SOLID SURFACES and the Gas-Solid Interface - ACS Publications

Development of Surface in the Hydration of Calcium Silicates DAVID L. KANTRO, STEPHEN BRUNAUER, and CHARLES H. WEISE Portland Cement Association Research and Development Laboratories, Skokie, Ill.

The paste hydration of β-Ca SiO , Ca SiO , and alite has been investigated at 5 ° , 2 5 ° , and 5 0 ° , for periods ranging from 1 to 400 days. A high— surface calcium silicate hydrate, tobermorite, was formed in all these reactions, and the surface development was primarily dependent on the degree of hydration of the paste. The stoichiometry of the reactions was variable, depending on the original silicate, time, and temperature; and this variation had important effects on surface development. A linear relationship was obtained between the surface area and the CaO/SiO ratio of the tobermorite. From this relationship and other considerations, conclusions were drawn about the composition and structure of stable tobermorites as well as the composition of the tobermorite surface. The unstable tobermorites, obtained in the early part of the hydration, are briefly discussed.

Downloaded by UNIV OF PITTSBURGH on April 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0033.ch021

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Tricalcium silicate, C a S i 0 , and β-dicalcium silicate, /?-Ca Si0 , are the two calcium silicates found in portland cement and comprise the major portion of this important construction material. The hydration reactions of these calcium silicates are complicated processes now undergoing a thorough investigation with an aim toward understanding the even more complicated processes of cement hydration. Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions ( 8 ). The surface area and the surface energy of tober morite are briefly discussed by Brunauer ( 1 ). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 3

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Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

ADVANCES IN CHEMISTRY SERIES

In the previous investigations in this laboratory, most of the data were obtained on well-hydrated specimens prepared at room temperature. The present investigation includes specimens of this type, as well as specimens hydrated for shorter periods of time. Three temperatures were used—5°, 2 5 ° , and 5 0 ° . The hydration of alite, the form of C a S i 0 found in portland cement, was also investigated under the same conditions. In addition to time and temperature, the effect of stoichiometry on the development of surface area was investigated. The crystal structure of the natural mineral tobermorite was partly worked out by Megaw and Kelsey (22). Taylor and coworkers (15, 17, 26, 27) have carried out detailed investigations of tobermorite prepared synthetically, and correlated their structural features with those of the natural mineral structure. However, little was known about the nature of the surface, except what may be deduced from the surface energy results, the surface energy being approximately the geometric mean of the surface energies of calcium hydroxide and hydrous amorphous silica.


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Experimental Preparation of Silicates. The #-Ca Si0 and C a S i 0 were prepared from U.S.P. calcium carbonate and ground quartz, the latter containing 99.9% S i 0 . Approximately 0.5% B 0 , as U.S.P. boric acid, was added to the C a S i 0 raw mix as a stabilizer. The burning procedure was as described previously (9), but the burns were made in a six-burner gas-fired kiln which had a larger capacity than the two-burner kiln previously used. The /?-Ca Si0 mixture was twice heated at 1 4 5 0 ° for 3 hours, with intervening grinding and remixing. Since the mixing operations were carried out in a 22-quart dough mixer, the mixtures obtained were more homogeneous than those previously obtained and no "dusting" (inversion to the y form of C a S i 0 ) occurred at any time. The C a S i 0 was twice heated to 1 6 0 0 ° with intervening grinding and remixing. Alite—Ca Si0 to which small amounts of alumina, magnesia, and additional lime were added to give the mix the over-all composition 52Ca Si0 .6CaO.Al 0 .MgO—was prepared in exactly the same manner as C a S i 0 . Reagent grade hydrous alumina and magnesium oxide were used. All three preparations were ground in porcelain jar mills with flint pebbles. A portion of each preparation was ground to a Blaine surface area ( A S T M designation C-204-51) of about 4500 sq. cm. per gram. 2

