Quick lime-gypsum interactions in stabilized soil bases for concrete highways

CEMENT and CONCRETE RESEARCH. Vol. 14, pp. 529-532, 1984. Printed in the USA. 0008-8846/84 $3.00+00. Copyright (c) 1984 Pergamon Press, Ltd.

QUICK LIME-GYPSL~ INTERACTIONS IN STABILIZED SOIL BASES FOR CONCRETE HIGHWAYS

S. Schlorholtz and T. Demirel Department of Civil Engineering Iowa State University, Ames, Iowa 50011 USA

(Communicated by D.M. Roy) (Received Nov. 17, 1983)

ABSTRACT Lime stabilization of clayey soils is ve~, common throughout many parts of the world. Typically if heaving is observed after quick lime (Ca0) stabilization, then one may think that insufficient lime and/or poor slaking and mixing techniques may be the root of the problem, Actually, there are several other reasons for the observed heaving. One explanation is that the lime may have been hard burnt, thus rendering the lime inactive until months later. Another explanation is due to the interaction of the quick lime with gypsum (CaSO 4 2 H20) in the soil to be stabilized. One may conclude that lime, Type I portland cement, or Class C fly ash stabilization of high gypsum bearing soils would at best produce poor results because of the possibility of expansion due to the formation of ettringite. Introduction Sulfate attack is probably the most common form of chemical attack experienced by portland cement concrete. The purpose of this paper is to present comments on the nature of sulfate attack in lime stabilized soils. A portion of the paper includes the presentation of data and conclusions obtained from an actual construction project that experienced detrimental subbase heaving due to sulfate attack. The Nature of Sulfate Attack Sulfate attack in portland cement concrete is typically represented by equation 1 (i) although there are several combinations of reactants that may produce the same product (i.e., ettringite). 3CaO.AI203.CaSO4.12H20+2CaSO4.2H20+I8H20 + 3CaO.AI203. 3CaSO4.32H20 (monosulfoaluminate) + (gypsum) + (water) ~ (ettringite)

(i)

Actually the reaction may be much more complex than shown above and at least one author claims that this equation alone does not explain all the expansion problems normally associated with sulfate attack (2,3,4,5). In this paper we will follow a slightly different mode of thinking since the subject under investigation is sulfate attack in stabilized soils rather than sulfate attack 529

530

Vol. S. Schlorholtz

14, No.

4

and T. Demirel

in concrete. Assume we can idealize a lime-clay system as denoted in equation 2, where the vague term "clay" simply refers to an alumino-silicate of high specific surface that has no self-cementitious properties. This reaction is pozzolanic in nature and thus is a fairly slow reaction: CaO + H20 + excess

clay ~ CaaAlb.cH20 + CaxSiy.zH20 + clay

(2)

where a, b, c, x, y and z are variables that are dependent on temperature, pressure and the molar ratios of reactants. If gypsum is added to the system and a fairly high moisture content is maintained then reaction 3 may occur. CaaAlb.cH20 + CaSO4.2H20 ~ 3CaO.AI203.3CaSO4.32H20

+

...

(3)

If reaction 3 took place in a soil that had been compacted to a high density (90 to 95~ standard proctor density) then detrimental expansion may take place due to the formation of ettringite. The reactions that have been listed above are only very simplistic models, the real nature of the soil-limegypsum interaction is much more complex, depending heavily on soil properties, lime activity, and the soil moisture content. A Case History Figure 1 illustrates what appears to be the influence of sulfate attack on a lime stabilized soil. The soil samples were taken from borings made at several different locations in a road construction project. The project was located in the southern United States and the borings were made after localized heaving was reported to the consulting engineers (approximately eight months after the project was completed). The consulting engineers ruled out the possibility of freeze-thaw heaving. Ettrin$ite

