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SN2092-The Role of Fly Ash Composition in Reducing Alkali

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SN2092-The Role of Fly Ash Composition in Reducing Alkali PORTLAND CEMENT ASSOCIATION RESEARCH & DEVELOPMENT I N FO RM AT1 0N PCA R&D Serial No. 2092 The Role of Fly Ash Composition in Reducing Alkali-Silica Reaction by Rachel Detwiler Q Portland Cement Association 1997 Table of Contents 1.0 INTRODUCTION ..................................................................................................... 1 1 . 1 Alkali-Silica Reaction., .................................................................................. 1 1 .2 Why Class C Fly Ash? ................................................................................... 2 2.0 CHARACTERISTICS OF FLY ASH ..................................................................... 4 2.1 Alkali Content ................................................................................................. 4 2.2 Calcium Content ............................................................................................ 5 2.3 Magnesium and Periclase Content ............................................................ 5 2.4 Sulfur and Sulfate Content .......................................................................... 5 2.5 Carbon Content and Loss on Ignition ........................................................ 6 2.6 Glass Content and Mineralogy.................................................................... 6 2.7 Variations in Composition by Size Fraction.............................................. 6 2.8 Fineness and Particle Size Distribution .................................................... 7 2.9 Specific Gravity .............................................................................................. 7 3.0 EFFECTS OF FLY ASH ......................................................................................... 9 3.1 Pore Solution Chemistry .............................................................................. 9 3.2 Hydration Products ...................................................................................... 11 3.3 Alkali-Silica Reactivity................................................................................. 12 3,3.1 Mechanisms...................................................................................... 12 3.3.2 Expansive behavior......................................................................... 15 4.0 OTHER FACTORS AFFECTING ASR ............................................................... 17 4.1 Type of Aggregate ....................................................................................... 17 4.2 Cement Composition .................................................................................. 17 4.3 External Alkalis............................................................................................. 18 5.0 STANDARD TEST METHODS........................................................................... 21 5. 1 ASTM C 114 ................................................................................................. 21 5.2 ASTM C 227 ................................................................................................. 21 5.3 ASTM C 31 1 ................................................................................................. 22 5.4 ASTM C 441 ................................................................................................. 22 5.5 ASTM C 1260 (P 214)................................................................................. 23 5.6 CSA A23.2-14AlASTM C 1293 ................................................................. 24 5.7 PCA's Guide Specification ........................................................................ 25 6.0 CONCLUSIONS AND RECOMMENDATIONS............................................... 27 6.1 Effects of Fly Ash Characteristics on ASR ............................................... 27 6.2 Recommended Test Program.................................................................... 28 7.0 REFERENCES ...................................................................................................... 30 The Role of Fly Ash Composition in Reducing Alkali-Silica Reaction by Rachel J. Detwiler* 1.O INTRODUCTION Some Class C fly ashes have been shown to be effective in controlling the alkali reactivity of even highly reactive aggregates. The objective of the proposed work was to determine the relevant characteristics of such fly ashes and any limitations on their use. High quality Class F fly ashes are not readily available in all areas where reactive aggregates are found. As supplies of nonreactive aggregates are used up, other measures to control alkali reactivity will be needed. It is known that not all Class C fly ashes are suitable for this purpose; however, some have proven acceptable. Establishing criteria for determining the acceptability of Class C fly ashes for this purpose will provide an additional means of controlling ASR. This report reviews the literature on this topic and recommends a test program to provide information on the mechanisms by which some Class C fly ashes can help to control deleterious expansions due to ASR. 1.1 Alkali-Silica Reaction Nielsen [1979] lists the signs of deleterious alkali-silica reaction (ASR) as extruded gel on the concrete surface, popouts from porous reactive aggregate particles near the surface, and map cracking that can extend through the thickness of the concrete. The cracks can be up to 1-2 mm wide. He describes the reaction sequence as follows. Alkali ions (Nat and K') from the cement, mixing water, or environment dissolve into the pore solution, increasing its OH- concentration. The OH- ions dissolve any amorphous silica present, forming a gel. Since the gel is hygroscopic, it imbibes water from the surrounding pore solution, The gel, which would expand if unconfined, exerts pressure on the surrounding cement matrix. Nielsen gives three possible results of ASR: The concentrations of alkalis, silica, and water are unfavorable for the process. It takes place without causing damage. This is the most common situation. The concentrations of alkalis, silica, and water are favorable for the process. Expansive pressures from the gel generate cracks throughout the volume of the concrete. Reactions and swelling in aggregate particles near the surface of the concrete can cause popouts. If the mortar is strong enough to resist the pressure of the swelling gel, the gel is expressed through the surface of the concrete. In this case the gel can be extruded to form drops, or may cause dark patches to form on the surface. According to Taylor [19901, the necessary conditions for alkali-silica reaction in portland cement concrete are a sufficiently high content of alkali oxides in the cement, a reactive constituent in the aggregate, and a supply of water. The K+ and Na+ ions are present in the cement as sulfates and in the silicate and aluminate phases. When the compounds containing these ions react, their anions enter products of low solubility, such as ettringite, calcium silicate hydrates, or AFm phases, and equivalent amounts of OH' are formed. At this stage the alkali ions are involved only in the sense that, since their hydroxides arc soluble, they allow the OH- to enter the pore solution. Alkali cations may also be supplied from deicing salts, seawater, supplementary cementing materials, or aggregates. These alkalis also yield equivalent amounts of OH- unless they enter solid phases or are balanced by other anions that remain in solution. Taylor [19901 likens the chemistry of ASR to that of the pozzolanic reaction, except that the effect is different because of the difference in particle size of the siliceous material. Hydroxide ions attack the silica, causing 9-0-5bridges to be replaced by pairs of SiO- groups. This process ultimately * Principal Engineer, Construction Technology Laboratories, Skokie, Illinois 60077, USA. 1 fragments the three-dimensional silica framework into separate silicate anions. The negative charges are balanced by the most readily available cations, which are K+ and Na+. The damaged framework is sufficiently deformable that, at least for some compositions, the material can imbibe water molecules and expand. If it is sufficiently fragmented, it can be highly mobile. It is unstable in the presence of Ca2+, which reacts with it to form calcium silicate hydrate. This regenerates OH' in the solution, for example from Ca(OH)2 if that is the source of the calcium. The alkali silicate constituent of the gel can probably accommodate some calcium, but a point is soon reached at which calcium silicate hydrate precipitates as a separate component. In the pozzolanic reaction, the alkali silicate gel forms in an environment rich in calcium, and so converts quickly into calcium silicate hydrate. In ASR, the cement paste cannot supply calcium fast enough to keep the gel from persisting for long periods, particularly in cases where the gel forms within the aggregate particles. Diamond [1989] points out that ASR can take place without damaging the concrete. Visible or measurable distress associated with ASR results from physicochemical responses subsequent to ASR, and is not necessarily a result of the reaction itself. The driving force for expansion and cracking is the osmotically-derived imbibition of pore solution into the insoluble reaction gel. Farny and Kosmatka [1997] agree that the presence of ASR gel does not always coincide with distress. They point out that some gels expand very little or not at all. Diamond et al. [19811 state that a low-swelling gel might expand less than 4% in which case the pressures generated in
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