a study of the determination of the heat of hydration of ... · a study of the determination of the...
TRANSCRIPT
Journal of Research of the National Bureau of Standards Vol. 45, No. 5, November 1950 Research Paper 2152
A Study of the Determination of the Heat of Hydration ofPortland Cement
By Edwin S. Newman
Three relatively inexperienced operators made six determinations each of the heat ofhydration of a sample of portland cement. The results of their measurements were calculatedby the method described in Federal Specification SS-C-158b and by two simpler reduced-observation methods. By statistical analysis, no significant differences were found amongeither operators or methods. The precision (standard deviation of a single determination)of the heat-of-hydration measurements was about 3 calories per gram at 7 days and 2calories per gram at 28 days. Somewhat better precision can be expected from experiencedoperators.
I. IntroductionThe heat-of-solution procedure for determining
the heat of hydration of portland cement has beena part of the federal specification [I]1 for a numberof years. More recently, the method has beenaccepted by the American Society for TestingMaterials [2]. Excepting a paper by Shartis andNewman [3], there has been little published on itsprecision.
The method depends upon the determination ofthe heat of solution in acid of a cement and of ahardened paste made therefrom. The differencebetween these values is approximately the heatevolved during the preparation and hardening of thepaste and is very nearly equal to the true heat ofhydration of the cement.2 Auxiliary determinationsof the ignition losses of the cement and of the hard-ened paste are required to determine the quantityof ignited cement represented by a weighed amountof the hardened paste. Heats of solution andhydration are given in this paper on the basis of theweight of ignited cement.
For low apparatus cost and for simplicity in opera-tion, the calorimeter chosen for the routine accept-ance testing of cement [4] consisted of a 1 -pintvacuum flask and a Beckmann thermometer with areading lens, together with a motor-driven stirrer.This calorimeter was to he operated in an ordinarylaboratory with only superficial protection fromchanges in the ambient temperature. The difficul-ties that exist in mak ing de te rmina t ions with thisc a l o r i m e t e r a r i s e i n l a r g e p a r t f r o m t h e f a c t t h a it h e r e i s i n t h e c o r k a n d t h e g l a s s o f t h e flask c o n -siderable material with a. low thermal conductivityand a, relatively large heat capacity. This materialis at an indeterminate temperature between that, ofthe room and that within i\)c calorimeter , and whenthe liquid t empera tu re is suddenly changed severaldegrees, as in a hea t-of-solul ion de te rmina t ion , thelag caused by its presence in the path of heat Howi n t e r f e r e s w i t h t h e d e t e r m i n a t i o n o f t h e a m o u n t o f
1 Figures in brackets Indicate the literature references :ii 11»' end <>i this paper," i h r heal of hydration defined us the difference between the two heats of
voii11 inn ignores the heal of solution of the water contained in the paste, amount-ing in approximate^ 0.1 cal/g <>r cement. The effect of the absorptl >f COjduring i in' grinding <>i i he paste is also Ignored.
heat lost to or gained from the surroundings. Thisinterference has led to some doubt that the applica-tion of methods of precise calorimetry embodied inthe specification procedure is entirely valid.
The original method for the heat-of-solution de-termination was somewhat oversimplified [4]. Theprecision necessary in this test is somewhat greaterthan appears at first glance. It is required to de-termine the heat of solution of a dry cement, amonut-ing to approximately 600 cal/g, and the heat of solu-tion of the corresponding hardened paste, 520 to 550cal/g, with such precision that their difference, theheat of hydration of the cement, can be consideredreliable to a few calories per gram. If a precision(standard deviation) of 2.0 cal/g, not an impressivevalue, is desired in the heat-of-hydration determina-tion, a precision of 1.4 cal/g (2.0/V2) is required ineach of the individual heat-of-solution tests. Thisamounts to a precision of approximately 0.25 percent,a value not easily reached consistently in routinecalorimetry with relatively crude apparatus.
