STEPHEN BRTJNAUER, L. E. COPELAND m~ R. H. Baaaa
112
VOl. 60
THE STOICHIOMETRY OF THE HYDRATION OF TRICALCIUM SILICATE A T ROOM TEMPERATURE. I. HYDRATION I N A BALL MlLL BY STEPHENBRUNAUER, L: E. COPELAND AND R. H. BRAGG Portland Cement Association Reaearch and Development Labordoriee, Chicagol I U i h Radwd Julv 11.1066
When tricalcium silicate, CapSiOl, is hydrated in a ball miU, the stoichiome of the reaction at 23” in a saturated calcium 6 6 0 = Ca&%OECL-2H@ SCa(0Hh. An &O/CaoSiOr hydroxide Bolution is represented by the equation 2C@iO6 weight ratio of 9 waa used in the ball mill, and complete hydration waa attained in Bix daya The calcium silicate hydrate produced waa afwillite. The af ’llite waa colloidal in dimemiom; m& of the particlea were roughly apherical, and their diameter waa of the order of 300wjft The density waa 2.647 f 0.01 g./cc.; the mean index of refraction waa 1.62 f 0.01. No ditrerence, except in the dimedona of the particlea, wae found between the natural mineral and artificisl dwillite.
+
Introduction Primarily because tricalcium silicate, c88io6, is one of the important constituents of portland cement, its hydration has been extensively investigated. These,as well as other investigations of the CaoSi02-H20 system, were critically reviewed by Steinour.’ Although valuable information has been gathered by several investigators, the stoichiometry of the hydration of C&i05 has not been settled. When Ca8i06 is treated with solutions of lime in water, the reaction products are Ca(OH)2 and calcium silicate hydrates of various compositions. At a concentration of 2 mmoles of CaO per liter, the molar ratio of CaO to Si02 is around 1.0 (or possibly as low as 0.8) ; at or near the saturation concentration the ratio is about 1.5. Taylor,2who made the most complete investigation of this system, designated the hydrates as calcium silicate hydrate (I) or CSH (I). Because of their similarity to the nab ural mineral tobermorite,8 we shall call them tobermorites. If in a mixture of water and CaaSiO6 the weight ratio of water to CaaSi06is less than about 300,the water becomes saturated with Ca(0H)s. It follows from this that unless a very large excess of water is used, the hydration takes plat-r at least ends-in a saturated lime solution. Of especial importance is the hydration in saturated calcium hydroxide solution because in the hydration of portland cement lime saturation is quickly established; and thereafter the CaaSiOs component of portland cement hydrates in a saturated solution. To determine by chemical means the composition of the calcium silicate hydrate produced, the calcium hydroxide must be removed from the system, but methodq so far proposed usually remove also some of the lime from the calcium silicate hydrate. Because of this, the stoichiometry of the hydration can be reliably solved only if the chemical composition and nature of the reaction products can be determined in situ, without disturbing the reaction system. An in situ technique was used recently by Graham, Spinks and Thorvaldson,‘ who investigated (1) II. H. Steinour, Chm. Rma.. 40, 391 (1947): “Pracdinga of the Third International Symposium on the Chemistw of Cement,’’ Loodon. 1952, p. 261. (2) H. F. W. Taylor, 3. Ch6m Soc., 3682 (1960). (3) G. F. ClaringbuU and M. H. Hey. MiMlOl. M w . . S9, 960 (1952). (4) f‘/iem,
W.A. G. Graliain, .I. N’. T. S1,inL.s and T. Tliorvaldson. Fun. J . 92, 129 (1954).
