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

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 12, pp. 1793−1799. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.S. Brykov, A.S. Vasil’ev, M.V. Mokeev, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 12, pp. 1903−1909.

INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY

Hydration of Portland Cement in the Presence of High Activity Aluminum Hydroxides A. S. Brykova, A. S. Vasil’eva, and M. V. Mokeevb aSt.

bInstitute

Petersburg State Technological University, St. Petersburg, Russia of High-Molecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia e-mail: [email protected] Received July 26, 2012

Abstract—The effect of highly dispersed amorphous aluminum hydroxides on the hydration of Portland cement was studied by the solid-state 27Al and 29Si NMR spectroscopy. It was established that in the presence of aluminum hydroxides the decrease in the setting time of a cement paste is due to rapid formation of ettringite phase, with contribution of admixture material the main and contribution of aluminum-containing phases at this stage insignificant. DOI: 10.1134/S1070427212120014

As known, the modern shotcrete technology of air entrainment uses accelerating admixtures not containing alkalis and therefore having a number of advantages instead of alkaline admixtures [1–3]. One component of the accelerating admixtures of this type are highly dispersed amorphous modifications of aluminum hydroxides and oxides. These have high reactivity in a cement paste and form with its components a number of compounds, including high sulfate form of calcium hydrosulfoaluminate 3CaO·Al2O3·3CaSO4·32H2O (phase AFt, or ettringite), low sulfate form of calcium hydrosulfoaluminate 3CaO·Al2O3·CaSO4·12H2O (phase AFm, or monosulfoaluminate), and calcium hydroaluminates such as 4CaO·Al2O3·19H2O, which also belong to the AFm-like phases of layered structure. The diagram of the conversion involving high-activity Al(OH)3 is presented by an example of the ettringite formation: 2Al(OH)3 + 3Ca(OH)2 + 3(CaSO4·2H2O) + 20H2O → 3CaO·Al2O3·3CaSO4·32H2O.

Hydroxides and oxides may be main components of such accelerators, whereas more often they are incorporated into aluminum hydroxosulfates of variable composition in combination with aluminum sulfates [4, 5]. Despite a considerable experience has been accumu-

lated in application of alkali-free accelerating admixtures, the peculiar features of their interaction with components of the cement are still unclear. In particular, there is no clear understanding on the hydration of cement clinker phases in the presence of aluminum-containing alkali-free accelerators and on the contribution of clinker phases into the mechanism of setting a cement paste in the presence of the above admixtures. In the study, the peculiar features of Portland cement hydration in the presence of fine particles of amorphous aluminum hydroxides were determined by the solid-state 27Al and 29Si NMR spectroscopy. EXPERIMENTAL We studied highly dispersed amorphous aluminum hydroxides produced at two different plants. Their characteristics are listed in Table. 1. Compared to product no. 2, product no. 1 consists of coarser particles. At the same time, its specific surface area, probably owing to more developed surface structure, is larger. At the same time, the mass loss by calcination for product no. 2 is larger than for product no. 1, which may be indicative of its larger amorphization and higher reactivity. We used in the study a CEM I 42.5 N Portland cement. The cement has the phase composition (wt %):

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alite (3CaO·SiO2, or C3S) 52–53, belite (2CaO·SiO2 or C2S) 18–20, the 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 chemical analysis, the total content of Al2O3 in the cement is 4.9 wt %. The Al(OH)3 admixtures were introduced into the cement in amounts of 1, 3, and 6 % of the cement weight. Cement pastes with and without admixtures were prepared at the water–solid ratio [W/(Cement + Admixture)] 0.27, which, according to GOST 310.3, corresponds to normal density cement paste without admixture. Into cement pastes containing Al(OH)3, a Melflux F 2651 F plastisizer (0.1 % of the cement weight) was introduced by dissolving in water of mixing. The setting time of the cement pastes was determined according to the GOST (State Standard) 310.3. Cement stone samples were prepared by keeping 30×30×30-mm cube molds filled with fresh cement paste in a climate chamber at 100% humidity at 20°C for 1 day. Then, the samples were removed from the molds and kept under the same conditions. The compression strength of the samples 1, 3, and 28 days of age was determined. Samples for measuring NMR spectra were prepared by grinding set or hardened cement paste (about 5 g weight) to fine powder, washing in acetone (3 × 30 ml) to remove free water, and by drying in vacuum at ambient temperature.

