I
ALAN S. MICHAELS, PETER M. WILLIAMS, and KENDALL B. RANDOLPH Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass.
Acidic Phosphorus Compounds as Soil Stabilizers ples were placed in a 115' C. oven and quantity, placed in a Harvard miniature SOIL sometimes called dried for 24 hours to constant weight. Compaction mold (a cylinder, 7.15 solidifiers, are used to increase the loadSTABILIZERS,
bearing capacity of the soil for construction of highways, airstrips, dams, or building foundations. Their selection involves a number of sometimes complicated problems (5, 703. No completely satisfactory method has been found for solidifying fine-grained soils having a relatively high clay content, yet these soils are frequently encountered. They are typically cohesive, strong when dry, but weak when wet. Suitable compounds must either impart adequate strength to the wet soil or prevent water absorption. Typical examples are calcium acrylate ( g ) which causes wet soil to retain strength, and amine-containing asphalt cutback (72) which prevents water absorption. Phosphoric acid as a cold-setting binder for metal oxides and ceramics has been frequently described, yet only recently (77, 72) has it been considered as a soil stabilizer. According to the proposed theory, its cementation action apparently involves formation of an insoluble phosphate glass, and it seems to meet several requirements for an ideal soil stabilizer; it is usable at low concentrations, causes reasonably rapid cementation, and has low cost. In this work, therefore, suitability of phosphoric acid together with a few related compounds and secondary additives were explored as soil solidifiers.
Procedure Five representative soils were examined (Table I), none of which without treatment retained measurable compressive strength after 24-hour immersion. Soil a t its natural water content was blended with additional water in a small Baker-Perkins sigma-mixer until visibly homogeneous. A weighed quantity of phosphoric acid, in which was dissolved other additives, was added and mixed. When the mixture appeared uniform (usually 2 to 4 minutes), a weighed
cm. by 3.33 cm. in diameter), was subjected in a Carver hydraulic press to two-end static compaction. Usually, the compactive load used was that required to produce a soil density equal to that obtained by standard dynamic compaction (Standard AASHO) of the untreated soil a t its optimum moisture content. However, most of the additives greatly reduced the compactive effort required to achieve normal density; therefore, to achieve uniform specimens of reproducible strength, higher than normal densities and normal loading were used. In other words, densities reported were obtained using standard compactive force on freshly prepared mixtures. Normally, six samples of each formulation were prepared. Because additives reacted with the soils, each sample molded became progressively more difficult to compact than the preceeding one. To minimize density variations between successive samples, the compactive load was increased as the mixture aged. T h e strength of all samples prepared from a given batch agreed closely. After compaction, samples were extruded from the mold, weighed, and cured for varying periods a t room temperature-either a t ambient humidity (30 to 60%) or over water in sealed desiccators (100% R.H.). After this, samples were reweighed and their volumes were determined by mercury displacement or caliper measurement. After curing, three of every six identical samples were totally submerged in water a t room temperature for a specified period, then blotted dry, reweighed, and their volumes redetermined. All samples were tested in unconfined compression using a specially designed low-rate-of-strain instrument. The peak load prior to obvious failure was used to calculate ultimate compressive strength. Strains a t failure were low (1 to 2%) and not measured. After testing, sam-
Additives Compd. Reagent grade HaPO4 (91 wt. %); PZOSpowder; NazSiFa Armeen 16D (hexadecylamine, C I ~ H ~ ~ N H ~ ) Armeen ISD (octadecylamine CISHBNH~) Benzene phosphonic acid (CsHaPOsHn, crystalline) Butyl, phenyl, and iso-octyl acid phosphates Rosin amine silicofluoride
i
Manufacturer ..e
Armour Chemical Div. Victor Chem. Works Virginia-Carolina Chemical Co. Davison Chemical Co.
Standard compaction curves showing compacted density as a function of molding water content were determined for Massachusetts clayey silt. Soil and enough water to give the desired water content were premixed by hand, and after equilibrating for 24 hours in a sealed container, water content for a small sample was determined. Next, phosphoric acid was mixed by hand with the soil, and the mixture immediately compacted in the Harvard miniature mold by the standard dynamic procedure (three layers, 25 40-pound blows per layer). The specimen was extruded, weighed, dried at 115' C. for 24 hours, and reweighed.
