Sources of Self-Hardening Properties in Fly Ashes
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1987; Communicated by G.J. McCarthy
SUMMARY Laboratory investigations of the self-hardening properties of selected subbituminous fly ashes have been conducted. Chemical analyses of the fly ashes are given in Table I. The self-hardening value of the fly ashes was determined by conducting unconfined compressive strength tests on compacted samples of the moistened ashes. Various physical and chemical tests were performed to identify the reaction products, if any, of the hardened compacted fly ash paste, and to delineate the source of self-hardening properties. Results from x-ray diffraction analyses, scanning electron microscopic examination and differential thermal analyses indicated that the hydration products include calcium silicate and aluminate hydrates, and ettringite. Chemical and physical tests conducted to evaluate pertinent fly ash properties included chemical analysis, water soluble fraction, dilute hydrochloric and hydrofluoric acid soluble fractions, heat of solution on dissolving in dilute hydrochloric acid, specific surface area, and electrical conductivity tests. Results of chemical and physical tests suggest that calcium (in the form of free calcium oxide), water soluble fraction, and specific surface area of the fly ash play important roles in the self-hardening mechanism. Correlation of compressive strength and various other data suggest that self-hardening value of the fly ashes studied may be evaluated from a self-hardening number which is the product of specific surface in cm2 /g and the weight fraction of available free lime (Fig. 1). Table I. Constituents (Wt. %)
Chemical Analyses of Fly Ashes Montana oCstrip
Kansas LaCygne
so.61
38.98
32.28
46.40
54.72
22.41
21.51
18.72
16.53
16.91
4.50
3.70
5.06
19.65
7.21
16.83
25.16
31.25
9.06
8.11
1.72
4.73
3.99
0.48
2.36
0.29
0.28
0.67
2.75
1.23
0.93
3.16
2.94
0.65
0.60
1.97
0.61
1.38
1.06
0.69
0.97
0.35
0.39
1.58
1.27
1.0
1.0
0.46
2.8
2.S1
5.3
4.4
Forestburg
Sundance
Sio 2
48.41
48.86
Al 203
23.47
23.89
Fe203
4.95
3.40
CaO
16.99
15.30
lgo
0.53
0.49
so3
0.27
Na2 0 K20 TiO2
1.0
LI (950%C)
free CaOi Determined
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