Development of low-pH cementitious materials based on CAC for HLW repositories: Long-term hydration and resistance against groundwater aggression

Cement and Concrete Research 51 (2013) 67–77

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Development of low-pH cementitious materials based on CAC for HLW repositories: Long-term hydration and resistance against groundwater aggression J.L. García Calvo a,⁎, M.C. Alonso a, A. Hidalgo b, L. Fernández Luco c, V. Flor-Laguna a a b c

Institute for Construction Sciences Eduardo Torroja, CSIC, Serrano Galvache 4, 28033 Madrid, Spain CSIC Delegation in Andalucía, Seville, Spain INTECIN-Universidad de Buenos Aires, Av. Paseo Colón, 850, C1063ACV Buenos Aires, Argentina

a r t i c l e

i n f o

Article history: Received 3 August 2012 Accepted 11 April 2013 Keywords: Low-pH cements Hydration (A) Long-term performance (C) Calcium Aluminate Cement (D) Radioactive waste (E)

a b s t r a c t One of the most accepted engineering construction concepts of underground repositories for high level waste considers the use of low pH cementitious materials. This research is focused on the development of those based on Calcium Aluminate Cements (CAC) with high mineral admixture contents that significantly modify their microstructural properties. Once their short-term hydration is known, this paper deals with the modifications generated in the pore solutions and in the solid phases of low-pH cement pastes based on CAC after 2 years of hydration, observing a high stability of the solid phases formed in the short-term. This paper also deals with the resistance of these materials to long term groundwater aggression using two types of aggressive agents: deionised water and groundwater from the real site of Äspö. Leaching tests show a good resistance of low-pH concretes against groundwater aggression but dependent on the leaching agent and on the concrete composition. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Cementitious materials play an important role in the structural stability and integrity of a purpose built repository for the geological disposal of high level waste (HLW). When conventional Ordinary Portland Cements (OPC) are used to produce concretes for these underground repositories, the contact with the groundwaters would create pore water leachates with a pH as high as 13.5. The generation of this alkaline plume from the concrete by the ingress of groundwater would have detrimental effects on the intended use of a bentonite buffer. To limit this risk, low-pH cementitious materials are being developed to have a target pH b 11 corresponding to the upper stability limit of bentonite [1,2]. The research on low-pH cementitious materials was initially developed in Canada and Japan [3,4], and it has been addressed from different approaches depending on the type of cement used: 1) Calcium Silicate Cements (OPC based), 2) Calcium Aluminate Cements (CAC based), 3) Phosphate Cements (PC) and 4) Magnesia Cements (MC) [3–18]. CAC materials represent an interesting alternative because their pore water pH, ranging from 11.4 to 12.5, is reduced as compared to the conventional OPC one [17,19]. However, direct use of these binders comes up against one main difficulty, the so-called “conversion process”: the hydration reaction of CAC at environmental temperatures ⁎ Corresponding author. Tel.: +34 91 302 0440; fax: +34 91 302 0700. E-mail address: [email protected] (J.L. García Calvo). 0008-8846/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2013.04.008

produces initial hexagonal hydrated calcium aluminates CAH10 and C2AH8, and gibbsite (AH3; that crystallises in a monoclinic system); but the stable phases are C3AH6 (cubic hydrate) and AH3, and the initial hexagonal phases (CAH10 and C2AH8) will inevitably convert to these, decreasing the mechanical properties of the material due to the lower density of the stable phases [20–22]. The conversion reactions of CAC are shown schematically below: 3CAH10 →C3 AH6 þ 2AH3 þ 18H

ð1Þ

3C2 AH8 →2C3 AH6 þ AH3 þ 9H

ð2Þ

where CAH10: CaAl2O14H2O, C2AH8: Ca2Al2O13H16, C3AH6: Ca3Al2(OH)12, AH3: Al(OH)3, and H: H2O. Some authors have found that an interesting way to reduce the conversion of the initial hexagonal hydrates, thus avoiding the decreasing of strength, is to replace some of the CAC by mineral admixtures with high silica content as silica fume (SF), fly ashes (FA) or blast furnace slags [23–30]. The reaction that avoids the conversion of CAC hexagonal hydrated phases could take place in the following way: the silica content of the mineral admixtures would react with the calcium aluminates, initially avoiding the formation of the hexagonal form C2AH8 and, subsequently the conversion in the cubic form C3AH6 (hydrogarnet). Therefore, instead of this cubic phase, a hexagonal aluminate hydrate containing silica, called strätlingite (C2ASH8) is proposed to be formed. But this is not the only calcium aluminosilicate

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formed in the cement pastes based on CAC with high mineral admixture contents, as the low-pH ones. Some authors [28–30] also reported that in the systems CAC + SF and CAC + FA, silica reacts with the calcium aluminate phases in the cement and water to form different crystalline hydrates (with variable proportions of Ca, Al, Si) such as C2ASH8 (strätlingite; abovementioned), members of the hydrogarnet solid solution with general formula Ca3Al2(SiO4)3 − x(OH)4x or C3AS3 − xH2x (0 b x b 3) and not very well defined and complex zeolite-type phases. The Ca(OH)2, Al(OH)3, CaAl2O4·10H2O, Ca2Al2O5·8H2O, Ca2Al2SiO7· 8H2O and a series of solid solutions, with a general formula of Ca3Al2(SiO4)3 − x(OH)4x, can be located in the CaO–Al2O3–SiO2–H2O system for temperatures below 100 °C. Indeed, the whole series exists with the end members known as grossular/garnet when x = 0 and hydrogrossular/hydrogarnet if x = 3. Since these solid solutions exist also in nature, mineralogists classify them as hibschite for the minerals with 0.2 b x b 1.5 and as katoite for the ones with 1.5 b x b 3. The identification of the calcium carboaluminate hydrate C4AcH11 as a precursor of hydrogarnet–garnet solid solution members with silica in their composition, C3AS3 − xH2x, when SF and/or FA are used, has been also proposed in literature [29,30]. It is also remarkable that, as well as it occurs in low-pH cements based on OPC, the percentage of alkalis does not alter significantly the pore fluid pH of these materials, and an alkali binding process is observed [17]. Therefore, once the short-term hydration of these special cement formulations is known, this paper deals with the modifications generated in the chemical composition of the pore solutions of low-pH cement pastes based on CAC and in their solid phases after 2 years of hydration. The long-term mechanical strength evolution of mortars fabricated using low pH cement formulations based on CAC has been also analysed till 3 years of hydration. Moreover, taking into account the long life expected in this type of repositories, parameters related to the durability of the cementitious materials must be analysed. In HLW underground repositories the degradation of concrete due to leaching will occur when the hydrates in cementitious materials dissolve into the surrounding water and the precipitation of new phases takes place. So there is an enormous concern on determining the low-pH concrete behaviour under the long-term action of water, in representative conditions of the real storage scenario. In previous work, the resistance of CAC + SF or FA concrete against groundwater aggression from the Äspö (Sweden) facilities was preliminary evaluated, resulting in a high concrete resistance but dependent on the concrete composition [17]. In the present paper, this evaluation is completed and also compared with that observed for deionised water attack, as this is the leaching agent commonly used in this type of studies. The groundwater from the underground facilities of Äspö was used to better reproduce the boundary conditions of a HLW underground repository. 2. Experimental 2.1. Studies in cement pastes Several low-pH cement pastes were fabricated using a low alumina content CAC as base cement which has been partially substituted by high contents of mineral admixtures (densified SF and/or FA), resulting in binary and ternary blends, using a water/binder ratio (w/b) = 0.5 in all of them. A reference paste, without mineral