+
the hydration of Ca&3i06 and &C&i04 @dicalcium silicate) by employing a radioactive tracer, Cas. They concluded that the calcium silicate hydrate obtained from both silicates in saturated lime solution had a molar CaO/SiOz ratio of 1.5. Taylor2and Bessey found that the CaO/Si02 ratio of the calcium silicate hydrate close to lime saturation was 1.5, but both investigators believed that at lime scrtzrrationthe CaO/SiO, ratio of the stable hydrate was 2.0. Our results confirm the result of Graham, Spinks and Thorvaldson for C&iO6, and give additional information on the stoichiometry of the hydration of Cs$i06 at room temperature in saturated lime solution. We employed a number of experimental approaches, among them in situ techniques; the main tool for identification and quantitative determination was an X-ray dillractometer. We investigated the hydration reaction at 23” under two conditions: (a) C&iOs was hydrated in a small steel ball mill; and (b) it was hydrated in the form of “paste”--imitsting its presumed mode of hydration in portland cement. The former is discussed in the present paper; the latter in the second paper. Experimental R e aration and Hydration of C%SiOn.-We used two lob o!CaSiO6. deeignated aa Cs$iOl(I) and ~ i O S ( I 1 ) . The fht lot was identical with that described by B m u e r , Hayea and Had; the second waa prepared in the aame manner, but it had a slightly dil€erent composition. The a roxjmate composition of both.lob waa 96% Cs$iOb, 3yCa$3i04 and 1%of other iqnpuntiea. Hydrabon waa carned out rn two s d stee! ball mills, designed b Grunwald.’ Each had a capauty of about 400 cc. anlwaa uaed with a proxunately 400 .of steel balls of assorted sizy (J/, to 8 l 4 v L + m ,& k e f hardnes~NO. 60-62). The internal dunemions of the rmlls were approximatel 6 diameter and 1’ width. The interior surfaces were L’ened to a depth of I/=‘ (Rockwell hsrdnegs No. 6062); the exterior surfacea were plated with chromium. The milla were rotated a t approximately 50 r.p.m. for 15 minutea of each hour. The mills were operated in a room ke t at 23 f 0.5”. The w?ter-bwlid weight ratio was 9 . &investigated two ball-mdl-hydrated batchea of Cs$iOb, prepared from Car Siob (I) and (II). Removal of Uncombined Water.-The bd-mill slurries. at the end of the grinding period, were spooned or pi tted for out of the mill ami transferred to erlenmeyer freezing. The practice of freezing the slurriea was adopted (5) G. E. h y . “Pmcednga of the Symposium on the Chemistry of Cements,” Stoakholm, 1938, p. 178. (6) 8. B m u e r . J. C. Hayea and W. E. H m . Tau,JOUBNAL. 68, 279 (1954). (7) Ernest hf. Gruowald. iinpuMb1ic.d Portland Cement Association
reports; Chem. Dept.. F{orida State Iloivemity. Tallahassee. Florida.
Jan., 1956
113
HYDRATION OF TRICALCIUM SILICATE IN A BALLMILL
by Copeland and Hayes8 to avoid sample losses owing to frothing and 'hiling," which occur when the slurry is laced on the vacuum line directly. After most of the water been removed, the samples were homogenized by thorough mixing to eliminate stratification; the rest of the-adsorbed water was then removed by the procedure deacnbed by B m u e r , Hayes and Haw.@ Drying was considered complete when the loss of water was less than 1 mg./g. of sam le En two days. operations, to the extent possible, were conducted in a controlled atmosphere manipulation cabinet equipped with Ascarite filters, to mhimke contamination of samples by COr. Extraction of Calcium Hydroxide.-This was done by a modification of the method of Frankes for the quantitative determination of lime. A solvent mixture, made u of 80 cc. of isobutyl alcohol, 9 cc. of acetoacetic &r ethyfacetoacetate) and 15 cc. of ethyl ether, was add to 1 g. of sample; the mixture was refluxed a t 70 to 80" for three hours, filtered, and the lime in the filtrate was determided gravimetrically or volumetrically. The extraction proceea was then repeated on the residue; and if the filtrate obtained in the second extraction contained a &able quantity of lime, a third extraction was performed. On some samples cold extractions were performed. Instead of refluxing the mixture, it was shaken on a rotating table for 24 hours. Preparation of Calcium Hydroxide.