I
ALAN S. MICHAELS and FREDERICK W. TAUSCH, Jr.
I
Department of Chemical Engineering and Soil Stabilization Laboratory, Massachusetts Institute of Technology, Cambridge, Mass.
Phosphorus Chemicals a s Soil Stabilizers Phosphate rock, sulfuric acid, ferric salts, and a new polymeric phosphoric anhydride may open up new prospects for phosphorus compounds a s low cost soil stabilizers
IN
RECENT years, the use of phosphoric acid as a soil stabilizer in highway, airfield. and related construction applications has commanded increasing attention (7, 3, 5). Its marked stabilizing abilit) in many aluminosilicate soils and its effectiveness a t low concentration levels have made it potentially competitive with conventional stabilizers such as cement, lime. and asphalt. Phosphoric acid has, however, certain limitations which have deterred its field utilization. Despite its effectiveness in moderately fine grained soils a n d its relatively low unit price, the cost of phosphoric acid stabilization is higher than that of conventional stabilization methods. I n heavy clay soils, where phosphoric acid in admixture with aliphatic amines has proved to be of unusual stabilizing effectiveness, the cost of amines makes stabilization economically prohibitive for all but rather special situations. Lastly, concentrated phosphoric acid presents problems in the field regarding transportation, handling, .and mixing with conventional processing equipment, where a noncorrosive, granular solid stabilizer is often preferred. This report briefly summarizes progress made in the authors’ laboratory toward circumventing some of the above limitations (4). Objectives of this work were to reduce the cost of phosphoric acid stabilization of moderately fine grained soils, to find a satisfactory granular solid substitute for phosphoric acid, and to find a low-cost substitute for amines in heavy clay stabilization with phosphoric acid. A mixture of ground phosphate rock a n d sulfuric acid shows promise as a low-cost substitute for phosphoric acid in the stabilization of low plasticity soils. Orthorhombic phosphoric anhydride is a satisfactory alternative to phosphoric acid for this purpose, if a safe, granular solid stabilizer is desired for field use. Trace amounts of this anhydride, used in conjunction with phosphoric acid, have been found to facilitate compaction of a low plasticity soil, and to improve strength.
-
In where phosphoric acid alone is inadequate to provide etabilization under wet conditions, incorporation of small amounts of ferric
chloride produces more satisfactory water-resistance than much more costly aliphatic amines. Further improvements in phosphoric acid stabilization of heavy clays are needed for economic processing in the field.
calciumphosphate-
or ~~~k
phosphate-Sulfuric Acid Mixtures Since the cheapest Source of PhosPhorUs is Phosphate rock-PredominantlY fluorapatite-it seemed approPriate to determine whether Soil could be successfullY stabilized by generation of Phosphoric acid in situ via reaction Of calcium Phosphates with sulfuric acid. The Soil selected for this study was a low plasticity (Plasticity index, p.I.2 = 6 ) clayey silt from Massachusetts, des$Fated MCS. Finely Pulverized Galcium PhosPhates-equivalent in PhosPhorus content to 2% HsP04 on dry Soil Weight-Were blended with dry Soil and water and a stoichiometric quantity of 98% sulfuric acid was added. The mixture was compacted under controlled conditions to essentially constant density, and allowed to cure in a water-saturated atmosphere for varying periods. Unconfined compressive strength of the samples, both as-cured, and after total immersion in water for 1 day, constituted the criteria of stabilization effectiveness. The effect of incorporation of a small amount of sodium fluosilicate-previously shown (5) to accelerate cure of phosphoric acid-stabilized soils-was also studied.
POTLNTIAL USES
Results (Table I) show that calcium phosphate-sulfuric acid mixtures are effective stabilizers, although less active than the corresponding concentration of phosphoric acid. Fluosilicate also accelerates cure with these systems as it does with phosphoric acid. T h e acid calcium phosphates are more active than tricalcium phosphates, evidently because of their higher water solubility and more rapid reaction with sulfuric acid. Of particular interest is the fact that pulverized rock phosphate (200mesh) is also a moderately effective stabilizer with sulfuric acid, especially when the rock and acid are preblended before incorporation with the soil. Addition of fluosilicate to rock-sulfuric acid mixtures was found to have little effect on stabilization, evidently because of the presence in the rock of adequate fluoride for cure-acceleration. Despite the fact that the stabilizing effectiveness of rock-sulfuric mixtures is about half that of phosphoric acid-per formula weight of phosphorus-the low cost of phosphate rock and sulfuric acid relative to phosphoric acid renders the former attractive. preliminary raw materials cost-calculations indicate that rock-sulfuric acid mixtures are competitive with cement and lime for low plasticity, clayey, or silty soils.