In an effort to improve the precision, and partlyas a result of work at the Bureau [3], the originalmethod was revised [1], primarily to allow more timefor the calorimeter to establish constant operatingcondit ions dur ing the test and also to introduce acorrection allowing for the time required to dissolvethe sample. The effect of the revision was to sub-stantial ly extend the time required for the tes t andto demand somewhat more attention by the operator.The procedure requires the recording of t ime-tempera tu re data during I wo rating periods, onebefore and one after the period dur ing which thesample is being dissolved. The t empera tu re of thesurroundings is assumed constant , and the thermal-leakage constant is calculated from these data, takenwhen the rales of I emperat lire change of the calorim-eter depend only on the flo\v of heat to or from theroom and on the energy of s t irr ing. Besides thereadings at 5-min intervals required by the originalm e t h o d , f o u r a d d i t i o n a l r e a d i n g s a r e t a k e n a t l - m i ni n t e r v a l s d u r i n g t h e f i r s t p a r t o f t h e s o l u t i o n p e r i o d .T h e s e d a t a p e r m i t t h e a p p r o x i m a t e c o r r e c t i o n o f t h eo b s e r v e d I e m p e r a I l i r e r i s e o f t h e c a l o r i m e t e r f o r t h ee n e r e v r e c e i v e d f r o m t h e s u r r o u n d i n g s a n d f o r t h e
411
energy of stirring and evaporation. These detailsare in accordance with precision calorimetry in whichan isothermal jacket is employed.
It has been suggested that some of these revisionscould be omitted without seriously affecting the valueof the calorimetric test. From an examination ofroutine test data, it appeared to the author that ex-cluding all the observations taken during the solutionperiod from the calculations would cause a systematicerror of possibly + 1 cal/g in the value for the heat ofhydration. The precision of 2.0 cal/g suggested in anearlier paragraph implies that one-third of the testswill differ by more than 2 cal/g from the true (un-known) value. In other words, with an acceptancevalue of 80 cal/g, one-third of the samples with an ac-tual heat of hydration of 82 cal/g will be accepted, andone-third of the samples with a heat of hydration of78 cal/g will be rejected. Only when the true heatof hydration is below 75 cal/g will the cement bealmost certain to pass the acceptance test. Onlywhen the true value is above 85 cal/g will the cementbe almost certain to be rejected. In the light of thisuncertainty it would appear that a systematic errorof + 1 cal/g could be permitted. The test could then"be considerably simplified, although the time requiredfor its performance could not safely be shortened.
If this apparent systematic error can be tolerated,it is possible to eliminate the preliminary 5-minrating period (distinct from the preliminary 20-minstirring period), to reduce the number of observations,and to avoid much of the calculation. In this sug-gested procedure only three readings of time andtemperature are required, one at the end of thepreliminary stirring period just before the introduc-tion of the sample, one at the end of the solutionperiod, and one subsequently to establish the finalrate of temperature change. If is necessary to extendthe solution period sufficiently to insure the calorime-ter's return to a steady state, after complete solutionof the sample, before the second temperature isrecorded. The time between the second and thirdreadings of the temperature is the ra t ing period. Ifthe solution and rat ing periods arc of the samelength, the calculation of the corrected temperatureis simple. T h e difference between the first, tworeadings is the uneorrecled rise. T h e differencebetween the second and third readings is the correc-tion to he added or subtracted according to whetherthe calorimeter t empera tu re falls or rises during therating period, [f the two periods differ in length, theobserved correction is adjusted to correspond to thedura t ion of the solution period. During the intervalbetween readings, the opera tor ' s lime can be utilizedto operate a second calorimeter, or for any o therpurpose not requiring undivided attention. T h ereduced-observation method was used in the investi-gation reported by Sharls is and Newman [3]. T h eI empera I lire of the ca Ion in el er was recorded a I 0, '20,and 40 min for the determinations presented in theirp a p e r . I t is t h i s a u t h o r ' s b e l i e f t h a t 2 0 m i n m a ys o m e t i m e s b e l o o s h o r t f o r a s o l u t i o n p e r i o d , a n d it, is
suggested that the reduced-observation method beperformed by taking readings at the end of the pre-liminary stirring period and twice thereafter atintervals of 25 min. A comparison of reduced-observation methods with the specification method ofperforming the heat-of-hydration test is a purpose ofthis paper.
II. Materials, Apparatus, and Procedure
A calorimeter meeting the requirements of theFederal Specification [1] was used. Three operatorswere available: A, with a moderate amount of experi-ence; B, with a small amount; and C, with a fewdays. Approximately 50 liters of 2.00 JV HN03were prepared and standarized. This quantity wassufficient for the entire series of tests, and therebyuncertainties were avoided that might be introducedby the use of different batches of acid.
