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Ind. Eng. Chem. Res. 1998, 37, 420-426
Development and Evaluation of New Multipurpose Soil Additives Shawqui M. Lahalih† Research Institute, King Fahd University of Petroleum & Minerals, P.O. Box 1127, Dhahran 31261, Saudi Arabia
New compositions of multipurpose soil additives were developed and evaluated. The compositions comprise three main components including a polyanionic component like sulfonated ureamelamine-formaldehyde (SUMF), a bridging agent like sodium chloride, and an aggregating and bonding agent like urea-formaldehyde (UF). The water- soluble compositions were tested on four types of soil including dune sand, Marl, and Gatch-type sandy soils with varying degrees of chemical composition. The compressive strengths of the various soils improved by a factor of 2-50 after they have been treated with the new compositions. The addition of 1.0% of SUMF improved the compressive strength of dune sand from 0.16 to 5.8 kg/cm2, Marl-A sand from 15.4 to 28.0 kg/cm2, Marl-B sand from 7.1 to 11.3 kg/cm2, and Gatch sand from 4.74 to 13.38 kg/cm2. The addition of 0.5% each of SUMF and UF with 0.25% of sodium chloride improved the compressive strength of dune sand from 0.16 to 9.1 kg/cm2. The erosion resistance of dune sands to water and wind improved significantly by the addition of small doses of the new compositions. When dune sands were treated with 30 g/m2, they resisted erosion by water for 6 h under a water flow rate of 6 L/min. And they resisted 60 km/h wind speed when they were treated with 20 g/m2 of the present compositions. The new compositions are water-soluble, and they are environment friendly chemicals. They are relatively cheaper than commercially available products, and they are more effective soil additives. They can be utilized for many applications including stabilizing sand dunes, and strengthening road beds and foundations and they can be used as good soil conditioners. A postulated mode of action of these compositions and their interaction and synergism with each other and with other organic and inorganic additives are also presented and discussed. Introduction Countries with arid climates and poor soil structures, like those in the Arabian Gulf Region, suffer from a multitude of problems concerning sand and soil erosion, sand dunes movements, high water salinity, limited water resources, and poor vegetative growth, among others. These problems are major contributors to the problem of desertification. In addition to these agricultural-related problems, there are also civil engineering problems which includes erosion of highways and road embankments and the premature failure of road bases under the combined action of loads and flooding. Other problems are noticed in sand storms around military and civilian airports, military airfields and installations, oil fields, farms, and residential areas. Not only these problems compound the problems of desertification but they are also becoming environmental and ecological issues. Natural and synthetic chemicals have been used to improve the soil structure. Chemicals that are added to soil are called additives, modifiers, or conditioners. They perform several functions when they are added to the soil. Some improve the soil’s strength, aggregate stability, and water infiltration. Others limit water adsorption, soil erosion by water and wind, water evaporation, and water runoff. In general, synthetic chemicals have been used for both engineering purposes and for agricultural-related applications. For engineering applications, interests † Tel.: (03) 860-4779. Fax: (03) 860-3989. E-mail: shawqui@ dpc.kfupm.edu.sa.
centered around developing soil stabilizers like bonding agents, water repelling agents, erosion control agents, and grouting agents. For agricultural applications, the interests were to develop synthetic polymers as surface active agents, aggregating agents, seepage control materials, and evaporation retardants. Because of the diverse nature of these applications, many chemicals have been synthesized and tested. Historical developments of these chemicals were provided in great detail by many reviewers (Brandt, 1972; Moldenhauer and Gabriels, 1972; Kinter, 1975; Payne et al., 1975; Chamberlain, 1988; DeBoodt, 1993). The following is a very brief review of the historical development of these soil additives as they were reported by the above reviewers. Up to the 1950s, polysaccarides, straws, and natural gum were used in agriculture as aggregating agents. Because of their ineffectiveness, large quantities were required which made them of limited use. During the 1950s hydrolyzed polyacrylonitriles and vinyl acetate-maleic anhydride were introduced by Monsanto as soil aggregating agents and as soil erosion control agents. These materials were too expensive, do not provide good strength to the soil, and were difficult to apply. Finally, there was a waning of interest in those products and consequently were pulled out of the market after they had been used for about 10 years. During the 1960s, polyvinyl alcohol (PVA), polyacrylamides, and acrylic polymers were actively pursued as aggregating agents and as erosion control agents. Those products also faced problems as being difficult to apply, and they were also too expensive. During the 1970s emulsions like bitumen, asphalt, styrene-butadiene, and polyvinyl acetate were also