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The analyses and compound compositions of these preparations are given in Table I. The compound compositions were adjusted to 100% by assuming that the missing percentage in the C a S i 0 preparations was C a S i 0 , and that in the C a S i 0 and alite preparations it was C a S i 0 . 2

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Preparation of Hydrated Silicates. The hydrated silicate specimens used were all in the paste form—that is, mixtures of one of the calcium silicates with a limited amount of water to form a slurry, which sets and hardens as portland cement itself does. These pastes were prepared by the vacuum mixing procedure described by Powers, Copeland, Hayes, and Mann (23), adapted so that the temperature of the mix upon removal from the mixer was the temperature at which the specimen was to be hydrated. The 5 ° specimens were made by starting with an ice-water mixture; t h e - 5 0 ° specimens by starting with preheated water. A manostat was incorporated into the pumping system to prevent the pressure from dropping below the vapor pressure of water at the desired final temperature. This was especially important for the 5 0 ° mixes, to prevent excessive cooling. The mixing of specimens consisted of two 1-minute mixing periods separated by a 3-minute standing period for the 5 ° and 2 5 ° specimens, and by a 2-minute standing period for the 5 0 ° specimens. In all but a few mixes, the water-solid ratio was 0.70. Four or five specimens were obtained from each mix. The Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

KANTRO ET AL.

Calcium Silicate Hydration

201

Table I. Compositions of Unhydrated Calcium Silicates Component

0-Ca SiO 65.43 33.56 0.12 0.05 0.47 99.63 2

GaO Si0 A1 0 MgO B 0 Total 2

2

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Material, % Ca Si0 73.57 25.69 0.20 Nil — 99.46

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Alite 73.62 24.93 1.01 0.37 — 99.93

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94.77* 98.18 2.36 Ca Si0 Ga Si0 97.01 — — Ga Al 0 0.32 0.53 2.68 MgO 0.05 0.37 — Free GaO 0.20 1.29 2.18 Insol. residue (Si0 ) 0.06 — — Total 100.00 100.00 100.00 « Total Ca Si0 taken as sum of Ga Si0 (94.65%) and B 0 expressed as 5 C a O B 0 (2.36%). Total alite (97.82%) taken as sum of Ca SiO „ Ga Al 0 , and MgO. 3

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specimens were transferred from the mixing vessel to temperature-resistant poly ethylene or polypropylene test tubes, in which they were allowed to hydrate. The 5 ° and 2 5 ° specimens thus obtained were placed in suitable temperaturecontrolled baths, and the 5 0 ° specimens in a temperature-controlled oven for the duration of the hydration period. Each specimen was removed from its test tube mold at the end of a prescribed hydration period. In most cases the specimen had set. If so, the top and bottom half inch of material were removed by means of a small diamond saw and dis carded. The remaining cylinder was sawed into two pieces axially. The specimen was then dried for 1 to 2 days in a vacuum drying apparatus of the type described by Copeland and Hayes (12), after which the material was placed in a gloved box containing a C0 -free atmosphere. Here the specimen was ground to pass a U.S. Standard No. 200 sieve. (A few specimens were ground to pass only a U.S. Standard No. 80 sieve. ) Each specimen was then placed in tared weighing bottles which were placed back in the vacuum drying apparatus and dried to equilibrium at the vapor pressure of ice at — 7 8 . 5 ° (5 χ 1 0 mm. ). 2

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This paper reports results obtained for a number of very well hydrated older specimens, prepared from other batches of C a S i 0 and C a S i 0 than those described above (5, 9). The compositions of these older unhydrated materials were given by Brunauer (Table I, 9). The paste preparation was identical with that described above, except in two respects: The specimens were hydrated in glass molds lined with a polyvinyl plastic paint, and the room temperature speci mens were hydrated in air in a temperature-controlled room. Specimens hydrated at higher temperatures were hydrated in temperature-controlled water baths. After 2 months of hydration at these higher temperatures ( 5 0 ° and 8 0 ° ) , the controls were turned off and the baths were allowed to cool to room temperature. Eventually the samples were removed from the baths and stored in air in the temperature-controlled room until they were used. Grinding and drying were accomplished as described above. Analysis and Surface Determination. Loss-on-ignition and carbon dioxide content were determined for each specimen. Complete chemical analyses were also performed on many of the pastes. These analyses were found to agree with those of the original unhydrated materials; in the end, one chemical analysis was used for all pastes made from the same original material. The analyses reported in Table I are the averages of those obtained for various pastes and respective un hydrated materials. 2