-Two preparations of calcium hydroxide were used. (a) Ca(OH)r(I).-CaO was prepared by the i 'tion of whiting (CaCG) a t 950 to 1OOO' for 24 hours. %e CaO was then hydrated by eyosure to an atm here of saturated water va r at 23 for two montha.%e contsiners were lined witgOceresin. The specific surface area of the calcium hydroxide was determined by the B.E.T. method,m using nitro n adsorption; it was found to be 36.8 m.*/g. (b) C 4 & II) -The same CaO was used, but it was hydrated m bo& water. The surface area of this sample was 5.8 m.'/g. Ignition loss and carbon dioxide determinations indicated complete hydration and very slight carbonation for both preparstions. Other Operations.-The ignition loss and carbon dioxide content were determined, the chemical analyses were formed, and the com 'tions of the substances were lated according to t r m e t h o d s described by Brunauer, Hayes and Hass.6 Density determinations were made at 27' by the liquiddisplacement method. The liquid used was a saturated solution of calcium hydroxide in water. Dr. Greenberg of the Johns-Manville Research Center examined several of our samples by the method@ of diBerential thermal analysis and Chevenard thermobalauce measurements. X-Ray I n v e s p . 4 Ka radiation was used in all cases. X-Ray raction pattern were obtained with a 114.6 mm. diameter Debye-Sherrer camera, a Norelco' Wide Range Geiger Counter DBractometer, or with a Norelco Counting Rate Computer used in con'unction with the diffractometer. his combiition perlorms **fixed count/* s+g automatically. Specimens were rotected from carbon dioxide and water vapor during use ofthe ditlractometer by a simple modifiestion of the scatter shield of the instrument. A thin sheet of polyethylene was sealed over the X-ray beam opening of the scatter shield, thereby providing a sealed chamber for the specimen during norms1 operahon, with negligible loas of intensity. A test in which a sample of Ca(OHK1) was held for 24 hours in the sealed enclosure showed no measurable change in carbon dioxide content, and a barely perceptible increase in ignition loss. Except for the materials which were already sufficiently fine-gmned. all samples were ground to pass a =mesh sieve and then further ground by hand for about 30 minutes in a mullite mortar. This procedure ensures good mixing of the constituents of the prepared mixtures, and it produee~
h
e6
(8) L. E. Copeland and J. C. Hayes. ASTM Bulletin, No. 194, December, 1953. (9) B. Franke. 2. anorg. dluem. Clum.. 147, 180 (1941). (10) 9. Brunauei. P. 11. Emmett and E. Teller, J . Am. Clum. Sot.. 60, 309 (1938). (11) 9. A. Greenberg. THIB JOURNAL, 68, 3G? (1954).
the he-grsined material necessary for reproducible intensity measurements.1* The grinding was carried out in the controlled-atmosphere cabinet. The most troublesome aspect of X-ray quantitative analysis, particularly of mixturea containing plfrty pr fibrous materials, is preferred orientation. A modification of the technique described by Swanson and Tatgel8 enabled us to ns for the difIractometer remarkably free produce from preerred orientation. Details of the s ecimenpre aration technique will be published later by 8opeland anzBragg. All quslititative analyses were performed with the diffmtometer. Intensity data were obtained either by means of rate-meter recording, with the goniometer scanning at '/so ?@/+ute, or by means of automatic %xed count" m n g mth the computer set for angular increments of 0.05O28. Corrections for Geiger counter lost counts were found to be unnece888py. The number of counts was such that the instrumental error in intensity measurements was usually less than 1%.