Three kilograms of Type 2 cement (moderate heatof hardening) were mixed for several hours in alaboratory ball mill containing a few pebbles, dividedinto three portions, and stored in tightly closedMason jars. The portions were numbered 1, 2, and3, and each portion was assigned to an operator.Each operator mixed a paste from each portionaccording to the procedure described in the FederalSpecification [1].
The three operators each determined the heatcapacity [1] of the calorimeter three times. Inaddition, during the time before the 7-day heat-of-solution test, each operator made a single determina-tion of the heat of solution of each of the three por-tions of dry cement. At 7 days and again at 28 days,each operator determined the heat of solution of thethree hydra ted pastes made from the one portion ofcement assigned to him. Thus each operator deter-mined the heat of solution of paste samples preparedfrom one portion of cement by himself and by eachof the other operators. This entire series of testswas repeated, so that each determination was madetwice. The entire elapsed time during the testswas about C> weeks.
III. Results and Discussion
The results of the heat-capaci ty and heat-of-solution tests are given in table 1, calculated asdescribed in the specification | 1 | . In addi t ion, thecalorimetric observat ions were used in calculat ingthe lest results by two reduced-observa t ion methods.Table I also shows these values as calculated fromtempera ture readings taken at 0, '20, and 40 min andat 0, 25; and 50 min, respectively. Throughoutthe tables and the paper these reduced-observat ionmethods w ill be called 0 20 40 and 0 25-50 methods.The heal capacit ies appearing in the table and usedin calculation of the heals of solution were calculatedin the same manner as (he hea l-of-soliit ion values forwhich they were used.
412
TABLE 1. Heat capacity of the calorimeter and heats of solution and hydration of a sample of portland cementIThe average heat capacity of the three values determined by each operator during a round was used to calculate that operator's heats of solution for that round]
Pastemixed
by
Calorim-eter
operator
Specification method
Heatcapac-
ity
Heat of solution
Drycement
7-daypaste
28-daypaste
Heat ofhydration d
7-day 28-day
0-20-40 method
Heatcapac-
ity
Heat of solution
Drycement
7-daypaste
28-daypaste
Heat ofhydration •*
7-day 28-day
0-25-50 method
Heatcapac-
ity
Heat of solution
Drycement
7-daypaste
28-daypaste
Heat ofhydration d
7-day d a y
a
b
c
(a. . .b
lc
(aJb.::::: : :lcfa*
[c
cal/degC394. 3395.9396.9
393.6395.1398.4
396.5396.9395.2
cal/g598.0594.9596.2
599.7596.4596.0
598.3596. 8598.9
cal/g533. 9539.8541.8
536. 4540.7537.7
538.2539.1542.2
cal/g522.0518.9519.3
519.1518.9520.3
519.1519.5523.5
cal/g64.155.154.4
63.355.568.3
60.157.756.7
cal/g76.076.076.9
80.677.577.7
79.277.375.4
ROUND
cal/degC394.3394. 3396.2
394.1394. 5394.8
395.6396.2394.7
cal/g598.1594.6595.0
600.6596.1594.8
599.4596. 5597.7
1
cal/g534.9539.6540.7
537.4540. 7536.6
539. 2539.0541.1
cal/g523.0518.7518.2
520.0518.7519.2
520.0519.4522.3
cal/g63.255.054.3
63.255.458.2
60.257.556.6
cal/g75.175.976.8
80.677.475.6
79.477.175.4
cal/degC393.5395.0396.4
394.1395.1394.8
396.5396.9394.7
cal/g598.9595.4595.6
600.6596.0595.1
599.4597.3597. 8
cal/g534.9538.7540.9
538.5541.9535.8
539.2539.7541.3
cal/g523. 0519.8517.3
520.0519.2519.3
518.8520.6522.6
cal/g64.056.754.7
62.154.159.3
60.257.656.5
cal/g75.975.678.3
80.676.875.8
80.676.775.2
a.
