High Strength Phosphate Cement Using Industrial Byproduct Ashes

During our effort to stabilize contaminated ashes in the U. S. Department of Energy's (DOE's) complex by a novel chemically bonded phosphate ceramic, we formulated a high strength concrete made of benign ashes (such as fly ash and coal bottom ash) that may be useful for specialized applications in construction industry. It is formed by a room temperature process in which MgO is reacted with a solution of one of the soluble hydrophosphates to form a dense matrix. Any ash or slag may be incorporated during the reaction. The slurry formed by mixing these components for 15-30 minutes is a pourable, low viscosity paste. Once it is left undisturbed, it sets into a hard mass in ≈45 minutes with a small amount of heat evolution. Its density ranges between 1.7 g/cc to 2.0 g/cc, open porosity is <5%. For a typical ash loading of 60 wt. %, it has a strength of ≈12,000 psi. The micro structure of the concrete is glass-crystalline. This product is not very sensitive to the ash composition. Unlike in portland cement, it is unaffected by the unburnt carbon in the ash or to any Cl ions, and thus a wide variety of combustion products may be incorporated to develop this product. The material cost is generally 50% higher than portland cement, but processing advantages, such as faster setting, setting in cold environment, and self binding characteristics may off-set some of the costs, or improved properties may justify slightly higher costs for specialized product development. INTRODUCTION The ever increasing industrial activity in the western world and in developing countries is depleting the natural resources and at the same time, producing wastes that need disposal. Much of the volume of the solid waste that is produced is non hazardous. For example the 1986 statistics (1) reveals that nearly 40 tons of waste per person was generated in the United States that year. Most of it was non hazardous. Preferred methods of handling the solid waste is to reduce its volume by incineration if possible (such as incineration of municipal solid waste). The incineration products, such as ashes, are then disposed at a reduced cost. In addition to the incineration ash, a large amount of ash is also produced by the utility industries that use fossil fuels. Approximately one third of this ash is recycled in the cement based products as an additive. Typically, cement products can incorporate ≈15-20% ash in them. Efforts are being made to improve this loading [2], but have not been successful because of several reasons. These include the following: 1) Cement chemistry is sensitive to the components of the ashes. Ashes, particularly those obtained from the incineration of municipal solid wastes, contain chlorides resulting from the destruction of polymers. Chlorides and other anions hinder setting of cement. 2) Carbon content of the ash is also a major factor. High carbon ashes are not suitable for cement setting. In particular, ashes produced by low-NOx burners produce high carbon ashes [2] and it is likely that, to meet the clean air act requirements, future industries may opt for low-NOx burners that will produce ashes unsuitable for incorporation in cement. 3) The ash loading in the cement products is generally low, and thus large scale utilization of ash in cement is not very economical. We propose development of a novel chemically bonded phosphate cement that is formed at room temperature that addresses these issues. This cement was developed initially for stabilization of hazardous and radioactive wastes (3). While exploring stabilization of contaminated ashes, it was realized that ash itself participates in the setting reaction, producing a high strength cement that sets like conventional cement. The end product was a low-open-porosity, light weight structural material with high strength. This article describes the process of forming the ash and slag based cement, its physical and mechanical properties and micro structure. This novel structural material with its improved properties should be particularly useful in those applications where conventional cement products have limitations. These properties are elaborated in this article. THE PHOSPHATE-CEMENT PROCESS The phosphate-cement process is based on an acid-base reaction between magnesium oxide (MgO) and a solution of alkali phosphate salt such as sodium or potassium phosphate. Here we describe the process based on monopotassium dihydrogen phosphate (KH2PO4). The reaction product is magnesium potassium phosphate (MgKPO4. 6H2O) that is formed by dissolution of MgO in the solution of KH2PO4 and its eventual reaction to form the product according to the reaction, MgO + KH2PO4 + 5H2O ---> MgKPO4. 