Improved Portland Cement Mortars with Polyvinyl Acetate Emulsions

The mortar containing a polyvinyl acetate to cement ratio of 0.2 was more workable and flowed more easily than did plain cement mortars with the same ...
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April 1953

INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED

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(1) Brimhall, B., IND. ENG.CHEM.,36, 72-5 (1944). (2) Chanda, S. K., Hirst, E. L., Jones, J. K. N., and Percival, E. G. V., J . Chem. Soc., 1950,1289-97. (3) Chem. Eng. News, 29,650-4 (1951). (4) Connell, J. J., Hirst, E. L., and Percival, E. G. V., J . Chem. Soc., 1950,3494-500. (5) Corn, 6,4 (1950). (6) Evans, T. H., and Hibbert. H., “Bacterial Polysaccharides.” Scientific Report Series No. 6, New York, Sugar Research Foundation, April 1947. (7) Graefe, G..Die Stdrke, 3,3-9 (1951). (8) Hehre, E.J., private communication. (9) Jeanes, A., and Wilham. C. A., J. Am. Chem. SOC.,72, 26557 (1950). (IO) Jeanes, A., Wilham, C. A., and Miers, J. C., J . Bid. Chem., 176, 603-15 (1948). (11) Katz, J. R., and Weidinger, A., 2. physik. Chem., A184, 10022 (1939). (12) Kerr, R. W.,ed., “Chemistry and Industry of Starch,” 2nd ed., pp. 174,345-55, New York, Academic Press, 1950. (13) Lansky, S.,Kooi, M., and Schoch, T. J., J. Am. Chem. SOC.,71, 4066-75 (1949).

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(14) Lockwood, A. R., Chemistry and Industry, 1951, 46-7; i l l f g . Chemist. 23. 49-55 (1952). Reoort of talk. (15) McCready, R. M.,Guggolz, J., Siiviera, V., and Owens, H. S., Anal. Chem., 22,1156-8 (1950). (16) Meyer, K.H., Noelting, G., and Bernfeld, P., Helv. Chim. Acta, 31,103-5 (1948). (17) Montgomery, E. M., Weakley, F. B., and Hilbert, G. E.,J . Am. Chem. Soc., 71,1682-7 (1949). (18) Northern Regional Laboratory, unpublished results. (19) Ricketts, C. R.,Lorenz, L., and Maycock, W. d’A., Nature, 165, 770 (1950). (20) Senti, F. R., and Hellman, N. N., Abstracts of Papers, 121st Meeting, AM. CHEM.SOC., Milwaukee, Wis., March 30 to April 3,1952. (21) Stacey, M., and Pautard, F. G., Chemistry & Industru, 1952, 1058-9. (22)Thorsen, G.,and Hint, H., Acta Chir. Scand., Suppl. 154 (1950). (23)Whiteside-Carlson, V.,and Carlson, W. W., Science, 115, 43 (1952). RECEIVED for review September 8, 1952. ACCEPTED December 15, 1952. Presented before the Division of Sugar Chemistry at the 122nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.

Improved Portland Cement Mortars with Polyvinyl Acetate Emulsions JACOB M. GEISTI, SERVO V. AMAGNA2, AND BRIAN B. MELLOR Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge 39, Mass.

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ORTLAND cementmortarsandconcretesareusedasbuilding materials because among other properties they have high compressive strengths, good bonding strengths, and resistance to weathering. Their principal weakness lies in a normally low tensile strength, a quality which has been corrected by various means. Most improvements of the properties of concretes and mortars have been made without admixtures, by controlling the mix proportion of the ingredients-the water to cement ratio being the most important factor ( 6 , 7 , 8, 13, 17, 19). Additives such as rubber latex have been incorporated whh portland cement t o produce floorings which are claimed t o have high resiliency (11, I d , 16, 20-25). Other additives such as resins, metallic oxides and salts, sugar, powdered metals and nonmetals, organic fibers, and gelatin have been used in an attempt to improve one or more properties of concrete mixes ( 5 , 1 5 ) . Polyvinyl acetate (PVA) emulsions are now commonly used in numerous industrial applications. Recently, polyvinyl acetate mixtures with portland cement were reported for surfacing concrete floors, for making joints between concrete blocks, and for mending broken cement surfaces (2, 9,10, 11), b u t apparently no work had been done to determine the exact nature of the new combination. The work described in this paper was initiated t o study the effects of using polyvinyl acetate emulsions as admixtures with portland cement mortars and concretes. GENERAL PROCEDURE

