HYDRATION-LIMITED, POLYMER=MODIFIED HYDRAULIC CEMENTS HER MA N
B
. W A G N E R , Drexel Znstitute of Technology, Philadelphia, Pa.
The rate of evaporative water loss from thin sections of compositions of portland cement mortar containing various polymers of different types and levels was studied. The polymer type and level do not affect the water loss rate. Moduli of elasticity and rupture were determined for various polymer types, levels, and hardening conditions, and found to depend primarily upon the extent of the cement hydration achieved and adhesion level developed. Results of microscopic studies of the structures of the compositions and fracture surfaces are given, and methods for evaluating adhesion levels in such compositions are described.
ARLIER
study (Wagner, 1965, 1966) of the behavior of
E polymer (latex)-modified hydraulic cements has involved
specimens of appreciable thickness, with correspondingly high degree of cement hydration. O n the other hand, such compositions are often applied in relatively thin sections-for example, as brick-mortar joints, floor surfacings, concrete patches, adhesives, and paints. In these applications thicknesses range from about 0.1 to 1 cm., and the environment during hardening of the compositions is commonly dry, in the sense that neither evaporative loss of water nor loss through imbibition by a dry substrate is prevented. Thus, data obtained upon bulkier specimens, such as 2-inch cubes, d o not fully evaluate the effects which follow the relatively rapid egress of water that is encountered with such thin sections. Studies have been made of the degree of hydration achieved in these thin sections and its relationship to such physical properties as the modulus of rupture, modulus of elasticity, creep behavior, and adhesion. Initial Rates of Hydration Reactions
T h e rates of hydration of various portland cement component compounds vary widely. Typically, tricalcium silicate ((23s) and beta-dicalcium silicate (CZS), which together constitute about three fourths of the composition, hydrate very slowly. T h e remaining fraction, consisting chiefly of tricalcium aluminate ( C A ) , and a ferrite solid solution of average composition generally close to (CaO), . A1203.Fez03, referred to as CaAF, ordinarily hydrates much more rapidly. T h e relative rates of hydration, however, may be affected by the composition and change with time. T h e “cement gel” that gradually develops as the product of these hydration reactions comprises chemical species of high specific surface area, which include within their structure the “gel pores” (Powers and Brownyard, 1947a, 1947b). Although the quantity of water required for complete hydration of each cement compound varies somewhat among the compounds, a n average, over-all value for water requirement for complete hydration is 0.25 of the weight of the portland cement powder for chemical reaction, with a n additional 0.15 fraction of water needed for concomitant inclusion within the gel pores. T h e former is often designated as reacted water, and the latter, as gel water. T h e “gel-space ratio” associated with any hardened composition has been found, by Powers and Brownyard (1947b),
to be a prime variable in determining strength properties. Because the value attained for this ratio depends directly upon the extent of hydration prevailing a t any given time during hardening, it is important to have information on the rate of hydration. In particular, the very early rate of hydration is of significance in thin-layer hardening because here the loss of water by evaporation and/or imbibition by substrate limits the time period during which water is available for participation in the hydration reactions. Unfortunately, few data are available to give directly the time dependence of the amount of reacted water (and associated hydration products) during this early hydration period. However, a very comprehensive body of calorimetric data has been obtained by Verbeck and Foster (1950), including this early period. From these data, combined with the values for total heat of complete hydration, obtained by Lerch and Bogue (1934), a single equation can be calculated to permit the estimation of reacted water as a function of effective reaction time and cement composition. T h e following equation is obtained, wherein w, is the weight of reacted water per gram of (initially) anhydrous cement powder and t is the time of curing, in hours. T h e coefficients given are applicable a t 75’F., and during the first 24 hours of hydration:
For a typical ASTM Type I portland cement having the potential composition below. the following w , values are calculated for the hardening times indicated (Table I): (CIA, 9.9%; G A F , 8.5%; C & 52.6%; CZS,20.2%; CaS04, 3.6%; MgO, 2.1; KzO, 0.7; Na20, 0.2; other (TiOz, SiOz), 2.1%.
Table 1.
Reacted Water Corresponding to “Early” Hardening Times, for Typical Cement Reacted Water Hardening (LO,,) Value, Time, Hr. G . WaterlG. Cement 8 12 24
0.061
0.073 0.090
VOL. 6
NO. 4
DECEMBER 1967
223
For example, after 24 hours, this cement has attained less than 40% of complete hydration (w, = 0.24), corresponding, for this composition to complete hydration. Evaporative Water Loss from Polymer-Modifled Compositions
T h e next phase of this study was the determination of the rates at which water is lost by evaporation from mixes of various polymer-modified compositions, which were spread in “thin” sections ranging in thickness from 0.3 to 0.9 cm. Here, 10- by 10-cm. square sections of the wet compositions were evenly metered over glass plates. Water loss was then determined, as a function of time, a t 78-80°F., in a “static” atmosphere, a t 35 to 50% relative humidity. T h e compositions listed in Table I1 were measured, and the data of Table I11 obtained. The “test number” identifies both the composition and its applied thickness.