Reaction

Figure la shows a X-ray diffraction (XRD) trace of the virgin soil (i.e., not treated with lime). Figure ib shows the XRD trace of a soil taken from a borehole in a zone that was stabilized with quick lime (CaO) and showed no measurable heaving. Figure ic shows a soil sample that was taken from a borehole located in a zone that was treated with CaO and showed severe heaving. All XRD traces were made using a Siemens D 500 diffractometer with copper Ks radiation. The diffractometer is equipped with a graphite monochromat~r. The obvious difference between the virgin soil (Figure la) and the treated soils (Figures ib,c) is the presence of gypsum in the latter. The consulting engineer acknowledged that gypsum rock was present in portions of the construction project. The gypsum rock was probably inadvertently pulverized and incorporated into the subgrade during the mixing phase of the project. The sample taken from the heaving zone (see Figure ic) also shows that a considerable amount of ettringite had formed. A sample taken from the zone that did not experience heaving (see Figure ib) may also show the presence of ettringite but if it is present then it is in much smaller quantities. Apparently, the formation of ettringite in the highly compacted subbase caused the heaving because no expanding clay minerals were evident in the diffraction traces. A point of major concern to the owner of the project would be how to attempt to eliminate further heaving without having to tear up the pavement and then remove and/or restabilize the subbase soil. Any attempt to restabilize the existing subgrade with lime or Type I portland cement would be very questionable since the soil still contains large quantities of gypsum. Actually, due to the current lack of knowledge of the mechanism of this type of chemical attack in soils it is impossible to predict how much (if any) future expansion may take place.

Vol. 14, No. 4

531 LIMIt, GYPSI5I, INTERACTION,

SOIL STABILIZATION

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Other Possible Reactions One interesting aspect of equation 2 is the rate at which it proceeds. As mentioned earlier typically it is assumed that thisreaction proceeds slowly (measured in terms of days) so one may speculate on the types of reactions that may occur in a quick lime-gypsum system before the clay-lime (pozzolanic) reaction. If, for example, insufficient water was used to slake the quick lime then reaction 4 is thermodynamically possible since the Gibbs free enero gy of formation (&G298) is -37 k-cal/reaction. Reaction 4 3CaO + 2CaSO4.2H20 ~ 3Ca(OH) 2 + 2CaS04.!~ H20

(4)

could then be followed by a subsequent reaction in which the gypsum hemi-hydrate would react with water from the environment to form gypsum (dihydrate). Both of these reactions result in a positive volume change (i.e., expansion). The end products of the reactions would be gypsum and calcium hydroxide (assuming total hydration occurs) and thus the pozzolanic reaction is still possible and as described earlier it could be followed by the formation of ettringite (see reaction 3). Reaction 4 is possible and it does occur in gypsum-quick lime mixtures stored at 55 + 5°C. However, the reaction is quite slow and requires several weeks for completion (see Figure 2). Figure 2a shows the gypsum-lime mixture after one day at 55°C. Figure 2b shows the same mixture after 18 days at 55°C; conversion from CaSO4.2H20 to CaSO4.½H20 is complete and the free water is readily used to convert CaO to Ca(OH) 2 Since the reaction is so slow it need not be considered further. Stabilization of soils with Class C fly ash may also create a system containing calcium oxide, calcium sulfate (anhydrite), and tricalcium

532

Vol. 14, No. 4 S. Schlorholtz and T. Demirel

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aluminate since these three compounds are often found in Class C fly ashes. The formation of ettringite is commonly the major reaction that occurs in the early stages of hydration of Class C fly ashes, whether or not this could lead to detrimental future expansion in the presence of excess gypsum is presently not known. Also, the free lime present in some fly ashes does not appear to hydrate readily, an indication of some type of inactivity in the calcium oxide that may lead to future problems (6). Summary and Conclusions In summary, it has been shown that sulfate attack can occur in stabilized soil bases if the proper conditions are met. The sulfate attack which appears to result in the formation of ettringite may cause detrimental expansion in subbases that are compacted to high densities. Several other reactions were considered which may contribute to expansion in construction projects when quick lime and/or Class C fly ash are used for the stabilizing agent and a free source of gypsum is available. References i.) 2.) 3.) 4.) 5.) 6.)

S. Mindness and J. F. Young. Concrete. Prentice-Hall: Englewood Cliffs, N.J., 1981. S. Chatterji and J. W. Jeffery. Mag. Conc. Res., 15144], pp. 83-86, 1963. P. K. Mehta. Amer. Ceram. Soc., 1514], pp. 204-208, 1967. P. K. Mehta. Cem. & C o n c . Res., 311], pp. 1-6, 1973. P. K. Mehta. Cem. & C o n c . Res., 1313], pp. 401-406, 1983. S. Schlorholtz. Unpublished M.S. Thesis, Iowa State University, 1983.