b
c
fab
[c
fab
lc
fa . . . .\blc
395. 3395. 0393.0
394. 5395.2395.3
395.0396. 3394.8
595.8599.9597.7
596.3595.1597.3
599.2598. 4598.5
536.8539. 0536.2
535. 2535.7536.0
540.7538. 3533. 5
517.3515. 6515.6
521.9520.5519.2
518.2518.1517.7
59.060.961.5
61.159.461.3
58.560.165.0
78.584.382.1
74.474.678.1
81.080.380.8
ROUND
394.9394.7394.6
394.1395. 4395.5
394.6394. 6393.1
596.0599.6600.6
596.7596.6598. 5
595.7598. 3599.6
2
537.1538.7535.5
537.5537.1536.7
542.3538. 5533.9
516.9514.9517.6
521.6520.1519.3
519.3517. 8517. 5
58.960.965.1
59.259.561.8
53.459.865.7
79.184.783.0
75.176.579.2
76.480.582.1
394.1394.9394. 6
394.9395.9394.8
394.6394.7393.1
596.0599.8600.3
598.3596.0598.2
599.1598. 6599.3
537.1539.1536.0
533.3536. 2536.5
541.0538.4535.5
516.9515. 6517.3
521.6520.2519.0
518.3518.3517.0
58.960.764.3
65.059.861.7
58.160.263.8
79.184.283.0
76.775.879.2
80.880.382.3
" Eeal of solution calculated according to Federal Specification SS-C-158b.f> Heat of solution calculated from the temperature of the calorimeter at zero time, 20 min, and 40 min.• Heat of solution calculated from the temperature of the calorimeter at zero time, 25 min, and 50 min.d Calculated from differences in the heats of solution of corresponding dry cements and hydrated pastes.
The data were subjected to a complete analysis ofvariance. Information was sought as to the re-prod ucibility of the methods of test, the variationamong the operators, and the deviations introducedby the reduced-observation methods.
The uncertainty of an individual determinat ion ofthe h e a t of solution is shown in table 2. There isevidence [3] to suggest that the hea t of solution ofdry cement changes when the container is openedeven briefly. To eliminate the possible influence ofthis phenomenon on the correlation of the heats ofsolution of the hydrated pastes, the data werearranged in groups of three in the order shown intable 1. Each member of a set of three was deter-mined on pastes mixed on a single day. From thedata of table 1 the standard deviations of individuald e t e r m i n a t i o n s of t h e h e a t s of s o l u t i o n of t h e d r yc e m e n t a r i d o f t h e 7 - a n d 2 8 - d a y h y d r a t e s w e r ec a l c u l a t e d a n d a r e s h o w n in t a b l e 2. It wi l l b e n o t e dt h a t t h e u n c e r t a i n t y of t h e d e t e r m i n a t i o n m a d e w i t ht h e 7 - d a y h y d r a t e d p a s t e is d i s t i n c t l y l a r g e r t h a ne i t h e r o f t h e o t h e r s . T h i s is p o s s i b l y d u e t o a m o r er a p i d g a i n of C O 2 f r o m t h e a t m o s p h e r e a n d l o s s o fw a t e r t h r o u g h e v a p o r a t i o n b y t h i s p a s t e d u r i n gerindine.
TABLE 2. Reproducibility of heat-of-solution tests
Method
Specification l)
0 20-40 «0-25-50 0
Standard deviation " of an indi-vidual test
Drycement
cal/g1.51.11.4
7-daypaste
callg2.72. 52.3
28-daypaste
cal/g1.41.51.6
Standard devial Ion, a,
w h e r e I he s u n in nil ion (1) t o (6) r e f e r s t o m i x e s b y A , B , a n d C in r o u n d s I a n d '_',.a n d t h e s u m m a t i o n i t o :i refers t o t h e I n d i v i d u a l d e t e r m i n a t i o n s w i t h i n a m i x
by A, B, etc. .v is the value of the single <iciciniin.ilion, ami .v is the averagevalue of i he group of three.
11 i leal of solui ion calculated by i lie method described in Federal Specifications s c L58b.
"These are reduced observation methods of calculating the heat of solutionf r o m r e a d i n g s t a k e n a t z e r o I l i n e , 2 0 m i n , a n d II) m i l l , m i d a t z e r o I i m e . 2 6 m i n ,and 50 min, respectively.