Table 2 Cement formulations used in the fabrication of the pastes. Sample

CAC (%)

SF (%)

FA (%)

SiO2 (%) in the formulation

Ref S-1 S-2 S-3 F-1 F-2 T-1 T-2

100 80 70 50 70 50 70 70

– 20 30 50 – – 20 10

– – – – 30 50 10 20

2.5 20 29 47 18 28 29 26

admixtures, was also fabricated. All the samples were cured in a chamber at 98% RH and 21 ± 2 °C temperature until the moment of testing. The chemical composition of CAC, SF and FA is presented in Table 1. It is obvious that the densified SF has the highest SiO2 amount and introduces low CaO percentages in the blend, with the alkali content a little bit higher than the CAC one. FA introduces much more alkalis in the composition and has the highest alumina content; indeed, it has also higher silica content than CAC. Deionised water was used for the preparation of the cement pastes. Table 2 summarizes the formulation used in the cement pastes evaluated, indicating the silica content of every blend. The pore fluid pH evolution of the cement pastes was determined at different days of hydration (up to 2 years) using an ex-situ leaching method described in [31] but also at 90 days and 2 years the pore solution was extracted using the Pore Fluid Expression Technique [32,33]. The ionic composition (Ca, Na, K, Al, sulfates and Si) of the extracted pore fluids was determined by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy). The long-term hydration of the cement pastes was determined by stopping the curing at different ages (90 days, 1 year and 2 years), by means of powdering the samples and removing the free water with ethanol and acetone. DTA/TG tests and XRD analyses have been made to study the evolution of the microstructural composition of the solid phases in the fabricated pastes. DTA/TG data were obtained with a resolution of 0.01 mg. The sample was taken in a platinum crucible and heated from room temperature to 1200 °C at a heating rate of 10 °C/min using nitrogen as a medium under static condition. Alumina powder was used as a reference material. XRD patterns were recorded at room temperature in the interval 5° b 2θ b 60°, with a step size of 2θ = 0.01973° and 0.5 s per step. 2.2. Studies in mortars In order to evaluate the long-term strength evolution of low-pH cementitious materials based on CAC, four cement formulations were evaluated in mortar form: Ref, S-2, F-1 and T-1. A binder:siliceous sand ratio of 1:3 was used in all the cases and the water/binder (w/b) ratios were determined for an equivalent consistency of 18 ± 1cm (measured according to UNE-EN 1015-3), adding 2% in weight binder of superplasticizer (Naphthalene Formaldehyde base from Sika). This consistency was chosen due to some of the applications of low-pH concretes in HLW repositories will imply the use of the shotcreting technique that involves the obtaining of this consistency in mortar form. The w/b obtained were as follows: 0.5 (Ref), 0.66

Table 1 Chemical composition (weight %) of CAC, SF and FA.

CAC SF FA

LI

IR

SiO2

Al2O3

Fe2O3

CaO (total)

MgO

SO3

Na2O

K2O

CaO (free)

0.42 0.09 2.19

2.24 0.06 0.52

2.52 91.8 54.3

43.86 0.59 26.9

14.2 3.74 5.38

35.7 1.31 4.52

0.69 0.93 2.24

– – –

0.15 0.15 0.61

0.1 0.37 3.17

0.11 0.01 0.15

LI: loss of ignition; IR: insoluble residue.

J.L. García Calvo et al. / Cement and Concrete Research 51 (2013) 67–77

mineral admixtures is almost constant during all the test period. At 2 years of hydration, the pH measured in the low-pH cement pastes is more than 0.5 points lower than the one of the reference paste. The highest drop in the pH value from 90 days to 2 years is observed in the binary pastes with FA due to the slow reactivity of this mineral admixture. Table 5 shows the pore fluid chemical composition of low-pH cement pastes after 90 days and 2 years of hydration. With the mineral admixtures (overall the FA) more alkalis were added to the binder than with the base cement; however, the alkali ion concentration in the pore solution is similar or lower in most of the low-pH cement pastes, indicating a possible alkali binding process during hydration that would remove the alkalis from the pore solution. Moreover, the alkali content at 2 years is lower than that measured at 90 days in the same sample, indicating a gradual incorporation of the alkali ions in the C–A–S–H matrixes (or C–A–H matrixes as this process is also detected in the reference sample). The calcium content, however, increases at 2 years of hydration in all the cases, although, at this age, the higher the mineral admixture content in the binder the lower the Ca content in the pore solution. The decalcification observed in all the pastes from 90 days to 2 years of hydration could be associated to the alkali binding process. As commented, the calcium content at 2 years of hydration is lower in the low-pH cement pastes than in the reference one. This phenomenon could explain why the pastes with high FA contents (F-1 and F-2), even showing more alkalis in the pore solution than Ref paste at 2 years of hydration, have a lower pH than that measured in the reference mix. It is also remarkable that if the charge balances of the expressed pore solutions are calculated, at 2 years of hydration they are almost 0 in all the cases, but at 90 days a slight excess of cations is detected. This should be related with the fact that at 2 years of hydration the equilibrium between solid phases and pore solution is more stable than at 90 days, as from 90 days to 2 years several modifications in the solid phases can occur, mainly in the pastes with FA due to its slower reactivity.

