Hydration of Portland Cement in the Presence of ... - Springer Link

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 6, pp. 793−801. © Pleiades Publishing, Ltd., 2013. Original Russian Text © A.S. Brykov, A.S. Vasil’ev, M.V. Mokeev, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 6, pp. 849−857.

INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY

Hydration of Portland Cement in the Presence of Aluminum-Containing Setting Accelerators A. S. Brykova, A. S. Vasil’eva, and M. V. Mokeevb St. Petersburg State Institute of Technology (Technical University), St. Petersburg, Russia e-mail: [email protected] b Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia a

Received February 15, 2013

Abstract—Portland cement hydration in the presence of two different aluminum-containing compounds, highly dispersed amorphous Al(OH)3 and aqueous solution of aluminum hydroxosulfate Al(OH)1.78(SO4)0.61, was studied by solid-state 27Al and 29Si NMR spectroscopy. DOI: 10.1134/S1070427213060013

concentration of aluminum ions in the pore fluid [8, 9]. At the same time, the role of the sulfate component, gypsum, which is introduced into Portland cement formulation for controlling the setting of the Portland cement paste within the required time limits, consists also in restriction of the concentration of aluminum ions in the pore fluid and thus in restriction of the effect of these ions on the hydration of silicate phases of Portland cement [8]. In view of the above facts, it seems interesting to perform a comparative study of Portland cement hydration in the presence of aluminum-containing accelerators with one of them containing sulfate ions. In this work we studied the specific features of Portland cement hydration in the presence of amorphous aluminum hydroxide and a solution of aluminum hydroxo sulfate, taken in amounts equivalent with respect to aluminum content. The rate and products of hydration were studied by solid-state 27Al and 29Si NMR spectroscopy.

Accelerating additives free of alkali metal compounds find growing use today instead of alkaline additives in concrete works requiring fast setting and hardening of concrete mix and cement mortar, with the aim to enhance the resistance of concrete to internal corrosion caused by the interaction of alkalis with fillers and to make longer the life of concrete structures [1–3]. Certain aluminum compounds are used as alkali-free accelerators, namely, highly dispersed amorphous modifications of aluminum hydroxides and oxides, aluminate cement, amorphous calcium aluminates, and aqueous solutions of aluminum sulfates and hydroxosulfates [1, 4–7]. In particular, they are used in concreting by dry and wet guniting [4], when setting of concrete mix or cement mortar should occur within seconds. High reactivity of these additives in cement paste is due to intense formation of high-sulfate calcium hydrosulfoaluminate 3CaO·Al2O3·3CaSO4·32H2O (AFt phase, or ettringite) with the paste components. On the other hand, it is known that aluminum ions present in the pore fluid and supplied with aluminumcontaining additives and components of Portland cement decelerate hydration of silicate phases of Portland cement [7, 8]. Some authors believe that poisoning of nuclei of the C–S–H gel with aluminum ions is responsible for the induction effect whose duration depends on the

EXPERIMENTAL We used amorphous highly dispersed aluminum hydroxide produced by Industrias Quimicas del Ebro Zaragoza S.A. (Spain) with the following characteristics: loss on ignition (900°С) 47.3 wt %, BET specific surface area 17.8 m2 g–1, particle size up to 20 μm. The same substance 793

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BRYKOV et al.