Phosphoric Acid All soils studied can be stabilized with phosphoric acid or its equivalent, phosphorus pentoxide (Tables I1 and 111). However, stabilization generally decreases with increasing fineness of the soil. With soils a t or near their optimum moisture content for compaction, increased phosphoric content seems to increase strength after curing, an effect which is more marked for coarser soils. Also, higher compacted densities are achieved. This dispersive action is evident during mixing-mixing-power input is greatly reduced. Initial water content of the soil appears to be an important variable. When initial water content is 1.3 times optimum for compaction, compressive strength after curing and subsequent immersion decreases three- to fivefold (Table 11). This reduction in strength seems directly related to decreased compacted density. Although few tests have been made, heavier soils seem to develop greater strength when molding water content is below optimum for compaction of the untreated soil. Another factor relating to initial water content which seems to influence rate and extent of strength development is concentration of phosphoric acid in the pore liquid. Presumably, the rate a t which the acid attacks aluminosilicates varies directly with its concentration in the surrounding solution, but the rate a t which cementing alumino- or silicophosphates precipitate varies more or less inversely. Hence, with a high initial acid-water ratio, a rather large quantity of aluminosilicate should be solubilized and produce a relatively high VOL. 50, NO. 6
JUNE 1958
889
~~
ultimate strength, although a t a slow rate. Conversely, a low acid-water ratio should cause a rapid strength build-up, but low ultimate strength. Table I1 seems to support this picture. At 1007, relative humidity, compressive strength for a 1-week cure is twoto fourfold greater than that for a 1-day cure. This increase appears to continue with age, and its rate seems to be greater for higher phosphoric acid concentrations. Also, when immersed in water after curing a t 100% relative humidity, samples molded at their optimum water content for compaction show little loss and sometimes even increase in compressive strength. This indicates that solidification caused by phosphoric acid can occur in essentially saturated soil. When water can evaporate during curing: all soils studied had lower immersed strength than those cured under saturated conditions (Tables I1 and 111). This marked influence of moisture during curing, more serious in heavy clay soils than in coarser soils, probably results from shrinkage during drying; thus, many interparticle attachments formed during cure are disrupted by the shrinkage, and a less tightly bonded network results. Since heavier soils shrink much more on drying than coarser soils, greater loss in strength of the heavier soils is expected. Table TI, where soils are arranged in order of increasing fineness, shows that shrinkage on drying, and swelling and
~
Properties of Soils Used VL MCS FBSC
Table 1. PSS 2.69
Specific gravity Liquid limit, % Plastic limit, % Plastic index, % Optimum water, % Max. as-molded derlsity Dry soil, lb./cu. ft.
2.77 20.0 14.0 6.0 12.3
... ... ...
14.2 115.0
121
VBC
2.80 40.8 26.0 14.8 18.2
2.72 32.0 18.0 14.0 16.5 110.0
2.67 60.0 27.8 32.2 22.0
105.0
105.0
16.0
30
4.6 0.2 1 . 6 i 0.1 1 . 8 f 0.1 32
4.6 0.3 1 . 9 i 0.1 1.1 f 0.1 65
3 0 1 3 20 i 10
2 0 f 3 20 i 10
1 5 i 3
2 5 f 3
2 0 1 3
2 5 1 3
Chem. Propertiesa 'Cat. ex. cap., meq./100 g. PH Sol. salts, yo FeaOa, % Organic matter, % Ethylene glycol retention, mg./g.
2.0 4.8 0.3 1 . 5 4 ~ 0.1 0.91 0.1 14
21
26
Compn.b, % 35 50
Quartz Feldspar Kaolinite Illite Chlorite Montmorillonoid
...
2.9
f 3 f 10
35 20
f 15
30
... ... ...
...
10 3
i
2
...
40
...
25
... ...
...
...
...
...
Particle SizeC
% finer than
85 47 12 3
1 mm. 0.1 mm. 0.01 mm. 0,002 mm.
95 62 22
100 100 28 3
100 61 28 18
10
100 100 55 35
Typical Strength Data Dry density, lb./cu. ft. 116 Dry compressive str., 80 i 5 p.s.i. As-molded compr. str., 22 psi. 0.9 Shrinkage on drying,
123 118 108 340 i 10 180 i 3 440 47
23
0.8
i 10
13
5.1
123 790
i 40
48
2.8
14.6
% PSS = Portsmouth silty sand: MCS = Mass. clayey silt: PBSC = Ft. Belvoir sandy clay; - 74 p fraction. - 74 p fracVL = Vicksburg loess: VBC = Vicksburg buckshot clay. tion: determined by DTA, x-ray, and microscopy. Sedimentation.