413
From the standard deviations of the heat-of-solution determinations in table 2, a predicted valuecan be calculated for the uncertainty of the heat ofhydration. In table 3 are shown predicted valuesand actual values of the standard deviation of theheat of hydration. The predicted and the observedvalues are generally in good agreement. An excep-tion is to be noted in the observed value for 7-daypastes calculated by the 0-20-40 method. Theprecision is somewhat less than that assumed earlierfor purposes of discussion. It is apparent from table2 that the variability of the 7-day determination isresponsible.
TABLE 3. Predicted and observed reproducibility of heat-of-hydration measurements of portland cement
Testedat—
7 days
28 days .
Standard deviation
Method a
[Specification•W-20-4010-25-50
(Specification{0-20-4010-25-50
Pre-dicted •>
callg3.12.72.7
2.11.92.1
Ob-served c
callg3.23.93.2
2.02.32.2
a The method of calculation of the heats of solution were according to theFederal Specification SS-C-158b: and from the temperature readings taken atzero time, 20 min, and 40 min; and at zero time, 25 min, and 50 min, respectively.
b Calculated from the values given in table 2 for the standard deviations of theindividual tests by the formula:
standard deviation (predicted) = -vVdry+o-^ydrated.c Calculated from the individual differences between the heats of solution of
the dry cements and the hydrated pastes given in table 1 by the formula:
3 _2 (X-X)*
tandard deviation (observed) ~ jj -
where the summation (1) to ((i) refers to mixes by A, B, and C in rounds 1 and 2and the summation 1 to 3 refers to the individual determinations within a mix.A' is the value for a single determination, and .V is the average of the mix.
The operators can he compared accurately only onthe results obtained with dry cement and duringcalibrations, as each operator had a different pastefor the tests with hydrated cement. In table 4 areshown the standard deviations calculated for theindividual operators arid for the test, when consider-ing all operators. To facilitate comparison, thecoefficients of variation have also been calculated.T h e beat-capacity determinations are slightly less
precise than the heat-of-solution measurements, asjudged from the coefficients of variation. Thedifferences shown in table 4 are insignificant, but itwas expected that any difference would be in favorof the calibrations. The zinc oxide, of analytical-reagent quality, is freshly heated before each de-termination, whereas the dry cement is subject toattack by whatever moisture and CO2 enters thecontainer during removal of test samples or betweentests. In table 5 are shown the averages of sixdeterminations by each operator on dry cement andon hydrated pastes at 7 and 28 days. There is, intables 4 and 5, no indication of significant differencesamong the operators either in bias or precision.The agreement among the results is as close as wouldbe expected if all the determinations had been madeby the same operator. This is true in spite of thedifferences in the operators' experience.
TABLE 4. Comparison of operator's precision
Operator
Standard deviation
Specifi-cation 0-20-40 0-25-50
Coefficient of variation
Specifi-cation 0-20-40 0-25-50
HEAT CAPACITY MEASUREMENTS
All
cal/deg C1.10.81.4
1.1
cal/deg C0.6
.81.0
0.8
cal/deg C1.20.9
.9
1.0
Of
0.28.20.35
.28
%0.15
.20
.25
.20
%0.30.23.23
HEAT OF SOLUTION OF DRY CEMENT
b
All
callg1.41.!)1.2
1.5
callq0.71.31.4
1.1
callg1.31.51.3
1.4
0. 23.32.20
.25
0.12.22.23
.20
0.22.25.22
.23
Standard deviation, a,
V 3 _ 3
where the summation 1 to 3 refers to determinations by the operator in a singleround, the subscript, refers to round 1 or round 2, A' indicates individual deter-minations, anand operator
'• Tneeoefflvalue of t he d
I A' indie lies the average value of the determination for the roundtidies ted.lent of variation is the ratio of the standard deviation to the averageterminations involved, expressed as percentage.