Table 3 Concrete composition of the samples used in the leaching tests. Constituents Water (kg/m3) Binder (kg/m3) Water/binder Medium agg. (4–8 mm) (kg/m3) Sand (0–4 mm) (kg/m3) Superplasticizer (kg/m3)

69

160 320 0.5 855 1033 3.2

(S-2), 0.44 (F-1) and 0.54 (T-1). Mortar prisms (40 × 40 × 160 mm) were fabricated and the compressive strength evolution was analysed from 2 days to 3 years. All the samples were cured in a chamber at 98% RH and 21 ± 2 °C temperature until the moment of testing. 2.3. Water aggression resistance of low-pH concretes The resistance against groundwater aggression was evaluated in two types of low-pH concretes, based on the binary cement formulations S-2 and F-1. Several samples with a ϕ = 5cm (test requirement) were fabricated using a “conventional” concrete mix composition. Siliceous aggregates from Äspö facilities were used. The granulometry of the two fractions was: 0–4 mm (sand) and 4–8 mm (medium aggregate). A Naphthalene Formaldehyde Superplasticizer from Sika was also added. Table 3 shows the nominal composition of both low-pH concretes. The concrete samples were cured in a humidity chamber at 98%RH during 90 days before testing. The material behaviour under the long-term action of water, in representative conditions of the real repository scenario, has been evaluated in both concrete types. The accelerated leaching test used helps to reproduce the sequence of degradation processes. Cylindrical samples (ϕ = 5 cm; h = 5 cm) were placed between two cylinders of methacrylate containing holes for water inlet and outlet. The device and the methodology used have been already described in a previous work [18]. Deionised (but not free CO2) water was used as a leaching agent (as it is the commonly used agent in this type of tests). Moreover, in order to simulate the real conditions, the leaching tests were also carried out using real groundwater from Äspö, whose chemical composition and that of the deionised water are presented in Table 4. Variables measured continuously in every case were: effluent flux (to determine the hydraulic conductivity of the concretes), chemical composition and pH of the leachates. At the end of experiments (14 months) concrete cylinders were divided in three similar size portions (≈1.5 cm.); the upper part was in direct contact with the water inlet. Each one of these three parts was characterised by BSEM with EDX analyses (samples were embedded into an epoxy resin, cut, polished and then coated with carbon), and Mercury Intrusion Porosimetry (MIP).

3.2. Long-term evolution of the solid phases in low-pH cement pastes based on CAC Comparing the results obtained in the low-pH cement pastes at 90 days [29,30] with those obtained after 2 years of hydration, a high stability of the solid phases formed in the short-term (90 days) is deduced. Fig. 2 shows the XRD of two low-pH cement pastes based on CAC plus 50% of mineral admixture (S-3 and F-2 samples), the ternary paste with less SF content (T-2) and the reference paste, after two years. As after 90 days of hydration, in the paste without mineral admixtures (Ref), the main hydrates are gibbsite and hydrogarnet. However, in the low-pH cement pastes the main hydrates are strätlingite (C2ASH8) and, instead of pure hydrogarnet, members of the hydrogarnet solid solution with Si in their composition (with general formula C3AS3 − xH2x). The introduction of Si in these members of the hydrogarnet solid solution is deduced from the shift in the theoretical C3AH6 pattern peaks (ICDD card 24-0217) observed in the pastes with high mineral admixture contents (see Fig. 2): in the low-pH cement pastes the C3AH6 peaks appear at higher 2θ values. For example, in Ref paste the main C3AH6 is detected at 2θ = 39.22° (equal to theoretical peak), in S-1 at 2θ = 39.42° and in S-3 at 2θ = 39.48°. In fact, it can be assumed that the higher the increase in the 2θ values of the hydrogarnet peaks, the higher the introduction of Si in the structure of the C3AS3 − xH2x members. In Fig. 2 the main peak

3. Results and discussion 3.1. Evolution of pore solution composition in low-pH cement pastes based on CAC Fig. 1 shows the pore fluid pH evolution of low-pH cement pastes based on CAC. It can be clearly seen that the pore fluid pH of the low-pH cement pastes slightly decreases from 90 days to 2 years of hydration. However, the pore fluid pH of the cement paste without Table 4 Chemical composition of the waters used in the leaching tests.

Deionised water [mg/l] Groundwater [mg/l]

Cl

SO4

Na

K

Ca

Mg

Si

HCO3

pH

– 2681

2.09 232

1.44 1129

2.09 9.36

1.78 356

0.03 78.2

1.43 7.32

25.0 30.0

7.70 8.20

70

J.L. García Calvo et al. / Cement and Concrete Research 51 (2013) 67–77

12,4

Ref S-1 S-2 S-3

pH

11,9

11,4

10,9

0

100

200

300

400

500

600

700

800

900

Curing time (days) 12,4

Ref T-2 T-1 F-1 F-2

pH

11,9

11,4

10,9

0

100

200

300

400

500

600

700

800

900

Curing time (days) Fig. 1. Evolution of the pore fluid pH of cement pastes (measured with method described in [31]). Up: Ref paste and binary pastes with SF; down: Ref paste, binary pastes with FA and ternary pastes.

is reduced, with strätlingite as the main hydrate formed (as it has been seen at shorter ages by many authors [24,26,29,30]) whereas in those with high percentage of FA the main hydrates are C3AS3 − xH2x, although the higher the FA content the higher the C2ASH8 content. Observing Fig. 2, in the S-3 paste only the main typical peaks of C3AS3 − xH2x hydrates are detected, whereas other peaks of these phases that are detected in the other pastes (e.g. 2θ = 17.27° and 2θ = 31.82°) are not present, confirming the lower formation of C3AS3 − xH2x hydrates (but the higher one of strätlingite) in the low-pH cement paste with the highest silica content (S-3). Therefore, it seems that the SF, due to its higher silica content, its more amorphous structure and its lower particle size, and thus its higher reactivity, promotes the formation of both C–A–S–H types (C2ASH8 and C3AS3 − xH2x) but mainly strätlingite with SF contents ≥ 50%,

of the hydrogarnet in the reference paste is marked in order to observe the shift present in the C3AS3 − xH2x hydrates of the low-pH cement pastes. It is remarkable that, even after 2 years of hydration, the hexagonal hydrates CAH10 are observed in the low-pH cement pastes with the highest mineral admixture contents (S-3 and F-2), corroborating a strong and effective control of the conversion process promoted by the addition of the mineral admixtures. In the T-2 paste, due to its lower silica content, the initial hexagonal hydrates are not detected at 2 years of hydration, although the presence of strätlingite and C3AS3 − xH2x hydrates is clear. Therefore, it is evident that the higher the silica contents in the cement formulation the higher the control of the conversion process. On this matter, it is also remarkable that in binary pastes with high SF contents (>30%), even the presence of C3AS3 − xH2x hydrates

Table 5 Chemical composition of the pore solution of the cement pastes: 90 days and 2 years of hydration. Sample

Time

pH

Ref

90 days 2 years 90 days 2 years 90 days 2 years 90 days 2 years 90 days 2 years 90 days 2 years 90 days 2 years 90 days 2 years

12.25 11.98 11.78 11.38 11.41 11.42 11.34 11.33 12.12 11.44 11.95 11.22 11.56 11.64 11.91 11.67

S-1 S-2 S-3 F-1 F-2 T-1 T-2

ND: not detected (detection limit = 0.05 mg/l).

