INDUSTRIAL A N D ENGINEERING CHEMISTRY
October, 1924
1063
Solubility of Portland Cement in Weathering Agents' By H. I(.Benson, J. S. Herrick, and T. Matsumoto UNIVERSITY OF WASHINGTON, SEATTLE, WASH.
H E N hardened cement and concrete are exposed to moist corrosive gases disintegration takes place. Numerous cases of this type have been studied,2 as has also the action of various salt solutions as decomposing agents.3 The disintegration of concrete when subjected to the usual weathering agents has also been a matter of discussion. It seemed desirable, therefore, to make an experimental study of the solubility of both fresh and hardened cement in water under varying conditions as a means of ascertaining the relationship between solubility and hydrolysis. The action of the commonly found impurities in water, such as carbon dioxide and dissolved organic matter, on hardened cement has also been studied.
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THEORETICAL CONSIDERATIONS According to Klein,4 the hydrolysis of tricalcium silicate proceeds in the following manner: 3CaO.SiOz 3.5H20 = CaO.Si02.2.5H20 2Ca(OH)z The quantity of tricalcium silicate in the average Portland cement is stated by Rankins to vary between 30 and 35 per cent. If this is totally hydrolyzed, as indicated by the equation, 0.2269 gram of calcium hydroxide per gram of cement will be formed. To dissolve this much calcium hydroxide will require 142.7 CC. of water a t 25" C. The hydration of dicalcium silicate is difficult to effect, as s h o w by Klein and Phillips.6 However, if it proceeds as in tricalcium silicate, we have 2CaO.SiOz 3.5H20 = CaO.Si02.2.5H20 4-Ca(0H)z It is estimated by Rankins that 90 per cent of Portland cement consists of tricalcium silicate, dicalcium silicate, and tricalcium aluminate. Hence, 55 per cent consists of the two latter components, and since the average cement contains 7 per cent alumina, the tricalcium aluminate content possible is 18.5 per cent. This leaves 36.5 per cent of dicalcium silicate. If the latter completely hydrolyzes in accordance with the foregoing equation, it will liberate 0.1568 gram of calcium hydroxide per gram of cement, for the solution of which 100 cc. of water a t 25" C. will be required. The further hydrolysis of calcium monosilicate would yield silicic acid and additional calcium hydroxide, but Klein and Phillipss have shown that this is very unlikely and that the calcium monosilicate is practically inert in water. Making the theoretical assumption that the average Portland cement contains 35 per cent tricalcium silicate and 36.5 per cent dicalcium silicate, we should obtain in the hydrolysis of 1 gram of cement, 0.384 gram of calcium hydroxide, which would require 242.7 cc. of water at 25" C. for complete solution. ANHYDROUS PORTLAND CEMENT
+
+
obtained from a local dealer were placed in glass containers, distilled water added, shaken on a mechanical shaker, and the clear supernatant liquid analyzed. EXPERIMENT I. AMOUNT OF SOLIDMATTER DISSOLVED IN A GIVENTIME OF AGITATIONTotal hours shaken 3.25 6.00 7.00 10.00
Presented before the Divison of Industrial and Engineering Chemistry at the 67th Meeting of the American Chemical Society, Washington, D. C . , April 21 to 26, 1924. 2 PYOC. P a c . N . W . SOC.Engrs., 15, 4 (1916). a Proc. A m . SOC.Ctr. Eng., 19, 1038 (1923). 4 T r a n s . Faraday SOC.,14, 22 (1919). 6 I b i d . , 14, 23 (1919). 8 Bur. Slandards, Tech. P a p e r 49 (1914). 1
Total hours Per cent of original settling sample in liquid 45.24 14.0 22.00 14.0 40.00 14.3 15.85 13.2
1-The results given are the average of four samples. 2-Temperatures ranged from 22.5' to 25O C. 3-Amount of sample, 2.5 grams. 4-Amount of distilled water, 250 cc.
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The results of this experiment demonstrated that the time of agitation did not materially affect the amount of material dissolved. EXPERIMENT 11. EFFECTOF DIMINISHING THE AMOUNT OF WATERON THE AMOUNT OF SOLIDMATTERDISSOLVEDAmount of distilled , water cc.
Notes:
Appearance of liquid
Per cent of original sample in liquid
1-The results given are the average of two samples. 2-Temperatures ranged from about 25' to 25.5' C. 3-Amount of sample in each case, 2.5 grams. 4-All samples were shaken for 4-hour periods.
