this second stage, the concentration of the external sulfates will have an important role, with more expansion when the sulfate concentration is higher. SEM images along wit EDX analysis of AAS specimens are provided in Fig. 11 for curing temperature of 23 °C (Fig. 11A), 35 °C (Fig. 11B) and 80 °C (Fig. 11C). In all the studied cases, as already observed with XRD results (Fig. 8a, b and c), no anomalous compound was found. The presence of anhydrous slag particles indicates a partial hydration of the material. Fine cracks observed on some particles could be due to the shrinkage that can undergo such systems [33–36]. Unlike the good continuity between the paste and aggregates for specimens cured at 23 °C, a disconnection is observed for specimens precured at 35 °C and 80 °C. This porosity could probably be due to a desiccation during the heat treatment. This could be responsible for the expansion related to water absorption observed in the water immersion test (Section 3.2).

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A

B

C Fig. 10. SEM-EDX images for OPC mortar bars exposed to 5% of sulfate concentration after 1 year of exposition. Curing temperature during the first 24 h: A) 23 °C; B) 35 °C; C) 80 °C.

EDX analysis reveals the presence of Si, Ca, Al, Mg and Na elements which are typical compounds of hydration products such as C-A-SH gel and hydrotalcite (HT) (Mg6Al2CO3(OH)16
4(H2O)) often obtained in alkali activated slag [25,26]. These products result from the hydration of the anhydrous slag with the activator, in this case, sodium silicate (Ms = 1.45). XRD analysis also confirmed the presence of these hydration products. As, both compounds (C-A-S-H and HT) are intermixed, it is difficult to differentiate them by SEM-EDX analysis. Si, Ca and Al are present in the main hydration product, the C-A-S-H gel. Al, otherwise, is also present in the hydrotalcite together with Mg. The extent to which the Al is present on the hydrotalcite or in the C-A-S-H gel is influenced by the Mg content [37,38]. Therefore, the excess of Al that is not incorporated to the hydrotalcite will be further incorporated in the C-SH gel. The presence of Na in the matrix is still controversial, but it is believed to be adsorbed on the C-A-S-H gel rather than incorporated in the gel structure [39]. Moreover, EDX spectra reveal the presence of S, probably present in the pore solution due to the ingress of sulfates. According to the results obtained in this part of the work, it is evident that AAS system is resistant to the sodium sulfate attack. These results are in accordance with other works found in literature [8,9,27,40]. Although no attack product was found after exposing to the sodium sulfate, this does not mean that AAS are always sulfate

resistant. As shown in other works [9,41], sulfate attack on AAS pastes depends also on the nature of the cations. Therefore, AAS binders has been found to be deteriorated by sulfate attack when they were exposed to magnesium sulfate. In this case, decalcification of the binder, gypsum formation, loss of mechanical performance and dimensional changes were found as a result of the magnesium sulfate attack of AAS. The degradation of the C-A-S-H gel was mainly attributed to the low solubility of magnesium hydroxide compounds (formed during the interaction of the magnesium sulfate with the gel) and the resulting low pH value [24,41]. Fig. 12 provides SEM images for the AAFA specimens with curing temperature of 23 °C (Fig. 12A), 35 °C (Fig. 12B) and 80 °C (Fig. 12C). No attack products resulting from the sodium sulfate exposure is found in the micrographs. In all cases, the EDX spectra obtained show the presence of elements of the sodium aluminosilicate gel (N-A-S-H), typical hydration product of such systems: Si, Al and Na [20,42]. Anhydrous FA particles (plenospheres, spheres) and white particles related to mullite/magnetite present in the original FA are also observed. Observing these images, it can also be deduced a relative hydration degree in terms of anhydrous FA particles (spheres). For the curing temperature of 23 °C and 35 °C (Fig. 12A and B), the quantity of anhydrous FA particles is higher than in the case of curing at 80 °C. Thus, in this particular system,

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A

B

C Fig. 11. SEM-EDX images for AAS mortar bars exposed to 5% of sulfate concentration after 1 year of exposition. Curing temperature during the first 24 h: A) 23 °C; B) 35 °C; C) 80 °C.