T h e s t a n d a r d d e v i a t i o n for a l l of t h e o p e r a t o r s is e q u a l t o
V-":
414
TABLE 5. Heat capacities and heats of solution and hydration
[Average of six determinations by each operator]
Operator
Method
Specifica-tion a 0-20-40 b 0-25-50 b
HEAT CAPACITY OF THE CALORIMETER
abc
RangeExpected range c
cal/deg C394.8395.8395.5
1.01.1
cal/deg C394.6395.0394.8
0.40.8
caljdeg C394.6395.4394.8
0.81.0
HEAT OF SOLUTION, DRY CEMENT
abc
RangeExpected range c
cal/g597.9596.9597.4
1.01.6
cal/g597.9597.0597.7
0.91.1
cal/g598. 7597.2597.7
1.51.4
HEAT OF SOLUTION, 7-DAY PASTE
abc
RangeExpected range c__
536.9538.8537.9
1.92.8
537.8538.9537.4
1.52.6
537.3539.1537.7
1.82.4
HEAT OF SOLUTION, 28-DAY PASTE
abc
Range . . .Expected range R . .
519.651H.fi519.3
1.01.5
520.1518.3519.0
1.81.6
519.8519.0518.8
1.61.7
HEAT OF HYDRATION AT 7 DAYS
abc
RangeExpected range c __
61.058.15 9 . r>
2.93. 3
59. 758.0(io. :i
1.74.0
61.458.260.0
3.23.3
HEAT OF HYDRATION AT 28 DAYS
abc
RangeKx peeled Hinge •
78.378.478. 5
0.22. 1
77.678.778.7
1. 12.4
79.078.279. 0
0.82.:s
«Calculated aooording to Federal Specification 8S C L58b.''These, are redueed-observat ion methods of calculating Hie heat Of Solution
from oalorimeter-temperature readings taken at zero lime, 20 min, and 40 mm,ami a t zero l ime, 25 mill, and ,ri() min.
ar Kxpeeted range for the average of six is equal to 2.634 , where a is Hie stand-V •'>
ard deviation of an Individual determination (given in table 2 for heal of solu-tion tests, in table t for heal capacities, and in fable \\ for observed heals ofhydration).
B y r e g r o u p i n g the d a t a in t a b l e I, the effect <>l'different operators as mixers can l>e tested. Thiswas done, and table 6 shows the results. Theaverages an* in good agreement, showing that thedifferences between mixers are not significant.
TABLE 6. Heats of solution of hydrated cement pastes
[Average of six specimens prepared by each mixer]
Mixer
HEATS OF
abc
RangeExpected range ".. .
Method
Specifica-tion a
SOLUTION
cal/g537.9537.0538.7
1.72.8
HEATS OF SOLUTION,
abc
RangeExpected range c-__
518.1520.5519.3
2.41.5
0-20-40 b 0-25-50 b
7-DAY PASTES
cal/g537.7537.4539.0
1.62.6
cal/ij537.8537.0539.3
2.32.4
28-DAY PASTES
518.2519.8519.4
1.61.6
518.3519.9519.3
1.61.7
» Heat of solution calculated according to Federal Specification SS-C-158b.t> Heat of solution calculated from calorimeter temperature readings made at
zero time, 20 min, and 40 min; and at zero time, 25 min, and 50 min; respectively.0 From table 5.
It was found unexpectedly that the heats ofsolution of the pastes determined during the secondround were lower than those determined during thefirst. For round one, cement was taken from thecontainers on July 7, 8, and 14, to mix the pastes.For round two, the dates were July 20, 21, and 22.The dry-cement determinations were made on July12, 13, and 21 for the first round and on July 22, 25,and 26 for the second. At these times the containerswere opened.
In table 7 are shown the results of calculationsmade on the tests grouped by rounds. In every case
TABLE 7. ffeats of solution
[Average for all operators for separate rounds]
Round 1Koillld 2Difference ( r o u n d
i round 2)Standard deviation
of difference b
t '
Method
Specification
Dryce-
menl,
cal/g597. 2597.6
-0.4
7-daypaste
cal/g538.'.»536.8
2. 1
0. Q•2.: ' .
daypaste
cal/g521). I518.2
1.9
0. 5•3.8
0-20-40
Dryce-
ment
.7.17. I5!I7. !l
. I2.1)
7-daypaste
cal/g
(I SL.9
28-daypaste
Clll/ll
•3.2
0-25-50
Dryce-
ment
cal/g597. 3MIS. I
- 1 . 1
o. 82. 2
7-daypasle
<•"//</
539. ()537.1
1.9
0.8•2. 1
28-daypaste
caljg520. 1518. 'J
I.I)
- Heals of solution calculated according to Federal Specification; from calori-meter temperatures ai zero time, 30 mil), and 40 min; and from ealorimelertemperatures ai zero time, 2.5 min, and 50 min; respectively.