was used for preparing aqueous solution of aluminum hydroxosulfate of the composition Al(OH)1.78(SO4)0.61 with the concentration of 15.1 wt % counting on Al2O3 The choice of the composition of aluminum hydroxosulfate was based on analysis of the patent literature given in [4]; the synthesis was performed by the procedure given in [10]. To prepare a solution of aluminum hydroxosulfate, a 250-ml three-necked flask equipped with a reflux condenser was charged with distilled water (43 g) and 40 g of granulated Al2(SO4)3·18H2O (ALG, Kemira Oyj); the mixture was stirred at 70°С until aluminum sulfate fully dissolved (5–10 min). Then amorphous Al(OH)3 (17 g) was added in portions over a period of 2 min. Heating and stirring were continued for an additional 30 min after adding the whole amount of Al(OH)3, after which the solution become transparent with the yellowish tint. The solution after cooling remained stable (lowviscosity fluid) for several days, but in approximately 1 week a precipitate started to form. Tests with liquid accelerator on cement paste were performed 24 h after preparation. In our study we used CEM I 42.5 Н Portland cement (Oskoltsement) of the following phase composition (wt %): alite (3CaO·SiO 2 , or C 3 S) 52–53, belite (2CaO·SiO 2 , or C 2 S) 18–20, intermediate phase (3CaO·Al2O3 + 4CaO·Al2O3·Fe2O3, or C3A + C4AF) 20– 22, gypsum (CaSO4·2H2O) 3–4, anhydrite (CaSO4) 1, and CaCO3 2. According to the results of chemical analysis, the total Al2O3 content in the cement was 4.9 wt %. Al(OH)3 was introduced into the cement in an amount of 1 and 3% of the cement weight (0.53 and 1.58% counting on Al2O3). The cement paste was prepared at a constant water-to-solid ratio [W/(C + additive)] of 0.27, corresponding to the normal thickness of the additive-free cement paste in accordance with GOST (State Standard) 310.3. The aluminum hydroxosulfate solution was introduced into the cement paste in an amount of 3 and 9% of the cement weight [0.46 and 1.36% counting on Al2O3, i.e., in approximately the same amounts counting on Al2O3 as Al(OH)3] after preliminary mixing with the water taken for preparing the paste; the amount of water introduced with the additive was taken into account. To ensure convenient workability, to the cement paste with aluminum-containing admixtures at a fixed W/C ratio we introduced Melflux 2651 F plasticizer in an amount of 0.1% relative to the cement weight. This additive was preliminarily dissolved in the water used

for preparing the paste. The setting time of the cement paste was determined in accordance with GOST 310.3. To prepare cement stone specimens, the freshly prepared cement paste was placed in cubic molds of size 30 × 30 × 30 mm and kept for 24 h in a climatic chamber under the conditions of 100% humidity at 20°С. Then the specimens were taken off from the molds and kept under the same conditions. The compression strength of the specimens was determined at the age of 1, 3, and 28 days. The data demonstrating the effect of the additives in various amounts on the setting time of the cement paste and on the strength of the stone formed are given in Table 1. To record the NMR spectra, a small amount of the set or hardened cement paste (about 5 g) was ground to fine powder, washed with acetone (3 × 30 ml) to remove free water, and dried under reduced pressure at ambient temperature. The high-resolution solid-state NMR spectra were recorded at magic angle spinning at room temperature with an AVANCE II-500WB spectrometer (Bruker). The operation frequency was 99.35 MHz for 29Si and 130.32 MHz for 27Al. The spectra were recorded with single-pulse excitation; the pulse length was 3 (π/4) and 0.7 μs (π/12) with the delay of 6 and 0.5 s; the accumulation number was 10240 and 2048 for 29Si and 27Al, respectively. Cement stone samples were packed in zirconium rotors (4 mm in diameter) and spun at a frequency of 10–13 kHz. The chemical shifts (ppm) are given relative to tetramethylsilane (TMS). The signals were assigned in accordance with published data [11–13]. The processing (deconvolution) of the NMR spectra were performed using Dmfit software. The NMR spectrum of aqueous solution of aluminum hydroxosulfate (with addition of D2O for stabilization and resolution tuning) were taken with an AVANCE-400 spectrometer (Bruker). The operation frequency for 27Al was 104.26 MHz. Figures 1–4 show the solid-state 27Al NMR spectra of the cement paste without additives and with addition of setting accelerators in various periods of hydration. The fragments of the spectra in the range 30–90 ppm, demonstrating changes in the content of aluminum ions, are given in the same figures in the enlarged scale. The signal intensities in the 27Al NMR spectra are given in the absolute scale (similar measurement conditions), which

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 6 2013

HYDRATION OF PORTLAND CEMENT in C3S and C2S

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Ettringite in alumosilicate gel

in C3A

δ, ppm Fig. 1. Solid-state 27Al NMR spectra of cement paste samples. (δ) Chemical shift; the same for Figs. 2–6. (1, 2) No additive (control sample) and sample with 3% Al(OH)3, age 35 min; (3) sample with 3% aluminum hydroxosulfate, age 70 min; (4) sample with 9% aluminum hydroxosulfate, age 20 min. Dashed line: 27Al signal in the spectrum of aqueous solution of Al(OH)1.78(SO4)0.61.