Table II. Thin-Section, Water Evaporative Compositions Composition, Parts ( Wt.) Test No. 100 portland cement 1, 4, 7, 10, 14, 18 300 Ottawa sand 42 water 100 portland cement 2, 5, 8 300 sand 16 poly(vinylidene, vinyl chloride),
film-forming type
32 water 100 portland cement 300 sand 16 poly(vinylidene, vinyl chloride),
3, 6, 9
non-film-forming type
32 water 100 portland cement 300 sand 16 polyethylene 32 water 100 portland cement 300 sand 16 polyacrylate polymer 32 water 100 portland cement 300 sand 17 neoprene 31 water
11, 15, 19
12, 16, 20
13, 17, 21
Table 111.
Test No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
224
Section Thickness, Cm . 0.341 0.324 0.353 0.494 0.480 0.612 0.853 0.817 0.857 0.341 0.387 0.316 0.329 0.513 0.486 0.458 0.448 0.744 0.757 0.734 0.723
The rate of water loss during the first hour is essentially independent of section thickness (Table 111). At later times, as expected, it is higher for the thicker sections. Analysis of the data shows also that water permanently retained at 7880’ F. and 35 to 50% relative humidity (chemically reacted) by the compositions ranges from about 6 to 10% of the dry cement weight, and bears no discernible relationship to the polymer type present. Cement-Gel Surface Area, at Terminal Evaporative Water Loss
After evaporative water loss had ceased, the specific surface area of the cement gel previously formed was determined for compositions 7 , 9, 19, 20, and 21. These contain, respectively, no polymer, poly(vinylidene, vinyl chloride), polyethylene, polyacrylate, and neoprene polymers. T h e adaptation of the BET method described (Brunaver et al., 1956) was used for these determinations. T h e adsorption data obtained show the following specific surface areas for the compositions indicated (Table IV). When specific surface area developed under non-evaporative conditions is calculated, the results of Table V are obtained. I n comparing these values with those of Table I V it is concluded that the period of effective hydration with these thin sections must be of only 1 to 2 hours’ duration. As smaller thicknesses are involved, this period of effective hydration is obviously even shorter. A more factual interpretation of the course of hydration during the process of evaporative water loss is that of a receding “water line” during the early period of hydration, with effective hydration occurring only a t depths below this water line. Thus, the specific surface areas developed should be regarded as mean values obtained across the thin-section thickness. Polymer Disposition within Thin-Section Compositions
I t is apparent from Table 111 that water lost by evaporation corresponds to about 70 to 85% of that initially present. Wagner (1965) reported that polymer coalescence does not typically occur until a t least hours following mixing and the beginning of the hydration reactions. T h e removal of sub-
Evaporative Water loss (Grams), for Various Compositions, at Different Section Thicknesses (78-80’ F.; 35 to 50% RH)
Initial Water Weight,
G.
6.4 3.6 4.5 9.5 5.9 8.3 16.9 9.9 11.4 6.4 5.6 4.1 3.7 9.9 6.7 6.1 5.3 15.9 11.1 10.2 8.8
Time, Hours
__ 0.25
0.79 0.61 0.69 0.83 0.67 0.87 0.82 0.72 0.85 0.79 0.70 0.63 0.63 0.91 0.71 0.68 0.51 0.77 0.85 0.75 0.75
0.50
1.37 1.19 1.25 1.51 1.44 1.57 1.44 1.47 1.34 1.37 1.27 1.19 1.24 1.54 1.31 1.16 1.00 1.57 1.44 1.38 1.22
1 2.74 2.10 2.34 3.00 2.84 3.05 2.81 2.52 2.55 2.74 2.21 2.23 2.13 3.06 2.33 2.09 1.75 2.76 2.53 2.48 2.38
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
2 4.89 2.66 3.45 6.17 4.43 5.26 5.17 4.49 4.29 4.89 3.47 3.17 3.23 5.51 3.94 3.64 3.20 5.31 4.54 4.66 3.88
4
5.32 2.66 3.63 7.76 4.89 6.39 8.31 6.48 7.43 5.32 4.18 3.11 3.38 8.29 5.03 4.78 4.09 9.05 6.87 6.69 5.87
6 5.32
...
3.63 7.76 4.89 6.45
... ... ...
5.32 4.25 3.11 3.38 8.34 5.25 4.88 4.10
... ...
... ...
8
24
144
...
...
... ... ...
...
...
... ... 6.45 12.2 7.56 8.69
...
4.25
... ...
8.34 5.33 4.88 4.10 12.08 8.42 7.67 6.84
.
I
.
...
...
... ...
... ... ...
12.9 7.56 8.83
12.9
... ... ... ... ... ...
...
... ...
...
... ...
...
...
8.83
... ... ...
