8018
Ind. Eng. Chem. Res. 2007, 46, 8018-8025
Water, Acid, and Calcium Carbonate Pretreatment of Fly Ash: The Effect on Setting of Cement-Fly Ash Mixtures Corina Lupu, Katherine L. Jackson, Sean Bard, and Andrew R. Barron* Energy and EnVironmental Systems Institute, Department of Chemistry, and Department of Mechanical Engineering and Materials Science, Rice UniVersity, Houston, Texas 77005
The treatment of class C, I, and F fly ash (FA) with water, HNO3, and aqueous CaCO3 has been investigated to develop a simple chemical route to change the morphology and surface chemistry of fly ash particles to enhance the setting properties of a cement/fly ash (C/FA) composite. The treatment of C-FA with an aqueous CaCO3 solution results in a dramatic improvement in the setting time and the setting profile on C-class FA; in contrast, the treatment has no effect on the set time for F-FA and reduces the set time and appears to result in an even more nonideal induction setting curve as compared to the untreated C/I-FA. The enhancement observed for the treatment of C-FA with aqueous CaCO3 solution is not a consequence of the water solution since simply washing with water (i.e., C-FAH2O) results in the extraction of Na and Ca with a concomitant increase in surface area and a performance similar to those observed for untreated I-FA and F-FA despite a much higher surface area. The acid (HNO3) treatment of I-FA and F-FA results in the formation of an inert filler-like material, while acid treatment of C-FA results in a material with completely undesirable setting properties. Clearly, the enhancements observed for the aqueous CaCO3 treatment are not as a result of simply either the aqueous or acidic nature of the HCO3 containing CaCO3 solution. Based upon the forgoing, we propose that the efficacy of the aqueous CaCO3 treatment on C-FA is associated with the availability of “reactive calcium”. Exposure of C-FA to dry CO2 does not affect the set time or set profile for C/C-FA mixture, but the retarding effect of the aqueous CaCO3 treatment on C-FA can be replicated by the exposure of the C-FA to a stepwise reaction with water and CO2. Exposure of C-FA to wet CO2 results in the improvement of the setting induction profile without significantly affecting the set time. We propose that this process should allow for the addition of fly ash to cement samples while the induction profile remains flat, providing predictability in pumping for down-hole applications. Introduction Fly ash is generated as a waste product from coal burning and consists of fine powders that contain polycrystalline oxide particles with sizes ranging from a few nanometers to micrometer levels (10-30%) as well as amorphous particles (up to 70%).1 Fly ash has a complex chemical composition being formed mainly from silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO), sulfates (e.g., Na2SO4), and small amounts of other oxides. The chemistry of a particular fly ash depends on the coal that it is generated from as well as the coal-burning conditions, which in turn dictates the composition and microstructure. There is a large range of variation of the particle sizes generated during the formation of fly ash and the response of the fly ash to different chemical treatments is governed by the response of each individual particle from its composition. Thus, it has been reported that upon hydration some of the particles react very fast and others are chemically inactive for longer periods of times (more than 2 years).2,3 The reuse of fly ash is a major issue that has recently attracted scientific and commercial interest. Enormous amounts of fly ashes (ca. 50 millions tons) are produced annually and its disposal or storage demands higher costs. As such, fly ashes have found applications in ceramic,4,5 road construction,6-8 and oil production related industries.1,9 However, only a small fraction of the yearly fly ash production is currently used in these industrial applications. The use of fly ash in larger quantities as a component of cement became even more * To whom correspondence should be addressed. E-mail: arb@ rice.edu.
problematic since after 1990 the Clean Air Act requires that all the power plants have low-NOx burners to reduce environmental emissions. Unfortunately, by application of these new regulations, the quality of the generated fly ash was dramatically affected, the amount of unburned carbon being increased negatively affecting the pozzolanic properties of the fly ash.10 Pozzolanic materials are any siliceous materials that combined with lime and water develop cementitious properties. Fly ash is a synthetic pozzolan material because is obtained by combustion of coal. Fly ash materials have been proposed as low-cost additives for down-hole cements in the oil industry.11,12 The addition of fly ash to a cement slurry results in the modification of its physical and mechanical properties including workability,13 strength,14 shrinkage,15 porosity,16 and permeability.17 However, it is desirable to understand the chemical role of fly ash and to find ways to control and tune its properties to obtain the response desired. In particular, methods are sought for extending the setting time for deep-well applications as well as providing enhanced stability during the induction period of cement hydration. Reaction of fly ash materials with free lime (or calcium hydroxide released during the hydration of cement particles) and water changes its properties and it behaves as a pozzolanic material. Unfortunately, the properties of this mixture when set are not sufficient for many applications. It is common to mix cement and fly ash; however, there is still a significant need to find new ways to control and tune the setting behavior. For example, it would be desirable to develop a simple chemical route to change the morphology and surface chemistry of fly ash particles to enhance the setting properties of a cement/fly ash (C/FA) composite.
10.1021/ie0709279 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/27/2007
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 8019 Table 1. Oxide Analysis (%) of the As-Received Fly Ashes fly ash
SiO2
Al2O3
Fe2O3
SO3
CaO
Na2O
MgO
K2O
P2O5
TiO2
C-FA I-FA F-FA
31.23 49.99 57.26
18.92 16.09 21.38
6.71 6.48 7.91
2.56 0.94 0.59
29.36 14.94 4.69
1.4 2.70 1.15
5.1 4.40 1.68
0.29 2.23 1.24
1.32 0.06 0.07
1.61 0.63 1.23
Table 2. XPS Analysis (at. %) for the As-Received and Chemically Treated Fly Ash sample
C
O
Na
Mg
Al
Si
P
S
Ca
Fe
6.94 2.92 4.72
61.77 63.15 65.12
4.23 1.65 1.53
1.34 7.71