Ettringite in alumosilicate gel

Calcium Monosulfoaluminate

δ, ppm Fig. 2. Solid-state 27Al NMR spectra of cement stone samples. (1, 2) No additive (control sample), age 35 min and 24 h; (3, 4) sample with 9% aluminum hydroxosulfate, age 20 min and 24 h.

allows them to be compared with each other. Figure 5 shows the solid-state 29Si NMR spectra for cement paste without additives and with addition of ac-

celerators in various periods of hydration. As seen from Table 1, the additives taken in equivalent concentrations exert similar effect on the setting time, but


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BRYKOV et al. in alumosilicate gel

Calcium Monosulfoaluminate

δ, ppm Fig. 3. Solid-state 27Al NMR spectra of cement stone samples. Age 24 h. (1, 2) no additive (control sample) and sample with 3% Al(OH)3; (3, 4) samples with 3 and 9% aluminum hydroxosulfate.

Table 1. Effect of additives on the setting time and strength of cement stone Additive

Additive content, wt %

Control specimen (no additives) Amorphous Al(OH)3 Al(OH)1.78(SO4)0.61 solution

Compression strength, MPa, at indicated age, days

Setting time, min onset

end

1

3

28

–

230

335

16.0

74.3

90.7

1

35

60

25.6

76.6

95.7

3

20

32

3.8

30.0

80.6

3

20

70

39.0

66.8

93.3

9

8

20

25.0

37.2

70.0

we cannot but note that the liquid accelerator is generally more effective, especially at high dosages. Even in the earliest period of hydration, a strong signal at 14.7–15.2 ppm, characteristic of ettringite, is observed in the 27Al NMR spectra of all the samples of cement paste (Fig. 1). Spectra 3 and 4, corresponding to cement paste with a solution of aluminum hydroxosulfate, indicate that the additive has fully reacted by the moment corresponding to the end of setting (70 and 20 min, respectively, for 3 and 9% additive), because the narrow signal of aluminum hydroxosulfate at 0.6 ppm was absent. In the cement paste with 3% Al(OH)3, large amount of the unchanged additive remains by the end of setting (32 min) [broad shoulder to the right of the ettringite

signal in spectrum 2, belonging to Al(OH)3]. Cement slurries with 3% Al(OH) 3 and 3% Al(OH) 1.78(SO 4) 0.61 (1.58 and 0.46%, respectively, counting on Al2O3) contain virtually the same amount of ettringite by the end of setting. However, in the paste with 9% Al(OH)1.78(SO4)0.61, ettringite is formed by the end of setting in a larger amount than in the other cases (Fig. 1). On the contrary, in the control sample whose setting is complete in 5.5 h after the start of hydration, the amount of ettringite remains low even after 24 h (Fig. 2, spectrum 2), as compared with the other samples by the end of setting. Probably, the setting of the control additive-free paste is affected by the hydration products of silicate phases to a greater extent, and the role of ettringite


HYDRATION OF PORTLAND CEMENT in alumosilicate gel

in C3S and C2S

797 Ettringite

Calcium Monosulfoaluminate in C–S–H

δ, ppm Fig. 4. Solid-state 27Al NMR spectra of cement stone samples. (1) No additive (control sample), age 28 days; (2) sample with 3% Al(OH)3, age 28 days; (3) sample with 3% aluminum hydroxosulfate, age 28 days; (4, 5) sample with 9% aluminum hydroxosulfate, age 24 h and 28 days.

is correspondingly lower. As for cement slurries with additives, for the slurries with very short setting times the setting completion is attained at a larger amount of ettringite formed than for the slurries characterized by longer setting time. This may be due to the fact that the juvenile ettringite particles formed under the conditions of too rapid crystallization do not exert sufficiently high structuring effect, which is compensated by the amount of the ettringite formed. The signal with a maximum at 83.4 ppm belongs to impurity aluminum ions with coordination number 4 in C3S and C2S phases of the cement. In spectra 1 and 3 (Fig. 1), corresponding to the control sample and to the sample with 3% liquid additive, to the right of this signal, in the range 55–75 ppm, there is a shoulder belonging to aluminum ions incorporated in aluminum-containing clinker phases. At high concentrations of additives (spectra 2, 4), this shoulder is masked by a new, relatively broad signal in the region of 65 ppm; its origin will be explained below. In the range 35–40 ppm in spectrum 2 (Fig. 1) of the sample with Al(OH)3, there is a signal suggesting the presence of five-coordinate aluminum ions belonging mainly to amorphous Al(OH) 3 . This signal fully disappears later (in 24 h). After 24 h, the ettringite amount in the cement paste