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ADVANCES I N CHEMISTRY SERIES

Free calcium hydroxide was determined by a modified Franke extraction procedure (24), most of the data having been obtained by the time-variation method ( T V M ) . The unhydrated material was determined by x-ray quantitative analysis, using magnesium hydroxide as an internal standard, as described previously (21). The calibration equations for all three calcium silicates were of the form L'-k"*

(l)

where I and I are integrated intensities of suitably chosen calcium silicate and internal standard diffraction lines, respectively, w and w are weight fractions of these substances, and k is a constant (10). The values of k for C a S i 0 , C a S i 0 , and alite, obtained from calibration mixtures, are 0.288 ± 0.006, 0.338 ± 0.006, and 0.332 ± 0.009, respectively. Surface areas were determined by the B E T method using water. vapor adsorption. The adsorption data were obtained gravimetrically in modified vacuum desiccators, such as that shown in Figure 1. x

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Figure 1. Water vapor adsorption apparatus The desiccator lid was manufactured with a ring stopcock through which evacuation was accomplished. A center tube was sealed on the lid, connecting it to a bulb. The bulb contained the saturated salt solution that maintained a constant water vapor pressure over the specimens. The bulb was isolable by means of a stopcock, so that the saturated salt solution could be stored under vacuum while samples were being changed. A ground joint connection made it possible to remove the whole upper assembly from the lid. Six such desiccators were employed, a different water vapor pressure being maintained in each. The relative Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

KANTRO ET AL.


pressures ranged from 0.07 to 0.33. pressure for 7 days before weighing.

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Duplicate samples were exposed to each

Table II. Compositions of Older /?-Ca Si0 and C a S i 0 Pastes 2

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(Grams per gram ignited weight)

Paste

Age

0

Temp, of Hydration, °C.

Ga(OH) G./G.

2

Unhydrated Material, G./G.

Minor Constituents, G./G.

0.306 0.147 0.355 0.310

0.024 0.028 0.040 0.028

0

Pastes of Ca Si04 2

C-23 C-51 C-29 C-57

17 mo. 4.25 yr. 21 mo. 5.2 yr.

Paste"

Age

Temp, of Hydration, °C.

C-23 C-51 C-29 C-57

17 mo. 4.25 yr. 21 mo. 5.2 yr.

25 25 50* 80*

Paste"

25 25 50* 80*

0.102 0.127 0.059 0.060

Tobermorite, G./G. 0.683 0.835 0.656 0.720

Temp, of Hydration, °G.

Age

17 mo. 5 yr. 21 mo. 5.2 yr.

Paste"

Age

C-17 C-52 C-27 C-55

17 mo. 5 yr. 21 mo. 5.2 yr.

Temp, of Hydration, °C. 25 25 50* 80*

25 25 50* 80*

2

1.15 1.09 1.19 1.29

1.65 1.63 1.73 1.75

Ca(OH)o

G./G. 3

C-17 C-52 C-27 C-55

2

2

Pastes of Ca Si0

Tobermorite Area, Sq. M . / G .

H 0/Si0 , Moles/Mole

GaO/Si0 , Moles/Mole

280.1 305.7 264.1 242.0

Unhydrated Material, G./G.

Minor, Constituents, G./G.

0.000 0.000 0.000 0.159

0.016 0.022 0.018 0.014

5

0.464 0.452 0.443 0.359

Tobermorite, G./G.