... 5.33
...
l2:08 8.88 7.97 6.86
Figure 5. Polymer profile in glass-intel._._ .-.=._.. film-forming poly(vinylidene,vinyI) composition
Figure 6. Figure 5
Polymer profile in region o
_. "on-
.,.__.. ._ .-- ._.._.
the interior by diffusion of these particles under the polymer concentration gradient set up very early in the evaporation process. Later when the "falling rate" (second stage) drying period begins, no further liquid movement occurs, and polymer coalescence must then ensue as this (water vapor diffusion) drying proceeds. I n any event, no evidence is seen of the formation of a polymer film a t the air interface, and the absence of such is consistent with the observed, essentia evaporative water loss from all of the at Modulus of Rupture in Relnlion lo Polyn Type, Level, a n d Hardening Condilions These determinations were made on bars measuring 30.4 by 5.1 by 0.63 cm. Three levels of each polymer type were employed and the compositions allowed to harden under the following conditions.
Figure 7. Figure 8
Polymer profile in region odjacent to region of
__
. ..,. ner profile in air-interface reaion of non._ film-forming polylvinylidene. vinyl) composition
. .
Sealing was accomplisnea by wrapping tne trays in several thicknesses of Saran wrap and coverins the top surface with a glass plate. Following each step in preparation and treatment each specimen was weighed, for later determination of evaporative water loss, degree of air entrainment, and water gain resulting from immersion in water. Specimens were formed in aluminum trays, lubricated for release with silicone jelly on their inner sides, and covered with T F E fluorocarbon film on the inner bottom surface. Hardening was allowed to take place in these trays, which could be disassembled to release the bars for physical testing. Modulus of rupture far freely. supported, centrally loaded .. bars is related to load and bar dimensions by the equation:
R=-
3LI
2ba2
where L, 1, b, and n are, respectively, the break ing load, length, I L. ... width, and thickness of bar. Since the dimenslons 01 TILT_ nstant in these determinations, the modulus of rupture were co is here 1?roportional to the breaking load found for any composition . These values are shown in Table VI, on a relative :&I_ L A_ A ^ _ _ l ^ _ _ l ^Pin """:""-A *,.+Irpr-.lnA -i., scale, WLLU LUG / I L U U ~ , ~ vaLIuc nrnsLLcY containing no polymer, at 14 days. After unsealed hardening (treatment A) the polymer-
~.
Tredment A
B C D
226
Descriptian 14 days unsealed, at 75' F., 4 0-5070 R H 14 days sealed, at 75' F. *" Treatment B, followed by 7 aays unsealea at. _I > - r., 40-50yu RH Treatment C, followed by 24 hours' immersionin water, at 75" F. 1
.
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I R E C P R O D U C T RESEARCH A N D DEVELOPMENT
_ 1 ~
~
ll.r
~
ILLILL.
modified compositions (except the polyethylene modification) show an increase in strength relative to that of the friable, unmodified composition. As polymer level is lowered, this effect diminishes. Following sealed hardening (treatment B) the strength of the poly (vinylidene, vinyl chloride) and polyacrylic latexes is increased over the (now much stronger) unmodified composition. T h e other latexes studied d o not increase in strength, under this hardening condition. Under sealed hardening conditions, followed by drying (treatment C ) , all polymer-modified compositions, except the polyethylene and poly (styrene, butadiene) ones, exceed the unmodified composition in strength. This treatment generally produces maximum strength. Here again, where the polymer at its highest level exerts a strengthening effect, this predictably diminishes with reduction in the polymer level. Rewetting (treatment D) decreases the strength below that obtained in treatment C. Modulus of Elasticity in Relation to Polymer latex Type, level, and Hardening Conditions
I n making the determinations reported above the maximum (midpoint) deflection was measured as loading was progressively increased. Since modulus of elasticity is given for these bars by the equation:
M=-
~ 1 3 s (4a3b)
where s is the deflection corresponding to any load L , and the other quantities are as noted earlier; the ratio Lls, taken from the slope of the initial, linear load-deformation curve, is a measure of this modulus. T h e results thus obtained for these compositions are presented in Table VII, again as related to the unmodified, sealedhardening composition taken a t a reference value of 10. Here, a lower value indicates a more “flexible” material. Treatment A modification with all the polymer latexes produces greater flexibility, although this effect must also be considered in relation to the relatively low strength values associated with this unsealed hardening condition ; treatment B also generally increases flexibility, except with the high polymer content levels of the poly(vinylidene, vinyl chloride) and polyethylene; treatment C either leaves flexibility at the treatment B level, or slightly decreases flexibility; trratment D generally retiirns flexibility to about the treatment B value. Creep Behavior and Additional Determinations on Preceding Compositions
Qualitative observation indicated that creep becomes more pronounced as the breaking load is approached, and a t the
Relative Modulus of Rupture, in Relation to Polymer Type, level, and Hardening Condition Hardening Harden1 ng Condition Condition and Relatiae and Kda!ive Polymer Modulus of Poljmer Modulus of Composition Parts, Weight Level Rujture Composition Parts, WPight Leoel Rupture 100 PC 100 PCa A