it is demonstrated that curing at elevated temperatures dissolves all the necessary elements present in the FA [42]. Table 6 shows the atomic ratios of the paste regions for each image. The atomic ratios are an average of a minimum of 5 points of the image (1.00 k of scale). It can be observed that the Si/Al ratio decreases with increasing the curing temperature during the first 24 h. This means that more aluminum elements are dissolved and thus fixed on the structure of the N-A-S-H gel. It is also observed that the Na/Al ratio is constant when curing temperature is 23 °C and 35 °C, and then decreases for 80 °C. This is also an evidence of the increase of the aluminum content in the structure at elevated temperatures. Therefore, the atomic ratios corroborate the high solubility of the elements at elevated curing temperatures. AAFA samples showed no expansion regardless of the curing regime and the sulfate concentration. Although curing temperature showed to be an important aspect on the mechanical performance of these systems (Fig. 2c), it seems that mechanical properties are not the most important aspect regarding the sulfate resistance but the chemical composition of the AAM precursor. The low Ca-content of the class F fly ash could be the reason that AAFA is highly resistant to sodium sulfate attack. XRD analysis performed on samples after 12 months of sulfate exposure did not show the presence of new crystalline products other than those present initially in the FA (mullite and magnetite). SEM-BSE images neither revealed the formation

of any crystalline product nor a degradation of the matrix. EDX analysis (for the three curing regimes) showed typical spectra of AAFA (when sodium silicate is used as activator) [42], with Na, Al and Si as the main elements attributed to the formation of the N-A-S-H gel. Therefore, the results showed very good resistance of AAFA to sodium sulfate attack, independently of the curing regime and sulfate concentration. These results are in accordance with those found in literature [7,43,44]. However, Bakharev [45] found different results depending on the nature of the activator and the type of the sulfate. Even though no visual deterioration signs were detected on AAFA samples after 5 months in 5% sodium or 5% magnesium sulfate solution, a slight mass gain and a compressive strength loss were observed with traces of ettringite. Mg sulfate appeared to be more aggressive than Na sulfate. However, in both cases, AAFA better resisted to sulfate attack than OPC samples [45]. It is important to note that the AAFA composition and the curing regime used by Bakharev were different from those used in this study. This can explain the difference observed in results, as, no trace of ettringite that is the main deterioration product and no expansion were observed in this work (Figs. 6, 9 and 12), unlike the one observed by Bakharev. This clearly demonstrates that AAFA is more resistant to sulfate than OPC. However, AAFA can somewhat be vulnerable to sulfate attack depending on several parameters which need to be appropriately defined according to required usage.

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A

B

C Fig. 12. SEM-EDX images for AAFA mortar bars exposed to 5% of sulfate concentration after 1 year of exposition. Curing temperature during the first 24 h: A) 23 °C; B) 35 °C; C) 80 °C.

Table 6 Atomic ratios obtained by the EDX analysis of the AAFA mortars exposed during 12 months to 5 wt% sodium sulfate solution for curing temperature during the first 24 h of 23 °C, 35 °C and 80 °C. Curing temperature

Si Al

SiO2 Al2 O3

Na Al

Na2 O Al2 O3

23 °C 35 °C 80 °C

5.22 4.07 3.56

10.44 8.15 7.12

0.54 0.53 0.38

0.54 0.53 0.38

6. Conclusion The resistance of OPC, AAS and AAFA systems to sodium sulfate attack was assessed using ASTM C1012 as basis. The effect of curing temperature and sulfate concentration was studied. Crosssectional dimension and mass changes were used as main indicators of the degree of attack. SEM-EDX and XRD experiments were performed to identify the corrosion products. Different behaviors were observed depending on the nature of the mortar. All the OPC bars exceeded the limit compliance established in the ACI201.2R for class 2 of exposure to sulfates. The only exception was the bars cured at 35 °C and exposed in 2 wt% sulfate solution, which passed the test at 6 months but failed at 1 year. The expansion was attributed in all cases to a gypsum and ettringite formation. OPC bars precured at 35 °C (as required in the standard) showed lower expansion than those cured at 23 °C. This could

probably be due to a higher porosity of the bars cured at 35 °C which could buffer the effect of the stress caused by ettringite crystals growth inhibiting expansion. Preheating OPC bars at 80 °C induced a fast expansion from the beginning, exceeding the limit compliance at 1 year (0.1%) after 50 days of exposure. This has been attributed to a double effect: internal sulfate attack and external sulfate attack. The ISA would be the result of the curing temperature at 80 °C. In AAS system, the expansion of the bars was quite below the limit compliance whatever the curing temperature and the sulfate concentration. However, a slight curing temperature dependence was noticed. Indeed, higher curing temperatures resulted in higher initial expansion (after 7 days) before remaining constant during time. This was attributed to a water absorption rather than a sulfate attack. Since large porosity surrounding the aggregates was detected for samples precured at 35 and 80 °C. And no presence of crystalline products other than the normal hydration products (C-A-S-H gel and hydrotalcite) was found. Therefore, AAS appear to better resist to sodium sulfate attack. AAFA specimens did not show any expansion regardless of attack conditions, suggesting a great resistance to sodium sulfate attack. Instead, slight shrinkage during time was found. In addition, microstructural characterizations showed no presence of crystalline products associated to sulfate attack, but a sodium aluminosilicate hydrate gel typical of these systems was observed.