CaO/Si0 , Moles / M o l e

H 0/Si0 , Moles / M o l e

0.725 0.734 0.744 0.646

1.54 1.56 1.58 1.63

1.10 1.14 1.13 1.29

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Tobermorite Area, Sq. M . / G . 285.9 267.6 296.5 289.3

A l l pastes made from 0.7 water-solid ratio mixes. * Paste hydrated 2 months at indicated temperature, remainder of time at room temperature. Minor constituents consist of G a C 0 , M g ( O H ) , Ca3Al 06.6H 0, Na SC>4. and in case of Ca SiC>4 pastes, H - B 0 also. a

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Results The compound composition of each paste was calculated from the chemical analysis in terms of oxides (CaO, S i 0 , A 1 0 , M g O , B 0 , C 0 , and H 0 ) , the 2

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free calcium hydroxide, and the unhydrated material present.

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The amount and

composition of the tobermorite present were obtained from the difference between the total analysis and the amounts of the oxides required to account for calcium hydroxide and the unhydrated material.

Allowance was made for minor impurities

such as calcium carbonate. Tobermorite has a variable composition.

The composition in any given paste

may be expressed as xCaO.Si0 .t/H 0, where χ and y represent the molar C a O / 2


Table III.

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Compositions of C a S i 0 Pastes 2

^ Age, days

Ca(OH) , G./G.

Ca Si0 , G./G.

L

1 1 2 4 7 9 11 14 17 21 28 50 74 100 199

0. 033 0. 035 0. 047 0. 064 0. 081 0. 078 0. 079 0.088 0. 102 0. 099 0. 099 0. 114 0. 111 0. 111 0. 151

0.903 0.877 0.910 0.858 0.841 0.821 0.826 0.820 0.782 0.759 0.768 0.643 0.591 0.470 0.300

F F Q F Q F Q H H Q C H H C C C'«

1 2 '5 8 11 14 18 21 28 41 72 100 200 330 400 127

0. .046 0.063 0 .091 0 .085 0 .104 0 .114 0 .104 0 .117 0 .114 0 .122 0 .132 0 .141 0 .157 0 .158 0 .160 0 .128

0.855 0.798 0.747 0.738 0.617 0.641 0.484 0.511 0.447 0.433 0.344 0.268 0.214 0.150 0.207 0.274

Ο

1 2 8 14 21 28 100 200

0 .072 0 .089 0 .099 0 .094 0 .104 0 .105 0 .111 0 .105

0.757 0.716 0.516 0.487 0.444 0.403 0.299 0.303

Mix

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Minor Constituents, G./G.

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CaO/ H 0 / Si0 , Si0 , Moles/ Moles/ Mole Mole 2

Tobermorite, G./G.

c

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Tobermorite Area, Sq. M./G.

Hydrated at 5 ° R V S Τ

s ν τ ν τ s ν S τL

0. 025 0. 025 0. 021 0. 019 0. 019 0. 017 0. 017 0. 017 0. 016 0. 016 0. 015 0. 014 0. 014 0. 018 0. 019

0.050 0.075 0.037 0.077 0.083 0.108 0.104 0.103 0.132 0.159 0.155 0.277 0.346 0.472 0.640

1. 04 1. 27 0. 57 0. 89 0. 76 0. 97 0. 93 0. 84 0. 92 1. 06 1. .04 1. .31 1. .43 1 .56 1. .55

0.00 0.00 0.05 0.09 0.00 0.12 0.23 0.20 0.16 0.24 0.39 0.47 0.75 0.71 0.94

169.2 94.1 224.9 392.6 478.3 397.2 488.6 537.0 457.8 408.2 497.8 364.0 358.7 260.3 299.6

1 .20 1 .26 1 .17 1 .27 1 .41 1 .30 1 .58 1 .49 1 .57 1 .52 1 .59 1 .60 1 .58 1 .62 1 .55 1 .65