Chemical composition (mg/l) Na

K

Ca

Si

Al

SO4

285 32.4 455 41.0 418 60.26 282 47.4 497 42.8 278 54.1 296 24.2 354 27.9

626 45.9 524 47.9 468 5.58 179 1.31 1550 62.1 908 81.2 425 40.2 711 53.7

22.6 168 14.0 95.9 1.99 117 1.87 104 1.91 68.9 15.5 78.0 20.1 176 19.1 73.8

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

321 109 112 149 75.8 89.9 42.1 48.4 116 144 80.3 91.5 137 210 124 97.8

10.4 23.7 154 27.9 205 74.4 48.3 44.4 9.03 41.9 15.1 36.4 12.6 24.0 7.59 33.9

J.L. García Calvo et al. / Cement and Concrete Research 51 (2013) 67–77

71

3600 3200 4

2800 4 3

I (cuentas)

2400

3 2

4 3

T-2

2

2000 1

2

1

F-2

3

1

S-3 Ref

3

2

6 7

3

4

6 7

3

4 4

5

3

2

4 4

4 3

400

2

4 5

4 3

800

3

4 4 3

4

1200 1 2

6 7

2 3

1600

4 4 5

4 3

43

5

1 = CAH10 2 = C2ASH8 3 = AH3 4 = C3AH6ó C3AS3-xH2x 5 = CaCO3 6 = C12A7(H) 7 = C4AF

3

6 7

4

0 5

15

25

35

45

2θ Fig. 2. XRD of Ref, S-3, F-2 and T-2 pastes after 2 years. The broken line plotted indicates the 2θ value of the main peak of the C3AH6 pattern.

while FA, due to its lower reactivity, mainly promotes the formation of C3AS3 − xH2x members. In the ternary mixes, due to the synergic effect of both mineral admixtures, the main hydrate phase cannot be easily identified, although it seems that in T-2, as a consequence of its low SF content (10%SF and 20%FA), the formation of C3AS3 − xH2x members is preferred. The thermogravimetric analyses confirm the results obtained by XRD. Fig. 3 shows the DTA analyses obtained in the reference paste and S-3, F-2 and T-1 pastes after 2 years of hydration. In Ref paste, the major endothermic effects recorded can be attributed to dehydration of gibbsite and hydrogarnet (C3AH6), indicating a total conversion of the initial hexagonal hydrates. In the low-pH cement pastes, the initial step in the thermal decomposition of samples is the endothermic effect at ~ 90 °C, which is induced by Al2O3·xH2O. This endothermic effect indicates that an amorphous aluminium hydroxide is produced as a product formed in the hydration reaction of calcium aluminates and SF and/or FA. Moreover, endothermic effects attributed to the dehydration of hexagonal calcium aluminate hydrates are also detected (~ 140 °C), indicating the control or the limitation achieved in the conversion process even after 2 years of hydration. But the most remarkable thing is that in low-pH cement pastes there is an overlapping of the AH3 and C3AH6 effects. This is due to the introduction of silica in the structure of hydrogarnet and the decrease in cristallinity degree, that produce a displacement of the endothermic effect of C3AH6 and the overlapping with that one of AH3. Therefore the DTA analyses

1000

Al2O3·xH2O+CAH10+C2AH8

800

AH3+C3AS3-xH2x

DTA / a.u.

600 400 200 0

confirm the presence of siliceous hydrogarnet, C3AS3 − xH2x, to be more stable than pure hydrogarnet [34], confirming that pure C3AH6 is not formed thus the conversion process is avoided from a microstructural point of view, even after two years at 98%RH (environments that could promote the conversion process [20]). At last, considering that siliceous hydrogarnet can include significant fractions of iron, the formation of C3(A,F)S3 − xH2x members cannot be dismissed in these low-pH cement pastes, as the raw materials used had important iron contents, mainly the CAC (see Table 1). 3.3. Strength development behaviour of low-pH mortars based on CAC Table 6 shows the compressive strength values determined in the fabricated mortars and Fig. 4 presents their strength development behaviour, showing relative values with respect to the compressive strength measured in each case at 28 days. A very fast strength gain is observed during the first 28 days in the four mortars. However, at 90 days there is a strength decrease in the mortar without mineral admixtures, a decrease even higher at 3 years, indicating a clear advance of the conversion process (total conversion of initial hexagonal hydrates, CAH10 and C2AH8, in C3AH6 and gibbsite). On the contrary, the low-pH mortars show a continuous and progressive but a slight strength increase from 28 days to 3 years, corroborating the suppression of the conversion process promoted by the inclusion of high total mineral admixture contents (>30%). Nevertheless, the compressive strength values of the low-pH mortars are lower than those measured in the reference one during the whole test period, thus the design of the low-pH cement formulations should be improved to minimize the obtained differences. Considering the low-pH mortars, the one based on the ternary formulation (T-1) showed the higher compressive strength so possibly the improvement in the mechanical strength must be assessed by means of ternary formulations that promote the synergy effects of both mineral admixtures (SF and FA). Moreover, chemical additives or mix procedures that generate better dispersion of the SF/FA particles would also promote higher strengths. According

2 years of hydration

-200

T-1 F-2 S-3 Ref

-400 -600

AH3

-800 0

200

C3AH6 400

600

800

Table 6 Compressive strength values of the fabricated mortars.

1000

Fig. 3. DTA of Ref, S-3, F-2 and T-1 cement pastes after 2 years of hydration.

Cement formulation

Curing time

Ref S-2 F-1 T-1

49.3 21.0 25.0 31.8

2 days ± ± ± ±

7 days 0.4 0.1 0.1 0.1

59.2 35.6 27.6 36.1

± ± ± ±

0.2 2.1 1.4 0.9

28 days

90 days

3 years

78.4 37.3 33.4 40.9

73.8 38.0 33.6 41.6

63.0 39.3 34.4 43.7

± ± ± ±

0.4 0.9 0.4 0.1

± ± ± ±

0.9 0.7 0.5 0.8

± ± ± ±

0.7 0.7 2.1 0.9

J.L. García Calvo et al. / Cement and Concrete Research 51 (2013) 67–77

Relative Compressive Strength (% respect to 28 days)

72

110 100 90 80 70 60

R

S-2

F-1

T-1

50 0

200

400

600

800

1000

1200

Time (days) Fig. 4. Compressive strength development of the fabricated mortars.