TESTSA N D COMPOSITIONS.Throughout the tests, A.S.T.M. (3) standard procedures were used wherever applicable. The procedures were modified when necessary, as dictated by the nature of the mortars and the tests. Tensile and compressive mortar specimens were prepared fol1 2

Present address, Hebrew Institute of Technology, Haifa, Israel. Present address, California-Texas Oil Co., Ltd., New York, N. Y.

lowing A.S.T.M. specifications and with ratios of sand to cement of 3 t o 1 for tensile briquets and 2.75 to 1 for compressive cubes and varying:

1. T h e ratio of polyvinyl acetate to cement from 0 t o 0.56 2. The ratio of dibutyl phthalate plasticizer t o polyvinyl acetate from 0 t o 0.2 3. T h e ratio of water t o cement for some tensile specimens from 0.32 t o 0.56 4. The particle size of the polyvinyl acetate (These ratios are weight ratios-e.g., 0.2 PVA t o cement would be 1 part by weight PVA to 5 parts by weight cement.) Selected compositions were further tested for impact strength, abrasion resistance, bond strengths of mortar t o steel and mortar t o concrete, corrosion resistance, air entrainment, water adsorption, and coefficient of expansion. Tensile and compressive specimens were also examined microscopically. CURINQCONDITIONS.The test specimens were cured a t different conditions and for different lengths of time. A.S.T.M. test methods for tensile and compressive tests (5)specify t h a t curing of mortar test specimens shall be under water after removal from the molds. Curing in a fog room a t 70 o F. with 100% relative humidity was originally selected for this work, since this represents the best possible curing condition, for plain cement mortars, which can be obtained in the field. It was observed, as was expected, t h a t the 28-day tensile strengths of plain mortars cured in the fog room were less (by 30%) than the tensile strengths of plain mortars under water. I n marked contrast t o the normal behavior of plain mortars, the 28-day tensile strengths of mortars containing polyvinyl acetate were equal or greater when cured in the fog room, than when cured under water. T o investigate this behavior further, specimens were cured in a room a t 70” F. with 50% relative humidity. Under these new curing conditions, although plain cement mortars were weaker, the mortars containing polyvinyl acetate were stronger. A few additional tests were made in which specimens

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24 0 KEY DAYS HUMIDITY 0 28 100%

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POLYVINYL ACETATE TO CEMENT RATIO Figure 1.

Relative Tensile Strengths of Cement AIortars 1.0 = 290 lb./sq. inch

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POLYVINYL ACETATE TO CEMENT RATIO Figure 2.

Tensile Elongations at Rupture 1.0 = 9 X 10-5 inch/incli

0.1 0.2 0.3 0.4 0,5 POLYVINYL ACETATE TO C E M E N T RATIO

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Figure 3.

Relative Tensile Moduli of Elasticity 1.0 = 7,050,000 Ib./sq. inch

were cured in a room with varying temperatures and humidities, and others were cured outdoors during the months of December and January where they were subjected t o extreme freezing and thawing conditions. MATERIALS.The polyvinyl acetate emulsions used were American Polymer Corp. Polyco 469, 470, and 471. The cement %as Type I1 manufactured by the Catslcill Plant of the North American Cement Corp. The Ottawa sands were obtained from t h e Ottawa Silica Co. and the dibutyl phthalate plasticizer was identified merely as technical grade. RESULTS

A polyvinyl acetate emulsion with a nonionic emulsifier and with particle diameters from 1 t o 5 microns (Polyco 470) was found t o be more effective as an admixture than a similar emulsion with particle diameters from 2 to 10 microns (Polyco 469) or an anionic emulsion with particle diameters from 0.5 to 2 microns