On Dry Soil HaPOa HzO, (91yo),%% 11 14.3 11 11 15
2 2 5 10 10 2
16.5
Table 11. Phosphoric Acid Soil Stabilization (Unconfined compressive strength and wt. % volatiles at test on dry soil) Cured 1 Wk. at 100% R.H. Cured 1 Day at 100% R.H. No Imm. 1 Day Imm. 1 TTk. Imm. No Imm. 1 TVk. Imm. Volat., Volat., Volat., Volat., Volat., C.S.,p.s.i. % C.S.,p.s.i. % C.B.,p.a.i. '3% C.S.,p.s.i. 7% C.S.,p.s.i. % Mass. Clayey Silt, 121a 110 42 250 335 i 14 36
10.0 12.5 12.6 9.7 15.6
70
...
... . . a
215 i 10
... ...
...
16i 4
16.2
... 12.6 ...
84 16 i 4
12.8 14.2
502 ==! 93 108 i 8
10.1 17.7
...
...
Ft. Belvoir Bandy Clay, l l O b 16 i 6
...
...
92 3z 98 f 373 f 1290 -I. 137 f
2 23 63 190 26
11.2 13.0 12.1 9.7 14.9
149 i 18
94 25 383 605 122
5~ 10 i 1 f 65 i 33
Av.
Dens.a
i 33
12.4 14.2 12.2 11.6 17.8
128 124 130 134 115
15.0
49 rt 2
16.2
118
21.7 29.6 15.5 27.0 28.4
45 i 1 17 i 4 130 f 10 81 i 15 27 i 1
24.6 32.2 20.2 25.3 35.5
107 94 116 90 90
Vicksburg Buckshot Clay, 105b
a
... ...
108 i 32 19.3 45 i 1 24.6 172 f 17 f 2 30.2 13 f 0 32.8 29 i 206 20.2 D D 418 f 33 i 2 22.2 16 f 1 30 f 0 78 =t 10 i 1 30.0 18 i 0 37.7 19 i Lb. dry solids/cu. ft. b Density of untreated soil at optimum water content, lb. dry soil/cu. f t . 2 2 10 10 10
24 31.6 16.5 24 31.6
...
. I .
...
...
...
.
I
.
5 3 20 32 1
D
=
disintegrated.
Properties of Soils Treated with Phosphorus Pentoxide on dry soil (equivalent to 4.2% H3P04) ; data av. of 3 samples] 7 Days' Water Immersion 14 Days' Cure HnO HzO, H20, absorp., Dens." C.S.,p.s.i. % Dens." % C.S.,p.s.i. %
Table 111.
[ P206concn., 3%
At Molding
HzO,
Soil
%
Dens.&
PSS MCS FBSC
10.4 11.1 14.6 16.4 19.9
127 130 117 115 109
VL
VB C a
Lb. dry solids/cu. ft.
890
2.1 2.2 2.4 3.1 5.1
* Samples disintegrated.
INDUSTRIAL AND ENGINEERING CHEMISTRY
132 135 130 119 127
990 i 90 1165 i 130 950 i 50 880 f 80 1225 i 100
9.7 10.9 13.4 22.7 b
7.4 8.3 10.3 18.8
i 0.2 i: 0.2 i 0.3
f 0.4 b
130 130 121 106 b
282 153 71 10
i 11 f 13
f 6 i 2 b
C.S. Ratio Rewet: Cured 0.28 0.13 0.08 0.01 b
. S O I L STABILIZERS water absorption on immersion, tend to increase accordingly. Thus, the question arises whether phosphoric acidstabilized soil cured for a long period under humid conditions and subsequently dried and reimmersed, would retain appreciable strength. Experiments aimed at answering this question are presently under way. When VBC was stabilized a t 16.5% molding water content with 10 weight yo of phosphoric acid, the system had a total liquid volume equal to that of the water a t optimum content (24 weight yo). No immersion strength was developed after the l-day humid cure but the strength (130 p.s.i.) measured after a I-week humid cure followed by 1-week immersion was the greatest of any VBC-phosphoric system tested. This is undoubtedly caused by higher compacted density of the sample (116 pounds per cubic foot compared with 105 for plain soil a t optimum water content) and suggests that optimum strength will be achieved when the water content at molding is adjusted to give maximum compacted density The water content is evidently lower than that for optimum Compaction of untreated soil. This formualtion failed to retain strength on immersion after 1-day humid cure probably because when compacted a t this water content, the soil is considerably below saturation and therefore imbibes water by capillarity. High capillary pressures within the soil mass when cementation has barely begun may cause disintegration. That the same soil, similarly treated but containing optimum water content retains strength, further supports this hypothesis. Thus it appears that phosphoric acid or phosphorus pentoxide at concentrations between 2 and 10 weight % can substantially improve compressive strength and water resistance of a wide range of aluminosilicate soils. Finergrained soils develop less strength and water resistance than coarser-grained soils; cementation by the acid occurs slowly, with strength increasing over an extended period of time; curing under humid conditions results in higher strength and strength retention on immersion than curing under drying conditions, particularly for fine grained soils. Best results are achieved when moisture level is that which gives maximum density on compaction. Compaction Characteristics. In Figure 1, the abscissa is total liquid content of the soil during compactionLe., water plus phosphoric acid (5.1 grams of 91% phosphoric acid equals 2.9 cc.). Over the liquid content range studied, phosphoric acid increases compacted density by about 1.5 pounds per cubic foot, and appears to shift the peak density to a slightly lower liquid content.