Orthorhombic Phosphoric Anhydride One dry, granular solid which has in the past been used as a phosphoric acid
OF PHOSPHORUS CHEMICALS
Phosphoric acid
Stabilization of clay soils for highway, airfields, and other constructions
Phosphoric acid plus aliphatic amines
Stabilization of heavy clay soils
phosphoricacid plus ferric chloride
Stabilizer for heavy clay soils
Ground phosphate rock and sulfuric acid
LOW cost substitute for phosphoric acid in lean
Orthorhombic phosphoricanhydride (Victor Chemical Works)
Solid substitute for phosphoric acid; also improves phosphoric acid stabilization of lean clay soils
clay; competes with lime and cement
VOL. 52, NO. 10
0
OCTOBER 1960
857
Table I. These Stabilizers Are Satisfactory for Silt and Clay Compressive
_.-
strength, P.S.I., 1-Day Humid Cure
+ 1-Day
Stabilizer immersion Ca3(PO& iHzSOa Mixture" 2% 270 H3POd f 0.5% NazSiFs 2.07Y0 Ca(HzP04)z 0.85% H2S0, Same 0.57, NasSiFfi 2.36% CaHPO4 1.707, &SO4 2.72% Caa(PO& 2.58% H2S04 Same 0.5$Zo NazSiFc 3.87, Rock Phosphate 2.6%
+ +
+
+
+
H2SOi
+
Same, preblended before adding t o soil
242 f 9
160 f 8
410 i 10
325 k 20
270 i 30
1 O O i 0
* 50
220 i 20
300 i- 10
95 i: 10
313 i 3
87 =k 8
487 i 20
150 i 10
222 1 12
90 It 10
270 i 10
125 i 5
655
Orthorhombic P2O8
+
2% 0.570 NazSiF6 1.25% o-PzOsb 0.5% Na?SiFs
+
305 1 17
235 i 15
340 1 5
225 zt 15
FeC13 and hlClrC 270 2% HaPo4
+ 0.5% CsHiiNHa 2% Hap04 + 1.62% FeCh 2% + 1.33% A1Ch 2% HaPo,
CrCL
+ 1.58%
240 f 5
0.i)
820 i 20
50 i 5
196 i 5
80 i 15
360 f 10
32
258f8
=k
1
7 1 2
Equivalent t o 2% H3P04. Satisfactory waterproofing additives for phosphoric acid-stabilized Vicksburg buckshot clay. a
Mass. clayey silt.