12 S-2 deionized water F-1 deionized water S-2 groundwater F-1 groundwater

1,E-07

K (m/s)

1,E-08 1,E-09

Acceptable

11 Acceptable

10

pH

1,E-06

9

1,E-10

8

1,E-11

7

S-2 deionized water F-1 deionized water S-2 groundwater F-1 groundwater

6 0

100

1,E-12

0

100

200

300

400

200

300

400

500

Time (days)

Time (days)

Fig. 5. Evolution with time of the hydraulic conductivity (left) and leachate pH (right) of the tested low-pH concretes with both leaching agents.

to these data, in a microstructural level the conversion process is not only controlled (or even avoided) by the strätlingite formation (as postulated by many authors [23–27]) but also by the inclusion of

silica in the microstructure of the C3AS3 − xH2x members, as in the F-1 the C2ASH8 content is quite low, with C3AS3 − xH2x members as the main hydrates.

120

60 S-2 deionized water

100

S-2 deionized water

50

80

Na

60

sulphates

Si

30

40

20

20

10

0

0 0

100

200

300

Ca

40

mg/l

mg/l

K

400

Al

0

100

Time (days) 600

200

300

200

S-2 groundwater

S-2 groundwater

Il-Iw (mg/l)

400

Il-Iw (mg/l)

400

Time (days)

200 0 K

-200

Na

Ca

Si

Al

Mg

0

-200

sulphates

-400

-400 0

100

200

300

Time (days)

400

0

100

200

300

400

Time (days)

Fig. 6. Evolution with time of the leachate composition of S-2 concrete. Il = ion content in leachate; Iw = ion content in the groundwater used.

J.L. García Calvo et al. / Cement and Concrete Research 51 (2013) 67–77

1000

60 F-1 deionized water K Ca

40

Na

600

mg/l

mg/l

F-1 deionized water

50

800

sulphates

400

Si

30

Al

20

200 0

10 0 0

100

200

300

400

0

100

Time (days) 600

300

F-1 groundwater

Il-Iw (mg/l)

200 0 K

300

400

F-1 groundwater

200

-200

200

Time (days)

400

Il-Iw (mg/l)

73

Na

sulphates

100

Ca

Si

Al

Mg

0

-100

-400

-200 0

100

200

300

400

0

100

Time (days)

200

300

400

Time (days)

Fig. 7. Evolution with time of the leachate composition of F-1 concrete. Il = ion content in leachate; Iw = ion content in the groundwater used.

3.4. Resistance of low-pH concretes against water aggression 3.4.1. Characterization of the leaching solution evolution According to some waste management agencies [35–37], in a granitic HLW repository the hydraulic conductivity of the low-pH concretes used has to be similar to that of the surrounding rock (which is in the order of 1 · 10 −1 m/s). The analysed low-pH concretes fulfil this requirement with both leaching agents and the obtained values are stable along the test period (see Fig. 5, left), also indicating a significant material stability. The pH values measured in the leachate fluids obtained (see Fig. 5, right) are never above 11 regardless of the leaching agent, except for the first 50 days in the concrete based on CAC + SF (S-2) attacked by groundwater, being between 8 and 9 at the end of the tests. The initial pH decrease observed in the concretes based on CAC + SF will be associated to the leaching processes of alkaline phases. Therefore, the low-pH CAC concretes designed do not produce the alkaline plume generated by the interaction of the groundwaters with the conventional concretes based on OPC.

The modifications generated in the chemical composition of the leachates of S-2 concrete are shown in Fig. 6 and those of the CAC + FA concrete (F-1) in Fig. 7. These modifications depend on the leaching agent used and on the evaluated concrete. In the S-2 concrete, when using deionised water as the leaching agent, there is a release of alkalis during the first days and of sulfate ions during the whole test period. A slight release of Ca 2+ is also observed after 200 days of test. When using granitic water from Äspö site as the leaching agent, the release of alkalis is again detected (mainly of Na ions) and there is a lack of sulfates along the test period. Besides, retention of calcium ions is detected (and Si but in less proportion), with this retention associated to secondary hydration processes or the formation of carbonated phases (this is corroborated in the next section). With respect to the concrete based on CAC + FA, the leachates show a similar evolution than that observed in the S-2 leachates, showing some differences, as a higher release of alkali ions during the whole test period when using deionised water and a release of calcium with both leaching agents. When using the Äspö groundwater, apart from sulfate

Table 7 Total porosity and pore size distribution of the evaluated concretes before and after the leaching tests. Upper: sample part in direct contact with the water inlet; lower: bottom part of the sample (leachate outlet); centre: sample part between the upper and lower ones. Leaching agent

Sample part

Total porosity (%)

% pores >10μm

% capillary pores 10–1 μm

% capillary pores 1–0.05μm

% gel pores b0.05μm

S-2 Concrete (70%CAC + 30%SF) Sample before testing Deionised water Upper Centre Lower Groundwater Upper Centre Lower

9.38 10.8 9.69 9.76 11.6 10.4 9.63

0.64 1.29 0.75 0.65 0.58 0.62 0.71

0.85 0.63 1.23 0.43 0.70 0.97 0.99

4.33 5.14 4.53 5.36 6.47 5.29 4.20

3.56 3.74 3.18 3.32 3.85 3.52 3.74

F-1 concrete (70%CAC + 30%FA) Sample before testing Deionised water Upper Centre Lower Groundwater Upper Centre Lower

17.1 15.4 16.0 16.7 14.8 14.9 16.2

1.88 2.48 2.22 1.83 2.38 2.06 1.77

3.23 3.04 3.46 3.03 2.92 3.23 2.94

9.81 8.53 8.49 9.20 8.20 7.91 8.92

2.18 1.35 1.83 2.64 1.30 1.70 2.57

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SF

SF SF

C2ASH8

C2ASH8

Fig. 8. Strätlingite platelets near SF grains in S-2 concrete attacked by deionised water (left; ×250) and groundwater (right; ×200).