Results and Discussion The stoichiometry of the hydration of CaaSiOs at 23" in a saturated calcium hydroxide solution can be represented by the equation
+
+
2CarSiO~ 6HtO = CsSir@-3H~0 3Ca(OH)r (1)
The formula, Ca&3i207.3H20, is not intended to imply here anything about the structure of the calcium silicate hydrate. The water may be present in the hydroxylic form or as molecular water in the compound. Both ball-mill hydration and paste hydration can be f m U y represented by the same equation, but the calcium silicate hydrates are different. In ball-mill hydration the hydrate produced is afwiltite; in paste hydration the product obtained is similar to tobermorite. To distinguish between the two calcium silicate hydrates, we shall adopt the notation Ca3Siz07.3H20 (A) for afwillite and CaSizO7.3H20(T) . . for the tobermorite-like hydrate. The Water of Hvdration.-Jac~uernin~~hydrated Cadi06 in a ballkill, and he concluded that the water of hydration waa 3 molecules per molecule. Grunwald,' who investigated ball-mill hydration at our laboratory a few years ago, arrived at the same conclusion. We hydrated several batches of CajSiO, in the ball mill and obtained complete hydration in about 6 days, as will be shown later. The water of hydration corresponded almost exactly to 3 molecules of water per molecule of C&iOs. We may mention two examples of this. The total water of hydration of one batch of hyhated CaaSiO,(I), calculated on the assumption that Ca3Si0, takes up 3 and C a r Si042 moleculea of water, was 23.2170 on ignitedweight basis. The experimental value was 23.11%. The total calculated water of hydration of another batch of hydrated CaaSiOi(I) was 23.25%; the experimental value was 23.16%. It is somewhat surprising to obtain such close agreement between theoretical and experimental values in a colloidal system. Afwillite, the calcium silicate hydrate produced in the ball-mill hydration of CadiOr, is obtained in the form of particles hav(12) L. Alexander, H. P. King and E. Kummer, J . Appl. Phus.. 742 (1948). (13) H. E.Swanson and E. Tatge. J . Rcssorch N d l . Bur. Standards 46, 318 (1951). (14) R. Jacqucmiu. "Recherche8 sur I'hydration des liants Iiydrauliques," L)issertation, University of Liege. Desoer. Liege. 19.14.
1 , .
STEPHEN BRUNAUER, L. E. COPELAND AND R. IS. BRAG*
114
ing an average dimension of less than 300 A, Nevertheless, afwillite even in this size range is stable at a water vapor pressure of 5 X mm., indicating that its dissociation pressure a t 23" hm a lower value than that. The Reaction Fr0duct.s.-There was no evidence of unhydrated CarSiOs on any of our X-ray diffractometer charts after 6 days of ball-mill hydration. All lines on the charts were attributable to calcium hydroxide and the calcium silicate hydrate. The amount of Ca(OH), in the products was first determined by the modified Franke method, described earlier. The result corresponded almost exactly to 1.5 molecules of Ca(OH), per molecule of CasSiOb. Three examples are given below. 1. Batch T of hydrated CaaSiOs(1) contained a total calculated value of 35.79% uncombined CaO. We assumed that the hydration of Ca3SiOs produced 1.5 molecules of Ca(OH)2 per molecule, and the hydration of CazSiOI 0.5 molecule of Ca(OH), per molecule. The experimental result was 35.76% uncombined CaO. 2. Batch I1 of hydrated Ca3SiOs(I) had a calculated uncombined CaO content of 35.30%. The experimental value was 35.42%. 3. The above analyses were performed by hot extractions. Cold extraction of batch I1 of hvdrated Ca,SiOb(I) gave an experimental value bf 35.33%. In- &ite of the extraordinary agreement between the calculated and experimental values, chemical analysis did not settle the stoichiometry of the hydration of Ca3Si06. It did not seem impossible, or even unlikely, that the extraction process had altered the hydration products. For example, it seemed possible, especially on the basis of prior information12 that the hydration proceeded according to the reaction CacGiO,
+ 3Hz0 = C ~ . J ~ ~ O ~ . +~ HCa(0H)r IO
(2)
and that during extraction the higher-lime hydrate decomposed 2(Ca&04.2H~O) = Ca3Si207.3HgO
+ Ca(0H)Z
(3)
Vol. GO
(OH)1I] and mixture B [Afwillite (C) plus Ca(0H)s-
111.