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tested as bonding agents and erosion control agents as well as grouting materials for road beds and foundations. Urea-formaldehyde-based materials, chromium lignosulfonates, and polyurethanes were also tried for the same purposes. The problems with these materials were many. For example, bitumens and asphalts gave bad colors and therefore were not acceptable aesthetically; hence, they were of limited use. In addition, these materials are hydrophobic and they tend to reduce water infiltration into the ground which results in more water runoff and poor vegitative growth. Bitumens and asphalts also cannot improve the load-carrying capacity of the treated soil. Other materials gave toxic chemicals like chrome from chromium lignosulfonates, and the organic solvent used with polyurethanes could be harmful to the applicator. Polyacrylamides are also thought to give a toxic acrylamide monomer. All of the abovementioned products gave poor compressive strength when they were mixed with soil, and high dosages were required to achieve reasonable compressive strength values which made them very expensive to apply. Finally, during the 1980s and 1990s most research interests centered on the development of polymeric hydrogels to conserve irrigation water consumption in agriculture. Materials like cross-linked polyacrylamides and starch-grafted polyacrylonitriles appeared in the market. However, these materials were considered to be very expensive and are of limited use. They could not be used to bond the soil aggregates nor aggregate the soils they are applied to, and hence these materials could not be used as soil stabilizers. Of special interest is the development of soil stabilizers that can be used as bonding agents, erosion control agents, and grouting agents for sandy-type soil-like dune sand. Urea-formaldehyde-based materials have been used quite extensively to consolidate sandy soil. They were used with cross-linking agents like calcium hydrogen phosphate (Gopal, 1982), in combination with polyvinyl alcohol and HCl as a cross-linking agent (Sakata et al., 1970), or without any cross-linking agent (Agarwal, 1981). Modified urea-formaldehyde like ureaphenol-formaldehyde (Prakash and Kaporr, 1981) and urea-furfural-formaldehyde (Lukania, 1968) were also used to stabilize and consolidate sandy soils. Other materials were also used to stabilize sandy soils. Lahalih and co-workers reported on the development of a novel soil stabilizer that was effective in stabilizing Gatch-type sandy soil (Lahalih et al., 1988). They used a mixture of a polyanionic polymer with polyvinyl alcohol and achieved very good stability for this type of soil. However, the effectiveness of this composition on very fine dune sand was limited; in addition, the use of polyvinyl alcohol poses the problem of not being easy to dissolve in water and being too expensive. Therefore, the objective of this paper is to report on the development of a new chemical composition that can be used for several applications. Some results on its performance to improve the load-carrying capacity of several sandy soils and some discussion on its mode of action will be presented. Experimental Section Materials. Commercial grades of urea, melamine, formaldehyde (37%), sulfuric acid, and caustic soda were used as received. Urea and melamine were supplied by Saudi Arabia Basic Industries (SABIC) while Saudi
Formaldehyde Company supplied the formaldehyde solution. Off the shelf salts and acids used were sodium chloride, disodium tetraborate, potassium nitrate, potassium dihydrogen orthophosphate, phosphoric acid, and boric acid. Inorganic additives tested included commercial grades of barite (barium sulfate) and bentonite (sodium montmorillonite) and were supplied by Baroid Saudi Arabian Ltd. Portland cement type-1 was also purchased from the local market. Dune sands and Marl-type sands were obtained from the desert in the Eastern Province of the Kingdom of Saudi Arabia. Commercial polymers like carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) were supplied by BDH Chemicals Ltd., England. Equipment. A standard 1-L jacketed reactor with stirrer and a condensor and a heating and cooling bath was used to prepare the two main components of ureaformaldehyde and sulfonated urea-melamine-formaldehyde condensates. The reactor is also equipped with a pH meter. The viscosity of the solutions was measured using Cannon-Fenske tube viscometers. Standard compression test molds were also used to prepare the test samples. Compressive strength measurements were made on a material testing system. Procedures. Preparation of the sulfonated ureamelamine-formaldehyde (SUMF) and urea-formaldehyde (UF) condensates were prepared according to a procedure outlined before (Lahalih and Absi-Halabi, 1987). Preparation of the various compositions with other admixtures were prepared in a normal way, depending on the end application. Performance Evaluation of the Soil Additives Unconfined Compressive Strength Test. For control specimens, water (160 g) is mixed with 1350 g of sand (