on the contrary, a decrease in the total porosity values was measured with both leaching agents; this decrease is more significant in the parts in direct contact with the water inlet and when using real groundwater from Äspö. This decrease in total porosity is related to a decrease in the capillary pores with sizes lower than 1 μm, due to the smaller pores clogging as a consequence of two phenomena: the material dragging due to the continuous water flow and, in the case of Äspö groundwater, the formation of new hydrated phases inside the pores. However, an increase in the higher pores (∅ > 10μm) is also observed, being related to the generation of microcracks in the concrete cement paste. The conclusions of the MIP tests are extracted considering the BSEM results showed below. After the leaching period, 14 months, the concrete samples were again characterised using BSEM with EDX analyses. In the case of concretes with silica fume, similar aspect and similar C–A–S–H paste phases than those observed in the same concrete before testing were observed regardless of the leaching agent, with only the decrease in the anhydrous cement grains presence being remarkable. In fact, it is possible that the leaching process, with both leaching agents, promotes a secondary hydration as the presence of hexagonal platelets of strätlingite seems to be higher after the water attack. As shown in Fig. 8, it is usual to detect these hexagonal platelets surrounding the silica fume grains. The high amount of strätlingite detected indicates its high stability against the water attack. Fig. 9 shows the EDX microanalysis profiles obtained in the pastes of the S-2 concretes attacked by each leaching agent, presenting the modifications occurred with the depth of the samples in the CaO, Al2O3 and SiO2 contents. In the case of the concrete attacked by real groundwater no significant changes are observed in the C–A–S–H matrixes but in the samples attacked by deionised water there is a progressive decalcification in the C–A–S–H matrixes near to the water inlet (that agrees with the calcium release detected in the leachates), although this decalcification was not very depth (≈ 300 μm). At last, with both leaching agents, calcite precipitation is observed on the surface in direct contact with the leaching agent inlet (see Fig. 10) that could be playing a

retention, retention of Mg ions is also detected. This phenomenon could be related with the incorporation of these ions in the C–A–S–H matrixes of the cement paste. The decalcification processes detected in S-2 concrete when using deionised water and in F-1 with both leaching agents, have been also reported in low pH cementitious materials based on OPC using pure water [12,16] or real groundwater [11]. 3.4.2. Evaluation of the modifications generated in the solid phases Before the test, the concrete samples were characterised using BSEM. Concretes with a dense and homogeneous paste texture, good aggregate–paste interfaces and with presence of anhydrous phases (both anhydrous cement phases and non-reacted grains of SF or FA) were observed. Moreover, in the S-2 concrete, hexagonal platelets of strätlingite were detected surrounding silica fume grains. The molar ratios CaO:Al2O3:SiO2 of hexagonal platelets obtained from EDX results (e.g. 0.30:0.25:1), do not really fit with that of strätlingite, with the excess of Si attributed to the signal from the surrounding unreacted silica fume. However, in the concrete based on CAC + FA, these hexagonal platelets were not easily observed, being mainly composed of C3AS3 − xH2x members and AH3. Before and after the leaching test period, the total porosity and the pore size distribution were measured using MIP and the results are shown in Table 7. The porosity parameters were measured in each of the sample portions (the upper part was in direct contact with the penetrating water). Observing this table, a lower total porosity and a more refined pore structure in the S-2 concrete is deduced, premonitory of its higher resistance against water aggression. In this concrete type, both leaching agents generate an increase in the total porosity, mainly in the material in direct contact with the water inlet (upper part). Similar increases in total porosity were also observed in concretes based on CAC without mineral admixtures [38]. This increase in total porosity is mainly due to increases in the amount of pores with diameter sizes between 10 and 0.05 μm, although in the upper parts there is also an increase of the smallest pores. In the F-2 concrete,

60

60

S-2 deionized water

50

40

(%)

(%)

40 30

30 20

20 10 0

S-2 groundwater

50

Al2O3

0

10

20

SiO2

10

CaO

30

Depth (mm)

40

50

0

Al2O3

0

10

20

30

SiO2

40

CaO

50

Depth (mm)

Fig. 9. EDX microanalysis profiles in S-2 concrete samples after the leaching test. Left: S-2 attacked by deionised water; right: S-2 attacked by groundwater. Depth relates to the direction of the water penetration through the sample;depth = 0 mm corresponds to the sample surface in direct contact with the water inlet. Leaching period: 14 months.

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75

Calcite precipitation

CAC+SF concrete ∼50µm

Fig. 10. Calcite precipitation in the surface of CAC + SF sample. BSEM images. Left: deionised water (×500); right: groundwater (×350).

ettringite) in pores that agrees with the sulfate ion retentions observed in the leachates (see Fig. 7). Although these phases are not typical hydration products of CAC based materials, their formation has been observed in conventional CAC concretes placed in environments rich in sulfates and with an excess of water [39,40], as the agent used for the leaching test in this study. Although the delayed ettringite formation is a known expansive problem that can promote microcracking [41], in the evaluated period the calcium sulfoaluminate phases have grown in available sites (as pores) and no microcracking associated to their formation has been detected, although longer term studies should be made to guarantee the material stability. Although the sulfoaluminate phases were mainly detected in the degraded zone, they were also present in the medium part of F-1 samples attacked by groundwater. In the F-1 concrete attacked by deionised water, a calcite precipitation on the surface in direct contact with the water inlet was also formed. Non-degraded zone: that corresponds with the majority of each concrete sample. This part has similar aspect and C–A–S–H phases than those observed in the same samples after testing, with only the decrease in the anhydrous cement grains' presence and the increase in the strätlingite platelet amount being remarkable.

protecting role against the water aggression. These results indicate a significant resistance of low pH concretes based on CAC plus SF against water aggression. In the case of the F-1 concrete higher modifications are detected, as expected due to its higher and less refined porosity. Fig. 11 shows several BSEM images from F-1 concretes after the water attack and Fig. 12 the EDX microanalysis profiles obtained in the pastes of these concretes. Combining BSEM observations with EDX analyses two zones with different characteristics can be defined in all the samples tested: Degraded zone: near the surface with a mean depth of 1500 μm regardless of the leaching agent. In this zone a decrease in the density of the paste, material loss in the aggregate–paste interphases and formation of microcracks are observed. There are also less anhydrous cement grains than before leaching test and the decalcification of the C–A–S–H phases is also detected in this degraded zone. Moreover, when using groundwater from Äspö as leaching agent, this decalcification is followed by an inclusion of Mg in the calcium aluminosilicate phases (this element is supposed to come from the groundwater from Äspö; see the water composition in Table 4) forming C–(M)–A–S–H matrixes. Similar behaviours have been detected in low-pH concretes based on OPC attacked by the same water [18]. But the most relevant issue in this zone is the formation of calcium sulfoaluminate hydrates (probably

Although in the concrete based on CAC + SF attacked by groundwater from Äspö the sulfate retention was also observed in the leachates

Material loss in aggregate-paste interphases (x35)

Strätlingite in paste (x2000)

Calcium sulphoaluminates in pores (x350)

Strätlingite in paste (x750)

Fig. 11. BSEM images of F-1 concretes attacked by deionised water (up) and groundwater (down).