with 9% liquid accelerator remains approximately on the same level as in the period corresponding to the end of paste setting (Fig. 2, spectra 3, 4). The shoulder to the right of the ettringite signal, noticeable in spectrum 4 (paste age 24 h), suggests the onset of the formation of calcium monosulfoaluminate (monosulfate, 3CaO·Al2O3·CaSO4·12H2O) by the reaction of ettringite with the aluminate phase of the cement. In the spectrum of the cement stone with addition of 3% Al(OH)3 at the age of 24 h (Fig. 3, spectrum 2), the broad signal of the additive virtually fully disappears. At the same time, the ettringite signal becomes close in intensity to that in the spectrum of the paste with equivalent content of the liquid accelerator (spectrum 4), and the ill-defined narrow shoulder to the right of it also already belongs to monosulfate. The ettringite signal also becomes stronger in the spectrum of the sample without additive at the age of 24 h (Fig. 2, spectra 1, 2). The changes occurring in the region of 83 ppm (Al in silicate phases of the clinker) by the age of 24 h are particularly noticeable in the spectra of the additive-free paste and of slurries with additions of aluminum hydroxosulfate solution (Fig. 2, spectra 2, 4, and also spectra in Fig. 3); these changes consist in weakening of the signal relative to the state in the first minutes of hydration (Fig. 2, spectrum 1). However, no signal weakening is


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δ, ppm Fig. 6. Deconvolution of the 29Si NMR spectrum (with spectrum 8 in Fig. 5 as example).

δ, ppm Fig. 5. Solid-state 29Si NMR spectra of cement stone samples. (1, 6) No additive (control samples), age 24 h and 28 days; (2) sample with 1% Al(OH)3, age 24 h; (3, 7) sample with 3% Al(OH)3, age 24 h and 28 days; (4, 8) sample with 3% aluminum hydroxosulfate, age 24 h and 28 days; (5, 9) sample with 9% aluminum hydroxosulfate, age 24 h and 28 days.

observed in the case of the cement paste with the addition of 3% Al(OH)3 (Fig. 3, spectrum 2), which indicates, most probably, that the hydration in this system is blocked. Table 1 shows that, as a rule, small amounts of the additives enhance the strength of the cement stone, especially in the early period. However, an increase in the Al(OH)3 dosage to 3% leads to a decrease in the specimen strength relative to control tests at all ages; at the age of

24 h, the stone strength is as low as ~4 MPa, i.e., 4 times lower than that of the control specimen. In the presence of an equivalent amount of an aluminum hydroxosulfate solution (9%), the strength of the stone at the age of 24 h, on the contrary, exceeds that of the control specimen by a factor of approximately 1.5, although in the subsequent period the strength becomes lower than that of the control specimen. The results of strength tests of the specimens reasonably correlate with the solid-state 29Si NMR data (Fig. 5) allowing quantitative evaluation of the effect of aluminum-containing accelerators on the hydration of silicate phases. In the spectra, there are signals of silicon atoms in different surroundings: Q0 signal belonging to isolated silicon–oxygen tetrahedra of the initial silicate phases of the clinker; Q1 signal belonging to external silicon atoms in chains of the hydration product, C–S–H gel; Q2 signal belonging to silicon atoms in inner units of the chains; the Q2(1Al) signal belongs to those silicon atoms of inner units of the chains in which one of the two nearest neighbors is aluminum (–Si–O–Si–O–Al–). The distribution of silicon atoms with different surroundings, determined by deconvolution of the 29Si NMR spectra of the samples (an example of the spectrum deconvolution is shown in Fig. 6), is given in Table 2. Using these data, we calculated the degree of hydration of silicate phases of Portland cement αt (%) by the formula αt = 100 – Qt0, where Qt0 is the content of silicon atoms in isolated tetrahedra at time t (%) [14]. The mean length n of


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Table 2. Distribution of silicon atoms with different surroundings in cement stone samples and characteristics of hydration product Additive Control sample

