intramolecular attack of the free radical site on the adjacent phenyl ring and formation of a dibenzofuran structure. T h e experimental evidence suggests that the antioxidant activity of cobalt benzoate in the polyphenyl ethers is related to its ability to terminate the free radical species found in the fluid. T h e amounts of volatile organic products from the cobalt-containing fluid are much less than those from the uninhibited fluid as well as the yields of carbon dioxide and water. Further evidence of inhibition is seen in the fourfold decrease in over-all oxygen consumption of the cobalt benzoatecontaining fluid. Initially the cobalt-containing fluid experiences a brief and rapid uptake of oxygen, probably for oxidizing the cobalt ion to a higher valence state.
Acknowledgment
T h e authors thank Earl A. Ebach for samples of the polyphenyl ethers. Literature Cited (1) Archer, W. L., U. S. Patents 3,151,079,3,151,080,3,151,081,
3,151,082(Sept. 29, 1964). (2) Blake, E. S., Hammann, W. C., Edwards, J . IV., Beichard, T . E., Ort, M. R., J . Chem. Eng. Datu 6, 87-98 (1961). (3) Walling, C., “Free Radicals in Solution,” 1st ed., pp. 166-7, I$’iley, Yew York, 1957. (4) LVilson, G. R., Stemniski, J. R., Smith, J. O., Proceedings of USAF Aerospace Fluids and Lubricants Conference (April 16-19, 1963), P. M. Ku, ed., pp. 274-81, prepared under Contract AF 33(657)-11088 by Southwest Research Institute, San Antonio, Tex. RECEIVED for review July 22, 1965 ACCEPTED December 30, 1965
COMPRESSIVE STRENGTH OF POLYMERMODIFIED HYCRAULIC CEMENTS H E R M A N
B. W A G N E R
Drexel Institute of Technology, Philadelphia, Pa.
The principal variables determining compressive strength of polymer-modified hydraulic cements are the “gel-space” ratio and the degree of air entrainment, Secondary variables, of significance under particular conditions, are rate of evaporative water loss during the hardening period and degree of wetness. Polymer type can influence rate of evaporative water loss and also the magnitude of the compressive strength decrease due to wetting. A quantitative correlation of compressive strength i s made with the two principal variables.
THE general characteristics of polymer-modified
hydraulic cements have been discussed (7). T h e present paper treats specifically of one important physical property of such compositions, compressive strength. When a polymer latex is incorporated in the cement composition, a large number of potentially significant new variables are introduced. Among these are effects on hydration rate, adhesion effects, effects upon elastic properties, workability a t a given cement-water ratio, air entrainment, and chemical reactions involving the polymers and cement constituents. The objective here has been to identify the most significant variables and to ascertain the extent to which “conventional” theory must be modified to accommodate the polymer. Compressive Strength of Conventional Cement Compositions
T h e ultimate particles in conventional, hardened portland cement paste have a high specific surface area and a crystalline structure. There is some disagreement as to the mechanism of bonding. Bernal (7) and Jeffery (4) picture a fine meshwork of calcium silicate hydrate crystals, growing outward from the cement grains as hydration proceeds. T h e crystalline products of this growth interlock and fill the voids among the original cement grains and between cement grains and aggregate particles, binding these together. Electron diffraction diagrams of transitional states of hydration indicate the presence of structures related to tobermorite and hillebrandite. Complete hydration, however, shows some
coagulated mass of very small particles having a crystal structure very similar to afwillite ( 6 ) . Brunauer (2) regards the adhesion of tobermorite particles to each other as the most important factor in the strengths of hardened portland cement pastes and concrete ; to produce failure in compression one must work against valence forces within the tobermorite crystallites, perhaps aided by imperfections within the crystallites. Relation of Compressive Strength to “Gel-Space’’ Ratio
Powers and Brownyard (5): from extensive measurements, find that the compressive strength of hardening portland cement pastes is a linear function of the ratio of the volume of the cement gel existent a t any time to the original space available. They term this quantity the “(cement) gel-space ratio,” and find the relationship to hold regardless of age, original cementwater ratio, or identity of the cement. The quantity of cement gel formed a t any time during hydration is considered to be measured by the surface area of the gel; this, in turn, is proportional to the quantity V , in the BET equation, and is evaluated from the experimentally determined water vapor pressure isotherm. Thus,
f c = M s + B
(1)
w,