X-Ray diffractometer charts of the two mixtures were prepared, and they were compared with charts of completely hydrated, unextracted CaaSiOs (Hydrated Cassios). The comparisons were based on the intensities of the four strongest lines of afwillite and the two strongest lines of calcium hydroxide. The line intensities obtained on seven charts are given in Table I. TABLE I LINEINTENSITY MEASUREMENTS Ca(0H)r lines 4.90
A.
Alfwillite (H) Mwillite (C) Hydrated Ca&3iOs Mixture A Mixture B Mixture B
2.63
A.
6.61
2.83
2.73
168 137
457 424 311 291 260
157
264
680 604 473 435 377 488 392
423 391 293 267 243 288 248
269 246 718 547 501
981 914 986 885 823
Afwillite lines 3.18
A.
A.
A.
A.
The intensities are given in arbitrary units (actually the number of squares on our charts). To ascertain whether the relative amounts of calcium hydroxide and afwillite were the same in hydrated Ca3SiOsand in the two mixtures, we calculated the ratios of the intensities of each of the afwillite lines to each of the calcium hydroxide lines for each chart and compared the ratios obtained for hydrated Ca3SiOswith the ratios obtained for the mixtures. The intensities shown in Table I enabled us to make 26 such comparisons. The result was that the afwillite content of hydrated Ca3SiOswas 98.7% of the afwillite content of mixture A or B (or, what IS the same thing, the calcium hydroxide content of hydrated Ca3SiOswas 101.3% of the calcium hydroxide content of mixture A or B). The standard error of the mean was 3.4%. It will be noted that in the comparison we used relative intensities, the calcium hydroxide serving as an internal standard for the afwillite and vice versa.16 We can definitely conclude from these data that, the stoichiometry of the hydration of Ca3SiOs in the ball mill at 23" in saturated calcium hydroxide solution is represented by the equation
X-Ray investigation of the hydration products before and after extraction settled the question definitely. Diff ractometer charts of the hydration + 3Ca(OH)2 (4) products established the fact that in both cases 2CarSiO6 + 6H10 = CazSip0~3H~O(A) Calcium Hydroxide.-The calcium hydroxide the calcium silicate hydrate was afwillite, CasSizO,. 3Hz0 (A). Other than the calcium hydroxide lines produced in the hydration of Ca3SiOs appeared in only the lines of afwillite were present, with one the form of crystals having larger than colloidal possible exception. On some charts there was a dimensions. The calcium hydroxide in mixture A vague indication of the strongest line of the tober- had a surface area of 36.8 m.2/g.; that in mixture morite-like hydrate, Ca3Siz0,.3Hz0 (T), the one a t B a surface area of 5.8 m.2/g. Comparison of the 3.03 8. Although a quantitative estimation could half-intensity widths of the calcium hydroxide not be made, probably less than 5% of the calcium lines obtained for these mixtures with those of the calcium hydroxide lines obtained for hydrated silicate hydrate was tobermorite. The material from which the calcium hydroxide CarSi06 indicated that the dimensions of the calhad been extracted showed complete absence of the cium hydroxide crystallites in the latter were intercalcium hydroxide lines on the charts; only afwil- mediate between the dimensions of the crystallites lite lines were present. This was true of both the in the two mixtures and closer to the dimensions of hot-extracted [Afwillite (H)] and the cold-extracted the low-surface calcium hydroxide. The line widths [Afwillite (C) 1 material. We mixed Afwillite (C) also clearly indicated that the crystallites of calcium with Ca(OH)2 in the proportion of three moles of hydroxide were stubby. Surface-area determinaCa(OH)z to one mole of afwillite. Two mixtures tions by water-vapor adsorption indicated about mere prepared: mixture A [Afwillite (C) plus Ca(15) L. Alexaoder and I T . P. Klu2, A d . Clrem., 20, 880 (1948).