(Polyco 471). Maximum improvement in physical propei ties wap obtained a t a polymer to cement M eight ratio of 0.2 without plasticizer, by curing in a room a t an average relative humidity of 35%. At 28 days, tensile strengths were ten times that of normal cement mortars cured under similar conditioiis and three times that of normal cement mortars cured under water. Some properties of mortars containing polyvinyl acetate emulsion (Polvco 470) and cured a t various conditions have been compared with the properties of plain cement mortaIs cured in a fog room a t 100% relative humidity and 70" F. in Figures 1 to 6. The fog room curing represents the best possible curing conditions, for plain cement mortars, which can be obtained in the field. Each point represents the average of at least three specimens and in most cases of six specimens made from the same batch of mortar. Some tests, such as abrasion resistance, corrosion resistance, and impact resistance were made only with specimens which were cured in a fog room a t 100% relative humidity. It is probable t h a t for the m6rtars containing polpin: I acetate these properties mould have been improved even mole if cuiing h:rd been carried out in a dry atmosphere. For mortar specimens containing a polyvinyl acetate to cenirnt ratio of 0.2, the following observatione have been recorded ftoni the detailed data published in several theses (1, 4, 14, 18). 1. The tensile strengths of mortars cured in a dry atmosphere were three to four times that of plain cement mortars cured in afog room and eight to ten times that of plain cement moItars cuied in a drv atmosphere (Figure 1). 2. The elongations a t rupture for tensile specimens cured in a dry atmosphere were about twenty times the elongation a t rupture of plain cement mortars, cured either in a dry atmosphere or in the fog room (Figure 2 ) . 3. The compressive strengths of mortars cured in a room a t 50% relative humidity M-ere only 70% of the strength of plain cement mortar cured in a fog room but more than three times that of Dlain cement mortar cured in a room a t 50% . - relative humidity (Figure 4). 4. For specimens cured in a dry atmosphere, the strength of the bond to steel was one and a half times and the strength of the bond to concrete surfaces was ten times that of plain cement mortars cured in a dry atmosphere. 5 . Since 50% relative humidity a t 70" F. approximates usual indoor drying conditions, the mortar can he left t o set and harden by itself, without the wetting required for plain cement mortars t o obtain high strengths. This was confirmed by the strength of the tensile specimens cured in a room with relative

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12 C U R I N G AT 7 0 ° F K E Y DAYS HUMIDITY 100 Ye 5 0 Ye

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I n considering the properties of plain cement mortars when cured in a fog room, i t should be realized that these curing conditions are better than those obtained in the field and t h a t the actual field strengths of the usual plain portland cement mortars are considerably less. MICROSCOPIC STUDY

EXAMINATIOX OF EMULSIONS BY TRANSMITTED LIGHT. The particle sizes of the emulsions as estimated by a microscale (Figure 8) tallied very closely with the particle size determinations made by the American Polymer Corp. with the aid of an ultramicroscope. These were Polyco 471, 0.5 t o 2 microns; Polyco 470, 1 to 5 microns; and Polyco 469, 2 to 10 microns. The particles were spherical in shape and appeared t o be well dispersed. In dilute emulsions with plasticizer, a distinct layer of plasticizer was evident around the particles. This fact was verified b y a microscopic view of the dispersion, using a red oil-soluble Calco dye, which imparted color only to the layer of dibutyl phthalate. The photographs were taken after the smears on the slides were diluted to eliminate, as much as possible, all b u t one layer of the particles. EXAMINATION OF GROUNDMORTARSURFACES BY METALLOGRAPHIC MICROSCOPE AND CAMERA. Figure 9 represents mortars

0.3 0.4 POLYVINYL ACETATE TO CEMENT RATIO 0.1

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Figure 5. Relative Compressive Qeformations a t Rupture

humidities varying from 25 t o 45% and with temperatures varying from 65" to 85" F. 6. The mortar containing a polyvinyl acetate to cement ratio of 0.2 was more workable and flowed more easily than did plain cement mortars with the same water to cement ratio. It did not require the extra tamping and ramming essential t o compact the plain mortars. The water to cement ratios in most of the tests were dictated by A.S.T.M. consistency standards t o be 0.6 for compressive cubes and 0.43 for tensile briquets. Figure 7 shows t h a t decreasing the water content within the limits of workability increased the tensile strength of mortars containing polymer. This was not possible with the plain cement mortar which was a t the lower limit of workability and was a "dry-mix." 7. The modulus of rupture for the mortar cured in the room at 50% relative humidity was forty times t h a t of plain mortars cured in the fog room. The impact strength of mortars cured in the fog room was 20% more than the impact strength of plain mortars cured in the fog room. Thus the new material is tougher and can absorb more energy from impacting bodies before breaking or cracking. 8. Mortars cured in a fog room when compared with plain cement mortars cured in a fog room had about ten times the resistance t o abrasion, at least the same resistance to freezing and thawing, and the same or greater resistance t o corrosion by alkali, organic solvents, and dilute inorganic acids. They were more resistant t o concentrated hydrochloric acid, but less resistant to boiling water.