6H2O This product (refered to as MKP here after) is a binder that can be used as the matrix material to host any inorganic waste material. Wastes such as ashes and slags are mixed with the binder powders and reaction is allowed to occur by mixing the components for 30 min. in a concrete mixer. The resulting slurry is a smooth paste that can be poured in the molds to form any desired shape. The exothermic reaction between the components starts heating the slurry. The slurry starts thickening and when the temperature reaches 55°C it sets into a hard mass. The exothermic reaction proceeds to heat the monolith. Typically the maximum temperatures of 60°C in small samples of 100 g and 82°C in 55 gal. size monoliths have been noted. In approximately 2 hours, the samples attain sufficient structural integrity and hence may be removed from the molds, but actual curing continues for several weeks. FABRICATION OF TEST SAMPLES Using the process described above, we made several cylindrical test samples incorporating various amounts of ash loading. To demonstrate the versatility of the process, we made samples of several different ashes that included, utility class F and C fly ashes, high carbon ash, municipal solid waste (MSW) incinerator ash, steel industry slag, and simulated ash compositions that represented U. S. Department of Energy's contaminated ash inventory. In all the cases, the reaction slurry mixed and set the same way and the temperature rise was also similar. Some additional heating was observed wherever calcium content in the ash was higher. This must be due to additional exothermic reaction between calcium oxide and the phosphate. Other than this heating, no other variations were observed during formation of the samples, and in all cases equally good samples were formed. This indicates that the phosphate-cement process may produce a very versatile technology for manufacture of ash products. We made test samples in cylindrical form of diameter 2cms and length of 4-5 cms. Each sample was made by pouring the slurry in a polyethylene syringe and allowing it to set for a week. It was then taken out of the syringe by cutting the narrow end of the syringe and then extruding the set sample out. Each sample was then cut on a diamond saw to the desired size, and the two end surfaces were polished to make them parallel to each other. Though samples of various ashes were made and studied, we focus here on three ashes that were representative of most of the ashes. They were class F fly ash supplied by Monex Corp., and class C fly ash and a mixture of class F and C fly ashes supplied by American Fly Ash Company. The elemental composition of these ashes is given in Table 1. Cements of class F and class C ashes were made at different loadings (ash loading is defined as its weight % in the total powder mixture) and physical and mechanical properties were measured. Realizing that optimal strength in both cases is obtained at a loading of 60 wt%, only samples of 60 wt% loadings of class F + class C were made and the strength was studied. Table 1: Elemental compositions (wt.%) of the ashes used in this trial Element Class F Class C Class F + Class C Al 11. 5 9. 74 11. 6 Ca 1. 54 16. 8 5. 71 Fe 4. 16 3. 44 5. 11 K 2. 31 --0. 69 Si 21. 8 16. 5 21. 9 C 8. 78 0. 08 5. 56 To gain an insight into the micro structure of the cements, the one made with class F was investigated in detail using X-ray diffraction method, differential thermal analysis (DTA) and scanning electron microscopy (SEM). PHYSICAL AND MECHANICAL PROPERTIES We measured the density, open porosity and compressive strength in each case. The density was measured by weighing the samples and measuring their dimensions. The open porosity was estimated by water intrusion method. To do this, each sample was first dried at 70°C in an oven and weighed. It was then immersed in water and the water was heated and maintained at 70°C for 2 hours so that the air in the open pores expanded and left the samples. When cooled, these pores would be filled with water. The samples were taken out, wiped lightly using a paper towel and weighed. The difference in two weights provided the weight of water in the pores. That allowed us to calculate the open porosity. The compressive strength was measured on an Instron machine in an uniaxial mode. We obtained an average of 5 samples for each reading. The results of these measurements are given in Table 2. The density measurements show that the class C ash products are slightly heavier than class F products. This may be due to the fact that class F ash contains more carbon and hence may be slightly lighter. Since the ashes and the binder powder have nearly the same density, there is hardly any effects of the ash loading on the density. Overall the ash products are approximately 25% lighter than corresponding cement products. The open porosity, that affects water absorption, also is comparatively less in this cement than in portland cement. The open porosity in cement products is approximately 20%. Comparatively, phosphate-cement products exhibit much lower open porosity. Table 2: Physical and Mechanical Properties Sample Description Density (g/cm3) Open Porosity (vol%) Compression Strength (psi) Phosphate Binder 1.73 2.87 3337 Class F loading (wt%) 30 1. 67 5. 22 5651 40 1. 77 4. 09 6207 50 1. 80 2. 31 7503 60 1. 63 8. 15 5020 7