Xay. 1913
T H E J0I:RTAL O F I\-/)CSTRI.AL
A.YD E.YGISEEII/.YG
CHE-WISTRY
369
before the Eighth International Congress of Applied bciw the subject of considerable investigation, for Chemistry, was copied not only by every paper in i t is scli-evident that the quality of the set of a cement the United States but was heralded throughout Europe; determines the ultimate strength of the concrete of and those who a a n t t o know a h a t excellent progress, which it is a constituent. Experimenters do not seem to agree very well as has been made in this branch of the Government, I to the mechanism of this setting, nor as to the factors t h a t exert the greatest influence during the time t h a t the hydrolysis is taking place. Consequently there is a diversity of opinion a s t o the method t o employ d u r i n g the processes of manufacture or as t o what subsequent treatment t h e cement must undergo. in order that the manufacturer may a t all times put upon the market a cement whose behavior may be predicted, and whose quality will show no deterioration d u r i n g long-time storage. The consensus of opinion seems t o be that the initial set of a cement is due to some action for which the aluminates are responsible, 01to which t.hev at l e a d
would refer to his excellent treatise on the subject which is to bc found in the transactions of the Eighth International Congress of Applied Chemistry. 320 Firm Avr., Nsw Yaxh
-.
~
.
CONTROL OF INITIAL SETTING TIME
PORTLAND
CEMENT' lay 1:. E. U'nnh'
I t is well known that Portland cement, as burned in the rotary kiln, is so quick setting that it cannot be used without the addition of some retarding material. such as gypsum or plaster of Paris. I t is not necessary to make this addition of retarding agent when dealing with thc product of a set kiln, probably for the reason that the ccment contains the ash of the fuel as well as most of the sulfur. Occasionally there has been reported a cement of such a nature as t o he quick setting evrn after the addition of the regular amount of retarder, and this paper is in thc nature of a report on the commercial manipulation of a 300,ooo barrel lot of such quick setting clinker. The manufacturer was interested, f r s t , in correcting the mat&al on hand, and second in establishing a routine of operation that w-ould prevent a recurrence of t h r trouble. The setting and hardening of hydraulic mortars has 1 Paper presented at Piith ~ n u a.\lcefing l of the hmerican Institute 01 Cbemicd Enriaeeis, Detroit, Drrnnbcr, 1912. 2 Credit is due to L. C. Nudeli and P. H . C h a w for the experimental work in connection rich this p a ~ e r .
Also i t seems to he quite me11 agree(1 that the retarding action of gypsum is due, if not to the formation of a double salt with the aluminates, at least to the fact that i t slows down their hydrolysis and consequently delays thc initial set of the cement. It has been thc writer's good fortune to have heen a t various times connected with the operation of f'ortland cement plants using materials abnormally high in alumina, and hc invariably found that it was u,,dc,r thiise conditions, to vary the lime Conti'ilt of tlic cement over any extended range withcarried high (63out causing trouble, If the lime 64 pcr ccnt.), the cement approached too closely the line of unsoundness, while if it dropped trio with quick ( 6 0 , j-61), factory was cement, Quick setting ccmcnt resulting from such operation is not so rrsoonsive to the retardin.- action of gypsum as one more nearly normal in composition. Somctimes it will be quick setting direct from the grinding mills, while at others it will develop a quick set after short storage. Quite often i t will show a rcversion to quick set if an excess of gypsum is added. The writer had his attention called to a condition where two sections of a plant were operated with differences of 30 per cent. in the gypsum added. None of the cases of quick set in the writer's operating experience e w x developed serious diiiiculties, as the setting times were watched very closely, and a t a suspicion of trouble in the stored materials a quick I
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.