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POLYViNYL -ACETATE TO CEMENT RATIO Figure

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Relative Compression iModuli Elasticity

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with polyvinyl acetate t o cement ratios from 0 to 0.5. No plasticizer was used in these mixes. Specimens were cured at 100% humidity and 70' F. for 28 days and dried under room conditions for a n additional 28 days. I n the plain cement mortar (Figure 9, A ) the large surfaces are sand grains, The masses between them are the products of cement hydration which form a gel structure. T h e voids in the gel are fairly well defined. I n the mortar with a 0.2 polyvinyl acetate t o cement ratio (Figure 9, B ) the shaded areas are polyvinyl acetate and the voids have been filled with polymer. T h e preponderant continuous phase is still cement gel. The bond between the sand grains and the cement mass is mainly between the hydrated cement and the silica surface, with some direct contact between the polyvinyl acetate and the sand where voids have been filled up by polymer. I n the mortar with a 0.3 polyvinyl acetate to cement ratio (Figure 9, C) there is apparently more polymer within the

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cement matrix in direct contact with the sand grains. The mortar with a 0.4 polyvinyl acetate to cement ratio (Figure 9, D ) shows a continuous phase of more than three-quarters polyvinyl acetate. I n the photograph which illustrates a mortar with a polyvinyl acetate t o cement ratio of 0.5 (Figure 9, E ) the cement hydration products can be seen dispersed in the continuous polymer phase.

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was a result of the rapid escape of water from the mixture. The briquet containing 0.2 polvyinyl acetate to cement ratio (Figure 12, B ) had tensile strengths ranging over 1000 pounds per square inch. The photograph shows a well compacted mass of hydration products and polyvinyl acetate, with the cement gel forming the bulk of the mass. A few bubbles are present. The mortar containing 0.2 polyvinyl acetate to cement ratio and 0.1 dibutyl phthalate t o polyvinyl acetate (Figure 12, C) is less compact, apparently due to the presence of plasticizer. AIthough this mixture had less strength than t h a t with no plasticizer, its tensile strengh was still twice that of plain cement mortar cured under the best conditions,

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Effect of Water to Cement Ratio on Seven-Day Tensile Strengths 1.0 = 270 lb./sq. inch

EXAMINATION O F FRACTURED SURFACES B Y ULTROPAK hfICROFigure 10 shows, a t a magnification of 65 diameters, the surfacesof briquetscured for 28 days a t 100% humidity and 70 O F . , and then dried under room conditions for another 28 days. With polyvinyl acetate t o cement ratios of 0.1 and 0.2 (Figure 10, B , C) the polymer phase was discontinuous. At a ratio of 0.3 the polymer phase showed some continuity (Figure 10, D ) and the hydrated cement was almost totally dispersed in the polymer phase a t higher polymer to cement ratios. The presence of polyvinyl acetate is generally indicated by a graying or darkening of the normally light colored cement gel structure. With plasticizer, Figure 11, the same phase change was noted. The plasticizer seemed t o introduce more voids, and the presence of plasticizer also caused internal cracking when the specimens were dried. Figure 12 shows the surface of briquets cured for 28 days at 50% humidity and 70” F. The first photomicrograph (Figure 12, A ) shows plain cement mortar which is largely unhydrated, with most particles unchanged from the original clinker. This SCOPE.

Figure 8.

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Workability. BEHAVIOR DURING MIXING. Workability is an intangible property describing the ease with which the mortar or concrete can be handled. Plain cement mortars usually exhibit maximum strength a t the lowest water to cement ratio consistent with the minimum workability. This is usually the water t o cement ratio determined by A.S.T.M. consistency tests (Figure 7 ) and is several times the amount of water required to hydrate the cement fully (19). Descriptions of the workability of mortars with a constant water to cement ratio of 0.43 and with various polyvinyl acetate to cement ratios are given in Table I. The most workable mortar had a polymer to cement ratio of 0.2. Subsequent measurements of physical properties showed that this polymer to cement ratio produced mortars with optimum properties. This implies that workability may be a very important factor in determining the final properties of mortars.

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TABLE I, WORKABILITY OF MORTARS Tensile specimens, water t o cement ratio 0,43 Polyvinyl acetate emulsion, Polyoo 470 Particle diameters 1-5 microns, nonionic emulsifier Polymer t o Cement Workability of Freshly Ratio Mixed Mortar Mixture No. 0 Workable h u t nonflowing 0.1 Workable, soft butterlike consistency 0.2 Very workable, thin, and easy flowing Workable. soft butterlike 0.3 consistency 0 4 Stiff, h u t more workable t h a n No. 6 0.56 Very stiff, just workable

The polyvinyl acetate emulsion, when diluted with sufficient water, probably acted as a dispersing agent for the cement particles and for the sand. Increasing the polyvinyl acetate to cement ratio to more than 0.2, while maintaining the water to cement ratio the same as in preceding mixes, produced more tacky mortars. I n these cases, there was not sufficient water in the mix to dilute the emulsion and the resulting mortar became too sticky. Strength after Curing. DRY ATMOSPHERES. In the case of

Polyvinyl Acetate Emulsions

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B. 0.20 PVA to Cement Ratio

Figure 9.