' ' S t andard deviation of iiie difference Is equal to:
* 2 „ , „ f r o m t a b l e 2 .v i s
' I d i H e r e n e e d i v i d e d b y I l ie s i a m l a r d d e v i a t i o n o f t l ie d i f f e r e n c e . T h e r> l»'icent level of significance is 2.2. Differences exceeding this value are markedWit h an asterisk.
415
the average heat of solution of the hardened paste isless for the second round of tests than for the first.With one exception, this difference appears to bereal for each age and for each method of calculation.The average value of the heat of solution of the drycement was higher for the second round than for thefirst, the difference in one case approaching signifi-cance. This difference is in the wrong direction tobe the effect of CO2 and H2O, apparently the onlyavailable agents, and must be considered accidental.
Up to this point, the determination of the ignitionresidues of the cement and cement pastes has notbeen discussed. Every heat-of-solution determina-tion depends on a companion determination of theignition residue of the calorimeter sample. Whenthe hardened paste is removed from the sealed vialand ground, part of the water is lost, and CO2 isabsorbed from the atmosphere. The extent of thesechanges depends on the temperature, humidity, andCO2 content of the air as well as on the length oftime the sample is exposed during grinding. The lossof water affects mainly the ignited weight of thecalorimeter sample, but the absorption of CO2 resultsin a reduction in the heat of solution, as the heat ofsolution of CaCO3 is markedly less than that ofCa(OH)2. Carlson and Forbrich [5] have discussedthe drying and carbonation of the hydrated pastein considerable detail.
The ignition-residue data obtained in these testswere analyzed in the same manner as the heat-of-solution data. The average ratios of ignited residueto sample taken were 0.9807, 0.7218, and 0.7189 forthe dry cement, the 7-day paste, and the 28-daypaste, respectively. The standard deviation of asingle determination for the corresponding sampleswas 0.0008, 0.0050, and 0.0029. The theoreticalratio for the pastes mixed from this cement is 0.7005.Experiment lias shown that there is negligible lossof water from the pastes while stored in the sealedvials. I t is apparent that appreciable water waslost during mixing of grinding. The statisticalanalysis shows no significant differences among theoperators either in mixing the paste or in determiningthe ignition residue of the dry cement. There1 weredifferences, perhaps significant, among the valuesobtained by the operators on the hydrated pastes.The largest difference, however, was between the,average residue at 7 days found in the first roundand that found in the second. The average valueswere 0.7179 and 0.7256, respectively, and the /-value(see table 7, footnote c) was calculated to be 4.5.At, 28 days it was only 0.9. This indicates thatt h e r e w a s a. r e a l a n d s i g n i f i c a n t d i f f e r e n c e in t h egrinding of the pastes for rounds I and 2, althoughthere was perhaps none at 28 days.
W h e n t h e h e a t s o f s o l u t i o n o f t h e 7 - d a y s e c o n d -r o u n d h y d r a t e s w e r e b e i n g d e t e r m i n e d , t h e r o o mt empera 1 ure rose from .!()° to 34° (! during 1 he work-ing days. This occurred at a comparatively uniformra le OI about 0.5 deg C/hr. When the heats of solu-tion of the 7-day first-round hydra tes were beingdetermined, the daily temperature range was from27° to 30° C on the first two days and from 30° (<>
32° C on the third. During the determination ofthe heats of solution of the 28-day hydrates, theroom temperature varied between 24° and 26° C.The higher average temperature for the second-round hydrates probably caused greater loss of waterduring grinding and hence the higher values for theresidue on ignition. This difference in water lossand room temperature may have caused the lowerheats of solution of the 7-day pastes in the secondround, although no such difference in either roomtemperature or ignited residue occurred in the 28-day tests, in which the first-round heats of solutionalso exceeded those of the second round by a signifi-cant amount.