The high-resolution solid-state NMR spectra were recorded at room temperature on an Avance II-500WB (Bruker) spectrometer [“magic” angle spinning, working frequency 99.35 MHz for 29Si and 130.32 MHz for 27Al, single pulse excitation, pulse duration 3 (π/4) and 0.7 ms (π/12) with a delay of 6 and 0.5 s , the number of accumulations 10,240 and 2,048 for 29Si and 27Al, respectively]. Samples were packed into zirconia rotors (4 mm diameter) and spinned at a frequency of 10–13 kHz. Chemical shifts were counted from the position of the TMS signal. The signals were assigned in accordance with the literature data [6–8]. Data on the effect of amorphous aluminum hydroxide on the setting time of a cement paste and the strength of the cement stone are listed in Table 2. As seen, the setting rate decreases from 2 to 30 times, depending on the admixture content. The largest acceleration efficiency has admixture no. 2, which, as noted above, may be due to its higher amorphization. Table 2 also shows that the small dosages (1% of the cement weight) of amorphous aluminum hydroxides have no adverse effect on the cement hardening in the early period (1 day), whereas, on the contrary, cause somewhat increase in the 1 day strength. In the presence of the admixture no. 2, a slight increase in the cement strength is also observed in the following period of the hydration. The admixture no. 1 decreases the stone strength at the age of 3 days, but after 28 day aging the stone strength in the presence of the admixture no. 1 coresponds to that of the reference sample.

Table 1. Characteristics of amorphous aluminum hydroxides

Table 2. Effect of admixtures of amorphous aluminum hydroxide on the setting time and strength of the cement stone

Admixture

Parameter No. 1 Manufacturer

No. 2

OJSC Boksi- Industrias Quitogorsk alumina micas del Ebro, plant, Russia Spain

Mass loss by calcination at 900°С, wt % (m.l.c.)

41.3

47.3

Particle size by data on petrographic analysis, μm

1–10 (80%) 10–20 (8–10%) 20–25 (6–8%) 25–30 (3–4%)

1–10 (80%) 10–15 (15– 16%) 15–20 (4–5%)

Specific surface area by (BET), m2 g–1

27.0

17.8

Admixture Setting time, min Admixture dosage, wt % beginning end

Compression strength, MPa, in age 1

3

28

Reference sample (no admixture)

–

230

335 16.0

74.3 90.7

No. 1

1

135

285 18.8

53.7 90.0

3

42

70

24.0

46.5 80.7

6

40

68

4.4

14.3 64.9

1

35

60

25.6

76.6 95.7

3

20

35

3.8

30.0 80.6

6

9

11

4.0

No. 2

–

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 12 2012

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HYDRATION OF PORTLAND CEMENT

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in C3S and C2S Al(IV) in C3A

(monosulfoaluminate) Al(IV) in aluminosilicate gel

in C–S–H gel

Fig. 1. The solid-state 27Al NMR spectra of the samples. (δ) Chemical shift (ppm); the same for Figs. 2–4. Sample: (1) Initial cement mixture with 3 % of the admixture no. 2, cement paste without admixture after hydration for (2) 35 min and (4) 1 day, cement paste with 3 % of the admixture no. 2 after hydration for (3) 35 min, (5) 1 day, and (6) 1 month.

in C–S–H gel

Fig. 2. The solid-state 27Al NMR spectra of the cement paste.Sample: (1, 3) without admixture and (2, 4) in the presence of 3 % of the admixture no. 2. Age (month): (1, 2) 1 and (3, 4) 3.

With dosage of amorphous aluminum hydroxides increased to 3 and even more to 6%, 1 day strength is mainly decreased several fold in comparison with the reference sample without admixture. Exception is a 3% dosage of the admixture no. 1, for which 1 day strength was higher than for reference. The presence of amorphous aluminum hydroxides at a dosage of 3–6% decreases the strength of the cement stone in later age too, although differences become lesser with increasing age; in this case, the higher the dose, the greater the loss of the strength by the stone.