HYDRATION OF TRICALCIUM SILICATE IN
Jan., 1956
the same average particle dimension as X-ray line broadening did. M a t e . - T h e d spacings of the X-ray diffraction lines of the afwillite Droduced in the ball-mill hydration of Ca3SiOs showed a very close correspondence to the d spacings published by various investigators for the natural mineral afwillite, especially to those published by Switzer and Bailey.l6 Interestingly, however, the relative intensities of our lines did not show such a close correspondence to published values. A comparison between our results and those of others for the four strongest lines of afwillite is given in Table 11.
TABLE I1 RELATIVE INTENSITIES OF AFWILLITE LINES
6.61 A.
Afwillite lines 3.18 2.83
A.
A. Our Values Afwillite (H) 40 67 Afwillite (C) 41 70 Hydrated CaaSiOb .. 66 Hydrated CarSiOb 39 67 Mixture A 36 69 Mixture B .. .. Mixture B 40 67
A.
2.73
A.
100 100 100
62 65 62 61 64 59 63
100
62
100
90
100 100
90 80
100
87
100 100 100 100
~~
Av. 40 67 B. Values of Others Switzer and Bailey16 90 90 Imperial Chemical Industries, Ltd.17 70 90 McMurdie and Flint’s 80 100 Av. of others’values
80
93
The discrepancy between our results and those of others is explainable on the basis of preferred orientation in the samples of other investigators. Our artificial afwillite was obtained in colloidal dimensions, after six days of grinding; we believe, therefore, that the particles had random orientation. This is evidenced also by the close agreement between the relative intensities obtained on seven charts, shown in Table 11. We received from the U. S. National Museum a sample of natural afwillite, which came from the Dutoitspan mine, Kimberly, South Africa. DebyeSherrer patterns %ade on parts of the sample showed that (a) the 2.83 A. line was the strongest line; (b) the 3.18 and 2.73 8. lines had about the same intensities, and their intensities owereabout two-thirds of the intensity of the 2.83 ,A. line; and (e) the intensity of the line at 6.61 A. had about one-half the intensity of the 2.83 1%. line. The Debye-Sherrer patterns of our artificial afwillite were indistinguishable from those of natural afwillite. We prepared six X-ray diffractometer charts of aatural afwillite and calculated the relative intensities. The results are shown in Table 111. A comparison of Table 111 with Table I1 shows ,hat the relative intensities of the four strongest (16) G. Switzer and E. H. Bailey, Am. Mineralogist. 38, 629 (1953).
(17) Imperial Chemical Industries, Norwioli. (18) H. F. McMurdie and E. P. Flint, J . Research N a l l . Bur. ’andards, 31, 237 11843).
A
115
BALLMILL TABLE 111
OF NATURAL AFWILLITE LINES RELATIVE INTENSITIES
Chart no.
1 2
3 4 5 6 Av. Standard error of mean, %
6.61 A.
Afwillite lines 3.18 A. 2.83 %.
2.73
k.
65.1
100 100 100 100 100 100 100
69.0 78.7 79.1 56.5 79.3 51.6 69.0
2.7
..