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Photographs of Ground Cube.Surfaces

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plain cement mortars cured a t 50% humidity, the water evaporated before the cement could fully hydrate (Figure 12) and the resulting mix was porous and weak in both tension and compression. A microscopic examination of the cement which was used revealed particle sizes ranging from as small as '/z micron upwards. When sufficient polyvinyl acetate was added t o the mortars, it filled the voids between cement particles and formed a continuous phase around some individual or groups of cement particles. A 0.2 polyvinyl acetate t o cement mix provided enough polymer so that, upon hydration of the cement particles, the voids usually left within the cement gel structures were filled with polymer. The polymer did not, however, fill the air spaces which result from air entrainment. The polymer in the mix, b y virtue of the attraction of the water on the surface of its hydrophilic particles, prevented the water from evaporating too rapidly, and allowed the cement to hydrate fully (Figure 12, B ) . I n the 0.2 polyvinyl acetate to cement mix, the main strength was probably due to the cement gel structure, reinforced by the polyvinyl acetate in the voids which also contributed to the adhesion with the cement particles. As a result, tensile strengths three t o four times t h a t of plain cement mortar cured a t lOOyohumidity and eight t o ten times t h a t of plain mort a r cured under dry conditions were obtained. For polyvinyl acetate to cement ratios less than 0.2, the voids became less filled with polyvinyl acetate, and the resulting mixtures were weaker than the 0.2 mix, although stronger than plain cement mortars. For polyvinyl acetate to cement ratios greater than 0.2, the continuous polymer phase became predominant, and the polyvinyl acetate matrix was probably weaker than the continuous gel structure. With dry curing conditions, the compressive strength of mortars with a polyvinyl acetate t o cement ratio of 0.2 was more than

E. 0.50 PVA to Cement Ratio

three times greater than plain mortar cured at the same condition, because the polyvinyl acetate kept the water from evaporating too rapidly. 100% HUMIDITY AND PARTIAL DRYING.Mortar specimens fresh from the moist curing room contained the same general phase distributions as described for curing under 50% humidity conditions. The polyvinyl acetate, however, not having been allowed to dry, remained in a weaker form. The resulting tensile and compressive strengths of all polymer-containing mortars were lower than for plain cement mortars cured in the moist room. When the specimens cured in the moist room were partially dried in the 50% humidity room, the structure of the rigid cement gel matrix, where continuous, remained unchanged. The polyvinyl acetate phase shrank, due t o the loss of water. For mixtures containing polyvinyl acetate t o cement ratios of 0.2 and lower, in which the cement gel phase was continuous, the partial drying of the polymer added strength by contributing some adhesive power in the cement gel pores. This increased the strength so t h a t the briquets with a polyvinyl acetate to cement ratio of 0.2 were twice as strong as fully hydrated plain cement mortar, cured under water, although only one half as strong as the same mix cured a t 60% humidity. For polyvinyl acetate to cement ratios of 0.3 and greater, the continuous polyvinyl acetate phase shrank when dried. Although the drying hardened the polyvinyl aeetate, the cracking which occurred and the internal formation of voids counteracted this strengthening effect. The result was a mixture as weak as when the mortar was still moist. UNDERWATER, Underwater curing allows the fullest possible hydration of the portland cement. It was found that as polyvinyl acetate was added, the resulting mortars cured under water were weaker. Up t o the 0.2 polymer t o cement ratio, it is felt t h a t the cement gel phase was predominant and accounted for

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Figure 10.

Fracture Surfaces after Curing for 28 Days at 100% Humidity and 70' F. No plastioizer (Taken by Ultropak, 10 X 6.5)

the main strengths of the mortars With increasing polymer content, the polymer phase began to predominate, and curing under water had practically the same effect as curing in the fog atmosphere. Deformation Properties. Plasticizer lowered the strength both in tension and compression, probably due t o a decrease of attractive forces between polymer particles and between the polyrnei and aggregates. In both tension and compression, unit deformation was increased and the modulus of elasticity was decreased with an increase in polyvinyl acetate and plasticizer. The modulus of resilience and the modulus of rupture are expressed as the work required to deform a unit volume of material t o the elastic limit and to the rupture point, respectively. Thrse quantities are determined by the stress and the deformation a t the elastic limit and a t rupture. The plasticizei contents affected these moduli, but the polyvinyl acetate content was the controlling factor. An increase in polymer content resulted in an increase in both the modulus of rupture and of resilience. Abrasion Resistance. The polyvinyl acetate acted as a binder for the cement hydration pioducts and prevented them from being torn away by abrasive forces. The resistance to abrasion increased with increasing polyvinyl acetate content, and the amount of plasticizer did not affect this property to any significant degree. The loss of weight of a/{-inch cubes, after abrasion for 5 minutes with a D u Pont abrasion machine was 1 6 grams for a plain cement mortar and 0.2 gram for a mortar with a polyvinyl acetate to cement ratio of 0.2. The specimens were cured in a 1 0 0 ~humidity o room. I t is believed that the abrasive resistance would be improved by dry curing a t usual atmospheric conditione, since the resulting polyvinyl acetate structure is stronger and tougher. Shrinkage and Thermal Properties. Mixes containing polyvinyl acetate showed greater shrinkage than did plain cement mortars, the amount of shrinkage and the rate of shrinkage varying with curing conditions. Shrinkage during 28-day curing a t