Figures 1–3 demonstrate solid-state 27Al NMR spectra for a mixture of cement and admixtures nos. 1 and 2 prior to mixing with water and for cement pastes without admixture and in the presence of admixtures nos. 1 and 2 at different periods of the hydration. Scaled-up region at 30–80 ppm corresponds to changes in contents of the four- and five-coordinated aluminum ions. The solid-state 29Si NMR spectra for cement pastes without admixtures and in the presence of 1 and 3 wt % of the admixture no. 2 are shown in Fig. 4 for different periods of the hydration.


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Al(IV) in aluminosilicate gel

in C–S–H gel

Fig. 3. The solid-state 27Al NMR spectra of the samples. Sample: (1) Initial cement mixture with 6% of admixture no. 1, cement paste without admixture in age of (2) 1 day and (5) 1 month, cement paste with 6 % of admixture no. 1 in age of (3) 1 day and (6) 1 month, cement paste containing 3 % of admixture no. 2 in age of (4) 1 day and (7) 1 month.

The 27Al NMR spectrum of the initial cement mixture containing 3% of the admixture no. 2 (Fig. 1) has signals attributable to the aluminum atoms of the cement and Al(OH)3 in different coordinations with oxygen. The most intense signal with a peak at 8.6 ppm corresponds to six-coordinated aluminum in Al(OH)3. The second in intensity signal peaking at 83.4 ppm refers to the impurity ions of four-coordinated aluminum in C3S and C2S cement phases. A shoulder on the right of this signal at 55–75 ppm belongs to aluminum ions of the aluminumcontaining clinker phases. Signal around 30–40 ppm is due to five-coordinated aluminum nuclei, belonging mainly to amorphous Al(OH)3. The data presented show that for the systems considered, the signal intensities of four- and six-coordinated aluminum ions do not correspond to the true content of these ions in the material. In the given mixture of cement and Al(OH)3, according to the calculation, the aluminum ions in coordination 4 and 6 must be in a proportion of about 3 : 1. The observed discrepancy is due to quadrupolar 27Al nuclei [8, 9]. In this regard, although the intensities of the signals in the 27Al NMR spectra are given in absolute scale (recording conditions the same) and therefore can be compared, their consideration can be principally qualitative. Spectra of the cement paste samples at different periods of the hydration are characterized by appearance of new signals at 0–15 ppm and simultaneous gradual disappearance of the admixture signal at 8.6 ppm. Intense narrow signal at 14.7–15.2 ppm, present in all

the spectra of the reference paste and in the presence of Al(OH)3, belongs to ettringite. To the time corresponding to the end of setting in the presence of Al(OH)3 (35 min), the admixture has reacted incompletely, however the amount of ettringite formed by the admixture already exceeds several times the ettringite amount in the reference sample of the same age (Fig. 1). The Al(OH)3 signal in the spectrum of the cement paste with the admixture disappears, whereas, the ettringite signal markedly increases by the age of 1 day. Some increase in the signal intensity is also observed in the absence of the admixture. The cement spectra in the presence and in the absence of the admixture show that at this stage of the hydration the only phase containing six-coordinated aluminum is ettringite. The signal at 30–40 ppm belonging to fivecoordinated aluminum ions completely disappears by the age of 1 day. By the age of 1 day changes on the spectrum of the reference paste and, to lesser extent, on the spectrum of a paste containing admixture are clearly resolved in the region attributable to aluminum of the clinker silicate phases around 83 ppm. Also, on the reference sample spectrum the intensity of the shoulder belonging to Al in alumino-containing cement phases around 55–75 ppm considerably decreases. Consequently, cement hydration in a paste containing 3% of the admixture is slower than in the reference sample without Al(OH)3. As will be shown from the analysis of the solid-state 29Si NMR spectra, in the