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70

35

F-1 deionized water

60

CaO

30

%CaO before test

MgO

25

40

(%)

(%)

50

20 15 10

20 10 0

F-1 groundwater

30

Al2O3

0

10

20

SiO2

CaO

30

40

5 50

0

0

Depth (mm)

2

4

6

8

Depth (mm)

Fig. 12. EDX microanalysis profiles in F-1 concrete samples after leaching test. Left: F-1 attacked by deionised water; right: F-1 attacked by groundwater. Depth relates to the direction of the water penetration through the sample;depth = 0 mm corresponds to the sample surface in direct contact with the water inlet. Leaching period: 14 months.

(see Fig. 6), no formation of calcium sulfoaluminates was detected. This can be explained in two ways: 1) Previous studies have demonstrated that the sulfate ions react with the calcium aluminates of the CAC based concretes [39,40]; in the S-2 concrete, no calcium aluminates but calcium aluminosilicates are formed, with a very high silica content in the C–A–S–H matrixes [29,30] so probably this high silica inclusion increases their resistance against the sulfate attack. Moreover, FA introduces more Al2O3 in the cement formulation (26.9%) than the SF (0.6%), necessary compounds to generate the formation of calcium sulfoaluminate phases. 2) The hydraulic conductivity of S-2 concrete is lower than that of the F-1 concrete, so fewer sulfates have come into the material; thus, although observing Figs. 6 and 7 it seems that the same sulfates have been retained by both concretes attacked by groundwater, it must be taken into account that the represented values refer to the content present in the leachates collected at each test time but the quantity of leachates collected from CAC + FA samples (F-1 concrete) was very much higher than that collected from CAC + SF concrete (S-2), so it is clear that the amount of sulfates fixed by CAC + FA concrete is quite higher. Therefore, it must be supposed that the amount of sulfates fixed by CAC + SF concrete was not as high as needed to detect the formation of calcium sulfoaluminates. 4. Conclusions From the results of this work several conclusions can be highlighted: ▪ The low-pH cement pastes based on CAC with high amounts of mineral admixtures show a good stability in their solid phases after long-term hydration. The main modifications in the pore solution composition and in the solid phases occur before 90 days of hydration and, after that, their properties were quite stable although some changes are detected, mainly in the mixes with high FA contents. ▪ The formation of C–A–S–H matrixes with different chemical compositions promotes the limitation of the conversion process, avoiding the strength loss during the period evaluated in this study. ▪ The resistance against groundwater aggression of low-pH concretes based on CAC clearly depends on the concrete formulation (with the SF more effective than the FA in binary blends) and the leaching agent. Results from leaching tests show, in general, a good resistance of the produced low-pH concretes against groundwater aggression, although an altered front can be observed from the surface in all the tested samples, with this altered front being higher in the concrete with FA. In the altered front of this last concrete type, the incorporation of magnesium ions from groundwater into the C–A–S–H matrixes and the formation of calcium sulfoaluminate phases are detected. The CAC + SF low pH concrete has not suffered from

any significant modification after the leaching tests, indicating a high resistance against both leaching agents' attack.

Acknowledgements Authors are grateful to EU Project ESDRED (FI6W-CT-2004-508851) and ENRESA for their support for this work. Authors are also grateful to Ciments Molins Industrial S.A. for supply the cement used in this study. References [1] D. Savage, D. Noy, M. Mihara, Modeling the interaction of bentonite with hyperalkaline fluids, Appl. Geochem. 17 (3) (2002) 207–223J. [2] S. Ramírez, J. Cuevas, R. Vigil, S. Leguey, Hydrothermal alteration of “La Serrata” bentonita (Almeria, Spain) by alkaline solutions, Appl. Clay Sci. 2 (2002) 257–269. [3] M.N. Gray, B.S. Shenton, For better concrete, take out some of the cement, Proc 6th ACI/CANMET Symposium on the Durability of Concrete, Bangkok, Thailand, May 31 to June 5, 1998. [4] K. Iriya, A. Matsui, M. Mihara, Study on Applicability of HFSC for Radioactive Waste Repositories, Radioactive Waste Management and Environmental Remediation, ASME Conference, Nagoya, Japan, September 16-30, 1999. [5] A. Hidalgo, J.L. García, M.C. Cruz, L. Fernández, C. Andrade, Testing Methodology for pH Determination of Cementitious Materials. Application to Low pH Binders for Use in HLNWR. R&D on Low-pH Cement for a Geological Repository. Workshop June, 2005 (Madrid). [6] U. Vuorinen, J. Lehikoinen, Low-pH grouting cements-results of leaching experiments and modelling. Workshop June R&D on Low-pH Cement for a Geological Repository, 2005 (Madrid). [7] T. Hugo-Persson, B. Lagerblad, C. Vogt, Selective stabilization of deep core drilled boreholes using low-pH cement. Workshop June R&D on Low-pH Cement for a Geological Repository, 2005 (Madrid). [8] C. Cau Dit Coumes, S. Courtois, D. Nectoux, S. Leclerq, X. Bourbon, Formulating a low-alkalinity, high-resistance and low-heat concrete for radioactive waste repositories, Cem. Concr. Res. 36 (2006) 2152–2163. [9] M. Nakayama, K. Iriya, A. Fujishima, M. Mihara, K. Hatanaka, Y. Kurihara, M. Yui, Development of low alkaline cement considering pozzolanic reaction for support system in HLW repository construction, Mater. Res. Soc. Symp. Proc. 932 (2006) 159–166. [10] L.R. Dole, C.H. Mattus, Low pH concrete for use in the US high-level waste repository: part I overview, Proc, R&D on Low-pH Cement for a Geological Repository, 3rd Workshop, June 13–14, 2007, pp. 31–39 (Paris). [11] J.L. García, M.C. Alonso, A. Hidalgo, L. Fernández Luco, Design of low-pH cementitious materials based on functional requirements, Proc, R&D on Low-pH Cement for a Geological Repository, 3rd Workshop, June 13–14, 2007, pp. 40–51 (Paris). [12] T. Yamamoto, H. Imoto, H. Ueda, M. Hironaga, Leaching alteration of cementitious materialsand release of organic additives, Proc, R&D on Low-pH Cement for a Geological Repository, 3rd Workshop, June 13–14, 2007, pp. 52–62 (Paris). [13] C.H. Mattus, L.R. Dole, Low pH concrete for use in a US high-level waste repository: part I:—formulation and tests, Proc R&D on Low-pH Cement for a Geological Repository, 3rd Workshop, June 13–14, 2007, pp. 72–82 (Paris). [14] M. Vuorio, J. Hansen, Long-term safety and durability related studies on low-pH grouting materials, Proc R&D on Low-pH Cement for a Geological Repository, 3rd Workshop, June 13–14, 2007, pp. 83–88 (Paris). [15] Y. Kobayashi, T. Yamada, H. Matsui, M. Nakayama, M. Mihara, M. Naito, M. Yui, Development of low-alkali cement for application in a JAEA URL, Proc R&D on Low-pH Cement for a Geological Repository, 3rd Workshop, June 13-14, 2007, pp. 98–106 (Paris). [16] M. Codina, C. Cau-dit-Coumes, P. Le Bescop, J. Verdier, J.P. Ollivier, Design and characterization of low-heat and low-alkalinity cements, Cem. Concr. Res. 38 (4) (2008) 437–448. [17] J.L. García Calvo, A. Hidalgo, C. Alonso, L. Fernández Luco, Low pH concretes based on CAC for underground repositories of HLRW, Resistance Respect to Ground