0.47 0.18 0.08 0.46 0.43 0.71 0.69 0.69 0.79 0.86 1.07 0.99 1.02 1.03 1.08 1.03

303.8 247.7 319.8 350.3 298.1 372.8 257.7 301.1 276.0 325.4 284.0 288.1 284.3 304.9 314.8 244.3

1 .32 1 .28 1 .56 1 .63 1 .62 1 .64 1 .68 1 .70

0.31 0.54 0.89 0.99 1.04 1.01 0.98 1.18

263.4 295.0 270.6 296.9 302.6 280.9 264.9 268.7

Hydrated at 25 ° 0 .025 0. .024 0. .021 0 .020 0 .018 0 .019 0 .022 0 .021 0 .021 0 .027 0 .017 0 .020 0 .020 0 .021 0 .032 0 .014

0.094 0.136 0.168 0.193 0.306 0.283 0.457 0.419 0.496 0.582 0.616 0.685 0.736 0.806 0.740 0.696

Hydrated at 5 0 °

ο ο οΡ Ρ Ρ Ρ

a b c

0 .023 0 .023 0 .024 0 .021 0 .021 0 .023 0 .022 0 .021

0.175 0.212 0.433 0.476 0.520 0.562 0.672 0.688

3500 sq. cm. Blaine surface Ca Si04 used. Total C a S i 0 includes C a S i 0 and 5 C a O . B 0 . Minor constituents include C a C O , M g ( O H ) , C a A l 0 . 6 H 0 , C a S i 0 , and H B O . 2

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KANTRO ET AL.


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S i 0 and H 0 / S i 0 ratios in the tobermorite. The compound compositions and the tobermorite compositions, the latter expressed in terms of C a O / S i 0 and H 0 / S i 0 ratios, for the older C a S i 0 and C a S i O pastes are given in Table II. The results for C a S i 0 , Ca SiO -, and alite pastes (compositions in Table I) are given in Tables III, IV, and V, respectively. The weights of the constituents are given in grams per gram of ignited weight, and not as grams per gram of paste. Thus, their sum is always greater than 1 gram. 2

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Table IV. Compositions of Ca SiO- Pastes Paste Age, Mix days

Ca(OH),. G./G.

GagSiOe, G./G.

Minor Constit uents^ G./G.


Hydrated at 5 Κ

κ κ κ L L L L

1 2 8 14 21 28 100 200

0. 0. 0. 0. 0. 0. 0. 0.

121 197 288 341 360 368 463 473

0. 0. 0. 0. 0. 0. 0. 0.

694 542 362 294 242 194 094 030

GaO/ Si0 , Moles/ Mole

Tobermorite, G./G.

H 0/ ToberSi0 , morite Moles/ Area, Sq. Mole M . / G . 2

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c

0. .012 0. 012 0. 010 0. 012 0. 010 0. 011 0. O i l 0. 009

0. .214 0. 332 0. 466 0. 495 0. 544 0. 587 0. 621 0. 683

1 .86 1 .72 . 1. 64 1 .53 . 1 .56 . 1..62 1 .43 1. 51

0. .32 0 .89 1 .08 . 1..01 1 .12 1..08 1 .09 . 1 .05

201 ..7 324. .7 340. 4 341. 1 326. .6 351. 8 357. 1 326. .3

1 .76 1 .56 1 .66 1 .62 1 .63 1 .57 1 .47 1 .49 1 .55 1 .44 1 .42 1 .53 1 .44 1 .52 1 .57

0 .63 0 .71 0 .63 0 .78 0 .91 0 .90 0 .92 1 .00 1 .00 0 .97 1 .01 1 .07 0 .98 1 .00 1 .00

306 .2 372, .8 312. ,5 359. .8 352 .2 354 .9 387, .6 355. 6 356. 8 357. 5 328. .6 357. .5 329 .7 352 .9 359 .8