There appears to be no significant difference in thevalues obtained for the heats of solution whethercalculated by the specification method or from thesame data by either of the reduced-observationmethods. From an examination of the tables, itseems also clear that on the basis of precision there1
is no choice among the three methods of calculation.Either of the shorter methods of calculation shouldgive acceptable results. Because of the possibilitythat some cements may dissolve more slowly thanthat used in this study, there may be an advantage
TABLE 8. Heats of hydration of routine
Brand
A
B
1!
('
1)
Bin
l
l
2
1
1
Avcrago
A vn: i iT differencefrom specification
At 7 days
samples
At 28 days
Method «•
Specifi-cation
cal/g( 58{ 58I 58
5150525456
565963.r),H
6163
6059ill(it57(ill
62
(ii58605780
{ 586152625N57
58. 2
0-20-40
cal/g535453
5651545458
5661l i l
556164
585661)63605860
62.ri!t
611685859605S635969
58. 6
2.3
0-25-50
cal/g585858
5651545457
565962536063
596061635860
63
6260lil5x58.r,.H
6066635859
68. 8
1.2
Specifi-cation
cal/g787882
7366687276
72'4'9'56
18
"6• 5
'6"7"1• ( >
* ! )
"7• 3
-'.1"6"8'610"7• 8
8077
76. 0
0-20-40
cal/g737471
7466697276
727179767K80
76717777787778
71711757576757674757676
74.7
2.1
0-25-50
cal/g787875
7467697275
737478757778
75737776767780
73(ii)757571727673747676
74.6
2.0
• I louts of solution were calculated (u) iioeonlinn l.o Koderal SpecificationSS C L58b, (b) from the calorimeter temperature at lero time, 20 mm, and 40min., and (c) from the calorimeter temperature at zero time, '-'5 mln, and 60niin.
416
ill using the method having observations made at25 or 50 min after the beginning of the solution test.
In table 8 are shown the heats of hydration deter-mined in the routine testing of five bins of cement offour different brands. The average heat of hydra-tion of thirty two samples is 0.4 cal/g lower at 7days and about 1 cal/g higher at 28 days when cal-culated by the specification method than when cal-culated by either of the others. At 7 days the aver-age deviation (without regard to sign) of the 0-25-50method from the specification method is about halfof the 0-20-40 method. At 28 days the averagedeviations are about the same, for either of thereduced-observation methods, about 2 cal/g. Adifference in the behavior of the cement or in theoperation of the calorimeter is evidenced by themuch larger deviations for the fourth brand.
IV. Summary
A study was made of the precision of the heat-of-solution test for determining the heat of hydration ofPortland cement. Three operators participated inmaking six determinations each of the heat of hydra-tion of a single portland cement sample. The resultsof their measurements were calculated by the specifi-cation method and by two reduced-observationmethods, and a complete analysis was made of thevariance of the data. It was found that there wereno significant differences among these measurementsobtained by three operators and that the reduced-observation methods each gave acceptable results inthese special tests. The agreement among the meth-
ods of calculation was somewhat poorer when theywere applied to routine tests. The special tests in-dicate that the heat of hydration of portland cementcan be determined by the heat-of-solution methodwith a precision of about 3 cal/g at 7 days and about2 cal/g at 28 days. Somewhat better precision maybe attained by experienced operators.
The heat-of-solution tests were made by R. B.Peppier, E. D. West, and J. V. Gilfrich. The sta-tistical analysis was performed by W. J. Youdenand J. M. Cameron, who also assisted in designingthe experiment. The author is deeply indebted tothem for their interest and assistance in this project.
V. References[1] Federal Specification for cements, hydraulic; general speci-
fications (methods for sampling, inspection, and testing)SS-C-158b (May 20, 1946).
[2] Standard method of test for heat of hydration of portlandcement. American Society for Testing Materials Des-ignation C-186-47 (Oct. 15, 1947).
[3] L. Shartsis and E. S. Newman, A study of the heat-of-solution procedure for determining the heat of hydrationof portland cement, Proc. Am. Soc. Testing Materials43, 905 (1943).
[4] Federal Specification for cements, hydraulic; general speci-fications (methods for sampling, inspection, and testing)SS-C-158 (Sept. 30, 1936).
[5] R. W. Carlson and L. R. Forbrich, Correlation of methodsof measuring heat of hydration of cement, Ind. Eng.Chem. 10, 382 (1938).
WASHINGTON, June 1, 1950.
417