substitute in soil stabilization is phosphorus pentoxide (2). However, the hygroscopicity and corrosiveness of this material make it unattractive for field use. Recently, a new rorm of phosphorus pentoxide, designated as orthorhombic phosphoric anhydride (Victor Chemical LVorks) has become available. whose properties make it much more suitable for field application. This compound is a granular, nonhygroscopic solid which dissolves relatively slowly in water to form a mixture of polyphosphoric acids. T h e stabilizing effectiveness appeared to warrant evaluation. The stabilization efficacy of orthorhombic phosphoric anhydride in Massachusetts clayey silt (Table I) shows that the anhydride is an entirely satisfactory substitute for phosphoric acid, insofar as performance is concerned. At present, however, the cost of orthorhombic phosphoric anhydride is so much greater than that of technical grade phosphoric acid that field use of the former compound could be justified only where logistical factors were
858
Table II. Trace Quantities of Orthorhombic PzOb Significantly Increase Density and Strength of Phosphoric Acid-Stabilized Massachusetts Clayey Silt 1-Day Cure, 1-Day Immersion ?-Day Cure, 1-Day Immersion Compressive Density, 1b.l Compressive, Density, lb.! Stabilizer strength, p.s.i. cubic feet strength, p.s.i. cubic feet 2% 2%
160 =t 8
HrPOi
+ 0.0570 o - P ~ O ~ 340 L 30
controlling. Whether the large-volume price of the anhydride might eventually be low enough to make its field use economical is difficult to ascertain. During trearment of soil with the anhydride, it was noted that the compound had a marked dispersive action on the soil, allowing much higher densities to be developed on compaction. Tests were therefore performed to establish whether trace amounts of anhydride, used in conjunction with phosphoric acid, might have a beneficial effect on density and thus, on strength. Results of these tests (performed with Massachusetts clayey silt) are summarized in Table 11. As little as 0.05y0 anhydride, based on dry soil weight permits a density increase of nearly 2 pounds per cubic foot, with an attendant increase in compressive strength of nearly twofold. I t is clear that the anhydride is most promising as a trace additive. particularly since the total cost of an acid-anhydride mixture needed to produce a given level of stability in this soil is substantially lower than that of the larger amount of phosphoric acid required to accomplish the same result. The dispersive action of orthorhombic phosphoric anhydride evidently arises from the clay-deflocculating action of the polyphosphoric acids formed when the anhydride dissolves in water. This suggests that technical metaphosphoric acid may be a satisfactory and cheaper alternative. Unfortunately, the dispersing and densifying action of the anhydride is nullified by the presence of fluorides, so that the addition of fluosilicate to accelerate cure is contraindicated in this system.
131.0 132.8
195 i 10 420 i 5
131.0 133.5
montmorillonoids, evaluation of salts of these metals as additives with phosphoric acid seemed desirable. The soil selected for this study was a high-montmorillonoid soil (PI = 32) designated T'icksburg buckshot clay (VBC). Measured amounts of the chlorides of trivalent iron, aluminum, and chromium were preblended rvith the wet soil, and 27, phosphoric acid, based on dry soil was added. Samples were compacted to essentially constant density, cured for 1 day under humid conditions, and immersed in water for 1 day. As before, unconfined compressive strength constituted the measure of stabilization (Table I). Ferric chloride is seen to provide significant ivater resistance at a concentration of about 1.6y0on the soil? and yields higher soaked strength than is observed with 0.5Yc octylamine. Aluminum chloride is somewhat less effective at an equivalent molar concentration, and chromic chloride, even less so. Of the three salts, ferric chloride is cheapest? and is significantly less costly than octylamine (5 LIS. about 30 cents per pound). Substitution of ferric chloride for the amine is estimated to reduce by 30% the cost of phosphoric acid stabilizarion of Vicksburg buckshot clay to the levels indicated. Despite this improvement: the cost of acid stabilization of heavy clays remains much higher than that of soils of low to moderate plasticity, and substantial further improvement in properties must be achieved before the economics or heavy clay stabilization is favorable. Literatwre Cited
l o w Cost Waterproofing Additives
I n an earlier study ( 3 ) , it was established that relatively short chain aliphatic amines (in particular, octylamine) were necessary secondary additives with phosphoric acid for successful stabilization of high plasticity clay soils. These additives were deduced to function primarily by replacing the exchangeable metallic cations in the expanding lattice (montmorillonoid) minerals in the soil, and thereby reducing swelling of the soil on immersion in water. Since polyvalent metallic cations-iron, aluminum, chromiumare also known to reduce swelling of
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Lyons, J. Mi., Eng. News-Record 159, h-0. 7, 101-6 (1957). (2) Michaels, A. S.: Puzinauskas, V.. H i g h w a y Research Board Bull. 129, 26-49 (1956). (3) Michaels, A. S.: Tausch, F. W. .Jr.. Hi,qhwap Research B o a r d Bull. 241 (Januarv 1960). (4) Michaels, A . S., Tausch, F. Mi. J r . . Hi,chulay Research B o a r d Bull., to be pub-
lished.
(5) Michaels. A. S., Williams. P. M.. Randolph, K. B., IND.ENG.C H E K 5 0 , 889-94 (1958).
RECEIVED for review December 11. 1959 A C C E P T EJune D 17, 1960 Presented at the Highway Research Board Meeting, LVashington. D. C . : January 1960.