This behavior is qualitatively comparable to, but less marked, than that for clay deflocculants such as the alkali phosphates. The density increase caused by phosphoric acid, although only about 1%, can often improve strength considerably on solidification. Water content of 9.4%, corresponding to maximum density in the presence of phosphoric acid, should yield the strongest product after compaction. Therefore, soil warer content should always be adjusted to optimum value. In the field, dry soils present no difficulty, but wet soils cannot be conveniently dried. Perhaps a practical solution to this problem is to treat the soil with phosphorus pentoxide in lieu of phosphoric acid-e.g., 3.4% is stoichiometrically equivalent to 4.7% of phosphoric acid. This amount of anhydride will consume 1.3% of the soil water on hydration. Hence, Massachusetts clayey silt (MCS) containing as high as 10.7% moisture can be treated optimally with phosphorus pentoxide. The total volatiles loss of the treated soil after oven-drying always exceeds the total water content of the soil as molded (Table IV). The difference between free water and total volatiles averages about 1.2 pounds per 100 pounds of soil; subtracting the water present in phosphoric acid (0.5 pound per 100 pounds of soil) leaves 0.7 pound of volatiles unaccounted for. This volatile component is certainly water; therefore, it is concluded that 0.7 pound of water is liberated by chemical reaction for every 4.6 pounds of phosphoric acid which reacts with the soil. Within experimental error, this corresponds to roughly 1 mole of water per mole of phosphoric acid. Assuming reaction of the acid with alumina, a stoichiometric relation would explain the observations : Al(OH),
+ HIPO~
+
Ai( OH)zPOz(OHz) SHzO
Aluminum dihydrogen phosphate (variscite) is produced by the reaction of kaolinite with orthophosphates (7). Rate of Cure Studies. For the data showing humid-cure strength as a function of time, all samples could not be
Table IV.
I30
121
120
I15
0
M21+ WATE
I
I
II
12
I3
14
TOTAL LlPUlDS CONTENT AT HOLDlNG ~001100G . DRY SOIL 1
Figure 1 . Phosphoric acid increases compacted density by about 1.5 pounds per cubic foot for Massachusetts clayey silt prepared at identical density; therefore compressive strength at each specified cure time was corrected to constant density by plotting compressive strength against the corresponding density, and interpolating or extrapolating to the reference density (Table V and Figure 2). Strength development continues over long periods of time, reaching extremely high levels after only a few days. T h e ultimate compressive strength of the soil system tested, although not determined, is conservatively estimated at 800 to 1000 p s i . ; this far exceeds the highest value necessary for most sot1 stabilizers. The sigmoid shape of the curing curve (Figure 2) also suggests a two-step reaction for the cementation process: first, a reaction of the acid with a soil component to form an intermediate noncementing compound, and second, a gradual conversion of this intermediate into a cementitious product. This mechanism is confirmed visually by the reaction of hydrated alumina with warm phosphoric acid: Alumina can be partially dissolved in the acid to yield a clear, viscous solution which, on aging or addition of more alumina, becomes turbid and solidifies with the evolution of considerable heat.
Other Acids Nitric and hydrochloric acids seem to stabilize moderately Vicksburg buckshot clay but their effectiveness is far less
Compaction Tests
(M-21 treated with HaPOa)
H3POP Concn.,
a
Tot. Liq., CC./lOO G. Soil
Wet Dens.*
Dry
Dry
%
Init. HxOa, %
Solids*
Dens.