507, humidity was 0.06yofor plain cement niorttLrs and 0.5% for mortars containing 0.2 polyvinyl acetate. The main part of the shrinkage occurred within the first few dam. However, when mortars containing 0.2 P V A to cement were placed between two rough concrete surfaces, the mortar mix remained unaffected by any shrinkage, and no cracking was evident. This apparent contradictory behavior can possibly be explained as follows. The mortar formed an adhesive bond between the old surfaces and when shrinkage set in to contract the mix, the adhesion a t the boundary was sufficiently strong t o prevent rupture. As a result, the central mix contracted while the material a t the boundary between the concrete surfaces and the mortar was elongated. Because of the high extensibility of the mixture containing polyvinyl acetate, the material !\as able to withstand this strain without rupturing. This important property, which helps to prevent cracking under usual conditions, makes polymercontaining mortars ideal for repairing cracks and for masonry mortars, and these mortars have been used successfully i n England, as reported by Farmer ( 9 ) . Within the accuracy of the experiments, the coefficients of thermal expansion of mortars with a 0.2 polyvinyl acetate t o cement ratio, with or without plasticizer, were practically the same as for pure cement mortar, a t about il X 10-6 inches per inch per C. Bond Strengths. When cured a t an average relative humidity of 35%, the bond strength of mortars with a polyvinyl acetate t o cement ratio of 0.2 to old mortar surfaces was 740 pounds for 4 square inches, about ten times the value of 75 pounds for plain cement mortars. The bond strength between mortars and old cement surfaces when cured in the fog room was practically the same a t 160 pounds for all mixes, including plain cement mortars. The bond strength t o steel was 1340 pounds for a 2-inch length on a 3/8-inch bar, about one and a half times the value of 840 pounds for plain mortars.

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Microscale, Millimeters, 10 X 6.5

Fracture Surfaces after Curing 28 Days a t 100% Humidity and 70" F. 0.20 Plasticizer to PVA ratio (Taken by Ultropak, 10 X 6.5)

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The use of plasticizer, however, caused loss of adhesive power, thus reducing the bond strengths of the polyvinyl acetate mixtures. Corrosion Resistance. The water absorption test with 0.2 polyvinyl acetate gave results showing that there was no significant variation in the water absorption as compared with plain cement mortar. Water a t or below 70" F. did not affect the strength or physical structures of polymer-containing briquets cured in a moist room even after considerable immersion. At 212' F., on the other hand, the briquets containing polyvinyl acetate spalled and d i s integrated after boiling for about 6 hours because the thermoplastic polyvinyl acetate softened a t this higher temperature. After 28-day curing in the fog room, four tensile briquets were immersed in a corrosive medium for 4 days, then removed and allowed to air dry for 2 days, and then tested for tensile strength. The strength and physical structure of mortars with polyvinyl acetate were not affected by benzene, dibutyl phthalate, brine, or concentrated or dilute sodium hydroxide. Dilute hydrochloric acid corroded both plain and polymercontaining cement mortars. Mortars containing polyvinyl acetate were more resistant to hydrochloric acid, both dilute and concentrated (Figure 13) than plain cement mortar, the reduction in strength being caused primarily by reduction of cross-section area due to corrosion on the surface. The corrosive action of d h t e sulfuric acid on the surface of the briquets was considerably less than that of dilute hydrochloric acid and the tensile strengths were apparently not affected. With concentrated sulfuric acid the polyvinyl acetate appeared t o have been carbonized on the surface. The strengths, however, of both pure cement mortar and polymer-containing mortar were increased by the action of concentrated sulfuric acid. This increase in strength is contrary to the results of usual sulfuric acid corrosion tests wherein specimens are immersed in the acid for