5.0
48.2 57.3 41.1 46.1 47.0 41.3 46.8
59.9 61.7 60.3 67.9 77.4 63.5
2.4
lines of natural and artificial afwillite are in very good agreement. On the basis of these results we conclude that there is no difference, detectable by the X-ray techniques employed by us, between the natural mineral afwillite and the artificial afwillite obtained in the ball-mill hydration of Ca3SiOs. Megaw l9 recently determined the crystal structure of afwillite and showed that the formula Cas(Si030H)z.2Hz0 best represented its structure. From the structure data she calculated the density of afwillite to be 2.643 i 0.005 g./cc. She used natural afwillite from the Dutoitspan mine for the structure determination. We determined the density of our afwillite, obtained from ball-millhydrated Ca3Si06 by hot-extraction of calcium hydroxide. The experimental density was 2.642 f0.01 g./cc., the density corrected for impurities (on additive basis) was 2.647 f 0.01 g./cc. For the indices of refraction of afwillite from Crestmore, California, Switzer and BaileylB reported the values (Y = 1.616, p = 1.619, y = 1.631. The average index of refraction of our sample of Dutoitspan afwillite was 1.62 i 0.01. The average index of refraction of our artificial colloidal afwillite was also 1.62 f 0.01. The specific surface area of afwillite obtained from the ball-mill hydration of Ca3SiOswas found to be 84 f 4 m.2/g. by the B.E.T. method.’O It was determined both by nitrogen adsorption a t the boiling point of nitrogen and by water vapor adsozption a t 25”. Using the customary value of 16.2 A.2 for the molecular area of nitrogen, we obtain identical surface area with water vapor, if we agsume that the latter had a molecular area of 11.8 A.2; a reasonable molecular area. If the afwillite particles were spherical and all the same size, the surface area would indicate a particle diameter of about 285 & Electron micrographs, obtained by Swerdlow, McMurdie and Heckman20from our artificial afwillite showed that the particles were roughly gpherical and had dimensions of the order of 250 A. Differential thermal analysis results were .reported by Moody21for natural afwillite from Kimberly. She stated that the chart “shows an endothermic reaction proceeding in several steps in the temperature range 250-450°, the most marked re(19) H. D. Megaw, Acto Cryst., 6, 477 (1952). (20) M. Swerdlow, H. F. McMurdie and F. A. Heckman, “Proc. of the International Conference on Electron Microscopy,” London, 1954: J . R o y . Micro. SOC.(in press). (21) K. M. Moody, Mineral. Mag., 29, 838 (1952).
STEPHEN BRUNAUER, L. E. COPELAND AND R. H. BRAW
116
VOl. 60
action occurring a t approximately 370”, and an afwillite had colloidal dimensions, whereas the q e exothermic reaction which takes place at about cific surface area of natural afwillite waa negligible 820”. It seems clear that these correspond to the compared with that of our art%cial afwillite. dehydration process and to the formation of ranAcknowledgments.-We wish to express our kinite, respectively.” great indebtedness to Dr. S. A. Greenberg for the The main features of the differentialthermal anal- thermobalance and D.T.A. measurements; to Dr. ysis curve of our artificial afwillite were the same D. L. Kantro for preparing most of the materials as those of natural afwillite. There were endother- used by us, for the density determinations and for mic minima a t 240,320 and 470’, which, doubtless, a part of the surface-area measurements; to Mr. correspond to the steps reported by Moody for the E. E. Pressler for all chemical a n a l y ~and ~ extractemperature range 250450”. The slight shifts in tions; to Dr. L. S. Brown for the index of refraction t.emperature are not significant; they may be caused determinations; to Miss Edith Turtle for a part of by differences in techniques. The minima at 240 the surface area measurements; and to Mr. T. C. and 320” probably correspond to the loss of the two Powers and Dr. H. H. Steinour for many helpful types of water in afwillite, Csa(SiO~OH)~~2.€€~0. discussions and suggestions. The most marked reaction occurs at 320’. There is We also wish to express our sincere appreciation a strong exothermic peak at 820°, which may pos- to Dr. A. B. Cummins,for permitting and encouragsibly be due to the formation of wollastonite, & ing the cooperation between the Johns-Manville C&iOa, rather than rankinite, Cdi20,. In addi- Research Center and the Portland Cement h c i tion to the peaks reported by Moody, Greenberg ob- ation Research and Development Laboratories, tained an endothermic peak at 125”, which may be and to Drs. hmington Kellogg, W. F. Foshag, and due to the loss of adsorbed water. The absence of George Switzer for the donation of a sample of the the peak in natural afwillite may be explained on natural mineral afwillite by the Smithsonian Instithe basis of difference in surface area; our artiiicial tution to the Portland Cement kssociation.