5.00 5.14 5.08 5.31 5.02 5.10
8.75 9.2 9.70 9.93 10.51 11.02
11.74 12.13 12.70 12.96 13.37 13.93
140.1 145.1 145.0 144.9 141.7 139.2
128.6 132.0 131.4 131.0 127.2 124.9
On dry soil.
* Lb./ou. ft. a8 molded.
VOL. 50, NO. 6
124.1 127.3 126.8 126.2 122.6 120.3
Total Volat.", % 9.61 10.62 10.64 11.11 11.75 12.30
JUNE 1958
891
COMPRESSIVE
+
AVERAGE VALUE CORRECTED TO 126.2 pcf DRY DENSITY
4 I 2 4 HR
450
STRENGTH, PSI
too
50
0
200
IMMEDIATE IMMERSION
- DISINTEGRATED
IMMEDIATE IMMERSION
-
//////////////A
l-
150
250
8 0
9 I 2 4 HR
I
I
2 4 HR
y///%l 2 4
HR
IMMEDIATE
DISINTEGRATED
IMMERSION
100% R H + 2 4 HR
IMMERSION
ONLY
A
Figure 3. Rewet strengths for amine-treated Massachusetts clayey silt stabilized with 5% of phosphoric acid
9 7 % WATER CONTENT 100 ?4 R.H. CURE
I
20
10
30
CURING
TIME,
40
4 Figure 2. The sigmoid shape of f
50
HRS
than that of phosphoric acid, despite their molal concentration (Table VI). Sulfuric acid is ineffective. Phosphoric acid increases compacted density, but the other acids cause a decrease. Nitric and hydrochloric acids probably react with clay minerals to form the corresponding water-soluble aluminum salts which, because of gradual hydrolysis and volatilization of the acids, revert to hydrated alumina which functions as an interparticle cement. Sulfuric acid is ineffective presumably because of its low volatility. Liberation of aluminum ion by this reaction can also account for reduction in compacted density of the soil because aluminum salts are active flocculants for clays. The unique ability of phosphoric acid to stabilize soil establishes that acidity alone is not adequate for strength development in fine-grained soils.
0.2 % AMINE ON DRY SOIL 9.8 % INITIAL WATER CONTENT
the curing curve for Massachusetts clayey silt stabilized with phosphoric acid suggests a two-step reaction
Secondary Additives Amines. Phosphoric acid can stabilize fine-grained soils but, particularly when partially cured, the soil loses appreciable strength when immersed in water. Furthermore, all phosphoric acid-treated soils disintegrated rapidly when immersed immediately after compaction. Therefore, it was presumed that if the cure process could be either accelerated or if capillary forces which draw water into the compacted soil could be reduced, significant increases in strength and/or water resistance could be achieved. Organic cationic compounds reduce wettability of soil particles (4). Therefore, primary aliphatic amines were dissolved (Table VI1 and Figure 3) or dispersed in phosphoric acid prior to its incorporation with the soil (MCS). Adding only 0.2% of octylamine on
Table V. Cure Rate Data (M-21 treated with 5% HsPO4; initial HzO content, 9.7%) Cure Time",
Av. C.S.,
Hr.
P.S.I. 60.5 79.2 102 112 153 144 176 210 288 410 452 816
11/2
4I/i 6 12 151/z
18 21 25 30 42 48 456
f 1.5 f 4.6 f 8 f 1 f 14 f 11 f 10 f 8 i 40 zk 10 f 21 f 18
Av. Dens. 127.4 f 0.8 131.0 f 0.7 131.8 f 0.7 131.4 f 0.4 131.8 f 0.7 129.2 zk 0.9 131.0 i 0.5 131.1 f 0.4 131.9 f 0.8 130.9 f 0.3 131.0 f 0.1 131.1 f 0.0
*
*
AV.
Volat.c, % 10.1 10.8 10.0 10.6 10.3 9.7 10.6 11.0 10.3 10.7 10.5 11.3
Corrected Str.d, P.S.I. e
80 94
107 138 e
176 206 260 412 452 800 Corrected t o 131.0
On dry solids. d Lb. dry solids/cu. f t . as molded. 100% R.H. lb. dry solids/cu. ft.; equivalent to 126.2 lb. dry soil/cu. f t . e Too low for correction.