presence of at least 3% Al(OH)3, the cement hydration may be temporarily blocked in the early beginning of the process. Since the hydration rate of the cement aluminate phases is limited by the rate of dissolution of the cement grains as whole, the ettringite formation in this period proceeds almost entirely from a material of the admixture, whereas the role of aluminate phase in its formation is insignificant. According to the estimate, the amount of gypsum in the cement is not sufficient to bind the whole substance of the admixture into ettringite. At the same time, in the spectrum of the 1 day cement paste with the admixture not only the admixture signal, but also the signal of monosulfoaluminate, less sulfated form than ettringite, and signal of calcium hydroaluminates are absent. Note, the spectrum of the 1 day cement paste containing the admixture has signal peaking at 66 ppm, which, however, disappears at a later age. This suggests that, in addition to Al(OH)3, the admixture forms instable, and, apparently, non-crystalline phase containing four-coordinated aluminum, which is responsible for temporary blocking of cement hydration and decrease in its 1 day strength. Along with ettringite signal, the spectra of cement pastes 28 day and 3 month of age have signal due to monosulfoaluminate at 11–12 ppm, irrespective of the presence of the admixture (Figs. 1, 2). In a cement sample containing admixture no. 2 the total content of the Aft and Afm phases and the AFm/AFt ratio are higher than in samples without admixtures. For samples of the same type at the age between 1 and 3 months, the content of the Aft and AFm phases and their ratio vary slightly. Evidently, in the formation of AFm phase in a cement paste in the presence of Al(OH)3 the aluminumcontaining clinker phases and ettringite formed by the admixture are already involved: 3CaO·Al2O3·3CaSO4·32H2O + 2(3CaO·Al2O3) + 4H2O → 3(3CaO·Al2O3·CaSO4·12H2O).

Aluminum released upon decomposition of instable aluminum-containing phase is also involved into the formation of monosulfoaluminate. In the region 60–80 ppm all spectra in Fig. 2 are practically identical. The spectra have signal with a peak at 73–74 ppm, related to the aluminum ions embedded in silicon–oxygen chains of the main product formed in hydration of the cement silicate phases, calcium–silica

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hydrogel (C–S–H) [7]. Similar shape of the spectra in the region 60–80 ppm shows that by the age of 1 month differences in the C–S–H compositions of the reference samples and samples containing Al(OH)3 admixture are not as significant as in the initial period of the hydration. As the content of highly dispersed amorphous Al(OH)3 in the cement stone 1 day of age increases to 6% (Fig. 3, admixture no. 1), the admixture partly remains in initial state and the content of the formed ettringite is about the same as in the cement paste with a 3% of the admixture no. 2. However, in the sample 1 month of age the admixture no. 1 is almost entirely transferred into the monosulfoaluminate and ettringite, with the AFm/AFt phase ratio and the sum of these phases much higher than in the stone with a 3% of the admixture no. 2 and even much higher than in reference sample. Cement paste in the presence of admixture no. 1 at the age of 1 day contains four-coordinated aluminum (in this case, signal has peak at 71 ppm, i.e., is located on the left of the corresponding signal in the spectrum of the paste containing admixture no. 2). The absence of this signal for sample of a later age confirms instability of this phase. The 29Si NMR spectra are demonstrated in Fig. 4. The spectra of the reference sample and a sample containing 1% of the admixture no. 2 at the age of 1 day are almost similar. The spectra have signal Q0 belonging to the monomer silicon–oxygen tetrahedra of the clinker silicate phases and signals of silicon atoms of the external (signal Q1) and internal [Q2, Q2 (1Al)] units of silicon–oxygen chains of C–S–H gel. The calculation of the hydration degree αt (%) by the equation αt = 100 – ω (Q0t), where ω (Q0t) is content of monomer silicon atoms at time t (%) [10], yields a value of 4.20 and 22.2% in the absence and in the presence of the admixture, respectively. The values of ω (Q0t) were calculated as the ratio of the Q0 area to the total area of all the signals in the spectrum: ω (Q01 day) is 79.6% for reference sample and 77.8% for a sample obtained in the presence of the admixture. Thus, a small (about 1%) dosage of amorphous Al(OH)3 does not slow, whereas, on the contrary, even assists cement hydration, which may be the reason, or one of the reasons, why 1 day strength of the cement stone increases. On the spectrum of the cement stone containing 3% of the admixture no. 2, signals attributable to the hydration product 1 day of age are still very weak. The hydration degree is only about 4%. This confirms the data on the 27Al NMR spectroscopy: cement hydration at high dos-