1 .69 1 .73 1 .66 1 .62 1 .66 1 .59 1 .61 1 .59 1 .58

0 .74 0 .74 1 .00 1 .24 1 .08 1 .10 1 .17 1 .20 1 .12

315 .2 290 .8 323 .2 358 .7 303 .4 326 .3 303 .4 310 .3 334. .3

Hydrated at 25 ° Q Q Q Q

F F H H C H H G G B

«

Ο

1 1.5 2 4 8 14 21 28 72 100 200 300 400 72 72

0. 148 0. 173 0. 189 0. .225 0. 264 0. 329 0. .380 0. .400 0. .444 0 .489 0 .504 0 .482 0..500 0 .440 0 .401

0. .654 0. .642 0. .580 0 .514 0. .422 0. .304 0. .245 0 .191 0..074 0..045 0 .024 0 .000 0 .000 0 .096 0 .143

1 2 8 14 21 28 100 200 90

0 .164 0. .199 0..270 0 .297 0 .325 0. .357 0. .420 0. .444 0..431

0 .628 0 .533 0 .388 0..345 0 .268 0 .232 0..082 0 .038 0 .080

0 .244 0 .238 0. .289 0 .338 0. .411 0 .487 0..512 0 .559 0. .655 0..649 0 .662 0 .709 0 .681 0 .633 0 .625

0 .010 0. .012 0 .011 0..009 0 .011 0 .012 0 .014 0 .014 0 .010 0 .012 0 .016 0 .013 0 .028 0 .012 0 .013

Hydrated at 50 ° Ρ Ρ Ρ Ρ

ο ο ο ο D« a 6 c

0 .012 0 .012 0 .014 0 .015 0 .012 0 .013 0 .013 0 .012 0 .011

0 .260 0 .335 0 .446 0..483 0 .539 0 .554 0 .675 0 .707 0..667

Water-solid ratio in mix: 0.57. Water-solid ratio in mix: 0.45. Minor constituents include CaC0 and C a A l 0 . 6 H 0 . 3

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The extent to which any paste is hydrated may be expressed by the ratio of the amount of unhydrated material that has disappeared to the amount present origi nally. This ratio will be called the degree of hydration. In the case of C a S i 0 , a correction was made for the small amount of C a S i 0 present. The latter was assumed to hydrate at the same rate as in C a S i 0 pastes under comparable condi tions. Thus, the amount of C a S i 0 remaining unhydrated in a C a S i 0 paste could be estimated and subtracted from the total amount of remaining unhydrated material, the difference being the amount of unhydrated C a S i 0 . 2

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ADVANCES I N CHEMISTRY SERIES

Table V. Compositions of Alite Pastes GaO/ ToberSi0 , Ga(OH) morite, Moles/ Alite, GaCOs, G./G. G./G. G./G. G./G. Mole Hydrated at 5 1 .79 0. 102 0. 781 0.002 0.,147 0. 217 0. 542 0.004 0. 318 1 .62 0. 348 0. 251 0.003 0. 544 1 .61 0. 385 0. 198 0.003 0.,582 1 .56 0. 451 0. 146 0.003 0.,577 1 .41 0. 454 0. 118 0.004 0. 605 1 .44 0. 452 0. 061 0.004 0. 676 1 .54 0. 477 0. 062 0.003 0. 661 1 .46

Paste Age, Mix days

2

b

c

H 0/ ToberSi0 , morite Moles/ Area, Sq. Mole M . / G . 2

c

2


c

a

M M M M Ν Ν Ν Ν

1 2 8 14 22 28 100 200

I I I I G G G G G A«

1 2 8 14 21 28 72 100 200 72

0..621 0..521 0..359 0 .256 0 .191 0 .139 0 .049 0 .033 0 .000 0 .102

Hydrated at 25° 0.004 0 .264 0.001 0 .328 0.004 0 .435 0.004 0 .502 0.006 0 .534 0.007 0 .582 0.007 0 .647 0.010 0 .650 0.010 0 .673 0.006 0 .652