THE STOICHIOMETRY OF THE HYDRATION OF TRICALCIUM SILICATE AT ROOM TEMPERATURE. 11. HYDRATION IN PASTE FORM BY STEPHEN BRUNATJER, L. E. COPELAND AND R. H. BRAW Portland Cement Asscwidh Reseurch and Devetqpmat Lcrbomtones . Chicago, ZUi& Rtueived July l l . l S 6 6
Evidence is presented that the stoichiometry of the hydration of tricalcium silicate,. Ca&3i06, in the form of hardened paste in a saturated calcium hydroxide solution at 23” n p y be represented by the equahon 2CarSi06 S a 0 = Ca&3ii07. 3Ha0 3Ca(OH)s. An initial H20/Ca&3iOs weight ratio of 0.7 was used, and the pas+ were hydrated for 2 to 2.5 years. X-Ray and microscopic examinations indicated almost complete hydration. The calcium silicate hydrate produced waa similar to the natural mineral tobermorite; ita density was 2.44 f 0.01 g./cc., and ita aversge index of refraction waa 1.56 f 0.015. Bernal proposed the structural formula C&[S~O~(OH),]I[C~(OH)~] for the hydrate. The srtificial tobermorite was colloidal in dimensi ns; nitrogen adsorption, X-ray h e broadening and electron micrographs indicated on average dimension of about 100 About 15% of the calcium hydroxide was adsorbed on the tobermorite surface; the rest of the calcium hydroxide appear‘ed in the form of relatively large.crystsls, visible under the micrcacope.
+
+
1
Introduction The stoichiometry of the hydration of tricalcium silicate, CaaSiOs, in a steel ball mill in a saturated calcium hydroxide solution at 23” was discussed in the previous paper.l -.The present paper discusses the stoichiometry of the hydration of C d i O 6in the f0r.m of “paste.” The term “paste” as used by cement chemists, and as used here, means a plastic or semi-fluid mixture of a hydraulic material, such as portland cement, CaoSiOr or Ca&iOr, with water. After a few hours the paste “sets” and then hardens; the present investigations were carried out on hardened pastes. Experimental
water-to-CstSiOs weight ratio waa 0.7. The pastes repared from CarSi06(I)were hydrated for 21 months; $os, repared from C@iOs(II) were hydrated for 30 months. !he hydration was conducted in a mom kept at 23 f 0.5” The uncombined water was removed by the method described by Brunauer. Hayes and Hass.’ The calcium hydroxide, Ca(OH)dIII), used in the resent experiments was merent from those described Lfore.1 Calcium oxide was prepared from “Baker analyzed” reagent grade CaCQ, low in +lies. by ippition at 900 to 950” for 16 hours. The hydrabon waa carned out m a polyethylene bottle, by letting the CaO stand in water at 2 5 O for three months and shaking the bottle once or twice a day.
Bo, 112 (1958). (2) S. Brunaiier. J. C. Hay- and W. E. Tlaw. itid.. 68, 279 (1954).
(3) J. D. Bernal, “Proceedings of the Third. International SYNposium on the Chemistry of Cement,” London. 1952. p. 216.
Results and Discussion X-Ray ditrractometer charts of the hydrated C&i06 pastes showed the presence of calcium hydroxide and tobermorite lines only. The calcium The experimental techniques employed were in most re- silicate hydrates, designated as tobermorites, vary spects identical with those deacribed before.’ Only a few in molar CaO/SiOz ratio from 1.0 (or 0.8) to 1.5. additional remarks are needed here. Pastes of CarSiOs were repared by the method described In a saturated calcium hydroxide solution the by Brunauer, Hayes and &as~,s except that the pastes were compound with the highest lime-bsilica ratio is cast as solid cylinders, without a hole in the center. The obtained.’ This compound has the formula ChSi20,-3Hz0according to Bernal,’ and the formula (1) 8. Brunauer, L. E. Copeland and R. H. Bra=. TEIEJOURNAL.