892
INDUSTRIAL AND ENGINEERING CHEMISTRY
dry soil appears to accelerate the early rate of cure, probably because the amine, by slightly elevating the p H of the pore fluid, increases the rate of precipitation of the cementing aluminophosphates. Also, retention in strength on immersion after 24-hour humid cure was significantly improved. and moderate compressive strength upon immersion immediately after compaction was developed. This amine also appears to reduce the ultimate humid-cured strength of this soil, although this is not of major consequence. On the other hand, the 16- and 18-carbon aliphatic amines at comparable concentration levels do not exhibit similar beneficial effects on early cure and water resistance-all samples disintegrated rapidly on immersion after compaction. There is. however, some evidence that these amines do promote strength increase on immersion after partial cure. The results are consistent with the postulated acid stabilization mechanism and the amine action on soils: Cementation depends on attack of the soil grains by the aqueous acid and adhesion between the acid reaction products and the soil surface. The amines, on the other hand, adsorb on the soil surfaces to produce a hydrophobic monolayer which tends to prevent acid attack of the soil and/or prevent adhesion of the cementitious reaction products. Thus, adsorption of amine would primarily reduce the number of particle contact points at which cementation or reaction with acid occurs, with consequent strength reduction. Because the longerchain amines adsorb more firmly and
SOIL STABILIZERS
(Using Acid Name HsPOa, 91%
Table VI. Stabilization of Vicksburg Buckshot Clay HC1, and H3P04; mixing Ht0, 24% on dry soil; untreated soil at optimum Ha0 content, 105 lb./cu. ft.) Cured 1 Wk. at 100% R.H. Cured 1 Day at 100% R.H. 1 Wk. Imm. No Imm. 1 Wk. Imm. No Imm. Wt. y* C.S. Volat., % C.S. Volat., % C.S. Volat., % C.S. Volat., %
2 "08, 95% 2.85 HCl, 37% 4.3 &sod, 96% 5.8 Lb. dry solids/cu. it.
108 f 32 50 65
...
45 f 1 9f1 7 f0
19.3 28.0 25.5
...
24.6 31.2 30.0
172 29 59 16
D
f5 f3 f2
49 f 9 14 f 1 12 f 2
21.7 23.7 23.7 27.0
f0
AV.
Dens.0
21.9 29.3 27.3
107 95 97 99
D
D = disintegrated.
form more hydrophobic monolayers, they should reduce cementation more than the shorter-chain amines. Strength retention on water immersion depends on both interparticle cementation and prevention of water absorption (more marked for the longer chain amines) ; optimum results are achieved when these two phenomena are in proper balance. Whether amines containing more or fewer than 8 carbon atoms are more beneficial than octylamine remains to be established. Acidic Organophosphorus Compounds. Because octylamine enhanced strength retention on immersion, it was believed that acidic organophosphorus compounds might be even more active (Table V I I I ) . None of the acid phosphate esters tested caused retention of strength on immersion directly after compaction, but butyl and iso-octyl esters did after 24hour cure. Only the butyl ester failed to reduce the 24-hour cure strength. Benzene phosphonic acid, on the other hand, does cause retention of strength on immediate immersion after compaction, and produces the highest immersed strength so far realized after 24-hour cure; nevertheless, this compound appears to reduce the ultimate strength attainable by phosphoric acid treatment. All of the organophosphorus additives, particularly benzene phosphonic acid, cause substantial reduction in compacted density. This behavior is typical of compounds which exert a waterproofing and flocculating action on soils. Surprisingly, however, this density reduction is not accompanied by the normal marked reduction in strength. Therefore, acidic organophosphorus derivatives, particularly benzene phosphonic acid, may be useful as secondary additives to rather wet soils which do not respond satisfactorily to phosphoric acid alone. Also, they may result in high strength and water resistance in soils such as MCS where high compactive loads can further densify the treated soil. Benzene phosphonic acid was further investigated (Table IX). As the proportion of this acid to phosphoric acid is increased, the cure rate under humid conditions is increased, but the ultimate cure strength is reduced; this
accounts for the maximum in 24-hour cure strength when both acids are present in equal weight concentrations. Immersion after this cure results in a 10% strength loss for phosphoric acid alone, but a 15% gain for specimens containing 0.570 benzene phosphonic acid, and no change for specimens containing higher percentages of this compound. Strength on immersion for 24 hours immediately following compaction is greatest for soils containing equal weight concentrations of both acids; whether this reflects the accelerated cure rate exhibited by the organic acid, or improved resistance to water attack, remains obscure. I n any case, mixtures of these acids produce more rapid strength development and greater water resistance in soils such as MCS than phosphoric acid alone. Fluorine-Containing Compounds. I n some spot tests, fluorides had a marked accelerating effect on the reaction of alumina with phosphoric acid. If this occurred in soil, perhaps the cure could
Table VII.