where fc is the compressive strength, w, is the weight of water VOL. 5
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149
added initially to the cement powder, V, is as indicated above, and M and B are constants. In Equation 1 w o is the measure of “space available” for cement gel formation, and is determined by the mixing proportions used. V, has been found, by Powers and Brownyard, to be related to the amount of water reacting with the various cement constituents in the following way:
V, V, V, V,
= = = =
0.23 w,, for 0.32 w,, for 0.32 w,,for 0.37 w,,for
tricalcium silicate dicalcium silicate tricalcium aluminate tetracalcium aluminoferrite
Here, each V, value corresponds to the surface areas associated with the cement gel formed by reaction of w, grams of water with tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite, respectively. Compressive Strength of Polymer-Modified Compositions
The compositions shown in Table I were provided a 28-day hardening period, a t 21’ C., after being hand tamped into 2-inch-cube, brass mold cavities. Water loss was completely prevented during this period, all sides including the top side being sealed. The values of compressive strength shown were then determined. Data reported earlier (7) show that after 28 days’ hydration, where water loss is prevented, hydration for various polymermodified compositions is about the same. Accordingly, it may be assumed that the volume of the cement gel formed a t this time is proportional to the weight of cement, and that, therefore, the gel-space ratio is here proportional to the ratio of dry cement to water. This ratio is shown in Table I1 for each of the above compositions. Also shown are the associated volumes of entrained air. Values are calculated from density measurements made on the various compositions, and are expressed as a percentage of total volume of the composition. The data were fitted, by the least squares method, to the equation:
fc=
c1+ C2 -
+ CIA
w0
where fc is the compressive strength, w, is the water-cement ratio, A is the volume percentage of entrained air, and CI, C?, and Ca are constants. From the data of Table I1 the values of these constants are found as 1420, 1396, and -123, so that Equation 2 becomes:
fc
(at 28 days)
Compressive Strength,
P.S.I. 3150 4500 3500 2950 4400 5500 3075 2850 3300 3250 5050 3450 4100 3100 2950 4025 2575
0.42 0.45 0.42 0.37 0.08 VeVca 0.35 0.35 VeVc 0.32 0.16 VeVc 0.33 0.15 PE 0.40 0.08 PE 0.32 0.16 Ac 0.40 0.16 Ac 0.32 0.16 VeVcb 0.40 0.08 VeVcb 0.32 M 0.16 SB 0.40 N 0 . 0 8 SB 0.39 0 0.09 s 0.32 P 0.16 VA 0 , 0 8 Vil 0.40 Q a VeVc = poly( vinylidene, vinyl chloride) ; PE = polyethylene; A c = polyacrylic; SB = poly(styrene, butadiene); S = polystyrene; V A = A B C D E F G H I J K L
poly( vinyl acetate). type.
150
None None None
b
Non-Jilm-formzng poly(uinylidene, vinyl chloride)
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
(3)
wo
Effect of Drying and Subsequent Rewetting
Certain of the compositions of Table I, after a 28-day period of hardening, during which water loss was prevented, were allowed to lose unreacted water (to an atmosphere a t 21’ C., and 40 to 50% relative humidity). Compressive strength was then determined (Table 111). Similar compositions, after undergoing the 28-day “wet” and the 28-day “dry” treatments, were immersed in water for 14 days, and their compressive strengths then determined (Table 111). There is generally a further increase in the compressive strength, following the 28-day drying period, attributed primarily to the mechanical effect of drying upon compressive failure, and only secondarily to the additional hydration that continues to some extent during the early part of the drying
Table II. Water-Cement Ratio, Entrained Air Volume, and Compressive Strength of Polymer-Modified Compositions
Composition A
B C D E F G
J
Composition Based on Cement WeightPolymer Water
1396 + __ -123A
In determining these data two specimens were measured for each composition; a standard deviation of about 225 p.s.i. for these means of two determinations is indicative of the variability of the compressive strength tests.
H I
Table I. Compressive Strength of Various Polymer-Modified Compositions
= 1420
K L M N 0
P
Q
Table 111.
Composition A
D c, H I J K L M N 0 P
Q
Water-Cement Ratio, wg
0.425 0.450 0.425 0.370 0.349 0,320 0.330 0.400 0.320 0.400 0,320 0.400 0.320 0.400 0.390 0.320 0.400
Entrained Air, Vol. A
x,
3 .0
2.6 12.3 21 . 0 14.8 4.5 13.5 14.3 16.7 15.9 8.1 12.5 17.8
Compressive Strength, P.S.I.
3150 4500 3500 2950 4400 5500 3075 2850 3300 3250 5050 3450 4100
Effect of
Drying and Subsequent Rewetting Compressive Strength, P.S.I. 28-day wet, 28-day wet and 28-day dry, 28-day wet 28-day dry and 14-day wet
3150 2950 3075 2850 3300 3250 5050 3450 4100 3100 2950 4025 2575
4800 3800 2800 4000 5250 4400 6600 4400 4750
4275 2575 2500 2700 3075 2925 5400 3450
,..
3150 2750 3175 1800
3050 6900 3825
...
~
~~
Table IV.