T H E J O U R N A L OF I N D U S T R I A L A N D EKGINEERING C H E M I S T R Y
37 0
cement was x i x e d out with a slower one, preferably one having a tendency toward unsoundness, the combination seeming t o remain perfectly stable and not to require any further additions of gypsum. Also at any indication of quick set in the material coming from the mills, the lime in the mix was immediately raised, a procedure that never failed to correct the trouble. This seems to agree with the experience of Meade,' who states t h a t quick setting cements t h a t have come under his observation are low lime cements. He states also, t h a t he has retarded the set of plastered cements that have gone quick, by addition of calcium hydrate or even calcium oxide. I t seems t o be, however, a direct contradiction t o the statements of Reibling and Reyes,' who state t h a t all quick setting cements contain free lime, remain quick setting so long a s the lime is in the form of oxide, become slow setting as the lime hydrates, again quicken when the hydrate changes to carbonate, and finally become slow setting as the hydraulic constituents become inert through long exposure. I n view of these interesting experiences with quick setting cement, i t mas with considerable interest t h a t the writer responded to an invitation from a cement company who reported a large stock of clinker as quick setting and beyond the influence of the ordinary corrective methods. The clinker was the regular fall run, stored over winter, the plant being one t h a t operated on marl, and followed the usual practice of burning a large stock of clinker during the late fall months t o supplement their stock for the early spring demand, which opens before the ice leaves the lakes from which they dredge their supply of
FIG.I
marl. An interesting circumstance in connection with the problem was t h a t although some cement ground in the late fall showed quick setting, the majority of it was perfectly normal except t h a t it would not stand any large additions of quick setting material without itself showing a n earlier set. Inquiry developed the fact t h a t this quick setting cement was ground during a short period that the kilns were out of operation, and t h a t when the kilns were started 1 2
"Portland Cement," ,Meade, p. 416. Philipbine Journal of Science, 1911, 207.
1'01. 5 , No. 5
the rest of the fall grind showed a normal setting time. Experiments were run t o try the effects of different added materials, such as plaster (instead of gypsum), hydrated lime, calcium chloride, and acids; but none of them seemed to be successful in retarding the set. At the same time other experiments were tried along the line of hydration, as recommended by Bamber. I These were highly satisfactory, the cement ground from clinker, which showed a set of 3-j minutes under ordinary procedure, being slowed to 2 1 / ~ - 3 hrs. when hydrated to the extent of less t h a n two per cent. I t seemed t o make little difference how this water was added, as is illustrated by the following experiments : I . The cement, after grinding with the usual amount of gypsum, was heated in a closed tube, the idea being t h a t the water resulting from the dehydration of the gypsum might prove sufficient for the hydration of the troublesome constituents. This proved t o be the case. 2. The clinker was heated t o approximately I O O C. and ground while hot, the result being the same. 3. The ground cement was dropped through a vertical tube through which a small cloud of steam was rising; subjection t o this atmosphere for even so short a time as t h a t necessary for it t o drop through a tube thirty inches long was entirely sufficient t o retard the set. 4. Water t o the extent of three per cent was added t o the ground cement, mixed rapidly by hand, and then placed in a laboratory pebble mill where it was mixed mechanically for a short time. The set was delayed, but not so uniformly as b y the other methods. 5 . Water was sprinkled on the cold clinker as it was fed t o the grinding mills a t the factory. This treatment was satisfactory so long as the water supply could be kept constant; but the mill operators could not be depended upon t o regulate the supply properly, and the idea was abandoned as impracticable. 6. Steam was turned into the conveyor leading from the mills. The results from this method were not dependable, and the scheme was dropped as being too uncertain to be safe to use. The method t h a t was finally adopted was t h a t of heating the clinker and grinding while hot. This method proved entirely satisfactory for the treatment of the greater part of the quick clinker, the remaining being left over until the kilns should be in operation, when the old clinker was ground with the new, the new being purposely not thoroughly cooled. The adoption of this method was largely influenced by the layout of the plant which, with the clinker pile lying alongside of the kilns and for their full length, made i t a simple matter t o send part of the clinker through one kiln, and mix it on its return with a quantity of cold clinker. The gypsum used was thoroughly wetted and added t o the clinker just before it reached the mill hoppers, these hoppers be1
Concrete and
Const. Eng., 1909,
( 4 ) 196.
May, 1913
T H E J O U R h - A L O F I N D L i S T R I A L A N D E!'\'GISEERI-YG C H E - I f I S T R Y
A cement with a setting time oi fifteen minutes was treated alternately with water and heat and showed a setting time curve as in Fig. I . At each stage in this addition of mater and subsequent heating to 3jo-400' C . a sample of the material was strongly ignited and showed losses corresponding to the dotted curve on Fig I From a consideration of the curve i t would seem as though the water must have been present in two different conditions, for although the cement showed a continuously increasing amount of water, the set was not correspondingly slowed This may be partially due to the fact that the heated cement retained some water in such a condition that it was not driven off when heated to 350° C.. in which condition the mater did not seem to exert much influence on the setting time I n a n effort to establish mhethw it was the gypsum or the cement t h a t was affected by heat, a cemcnt containing no gypsum was heated and afterward mixed with the normal amount It showed a slow
ing kept only partially filled in order t h a t the clinker might nct have time to cool or to dehydrate the gypsum before reaching the mills. From a consideration of the plant, i t was a simple matter to explain the quick setting cement t h a t was ground in the fall. As the stock of clinker grew larger, there was left only one place to discharge the kiln output, and t h a t was a t the part of the clinker pile farthest away from the kiln discharge, a point which is nearest t o the mills. This meant t h a t for at least the last few weeks the mills were grinding hot clinker; but for the few days that the mills were operated while the kilns were off fire they would be supplied with cold clinker, and so ground out a small amount of quick setting cement. The table of analyses shom-s four analyses of quick setting cements, 1-4, and three slow setting cements. 5 - j , from the factory stock. The set 8-12 belongs to a series of laboratory cements made from the same raw material, in an (endeavor to establish the safe limits for factory operation. TABLEI-ANALSSES LOSS
..................