Hydrated at 50° 1 R 0 .218 0 .583 0.003 0 .272 R 2 0..274 0..502 0.004 0 .318 R 8 0.,394 0.,254 0.005 0 .504 R 14 0..436 0. 195 0.006 0..536 22 0. 453 0. 130 0.006 0..597 Q 28 0. 453 0. 130 0.008 0. 591 Q 100 0. 494 0. 032 0.014 0. 669 Q 200 0. 492 0. 000 0.012 0. 707 Q Water-solid ratio in mix: 0.45. Sum of Ga Si0 , Ga Al 0 , and MgO. Si0 includes A1 0 which substitutes in lattice for Si0 .

b c

0. 179 0..237 0.,329 0,.392 0 .435 0 .452 0 .501 0 .511 0 .525 0 .427

3

5

2

3

2

2

0..46 0..77 1..04 1 .19 0,.98 1..01 1,.11 1..16

231.0 311.9 307.6 332.0 335.0 339.2 325.9 365.9

1 .65 1 .57 1 .47 1 .41 1 .37 1 .40 1 .40 1 .38 1 .39 1 .55

0 .80 0 .76 0 .90 0 .97 0 .92 1 .01 1 .05 1 .00 0 .96 1 .15

294.2 289.7 349.9 377.7 366.7 346.8 387.8 357.5 336.6 319.4

1 .48 1 .37 1 .40 1 .36 1 .42 1 .41 1 .43 1 .49

0 .69 0 .77 1 .02 1 .02 1 .08 1..15 1 .09 1 .10

333.9 377.7 389.5 385.0 385.3 390.7 328.2 313.7

6

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The degrees of hydration of C a S i 0 , C a S i 0 , and alite pastes as functions of time at the three temperatures are given in Figures 2, 4, and 6, respectively. The data, obtained from pastes whose compositions are given in Tables III, IV, and V, are plotted on a logarithmic time scale only as a matter of convenience. The V values for C a S i 0 , C a S i 0 , and alite pastes are plotted as functions of time at the three temperatures in Figures 3, 5, and 7, respectively. (The spe cific surface areas shown in the last columns of Tables III, IV, and V were calcu lated from these V values.) In these figures, too, the logarithmic time scale has been used for convenience. 2

m

4

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4

3

5

3

5

m

Discussion Progress of Surface Formation. During the early stages of reaction, the hy dration of /?-Ca Si0 depends on temperature in much the same manner as most chemical reactions. As can be seen in Figure 2, at any given time during the early stages, hydration of £ - C a S i 0 has proceeded to the greatest extent at the highest temperature of hydration. This temperature dependence is not maintained, how ever, in the later stages of reaction. The degree of hydration of /?-Ca Si0 at 5 0 ° is less than that at 2 5 ° at any given age beyond 60 days, although at 60 days only about 65% of the /?-Ca Si0 has disappeared. A comparison of Figures 2 and 3 shows that the degree of surface area de2

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KANTRO ET AL.

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velopment behaves in about the same manner. Because the specific surface areas of the calcium silicates are three orders of magnitude smaller than those of the hydration products, the V values are almost entirely dependent on the surfaces of the hydration products. As Figure 3 shows, the V data, like the degree of hydra tion data, are lower for 5 0 ° pastes older than 60 days than for 2 5 ° pastes of the same ages. This parallelism is only semiquantitative, however, for as can be seen from a comparison of Figures 2 and 3, the detailed shapes of the corresponding temperature curves differ in some respects. Relationships between V and other variables are discussed later. m

m

m


90 I

n

>~"

L I

ι

1

1

1

ι 7

ι 10

1

1

1

2

3

5

Figure 2.