Rate of Evaporative Water Loss for Various Compositions
(Fraction of dry cement wt.) Polymer Modifier
Cement-Polymer- Water” Rations5 1 0.00 0.44 1 0.17 0.33 1 0.17 0.33 1 0.17 0.33 1 0.16 0.34
None Poly(vinylidene, vinyl chloride) Poly(styrene, butadiene) Polystyrene Polyacrylic Poly(vinylidene, vinyl chloride), non-film-forming 1 0.17 0.33 All compositions contained Ottawa sand at 3/1 weight ratio of Q
7
2
0.08 0.05 0.04
... ...
3 0.12 0.08
0.05
...
...
...
0.112
0.13
0.145
...
0.07 ... sand to cement.
period. Weight determinations made on each of these compositions show that substantially all of the capillary (unreacted) water is lost during the first 7 to 14 days of drying. Upon subsequent immersion in water (Table I11 data) the measured compressive strength generally decreases (from the “dry” values) to a value approximating that a t the end of the initial 28-day period of hardening. A notable exception is found in the poly(viny1 acetate) modified compositions (P and Q), where a greater decrease in compressive strength results from the water immersion. Development of Compressive Strength, with Simultaneous Early, Evaporative Water Loss
Where water loss is not prevented, and it can evaporate from the composition during the critical early period of hydration, additional factors come into play. Under this condition cement gel formation can proceed only until water loss, by evaporation, precludes further reaction. I n Table I V is shown this rate of evaporative water loss, for the compositions indicated. These determinations were also made on 2-inch cubes, provided a 24-hour initial period during which water loss was prevented, followed by exposure to a “static,” 21” C., 35 to 40% RH atmosphere. In Table I V water loss is shown as a fraction of the dry cement weight, a t various periods following open exposure. Rate of water loss varies appreciably among the latexes. Correspondingly, the total amount of water permanently retained (reacted) varies considerably. For example, with the poly(vinylidene, vinyl chloride) and the polyacrylic modifiers this retained water is 0.25 and 0.17 of the dry cement weight, respectively. After 28 days the compressive strength of these compositions was determined. Additionally, from density data, the degree of air entrainment in the original, wet compositions was calculated. These data are shown in Table V , where the high degree of air entrainment of the polymer-modified compositions should be noted.
Table V.
Compressive Strength and Degree of Air Entrainment of Compositions of Table IV Compressive Entrained Air, Strength, Vol. %, Polymer Modi$er P . S . I . , fc A
None Poly(vinylidene, vinyl chloride) Poly(styrene, butadiene) Polystyrene Polyacrylic Poly’ .i.,!;l?ne, vinyl chloride) nonfilm-forming
221 5 3000 2860 1020 2300
11.8 22.7 17.9 29.2 21.7
3060
16.3
Time, Days 5 6 ... 0.13 ... 0.08 0.07 ... 0.116 ... 0.17 ...
..
7
73 0.145 0.08 0.085 0.121 0.17
... ... 0.08
... ... 0.11
23 0.15
... 0.086
... ...
0.11
Equation 1 was applied to these data by considering that
V, is proportional to the product (P)of (concentration of dry cement in the composition) times (reacted water, expressed as a decimal part of dry cement weight), while w,’ indicates the water initially added, on the basis of grams of water per 2-inch cube. T h e form of equation :
fc = Ci’
+ C*’ R‘ + CIA
(4)
was then fitted to these data, giving values of 750, 5000, and -80, respectively, for the constants CI’, C2’, and Ca’. A mean deviation of about 20% is found in comparing values thus calculated with observed data. Factors Affecting Rate of Evaporative Water Loss
I t is evident from Table I V that rates of evaporative water loss can vary significantly, with polymer modifier, even though water-cement ratio and degree of air entrainment show (Tables V and V I ) no correlation with rate of \vater loss. I t was considered that a migration of polymer to the air interface during the early stages of evaporative water loss, and variable water permeability, with the different polymers, might account for these differences. Determinations made by several methods (to be detailed in a subsequent paper) show that, in general, no such polymer migration occurs. Determinations were made of the rate at which hydration rf the cement occurs Ivith several of these compositions, whc re water loss is prevented by complete sealing. This rate \‘as followed calorimetrically on the poly (vinylidene, vinyl ch oride), polyacrylic, and unmodified compositions. Heat evc ution was followed, adiabatically, for the first 24 to 40 hours of
Table VI.