............ ..................
SiOdR203
Set
7
8
9
24.9
2.4 24.4
1. o 21 .4
2.4 22.4 3.6 7.4 61.6 2.1 0.74 2.05
3.0 22.7 4 .O 6.6 59.8 2.3 1.83 2.13
1.3 22.25 4.4 6.35 62.5 2 .o 1.57 2.07
0.4 21.9 4.8 7.3 61 . 7 2.0 1.54 1.8
0.03 22.48 4.2 7.3 62.9 2.2 1.58 1.95
11.7 59.4 2.11 2.21 2.13
10.9 58.5
1.74 1.85
0.7 22.0 3.9 7 .O 61.3 2.4 1.57 2.02
quick
quick
quick
quick
normal
normal
normal
quick
quick
4
The clinker from these burns was carefully sorted and all material t h a t showed any signs of underburning was discarded. The good clinker was then crushed and ground with gypsum. This set of analyses shows alumina in a fairly high percentage, b u t fails to show why this alumina has such a decidedly quickening effect under conditions not entirely accounted for by the composition of the cement. I n a n endeavor t o locate some of the influencing factors, a few further experiments were carried out. Quenched clinker from these high alumina samples showed quick setting if perfectly dried, but slow setting if only air-dried. Steamed clinker when airdried showed a retarding set when ground with plaster. All cements t h a t have come under observation develop quick set when heated t o 3jO-400' C. This includes a number of commercial samples of varying composition and compounded from widely different materials. Two commercial cements t h a t had been stored since 1899 and which were presumably in their last stage of slow set, had their initial setting time decreased from 3I/* hours t o I minute. There was a loss in weight during heating of only 0.1j per cent. J. A m . Chem. Soc.. 24,
6
3
These experimental burns were carried out in a small experimental kiln designed by Prof. E. D. Campbell,' and used by him in all his work on the composition of Portland cement and the influences t h a t affect its constitution and characteristics.
1
CLINKERS (PERCEXTAGES) 5
2
Silica. . . . . . . . . . . . . . . . . . Ferric o x i d e . . . . . . . . . . . . 5 . 1 6.4 Alumina . . . . . . . . . . . . . . . Lime . . . . . . . . . . . . . . . . . . 61 .O hlagnesia . . . . . . . . . . . . . . 2 . 3
SOa . . . . . . . . . . . . . . . . . . .
OF
I
NUMBER
248.
371
....
.... ....
1.58 2.23
10
11
....
....
23.7
23.7
.... 10.4 61.4 2.17 1 ..57 2.28 quick
....
10.1 63.3
....
1.36 2.36 3 hrs.
12 1.16 23.9
....
9.8 61.7 2.14 1.34 2.46 3 . 5 hrs.
set. Another example of the same material, unheated, was mixed with gypsum t h a t had been heated. While it required a larger amount (5 per cent)' t o retard the set, it showed a normal setting time. Any one of these slow setting samples would develop a quick set upon heating. In the case of the cement carrying five per cent dead burned gypsum, i t required a much longer time of heating than in the case of those samples carrying the smaller amounts. The times of heating varied from six hours to fortyeight hours in the different samples. Dry slaked lime showed no appreciable loss of water under the same conditions of heating. The whole set of quick setting cements was tested for free lime according t o the microscopic method described by Prof. A. H. White.* There seemed t o be no indications of free lime. Although it may be impossible t o draw any definite conclusions from this rather incomplete line of experiments, it seems t o be certain t h a t in this instance, a t least, the quick setting was not due to free lime. This experimental work is to be continued, in the hope of gathering further data that may assist in clearing away the uncertainties in regard t o the role t h a t alumina and gypsum play during the initial setting of Portland cement. UNIVERSITY OF MICHIQAN ANN ARBOR
Meade and Gano, Chem. Enp., 1, 92. THISJOURNAL, 1909, (1) 5.