much longer periods of time-6 months to a year-with resultant weakening. I n this case, the effect was probably due to a chemical reaction occurring within the cement mix, wherein the unhydrated cement particles and the oxides of calcium formed stronger sulfate particles, which reinforced the already existing hydrated gel structure. From these data i t is concluded that the polyvinyl acetate cement mortar is no less resistant to sulfuric acid than is plain cement mortar. Air Entrainment. The mechanical method of combving cement, aggregate, and polyvinyl acetate emulsions determines t o a large degree the air entrainment. Mortars reported in this paper were mixed carefully by hand in order to minimize and standardize the amount of air entrainment due to the mixing alone. Only a few measurements of air entrainment were made, and these could not be correlated simply, with either polymer contmk or polymer particle size. However, in all cases, mortars containing polymer had entrained air contents a t least 3% above that of plain cement mortars. Behavior during Curing and Drying. The delay in the initial setting time when the polymer emulsion was added to the mortar was significant only with polyvinyl acetate to cement ratios of 0.3 or greater, since a t these compositions the continuous phase was the polymer. For these polymer concentrations, the rigidity of the whole specimen depended mainly on the hardening of polymer. With polymer contents less than 0.3 where the cement phase was continuous, the initial set was due to the formation of the cement gel. For the polymer-containing mortar cured in air with a relative humidity of 50%, the development of the final strength required the same time as plain cement mortar. For polymer-containing mortars cured in air a t 100% humidity, in which the polyvinyl acetate phase did not completely dry, the initial and final setting time of the portland cement portion of the mixes was not affected.

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C. 0.20 PYA t o Cement Ratio; 0.10 Plasticizcr t o PVA Ratio

Figure 12.

Fracture Surfaces after Curing 28 Days at 50% Humidity and 70" F. (Taken by Ultropalr, 10 X 6.5)

Figure 13.

Corrosion Effects of Acid Immersion for 96 Hours at 70" F. a. Pure cement mortars b. 0.20 PVA t o cement ratio; no plasticizer C. 0.20 PVA t o cement ratio; 0.10 plasticizer t o PVA ratio

However, the polymer-containing mortars remained soft until the polyvinyl acetate phase had attained sufficient rigidity. Particle Size of Polyvinyl Acetate Emulsions. The emulsion with particle sizes from 0.5 t o 2 microns produced mortars relatively weaker than mortars prepared with emulsions with 1t o µn particles. This may have been due to the fact t h a t with the smaller particle sizes there was a greater proportion of continuous polyvinyl acetate phase which weakened these mortar specimens. T i t h emulsion particle sizes of 2 to 10 microns, there was a tendency to produce mortars slightly weaker than with particles of 1 t o 5 microns. This behavior may have been due to the fact that the large particles were too large t o fill the usualavoids left within the cement gel structure and the large sizes disrupted the normal gel arrangement of hydrated cement. Within the limits of this investigation and for the cement used, it is concluded that a polymer with a particle size of from 1 t o 5 microns imparts the best qualities desired in the mortar and con-

crete mixes. The emulsifier in the polymer may contribute t o the strength of the mixes. Resistance to Cracking. STEADY LOAD. When a steady load is applied t o any concrete structure, the first failure almost invariably occurs as soon as a part of the structure is subjected to a tensile stress exceeding its ultimate strength. With a tensile strength above that of a plain cement mix, it is expected t h a t the polyvinvl acetate-cement mortar with a polyvinyl acetate t o cement ratio of 0.2 would be more resistant to cracking caused by a steadv load. * IVPACT STRESSES.The mortar mix with 0.2 polyvinyl acetate t o cement ratio had greater impact strength, 2.1 inch-pounds as compared with 1.7 inch-pounds, using h o d test specimens. Another criterion to support this point is the higher value of the modulus of rupture. This means that the mortar can absorb more energy from impinging bodies before cracking, as compared with plain cement mortars. THERVAL STRESSES. Temperature gradients cause uneven

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INDUSTRIAL AND ENGINEERING CHEMISTRY

contraction or expansion of concrete, which in turn create internal stresses both tensile and compressive. Because of its superior tensile strength and only slightly reduced compressive strength, mortar containing polyvinyl acetate would probably be better able t o withstand stresses due to sudden temperature changes than would plain cement mortar. The thermal coefficients of linear expansion of plain cement mortars and of polyvinyl acetate cement mortars were practically t h e same. Because of the higher values of the modulus of elasticity for plain cement mortars, the stress caused by a linear change resulting from the same temperature change would be much greater than for the polymer-containing mortars. This fact, coupled with the higher tensile strength of the polyvinyl acetate mortars would seem to indicate that this new material should be more resistant to cracking. SIGNIFICANCE AND APPLICATIONS