J.L. García Calvo et al. / Cement and Concrete Research 51 (2013) 67–77

[18]

[19] [20] [21] [22] [23]

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

[28]

[29]

[30]

Water Aggression, Proceedings of the International RILEM Workshop NUCPERF 2009, RILEM Publications S.A.R.L., ISBN: 978-2-35158-072-1, 2009, pp. 99–108. J.L. García Calvo, A. Hidalgo, C. Alonso, L. Fernández Luco, Development of low-pH cementitious materials for HLRW repositories. Resistance against ground waters aggression, Cem. Concr. Res. 40 (2010) 1290–1297. S. Goñi, C. Andrade, C.L. Page, Corrosion behaviour of steel in high alumina cement mortar samples: effect of chloride, Cem. Concr. Res. 21 (4) (1991) 635–646. H.G. Midgley, A. Midgley, The conversion of high alumina cement, Mag. Concr. Res. 27 (1975) 59–77. K. Scrivener, J.L. Cabiron, R. Letourneux, High performance concretes from calcium aluminate cements, Cem. Concr. Res. 29 (1999) 1215–1223. C. Xiandong, R.J. Kirkpatrick, Hydration of calcium aluminate cements. A solid state 27Al NMR study J. Am. Ceram. Soc. 76 (1993) 409–416. A.J. Majumdar, B. Singh, R.N. Edmonds, Hydration of mixtures of “ciment fondu” aluminous cement and granulated blast furnace slag, Cem. Concr. Res. 20 (1990) 197–208. A.J. Majumdar, B. Singh, Properties of some blended high-alumina cements, Cem. Concr. Res. 22 (1992) 1101–1114. M. Collepardi, S. Monosi, P. Piccioli, The influence of pozzolanic materials on the mechanical stability of aluminous cement, Cem. Concr. Res. 25 (1995) 961–968. J. Ding, Y. Fu, J.J. Beaudoin, Strätlingite formation in high alumina cement-silica fume systems: significance of sodium ions, Cem. Concr. Res. 25 (1995) 1311–1319. K. Quillin, G. Osborne, A. Majumdar, B. Singh, Effects of w/c ratio and curing conditions on strength development in BRECEM concretes, Cem. Concr. Res. 31 (2001) 627–632. J.M. Rivas Mercury, X. Turrillas, A.H. de Aza, P. Pena, Calcium aluminates hydration in presence of amorphous SiO2 at temperatures below 90 °C, J. Solid State Chem. 179 (2006) 2988–2997. A. Hidalgo, J.L. García Calvo, J. García Olmo, S. Petit, M.C. Alonso, Microstructural evolution of calcium aluminate cements hydration with silica fume and fly ash additions by scanning electron microscopy, mid and near infra red spectroscopy, J. Amer. Cer. Soc. 91 (4) (2008) 1258–1265. A. Hidalgo, J.L. García, M.C. Alonso, L. Fernández, C. Andrade, Microstructure development in mixes of calcium aluminate cement with silica fume or fly ash, J. Therm. Anal. Calorim. 96 (2009) 335–345.

77

[31] M.C. Alonso, J.L. García Calvo, C. Walker, M. Naito, S. Pettersson, I. Puigdomenech, M.A. Cuñado, M. Vuorio, H. Weber, H. Ueda, K. Fujisaki, Development of an accurate pH measurement methodology for the pore fluids of low pH cementitious materials, SKB R-12-02. Stockholm, SKB. Svensk Kärnbränslehantering AB. Swedish Nuclear Fuel and Waste Management, 2012. [32] P. Longuet, L. Burglen, A. Zelwer, The liquid phase of hydrated cement, Rev. Mater. Constr.Trav. Publics 676 (1973) 35–41. [33] R.S. Barneyback, S. Diamond, Expression and analysis of pore fluids from hardened cement pastes and mortars, Cem. Concr. Res. 11 (1981) 279–285. [34] D. Damidot, Improvement of calcium aluminate cement resistance to sulphate and carbonate attacks by silica addition: thermodynamic approach. Kracow in: W. Kurdowski (Ed.), Corrosion of Cement Paste, 1994, pp. 49–63. [35] J. Ahn, M.J. Apted, et al., Geological Repository Systems for Safe Disposal of Spent Nuclear Fuels and Radioactive Waste, Apted, Woodhead Publishing Limited, Ed. Joonhong Ahn & Michael J, 2010. [36] J.L. García-Siñeriz, M.C. Alonso, M.C. P.Blümling, S. Pettersson, J.-P. Salo, F. Huertas, ESDRED, Input Data and Functional Requirements, Deliverable 1 Module 4 WP-1, EC Contract FI6W-CT-2004-508851, European Commissio, 2004. [37] A. Kromlöf, Injection Grout for Deep Repositories — Low-pH Cementitious Grout for Larger Fractures: Testing Technical Performance of Materials, Working Report POSIVA OY, 2005, 2004–45. [38] A. Hidalgo, M. Castellote, I. Llorente, C. Alonso, C. Andrade, ECOCLAY II. Effects of cement on clay barrier performance — phase II, final report EC contract no FIKW-CT-2000-00028, European Commission, 2004. [39] J. Bensted, J. Munn, A discussion of the paper ‘Effect of sulphates on the setting time of cement and strength of concrete’ by S. Kumar and C.V.S. Kameswara Rao, Cem. Concr. Res. 26 (1996) 641–643. [40] J. Bensted, J. Munn, A discussion of the paper ‘On the distinction between delayed and secondary ettringite formation in concrete’ by Y. Fu and J.J. Beaudoin, Cem. Concr. Res. 27 (1997) 1773–1775. [41] H.F.W. Taylor, C. Famy, K.L. Scrivener, Delayed ettringite formation, review, Cem. Concr. Res. 31 (2001) 683–693.