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formation of ettringite from amorphous Al(OH)3 in quantitative amount. In this case, product formed in hydration of the cement near the surface of its grains under the conditions of deficiency of calcium and excess of aluminum ions, apparently, should be the aluminosilicate gel with disordered structure. According to the data on the the 27Al NMR spectroscopy, the gel is instable. In view of the morphological features, such a product can prevent cement hydration (the composition and structure of the gel may be similar to the aluminosilicate structures described in [11]). Anyway, recovery of the cement stone strength in the presence of high dosages of Al(OH)3 is associated with disappearance of the admixture signal (at 66 and 71 ppm for admixture no. 2 and 1, respectively) on the 27Al NMR spectra of the samples 1 month of age and with the recovery of hydration, as evidenced by the 29Si NMR spectra of samples of the same age. The rate of the cement hydration in the presence of the admixture is still slow even after 3 months of hydration, although differences are not as significant as in the early age. Thus, the hydration degree in the reference cement sample to the age of 3 months reaches 49% [ω (Q03 month) = 51%], whereas in the presence of 3% of the admixture no. 2 it reaches 43% [ω (Q03 month) = 57%]. Slowing of hydration in the presence of high doses of active aluminum hydroxides can be explained otherwise, suggesting that the rapid formation of ettringite from the admixture material blocks the cement grains, thus impeding access of water molecules to their surface. The blocking of the cement grain surface is removed after the exhaustion of all the admixture material with the beginning of the ettringite transformation into monosulfoalyuminate. Fig. 4. The solid-state 29Si NMR spectra of the cement stones. Sample: Reference at the age of (1) 1 and (4) 28 days; containing (2) 1 % and (3) 3 % of admixture no. 2 at the age of 1 day, and (5) containing 3 % of admixture no. 2 at the age of 28 days.

ages of high-activity Al(OH)3 on the first day is significantly decelerated compared to reference sample. For the formation of ettringite from high-activity aluminum hydroxide and gypsum, it is necessary to have an additional source of calcium ions. As such source, only clinker phases may serve. According to a rough estimate, the amount of the released calcium during hydration of 4% of the cement (even from only one phase, alite), is sufficient for the

CONCLUSIONS (1) According to the data on the solid-state 27Al and 29Si NMR spectroscopy, a decrease in the setting time of a cement paste in the presence of highly dispersed amorphous aluminum hydroxides is due to their participation, as main material, in rapid formation of ettringite phase. At this stage, contribution of the aluminum-containing phases into the formation of ettringite is insignificant. (2) At the content of high-activity aluminum hydroxide about 3% and more the hydration of cement clinker phases, including aluminum-containing phases, is blocked at the initial stage of the hydration (1 day). The hydration degree of a cement paste in the presence



of the admixture is lower than without admixture even after 3 months. (3) In addition to the formation of ettringite, amorphous aluminum hydroxides at doses of 3% and above participate in the formation of instable aluminosilicate hydrogel with disordered structure, which temporarily blocks cement hydration. REFERENCES 1. Myrdal, R., Accelerating Admixtures for Concrete: State of the Art: SINTEF report N SBF BK A07025, Trondheim, 2007, p. 35. 2. Bracher, G., Int. Symp. on Waterproofing for Underground Structures, San Paulo, 2005, p. 11. 3. Rixom, R., and Mailvaganam, N., Chemical Admixtures

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for Concrete, London: E&FN Spon, 1999. 4. US Patent 20110017100. 5. US Patent 6537367. 6. Richardso,n I.G.T., Cem. Concr. Res., 1999, vol. 29, no. 8, pp. 1131–1147. 7. Andersen, M.D., Jakobsen, H.J., and Skibsted, J., Cem. Concr. Res., 2004, vol. 34, no. 5, pp. 857–868. 8. Mendes, A., Gates, W.P., Sanjayan, J.G., and Collins, F., Mater. Struct., 2011, vol. 44, no. 10, pp. 1773–1791. 9. Rawa, I.A., Smith, B.J., Athens, G.L., et al., J. Am. Chem. Soc., 2010, vol. 132, no. 21, pp. 7321–7337. 10. Brykov, A.S., Kamaliev, R.T., and Mokeev, M.V., Zh. Prikl. Khim., 2010, vol. 83, no. 2, pp. 211–216. 11. Cloos, P., Leonard, A.J., Moreau, J.P., et al., Clays Clay Miner., 1969, vol. 17, no. 5, pp. 279–287.