Al(OH)3

Al(OH)1.78(SO4)0.61

Al(OH)1.78(SO4)0.61

Additive content, wt %

Age, days

–

Content of Si atoms, %

α, %

n

Al/Si

4.03

20.38

3.28

0.074

5.01

8.86

41.72

3.18

0.060

14.89

3.24

4.14

22.27

3.21

0.073

95.61

2.61

0.36

1.43

4.4

3.51

0.041

28

66.78

20.59

5.04

7.59

33.22

3.47

0.076

3

1

73.66

17.12

4.62

4.6

26.34

3.35

0.088

3

28

54.8

24.33

9.91

10.95

45.19

4.12

0.110

9

1

86.3

7.2

3.2

3.3

13.7

4.25

0.117

9

28

63.19

19.65

8.68

8.48

36.81

4.19

0.118

Q0

Q1

1

79.61

13.35

3

28

58.28

27.85

1

1

77.72

3

1

3

aluminum–silicon–oxygen chains in the cement hydration product and the ratio of the Al and Si atoms (Al/Si) in aluminum–silicon–oxygen chains were calculated by the formulas given in [11]. The calculation results are also given in Table 2. As seen from Table 2, at small dosages both aluminumcontaining additives stimulate the hydration of Portland cement, with the solution of aluminum hydroxosulfate exerting a stronger effect, noticeable even after the lapse of 1 month. High dosages of aluminum-containing setting accelerators lead to the deceleration of the Portland cement hydration; in the presence of amorphous Al(OH)3 the hydration is almost fully suppressed in the early period. As noted above, the signal in the region of 60–70 ppm appears in the 27Al NMR spectra of cement slurries with high content of additives already in the earliest period (Fig. 1, spectra 2, 4). Most probably, the appearance of this peak is directly related to the deceleration of the Portland cement hydration. In [15] we suggested formation of a noncrystalline aluminosilicate phase temporarily blocking the cement particles and assigned to it this signal in the spectrum. As a result of ettringite formation with the participation of highly active Al(OH)3, the SO42– ions released in the course of gypsum dissolution rapidly become deficient. Under these conditions, the formation of the

Q2(1Al)

Q2

aluminosilicate hydrogel from the substance of the Al(OH)3 additive present in excess relative to gypsum starts to compete with the ettringite formation. A solution of aluminum hydroxosulfate is a source of not only aluminum ions but also, simultaneously, of sulfate ions which, in contrast to gypsum, are initially in the dissolved state. The total amount of sulfate ions from the additive and from gypsum, according to the calculations, appears to be sufficient for complete binding of aluminum ions from the additive into ettringite. Under these conditions, quantitative formation of ettringite from a solution of aluminum hydroxosulfate is complete within the time that does not exceed several tens of minutes, whereas the aluminosilicate gel is formed considerably less actively than in the presence of the Al(OH)3 additive (Fig. 3, spectra 2, 4). The presumed phase of the aluminosilicate gel is unstable, as in the subsequent period the signal at 60–70 ppm gradually becomes weaker (Fig. 2, spectra 3, 4), and in the spectra taken at the age of 28 days it is absent at all (Fig. 4, spectra 1–3, 5). Apparently, the substance of the aluminosilicate gel becomes incorporated in sulfoaluminate phases and C–S–H gel. Less active formation of the aluminosilicate gel and, correspondingly, its faster decomposition in the system with aluminum hydroxosulfate additive are responsible for the fact that the hydration delay in the cement paste with this additive


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is manifested to a lesser extent than in the paste with the addition of Al(OH)3. At a late age, the differences in the degree of hydration between the control sample and samples with aluminumcontaining additives are still noticeable, although they are somewhat leveled off (Table 2). Lower hydration rate in samples with high content of additives is responsible for lower strength compared to control samples (Table 1). However, in the early period (24 h), the strength of the stone with the liquid additive is higher than that of the control specimens, apparently because of the effect of ettringite participating in the formation of the structure jointly with the C–S–H gel. In the 27Al NMR spectra of the samples of 28-day age (Fig. 4), a broad signal appears with a maximum at 73–74 ppm. It belongs to aluminum ions incorporated in silicon–oxygen chains of the C–S–H gel (Si–O– Al–O–Si bonds). The Al/Si ratio in hydration products can be estimated quantitatively from the 29Si NMR data (Table 2). According to these data, the cement hydration product, C–S–H gel, formed in the presence of aluminum hydroxosulfate is characterized by higher Al/Si ratios than in the other cases. The aluminum ions of this additive are actively incorporated into silicon-oxygen chains of the hydration product, somewhat increasing their mean length n. In the 27Al NMR spectra of samples of 28-day age in the range 0–15 ppm (Fig. 4), along with the ettringite signal, there is a strong signal of calcium monosulfoaluminate (monosulfate); the ettringite content becomes lower compared to the samples of 24-h age. As already noted, monosulfate is formed by the reaction of ettringite with C3A phase. It can be shown by calculation that the ettringite to monosulfate ratio should be higher in the cement stone with the addition of aluminum hydroxosulfate, which is an additional source of sulfate ions for ettringite formation; the spectroscopic data confirm this fact. It is known that the presence of large amounts of monosulfate can lead to the formation of secondary ettringite, causing destructive expansion of cement mortars and concretes [16]. Hence, the structures formed in the presence of aluminum hydroxosulfate should be more resistant to sulfate expansion than those formed in the presence of Al(OH)3. CONCLUSIONS (1) The aluminum hydroxosulfate solution as

