Environ. Sci. Technol. 2003, 37, 3454-3457
Alkali Ash Material: A Novel Fly Ash-Based Cement HOSSEIN ROSTAMI* AND WILLIAM BRENDLEY Philadelphia University, School House Lane and Henry Avenue, Philadelphia, Pennsylvania 19144-5497
The United States generates 110 million t of coal ash annually. Approximately 70 million t of this coal ash is fly ash, of which 27% is recycled and the remaining 73% is landfilled. Disposal of such a huge quantity of ash poses a significant environmental problem. A new cementitious material has been developed, called alkali ash material (AAM), which is used to produce concrete for construction. AAM can be used to create a variety of concrete strengths and could revolutionize the concrete product manufacturing industry due to its economic advantage. AAM contains 4095% Class F fly ash and is used as cement to bind sand, stone, and fibers creating concrete. AAM concrete has been tested for strength, durability, mechanical properties, and, most importantly, economic viability. AAM concrete is economically and technically viable for many construction applications. Some properties include rapid strength gain (90% of ultimate in 1 d), high ultimate strengths (110 MPa or 16 000 psi in 1 d), excellent acid resistance, and freeze-thaw durability. AAM’s resistance to chemical attack, such as sulfuric (H2SO4), nitric (HNO3), hydrochloric (HCl), and organic acids, is far better than portland cement concrete. AAM is resistant to freeze-thaw attack based on ASTM C-666 specifications. Potential immediate applications of AAM are blocks, pipe, median barriers, sound barriers, and overlaying materials. Eventual markets are high strength construction products, bridge beams, prestressed members, concrete tanks, highway appurtenances, and other concrete products.
Introduction Approximately 950 million t of coal is consumed yearly for electric generation and industrial use in the United States generating about 110 million t of ash. Out of this 110 million t, about 70 million t consists of fly ash. Presently, 27% of fly ash is reused while the remaining 73% is landfilled or surfaceimpounded. The 50 million t of fly ash that is disposed of each year represents the waste of a valuable engineering material (1). The most common approach has been to use fly ash as a filler. Over 7.42 million t of fly ash was utilized in concrete in 1994 (1). The addition of fly ash to cement mixtures has been practiced for many years (2, 3). Currently, the amount of fly ash that can be added to portland cement concrete is limited because of its effects on strength gain and influence on air entrainment. In addition, the majority of fly ash used in this application is Class C fly ash or high-calcium fly ash. Lower quantities of Class F fly ash, or low-calcium fly ash, is used because of the greater impact on strength development (4, 5). There are significant benefits to the use of fly ash * Corresponding author phone: (215)951-2877; fax: (215)951-6812; e-mail:
[email protected]. 3454
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 15, 2003
in concrete including increased ultimate strength, better chemical resistance, and a number of other property improvements. Table 1 shows the chemical composition of both portland cement concrete and Class F fly ash. Both fly ash and portland cement concrete are made of the same oxides, while fly ash has higher SiO2 content and portland cement has larger CaO content. At the same time, the conversion of fly ash into cementitious material without portland cement has generated considerable interest. Recent work by Majiling and Roy (6) indicates the hydrothermal transformation of Class F fly ash into a cementitious material. It was reported that mixing fly ash and lime under hydrothermal conditions produces a new reactive fly ash cement (6). Other work has suggested the use of chemicals to convert fly ash into cementitious material without the use of cement (6). Roy’s state-of-the-art report on ceramic-based composites includes a variety of technologies including the alkali activation of latent hydraulic materials to produce ceramic-based composites (7-9). Recent patents by Lone Star Industries and Louisiana State University show methods of transforming Class C fly ash into cementitious materials (10, 11). Shi reports on alkaliactivated lime fly ash pastes with high levels of fly ash in an alkali-activated systems (12). On another front, much research has been devoted to the alkali activation of blast furnace slags (13-16). A characteristic of these efforts is the hydration of alkali-aluminosilicate materials to form strong and durable construction materials. These efforts have met with varying degrees of success (17-20). In the current work, Class C and/or Class F fly ash is mixed with chemicals to create a new type of cementitious material, alklai ash material (AAM). The compositions are mixed with sand and gravel to create concrete mixtures (20). For Class F fly ash compositions, the concrete needs to be cured at elevated temperatures of 40-90 °C for a period of 3-24 h. The resulting material is called AAM concrete. Current efforts have focused on using Class F fly ash-based compositions. Physical and chemical properties of AAM concrete are discussed in the following section. The economy of AAM as compared to portland cement concrete is an important factor that needs considerable attention. The cost of 35 MPa (5000 psi) portland cement concrete is approximately $80 per cubic yard. The cost of comparable strength AAM is about $60. The cost of higher strength AAM only marginally increases while higher strength concrete is considerably higher than that of 35 MPa concrete. AAM with 35 MPa contains about 10% activating chemicals. The activating chemicals are sodium silicate type N and 50% concentration sodium hydroxide.
Results and Discussions To date, some 1800 mixes have been made, and physical and chemical properties of AAM were compared to portland cement concrete. In the following section, compressive strength, freeze-thaw durability, and aggressive environment resistant of AAM with those of portland cement concrete will be compared. Methods and Materials. AAM mixture consists of 82-90 wt % solid, and the rest is liquid. The solid portion consists of low-carbon Class F fly ash (less than 4% C), fine aggregate, and coarse aggregate. The liquid part contains sodium hydroxide, sodium silicate, and water. The mixing procedure consists of (a) thoroughly mixing fly ash, fine aggregates, and coarse aggregates; (b) mixing sodium hydroxide, sodium silicate, and water together, which represents the activating chemicals, and (c) adding the activating chemicals to the 10.1021/es026317b CCC: $25.00
2003 American Chemical Society Published on Web 06/21/2003
TABLE 1. Chemical Composition of Class F Fly Ash and Portland Cement oxides
% content Class F fly ash
fly ash used in this work
portland cement
SiO2 Al2O3 Fe2O3 CaO MgO alkali SO3 LOI (% unburned C) heavy metals
45-65 20-45 3-12 3-10 1-3