Tabulation of Quantities Related to Gel-Space Ratio Data of Table V ( O n basis of grams per 2-inch cube)
Product
( 8of
Polymer Mod$er
None Poly(vinylidene, vinyl chloride) Poly(styrene, butadiene) Polystyrene Polyacrylic Poly(vinylidene, vinyl chloride) non-film-forming
(Cement) (Reacted Water Fraction) 18
Prtio (R’) 3f w,’
13.2 13.4 10.1 9.1
1-.G
18.1 16.0 18.4
P l Jo 0.66 0.75 0.74 0.63 0.50
12 8
19.1
0.67
27.3
~~
VOL. 5
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~
151
the hydration reaction. T h e data obtained are shown in Figure 1 ; the rate of heat evolution, and therefore, correspondingly, the rate of hydration vary significantly among the compositions, being most rapid with the unmodified composition, and least rapid with the acrylic polymer modifier. In a second method, extending to longer hydration periods, sand-free compositions, corresponding to the above three, were allowed to react, without loss of water. At the end of the periods shown in Table V I 1 sections of these were taken, accurately weighed, and placed under vacuum, over magnesium perchlorate. Substantially all (about 80%) of the evaporable (unreacted) water was removed within 4 hours, and constant weight attained within 24 hours. From these data the reacted water was calculated, and the values of Table VI1 were obtained. During the first three days hydration is most rapid with the poly (vinylidene, vinyl chloride) modified composition and approximately the same with the unmodified and polyacrylicmodified compositions. Under the earlier described conditions, where evaporative water loss is freely allowed, two factors may determine the rate of water loss (and consequently the value of P in Table V I ) : the quantity of water present as unreacted water a t any time and the density of the cement gel that has formed u p to that
-
2
35
30
Lo)
n
0
0 u
25
D
W
> J 0
20
> W
I-
d L
15
w
IO
LATEX: I- POLY(VINYL,VINYLIDENE CHLORIDE)
time. The first will depend upon the water initially present in the composition, as well as the rate at which this has reacted up to the time in question. T h e second will also depend upon the water initially present, which determines the “gel space” available, and upon the rate of hydration, which will, a t any time, determine the quantity of cement gel occupying this space. Conclusions
The most important single variable determining the compressive strength of polymer-modified mortars is the accompanying reduction in water requirement. This is the result of replacement of a fraction of the water volume with polymer, so that fluidity is thereby obtained a t lower water levels. In this way favorably higher gel-space ratios result. Degree of air entrainment is a second, significant variable, being generally greater with the polymer-modified compositions. Nearly all of the voluminous data earlier obtained with polymer-modified cements fail to determine, and take cognizance of, this important variable. Following the above, in order of importance, particularly under “dry-curing’’ conditions, are the effects of the particular polymer latex on the rate of the hydration reaction and on the rate of evaporative water loss. Additional effects, though probably of secondary significance in relation to compressive strengths, may be the level of adhesion of the coalesced polymer to the inorganic constituents, and the influence of this upon mode of compressive failure. The action of the polymer, and its associated latex ingredients, is thus considered to be primarily physical in nature. Chemical reaction between the polymer and inorganic constituents, while sometimes encountered, is not p e r se considered a prime factor in development of compressive strength, or in influencing the cement-gel structure. Two notable exceptions to this last generalization may exist with poly(viny1 acetate) and poly(vinylidene, vinyl chloride) modified compositions, where chemical reaction is involved. With the former, a substantial reduction in compressive strength is observed to accompany water immersion of dry-cured compositions. With the latter, chemical reaction involving the polymer is known to occur ( 3 ) ,and may favorably, though indirectly, influence the above physical effects.
ODlFlER 5
IO
20
30
40
50
CURE
Figure 1. tions
T I M E (hours) Rate of reaction exotherm for several composi-
Rate of Hydration, Water Loss Prevented Water Reacted at Given Time, Grams Water per Gram Cement 2 3 4 7 13 28 Composition hr. days days days days days
152
The assistance of Frank Jordan and Thomas McGinley in the laboratory work is gratefully acknowledged, as is that of Dallas Grenley and his associates a t the Saran Products Laboratory for many helpful discussions. This research was sponsored by the Plastics Department, Dow Chemical Co. Literature Cited
Table VII.
Unmodified 0.07 0,077 . . . 0.090 Poly( vinylidene, vinyl chloride) modified 0.09 0.115 , , , 0,125 Polyacrylic modified 0.053 , , . 0.090 0.105
Acknowledgment
.. .
...
... ...
.. .
...
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Bernal, J. D., Cement Symposium, London, 1952, p. 258. Brunauer, S.. Am. Scientist 50, 210 (1962). Dow Saran Products Laboratory, private communication. Jeffery, J. !V., Chem. Ind. (London) 53, 1756 (1955). (5) Powers, T. C . , Brownyard, T. L., Proc. Am. Concrete Znst. 43, 845 (1947). ( 6 ) Swerdlow, M., McCurdie, H. F., Heckmann, F. A . , Proceedings of International Conference on Electron Microscopy, London, 1954. ( 7 ) Wagner, H. B., IND.ENG. CHEM.PROD.RES. DEVELOP. 4, 191 (1965). (1) (2) (3) (4)
for review October 19, 1965 RECEIVED ACCEPTED February 7, 1966