accelerator of cement paste setting acts more efficiently than amorphous Al(OH)3, which is due to more intense formation of ettringite. (2) In small amounts (~0.5 wt % counting on Al2O3), both additives accelerate the hydration of silicate phases of the cement and enhance the strength of cement stone relative to the control specimen. An increase in the Al(OH)3 dosage (~1.5% counting on Al2O3), along with acceleration of cement paste setting, practically fully suppresses the hydration of Portland cement in the early period and leads to a decrease in the 24-h strength of the cement stone, which is several times lower than that of the control specimens. (3) The decelerating effect of aluminum hydroxosulfate is considerably weaker than that of Al(OH)3 at equivalent dosage. The strength of the cement stone with this additive at the age of 24 h is higher than that of the control specimens, apparently because of the effect of ettringite participating in the structure formation along with C–S–H gel. (4) The deceleration of the cement hydration in the presence of large amounts of highly active aluminumcontaining additives is due to formation (under the conditions of deficiency of SO42– and Ca2+ ions) of the aluminosilicate gel blocking the silicate phases of the cement clinker. This deficiency is more pronounced with Al(OH) 3 and less pronounced with aluminum hydroxosulfate, which is a source of sulfate ions. REFERENCES 1. Myrdal, R., Accelerating Admixtures for Concrete. State of the Art, SINTEF Report, Trondheim, 2007, no. SBF BK A07025. 2. Bracher, G., in Int. Symp. on Waterproofing for Underground Structures, Sao Paulo (Brazil), 2005. 3. Rixom, R. and Mailvaganam, N., Chemical Admixtures for Concrete, London E&FN Spon, 1999. 4. Brykov, A.S. and Vasil’ev, A.S., Tsement Ego Primen., 2012, no. 3, pp. 112–117. 5. Ilyasov, A.G., Medvedeva, I.N., and Korneev, V.I., Tsement Ego Primen., 2005, no. 2, pp. 61–63. 6. Xu, Q. and Stark, J., Zement Kalk Gips, 2008, vol. 61, no. 3, pp. 82–92. 7. Saout, G., Lothenbach, B., Hori, A., et al., in 18th Int. Baustofftagung “Ibausil,” Tagungsbericht, Weimar, 2012, vol. 1, pp. 474–481.


HYDRATION OF PORTLAND CEMENT 8. Matschei, Th. and Costoya, M., in 18th Int. Baustofftagung “Ibausil,” Tagungsbericht, Weimar, 2012, vol. 1, pp. 276–285. 9. Garrault, S., Nonat, A., Sallier, Y., and Nicoleau, L., Proc. XIII Int. Congr. on the Chemistry of Cement: Abstracts and Proc., Madrid, 2011. 10. US Patent 7699931. 11. Richardson, I.G., Cem. Concr. Res., 1999, vol. 29, no. 8, pp. 1131–1147. 12. Andersen, M.D., Jakobsen, H.J., Skibsted, J., Cem. Concr.

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Res., 2004, vol. 34, no. 5, pp. 857–868. 13. Mendes, A., Gates, W.P., Sanjayan, J.G., and Collins, F., Mater. Struct., 2011, vol. 44, no. 10, pp. 1773–1791. 14. Brykov, A.S., Kamaliev, R.T., and Mokeev, M.V., Zh. Prikl. Khim., 2010, vol. 83, no. 2, pp. 211–216. 15. Brykov, A.S., Vasil’ev, A.S., and Mokeev, M.V., Zh.. Prikl. Khim., 2012, vol. 85, no. 12, pp. 1903–1909. 16. Stark, J. and Wicht, B., Dauerhaftigkeit von Beton: der Baustoff als Werkstoff, Springer, 2001.