1

1

1

1

1

1

Γ

ι 20

ι 30

ι 50

ι 70

ι 100

ι 200

300

T i m e — days

Percentage hydration of Ca Si0 2

ll

1

1 500

vs. time

The degree of hydration results for C a S i 0 (Figure 4) have relationships different from those for /?-Ca Si0 . First of all, the temperature dependence of the hydration of C a S i 0 is considerably smaller than that of C a S i 0 ; the curves at the three temperatures are much closer to each other in Figure 4 than in Figure 2. In the second place, the C a S i 0 curves show only one crossing between 1 and 100 days of hydration, whereas the C a S i 0 curves show four crossings. The V curves for C a S i 0 (Figure 5) also exhibit four crossings between 1 and 100 days. The crossings do not occur at exactly the same ages in Figures 4 and 5, because V is a function not only of the degree of hydration but also of the stoichiometry of the hydration. Nevertheless, the 2 5 ° curve indicates a lower degree of hydra tion between 1.5 and 10 days and a higher degree of hydration after 20 days than 3

2

3

5

2

2

4

4

3

3

5

4

5

5

m


m

208



.07 1

-«

'

'

·

.

•

1

Time - Days

Figure 3.

Surface development in hydration of Ca SiO 2

vs. time

i

the 5 ° and 5 0 ° curves (Figure 4) and, similarly, the 2 5 ° curve indicates a lower surface between 1.7 and 4 days and a higher surface after 20 days than the 5 and 5 0 ° curves (Figure 5). Figure 4 shows a region in the vicinity of 14 days in which inverse correlation with temperature exists, the 5 ° paste being the most hydrated, the 5 0 ° paste the least. A similar region of inverse temperature dependence is found for V in Figure 5, at about the same age. The hydration of alite is of interest because of the similarity of this material to C a S i 0 . The alite used was stabilized by A 1 0 and M g O as described in an earlier investigation (II). Thus, alite is slightly different from C a S i 0 in composition, but more important is the fact that C a S i 0 and alite are crystallographically dissimilar. C a S i O is triclinic and alite is monoclinic, as was shown by Jeffrey (19) and confirmed by Yamaguchi and Miyabe (28). The curves of degree of hydration vs. time for alite (Figure 6) bear a strong resemblance to those of C a S i 0 (Figure 4). However, for alite the pastes hydrated at 5 0 ° are at all times more hydrated than corresponding 5 ° and 2 5 ° pastes. The 5 ° and 2 5 ° curves, however, behave relative to each other in much the same way as the corresponding C a S i 0 curves. Comparison of the V vs. time curves of alite (Figure 7) with the degree of hydration vs. time curves (Figure 6) reveals a similarity in behavior, as in the previous cases. No attempt is made here to explain the intricate temperature dependence of the hydrations of the three calcium silicates. Work on the kinetics of these hydration processes is not complete, so the authors could give at present only highly tentative and speculative explanations. The conclusion from Figures 2 to 7 for the purposes of this paper is that the surface area development in pastes of all three silicates is determined primarily by the degree of hydration of the silicate. For a simple reaction, in which the reactants have negligible surface areas compared with the reaction products, such a conclusion would be trivial. However, for as complex m

3

5

2

3

3

3

3

5

s

3

5

3

5

m


5


KANTRO ET AL.

20


209

L-

1 0 h-

I

0

2

3

5

10

7

20

30

50

70

100

200

300

500

Time — days

Figure 4.

Percentage hydration of Ca Si0 3

5

vs. time

reactions as the three discussed here, through all the intricate and not easily explainable crisscrossings of the hydration curves, the surface area development, by and large, faithfully follows the degree of hydration. Consideration of the secondary factors leads to more interesting conclusions, because these factors reveal important information about the mechanism of surface formation and the nature of the tobermorite surface. Stoichiometry of Hydration Reactions. The stoichiometry of the hydration reactions has a significant influence on surface formation. In previous investigations of C a S i 0 and /?-Ca Si0 pastes hydrated at room temperature for long times (3, 5), the stoichiometry of the hydration process was found to be represented by the following equations: 3

5

2

4

2Ca Si0 + 6H 0 = Ca Si 0 .3H 0 + 3Ca(OH) 3

5

2

3

2

7

2

2Ca

SOLID SURFACES and the Gas-Solid Interface - ACS Publications

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