One of the outstanding results of this work is that the portland cement mortars containing polyvinyl acetate emulsions as admixtures show maximum improvement in properties when cured in air a t ordinary temperatures and humidities. This is in contrast t o plain cement mortars which require a water or damp curing in order to achieve optimum properties. This self-curing, combined with improvements in tensile and bond strengths, extensibility, and resistance t o abrasion, impact, and corrosion, makes this material extremely interesting as a possible means of improving some of the qualities of portland cement mortars. Among the uses for which this material may offer particular advantages are:

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1. Floor toppings and road surfacings for which greater tensile and impact strengths, greater resistance t o abrasion and corrosion, and increased resiliency offer a considerable resistance t o cracking 2. Wall and ceiling cement plasters where increased bond strengths and increased resiliency, combined with self-curing properties, make it particularly valuable 3. Masonry surfaces where self-curing and bond strengths are extremely important 4. Limited structural uses, such as for tanks and pipes, where tensile strength is an important factor

It is particularly significant that the polyvinyl acetate to cement ratio of 0.2 which produces the optimum physical properties is the most workable mixture. It is highly probable that because of the excellent workability the physical properties can be even further improved by decreasing the water to cement ratio. This conclusion was supported by limited experimental evidence. FURTHER WORK

This work is being continued in order t o determine the usefulness of other polymer emulsions, and stmudiesare also under way on the effect of the water t o cement ratio and the workability

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of the mix and with concrete beams. The addition of pigments and various aggregates is also under investigation. Patent applications are being filed. ACKNOWLEDGMENT

The authors wish t o thank Sidney J. Baum of the American Polymer Corp. for suggesting this problem and for his opinions during the course of this investigation. They also wish to express their appreciation t o W. Tereshkevitch and B. R. Ashton for their assistance during the preliminary stages of this work and to E. A. Hauser for his assistance in taking the microphotographs. The American Polymer Corp. of Peabody, Mass., graciously contributed all the materials used. J. A. Murray and A. J. O’Neill of the Building-Engineering and Construction Department of the Massachusetts Institute of Technology contributed useful advice during the tests and deserve special thanks, LITERATURE CITED I

(1) Amagna, S. V., S. M. thesis in chemical engineering, Massachusetts Institute of Technology, 1951. (2) American Polymer Corp., Peabody, Mass., Tech. Data Sheet P-20. (3) Am. Soc. Testing Materials, Standards, Pt. 111 (1949). (4) Ashton, B. R., S. M. thesis in chemical engineering, Massachusetts Institute of Technology (1951). ( 5 ) Barbee, J. F., Concrete, 52, No. 1, 103-4 (1944). (6) Bogue, Robert H., “Chemistry of Portland Cement,” New York, Reinhold Publishing Corp,, 1947. (7) Brownmiller, L. T., J . Am. Concrete Inst., 14, 193-210, 212-13 (1943). (8) Colony, R. J., Columbia Univ., Eng. Scientific Paper, 3 (February 1951). (9) Farmer, N. W., personal communication, 1951. (IO) Farmer, N. W., PZastics (London),4, 89 (1950). (11) Griffiths, L. H., Soc. Chem. I n d . (London),1947, pp. 578-80 (12) Griffiths, L. H., Trans. Inst. Rubberlnd., 22,170-4, (1947). (13) Mason, P. N., and Manning, J. F., “Technology of Plastics and Resins,” New York, D. Van Nostrand Co., 1945. (14) Mellor, B. B., S. M. thesis in chemical engineering, Massa’ chusetts Institute of Technology, 1952. (15) Sindler, J., Mass. Inst. Technol., Chem. Eng. Dept. Papers, 186 (1916). (16) Stern, H. J., India Rubber WorEd, 102, No. 2, 37-40 (1940). (17) Swaze, M. A., J . Am. Concrete Inst., 19,317-31, (1950). (18) Tereshkevitch, W., S. M. thesis in chemical engineering, Massachusetts Institute of Technology, 1951. (19) U. S. Dept. Interior, Bur. Reclamation, “Concrete Manual,” 5th ed., 1949. (20) Van der Bie, C. F., India-Rubber J., 112,229 (1944). (21) Van Gils, C. E., and Clarkson, H., India-Rubber J . , 118, 773-4, 776 (1950). (22) Wren, W. G , I n d i a Rubber World, 99, No. 6, 29-31 (1939). (23) Wren, W. G., Trans. Inst. Rubber Ind., 13,189-229 (1937). RECEIVED for review April 1, 1952. ACCEPTEDDecember 22, 1952. Presented before the Division of Industrial and Engineering Chemistry a t the 121st Meeting of the AMERICANCHEMICAL SOCIETY, Buffalo, N. Y .