+
A number of experiments were conducted on the solubility of cement in distilled water. Weighed quantities of cement
Appearance of liquid Clear Clear Clear Clear
The curve (Fig. 1) plotted from these results is parallel to the solubility curve of calcium hydroxide although it is not identical with it. N
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FIG S SOLUBILITY CURVESOF CALCIUM RYDROXIDE A N D PORTLAND CEMENT
EXPERIMENT 111. EFFECT OF TREATING THE SAME SAMPLE CEMENTPROGRESSIVELY WITH SEPARATE PORTIONS OF WATER.-The original sample of 2.5 grams of cement was treated with 250 cc. of distilled water, and the material left after shaking for 4 hours was separated by decantation and filtration on a Bcichner funnel, the material being washed back into the flask with the fresh portion of 250 cc. of distilled water. A decreasing amount of material was found in solution after OF
INDUSTRIAL AND ENGINEEBING CHEMISTRY
1064
each treatment. The solution from this experiment was saved in clean, dry bottles. I n all cases amorphous bodies were observed, but all sett'led out on standing. Total amount of distilled water c c. 260 500 7 50 1000 1250 1500 1750
Notes:
Appearance of liquid Clear Clear Clear Clear Clear Clear Clear
Per cent of original sample in liquid 14.00 10.90 5.64 3.70 3.70 2.84 2.53
1-The results given are the average of four samples 2-Temperatures ranged from 25" t o 26O C. 3-All samples were shaken for 4-hour periods,
The results obtained in Experiments I and 11 are plotted in Fig. 2.
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nents is shown by this same curve, which does not flatten out until after 43.2 per cent of the original sample has been dissolved, and which is more than the total amount of calcium hydroxide predicted from the complete hydrolysis. It should be noted, however, that the soluble portion in this prolonged treatment contains also silica and alumina, thus indicating minor hydrolysis of the hydrated silicates and aluminates.
HYDRATED PORTLAND CEMENT Solubility studies were made on hardened cement pats obtained from the laboratory of the Engineering Department of the City of Seattle. These pats were of local brands of cement and varied in age from two months to two and onehalf years. The composition of the latter was as follows: Moisture a t 110' C . . ................................. Ignition loss......................................... Lime, CaO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron and alumina.. ................................... Silica.SiOr ..........................................
Per cent 1.53 13.98 53.12 10.71 18.39
EXPERIMEST I. EFFECTOF TREATING THE SAMESAMPLE OF HYDRATED CEMENT PROGRESSIVELY WITH SEPAR-4TE PORTIONS OF WATER-The sample was ground to the fineness of
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FIG.2-SOLUBILITY
1000 /200 /9w /e1800 D/sh//cd Wufer Used
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OF P O R T L A N D CEMSNT I N S U C C E S S I V E Q U A N T I T I E S OF WATER
ANALYTICAL DATA-The solution obtained in Experiment I was evaporated to dryness and the resulting dry salt, which was nearly pure white in color, was analyzed. The results show this sample to be almost pure hydrated lime. The analysis of the electrolyte in the solution obtained from Experiment I11 gave result's that show a,preponderance of lime in solution. Yo doubt the lime became slightly carbonated during the treatment of the samples and the evaporation of the solution. The analysis of material dissolved by shaking cement with distilled water in Experiment I is as follows: Per cent Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.00 0,70 Iron and alumina.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.86 Lime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lime expressed as hydrated lime, Ca(0Hjz.. . . . . . . . . . . . 100.20
The analysis of material in solution obtained from the combined sample saved in Experiment I11 is as follows: Silica,SiOn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.....