Amine Concn., Wt. % Control C.S., p.s.i. Volat. init., % Volat. at test, % Dry dens.a Octylamine, 0.2% C.S., p.s.i. Volat. init., % Volat. at test, % Dry dens." Armeen 16D, 0.2% C.S., p.s.i.
Volat. init., % Volat. at test, % Dry dens." 0.5%
be sufficiently advanced so that on immersion, breakdown from capillary pressure could be avoided. Of the fluorides tested, the effect of 0.5% sodium fluosilicate is dramatic; after 24-hour humid cure, compressive strength was nearly 2.5 times that attained with phosphoric acid alone. Of perhaps greater significance is the fact that immersion for 24 hours after this cure resulted in no strength loss. O n immersion directly after compaction, however, sodium fluosilicate-treated samples disintegrated. Treatment with rosin amine silicofluoride, on the other hand, showed only moderate strength increase over phosphoric acid after 24hour humid cure, but it did impart strength on immersion directly after compaction. As with benzene phosphonic acid, rosin amine silicofluoride causes a substantial reduction in compacted soil density; thus, under more favorable compaction conditions, strength development with this additive may be greatly improved. Nevertheless,
Effect of Primary Amines on Massachusetts Clayey Silt [Stabilized with 5% of HsPOa (91%)1 Cured at 100% R.H. 48 hr.
24 hr.
6hr.
24 hr. 424 hr. imm.
455 10.4 10.4 131.0
200 10.9 10.9 131.0
85 10.0 10.0 131.0
177% 10 11.3 11.3 131.1
365 f 5 11.0 11.0 129.8
239 f 40 11.1 11.1 130.1
88 f 5 10.9 10.9 130.8
229 f 1 11.6
410 f 30 10.8 10.8 130.5
147 f 10
11.1 11.1 129.0
... ... ... ...
C.S., p.s.i. 415 % 25 10.1 Volat. init., % Volat. at test, % 10.1 Dry dens." 131.2 Armeen 18D, 0.2% 335 f 40 C.S., p.s.i. 10.8 Volat. init., % 10.7 Volat. at test, % 130.5 Dry dens." Lb. dry solids/ou. ft. D = disintegrated.
... ... ... ...
... ... ... ...
11.5 128.5
24Hr. Imm.
-
D
6 4 f 12 11.7 12.9 128.8
172 f 1 11.8 11.8 129.0
D
... ...
... ...
... 9 . .
... ... VOL. 5 0 , NO. 6
JUNE 1958
893
Table VIII.
Table IX. Effect of Benzene Phosphonic Acid (MCS; 9.8% initial water content; HaPO4, 91%) Additive,
Effect of Organophosphorus Compounds
[MCS stabilized with 5% of H3POa (91%) ; initial H20 content, 9.8% 1 Concn.n, Cure, Hr. Av. Volat.*, Wt. 100% C.S., Additive % R.H.Imm. P.S.I. Dens.* % 126.2 10.7 None 24 0 210 f 20 10.7 48 0 455 i 40 126.2 126.2 11.0 175 i 20 24 24 0 24 0 CsHsPOaHz 0.5 24 0 210 f 20 122.0 11.1 48 0 345 35 122.0 11.1 122.0 11.2 24 24 244 i 3 0 24 3 3 5 1 122.0 12.6 Acid phosphates 123.8 10.3 ButylC 0.25 24 0 210i20 48 0 11.0 24 2 1 0 f 12 123.8 24 0 24 0 PhenylC 0.5 24 0 175 5 123.0 11.0 48 0 24 24 1 3 5 i 4 0 123.0 11.9 0 24 0 Iso-octylc 0.33 24 0 150520 124.2 10.2 48 0 24 24 185 i 42 124.2 10.9 0 24 0
...
I . .
BPA
%
'
Rk: Imm.
5.0
0
*
...
*
...
...
...
...
...
...
...
...
...
...
... ...
... ...
On dry soil. * Lb. dry soil/cu. ft. Mixed mono- and dihydrogen phosphates containing varying amounts of polyphosphates.
very small quantities of fluosilicates, used with phosphoric acid can contribute significantly to soil strength and water resistance. Lower aliphatic amine silicofluorides-e.g., octylamine silicofluoride-are likely to be particularly effective for phosphoric acid stabilization of fine-grained soils.