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This study clearly showed that curing regime (temperature and time) should not be a prescriptive parameter but selected according to the AAM precursor used. As curing temperature is an important parameter in the development of AAM performance, it should be considered to improve the resistance of such a system against sulfate attack. The sulfate concentration, as in the ASTM C1012, should also be selected depending on the real conditions simulated. Moreover, the expansion, caused by the calcium-sulfatebased products, seemed to be a suitable indicator for measuring the degree of sodium sulfate attack in OPC systems. But, for AAM systems which contain low amount of calcium, other indicators of the degree of attack should be considered, e.g. the compressive strength loss and the change in the corrosion depth through carbonation test. Microstructural characterization of samples after sulfate exposure should be performed to confirm the stability of the binder. And due to the great variability of AAM sources, the 20 MPa required in the ASTM C1012 standard prior to start the test should not be a requirement. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the financial support of the Industrial Chair on the Valorization of Glass in Materials provided by the Société des Alcools du Quebec (SAQ). References [1] J.L. Provis, J.S.J. van Deventer, Alkali Activated Materials: State-of-the-Art Report, RILEM TC 224-AAM, 2014. doi:10.1007/978-94-007-7672-2. [2] F. Pacheco-Torgal, Handbook of alkali-activated cements, mortars and concretes, Woodhead P, 2004. doi:https://doi.org/10.1016/C2013-0-16511-7. [3] S.A. Bernal, J.L. Provis, Durability of alkali-activated materials: progress and perspectives, J. Am. Ceram. Soc. 97 (2014) 997–1008, https://doi.org/ 10.1111/jace.12831. [4] J. Aliques-Granero, T.M. Tognonvi, A. Tagnit-Hamou, Durability test methods and their application to AAMs: case of sulfuric-acid resistance, Mater. Struct. Constr. 50 (2017) 36, https://doi.org/10.1617/s11527-016-0904-7. [5] O. Mielich, C. Ötll, Practical investigation of the sulfate resistance of concrete from construction units, Otto-Graf-J. 15 (2004) 132–135. https://www.mpa. uni-stuttgart.de/publikationen/otto_graf_journal/ogj_2004/beitrag_mielich. pdf (accessed February 21, 2019). [6] A. Palomo, M.T. Blanco-Varela, M.L. Granizo, F. Puertas, T. Vazquez, M.W. Grutzeck, Chemical stability of cementitious materials based on metakaolin, Cem. Concr. Res. 29 (1999) 997–1004, https://doi.org/10.1016/S0008-8846(99) 00074-5. [7] F. Škvára, T. Jílek, L. Kpecky´, Geopolymer materials based on fly ash, Ceram. Silikáty 49 (2005) 195–204. http://citeseerx.ist.psu.edu/viewdoc/download? doi=10.1.1.465.6987&rep=rep1&type=pdf (accessed February 22, 2019). [8] F. Puertas, R. de Gutiérrez, A. Fernandez-Jimenez, S. Delvasto, J. Maldonado, Alkaline cement mortars. Chemical resistance to sulfate and seawater attack, Mater. Constr. 52 (2002) 55–71. [9] T. Bakharev, J.G. Sanjayan, Y.-B. Cheng, Sulfate attack on alkali-activated slag concrete, Cem. Concr. Res. 32 (2002) 211–216, https://doi.org/10.1016/S00088846(01)00659-7. [10] L. Gu, P. Visintin, T. Bennett, Evaluation of accelerated degradation test methods for cementitious composites subject to sulfuric acid attack; application to conventional and alkali-activated concretes, Cem. Concr. Compos. 87 (2018) 187–204, https://doi.org/10.1016/j. cemconcomp.2017.12.015. [11] L. Gu, T. Bennett, P. Visintin, Sulphuric acid exposure of conventional concrete and alkali-activated concrete: assessment of test methodologies, Constr. Build. Mater. 197 (2019) 681–692, https://doi.org/10.1016/j.conbuildmat.2018.11.166. [12] K.O. Kjellsen, R.J. Detwiler, O.E. Gjørv, Development of microstructures in plain cement pastes hydrated at different temperatures, Cem. Concr. Res. 21 (1991) 179–189, https://doi.org/10.1016/0008-8846(91)90044-I. [13] J.J. Thomas, D. Rothstein, H.M. Jennings, B.J. Christensen, Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes, Cem. Concr. Res. 33 (2003) 2037–2047, https://doi.org/10.1016/S0008-8846(03)00224-2.

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