fresh cement and was treated as in the case of anhydrous cement. A 2.5-gram sample shaken for 4 hours in 250 cc. distilled water a t 25' C. dissolved to the extent of 13.3 per cent. For the purpose of ascertaining the effect of progressive treatment with water, 1-gram samples were placed in 100 cc. of water and shaken for 3 hours, twelve glass beads having been added to prevent a tendency to cake. The results of ten such treatments on one sample are as follows: No. of treatment 1 2 3 4
Per cent of sample dissolved 18.8 12.6
5
.6 7
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3.1 3.6 3.7 3.6 3.0 3.6 3.3
The composition of the total solids from the combined filtrates was as follows: Silica,SiOz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron and alumina.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lime, CaO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Per cent 0.18 4.42 65.99
EXPERIMEST 11. SOLUBILITY OF HARDENED CEMEKTIN WATER-Varying amounts of the sample were placed in constant volumes of water, agitated with air for 15 minutes, settled, and the solutions analyzed for lime, CaO. Weight of sample Grams 15.0
Ca(0H)z in 10 cc. of solution Grams 0.1342
Per cent of original sample in liquid as Ca(0H)z 1.18
Per cent 0.97 1.90
Lime expressed as hydrated lime, C a ( 0 H ) z . . . . . . . . . . . . . . 78.00
DISCUSSION OF EXPERIMENTAL RESULTS-In the discussion of the theoretica.1factors of hydrolysis, it was noted that 0.227 gram of lime, as Ca(OH)g, would result from the hydrolysis of t'he tricalcium silicate in 1 gram of cement. This would require 355 cc. of water to dissolve the amount found in 2.5 grams. By consulting the curve in Fig. 2 it is found that 355 cc. af water have dissolved approximately 21 per cent of the original sample, which is equivalent to 0.210 gram of calcium hydroxide per gram of cement. This close agreement of experimental and theoretical values would indicate that tricalcium silicate is extremely unstable in the presence of water. That the hydrolysis slowly proceeds with the other compo-
From the curve (Fig. 3) it is noted that the solubility reaches a constant a t the point where the lime content is approximately 0.133 gram, equivalent to 0.177 gram Ca(OHj2, in 100 cc. of water, which is also the limit of solubility for calcium hydroxide. At this point 1.4 per cent of the sample goes into solution, while at lower concentrations the hardened cement is much more soluble. OF HARDENED CEMENTIN EXPERIXENT 111. SOLUBILITY VARIOUSSoLvENTs-Since it was not possible to establish a comparable working basis with the different amounts of sample taken, 1gram of hardened cement from test pats about 2 months old was selected as a basis and treated with varying amounts of solvent. I n this way it was possible to find the
INDUSTRIAL AND ENG NEERING CHEMISTRY
October, 1924
total amount of solvent required to dissolve out all the lime in the cement. The solutions were distilled water, a saturated solution of carbon dioxide in water, 5 per cent sugar solution, a Douglas fir bark solution, and Douglas fir needles solution.
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FIG 3-SOLUBILITY
OF
HYDRATED CEMBNT
The bark solution was analyzed and the amount of tannin was estimated to be 2 per cent. The fir needles solution was prepared from young needles of fir growing on the campus. The solution showed marked acidity. By titrating with 0.01 N sodium hydroxide solution the equivalent acidity was found to be 0.0045 N . The difficulty of treating large v01umes of the solvents was overcome by taking a fraction of a gram of the sample in 200 cc., calculated proportionately; for example, in place of 1 gram of 5000 cc., only 0.04 gram in 200 cc. was taken. i
OF PROGRESSIVE TREATMENT OF EXPERIMENT IV. EFFECT SAMPLE-The process of disintegration that takes place in nature is a continual effect by solvents. When a solvent attacks concrete, it is more or less saturated with cement. The saturated solution is then replaced by the fresh solvent, and thus again and again the concrete is attacked by the circulation of the solvent. In order to determine these effects, a progressive treatment of sample was run. One gram of sample was agitated with 100 cc. of solvent for 15 minutes by a rapid current of compressed air or that of carbon dioxide from the cylinder. The solution was filtered and the residue washed back into the original flask with a fresh 100 cc. of solvent. The process was repeated seven times. The solutions employed were distilled water, saturated solution of carbon dioxide, and the bark solution. The following results were obtained and plotted (Fig. 5 ) : SOLUBILITY OF CEMENTI N TERMS OF LIME (CaO) IN EACH100 Cc. PROGRESSIVE TREATMENT No. of, Saturated Bark progresswe COZ Distilled water solution treatments . . . . ~ ~ . ~ ~s o h 0,0884 1 0.0506 0.0354 0.0717 2 0,0442 0.0247 0.0618 3 0.0330 0.0207 0.0164 0,0526 4 0.0274 0.0484 5 0,0211 0.0150 0.0139 0.0412 6 0.0180 0.0136 0.0378 7 O Z TOTAL 0,2108 0.1397 0.4019
BY
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DISCUSSION OF ExRESULTS- o) The analysis of the vari- ; ous solutions indicates that the concentration of lime gradually decreases with the progres0s sive treatment, the total solubility in the case of ;" water being 0.1397 gram t o + CaO, equivalent t o 0.1844 gram Ca(OH)2or \ O3 18.44 per cent of the ; original sample. The '' solubility is naturally increased by the presence *' of carbon dioxide and oo soluble organic matter, 4 b 7 P r o y r c J J "e irrafn?rnt3 nearly all the lime havFIG ~ - L I M Z (CaO) OBTAINED FROM ing been removed in HYDRATEDCEMENT BY PROGRESSIVE soluble form from the TREATMeNTs original sample. Since the total amount of lime in the sample was 62 per cent, the percentage of disintegration of hardened cement may be computed by dividing the amount of lime dissolved by 0.62. The following results were obtained and are plotted (Fig. 6). Of
PERIMENTAL