Economic Considerations Assuming an average soil density of 120 pounds per cubic foot (3200 pounds per cubic yard) phosphoric acid treatment costs about $1.74 per weight per cent of 100% acid used. I n areas where coarse aggregate can be obtained and placed for $2.50 to $3.00 per cubic yard, this treatment would have to be successful at a concentration of 1.4 to 1.7% by weight to compete. However, this yard-for-yard basis of comparison may be misleading, because the ability of acid-treated soil to carry loads without confinement may allow use of a lesser volume of soil than of aggregate. Furthermore, increasing acid concentration may reduce more than proportionately the total treated soil requirement so that cost may be further reduced. I n any event, phosphoric acid may prove competitive with existing methods of or alternatives to soil stabilization and may be one of the least costly methods for handling heavy clay soils where excavation and replacement is either impossible or undesirable. Of the secondary additives examined, only sodium fluosilicate would add but little to the soil-treatment cost. The organic additives, despite their low use concentrations, can greatly increase the over-all cost of treatment. Whether
894
0.5
5.0
7
0 0 0 0
1
1
0
1
0 1 2
0 0 0
7
0
1
1 1
0
3.0
3.0
1 1
0
0 1 2
0
3.0
1
0
Lb. dry soil/cu. ft.
improvements gained from their incorporation are enough to reduce more than proportionately the phosphoric acid requirement, treated-soil requirement, or both remains to be established. Physical requirements of the field may determine the desirability of using these additives, with cost but secondary. Unconfined compressive strength is believed a reliable index of solidifying ability, but it is seldom adequate to establish definitely the suitability of an additive for field use. Knowledge: therefore, of the resistance of phosphoricstabilized soils to shock loading, protracted leaching with water, cyclic freezing and thawing; and cyclic wetting and drying is needed before the true potentialities of this stabilizing system can be thoroughly established.
Conclusions Phosphoric acid or its anhydride in concentrations between 1 and 10 weight yo is a promising stabilizer for a wide range of fine-grained soils. Fine-grained soils require more acid for effective stabilization than do coarse soils. Under humid conditions, compressive strength reaches reasonable levels in 12 to 24 hours and continues to increase several fold over a period of several weeks. Stabilization with phosphoric acid is markedly dependent on water content of the soil. Strength is highest when the soil water content is adjusted to give maximum compacted density and drops rapidly with increasing water content. Treated soils develoD maximum strength and retain higher st'rength on immers%n when cured under humid conditions. A fraction of a per cent of a primary
INDUSTRIAL AND ENGINEERING CHEMISTRY
0 1 2
Av.
P.S.1'. 63 i 5) 210i20 455 i 40 720 i 40 175 i t 0 0 46 i. 10 210 i 201 345535 585 f 601 244 i 3 3 3 i 11
1 1
2 5 0 5 30 2 5 0 i 4\ 135i 7J
0 0 0 1 1
70 i 41 165 i 7 ZOO* 13 165 i 31 7 0 i 2,
0
* On dry soil.
i
Dens." 126.2
Volat.* % 10.7 10.7 10.7 10.7 11.0
...
122.0
11.4 11.1 11.1 11.1 11.2 12.6
115.3
11.3 12.0 12.6
118.4
12.5 12.4 12.3 12.6 13.3
aliphatic amine or an acidic organophosphorus compound used with phosphoric acid appears to accelerate the curing process and improve significantly strength retention on water immersion; however, ultimate strength is reduced. Octylamine and benzene phosphonic acid are particularly effective. Sodium fluosilicate in concentrations of less than 1% by weight greatly accelerates strength development in phosphoric acid-treated soil. Amine silicofluorides are also promising for both accelerating cure and improving water resistance.
References (1) Cole, C. V., Jackson, M. L., J . Phjis. Chem. 54, 128 (1950). (2) Comeforo, J. E., J . Am. Ceram. Soc.
31, 191 (1954). (3) Ibid.,p. 427. (4) \ , Hoover. J. M.. Davidson. D. T.. Highway Research Board Bull. 129; 10-25 (1956). (5) IND.ENG.CHEU.47, 2230-81 (1955). (6) Kingery, W. D., J . Am. Ceram. SOC. 33.239 11950). (7) Ibid.,'p. 242. 18) Ibid.. D. 247. Lam'ge, T. W., J . Boston SOC.Civil Engrs. 38, 200 (1951). Lambe, T. W., Michaels, A. S., Chem. Eng. News 32, 488 (1954). Lyons, J. W., Eng. Nems-Record 159, NO. 7,101-6 (1957). Michaels, A. S., Puzinauskas, V., Highway Research Board Bull. 129. 26-49 (1956). RECEIVED for review September 4, 1957 ACCEPTED March 3, 1958 Work conducted under contract with the
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