When incorporated in concrete mixtures, fly ashes are known to influence both its fresh and hardened properties. An accurate and quick technique to predict the extent of this influence based on the characteristics of fly ash would be highly beneficial in terms of field applications. The current study was an attempt to quantify the effects of fly ashes on the properties of pastes as a function of: (a) the mean particle size of the fly ash particles, (b) their fineness and (c) their chemical composition. In addition, since the type and the amount of glass present in the fly ash significantly affect its reactivity, this property was also included in the investigation.

Twenty different fly ashes (both, ASTM Class C and Class F), obtained from power plants in and around Indiana, were characterized during the Phase 1 of the study. The information collected included: physical characteristics, chemical composition and the amount and type of glass present. Phase 2 of the study consisted of evaluation of various properties of binary paste systems (portland cement with 20% of cement of fly replacement). The evaluated properties included: the set time, the heat of hydration, the strength activity index, the non-evaporable water content and the amount of calcium hydroxide formed at different ages.

These results obtained from both phases of the study were used to build statistical models for prediction of previously evaluated properties for any hypothetical fly ash with similar characteristics. The models included only the most significant variables, i.e., those which were found to most strongly affect any specific property. The variables to be included in the model were selected based on the adjusted R2 values.

As a result of the modeling process, it was found that the sets of statistically significant variables affecting the properties consisted of both physical and chemical characteristics of the fly ash and that the combination of these variables was unique for each property evaluated. When applied to a set of results from two additional (not previously used) fly ashes, the models provided the following residuals of predicted properties:

(a) Initial set time – 100 minutes for Class F ashes and over 300 minutes for Class C ashes

(b) Peak heat of hydration – 0.7 W/kg

(c) Time of peak heat – 375 minutes

(d) Total heat of hydration – 96 J/kg

(e) Calcium hydroxide content at various ages – 0.25% for early ages (1 and 3 days) and 0.5% for later ages (7 and 28 days)

(f) Non-evaporable water content – 0.7% for early ages (1 and 3 days) and 5% for later ages (28 days)

(g) Strength activity index – range of 1% in Class C ashes and 1% to 2% in Class F ashes (from 7 days to 28 days)

Phase 3 of the study consisted of evaluating the same set of properties but using ternary paste systems (cement and two different fly ashes). The goal for this study was to ascertain the applicability of the weighted sum of the models chosen for the binary paste systems to predict the properties of ternary binder systems. In addition, the analysis as to which of the chosen variables has the maximum effect on the properties was performed. It was found that the properties of the ternary binder systems were not additive in nature, except for strength activity index at 28 days. Lastly, the percent influence of each of the chosen independent variables, which affect the mentioned properties, was calculated along with the unexplained variation (error percentage). The error percentages varied for each of the properties, with set time having the maximum error (almost 50%).

Report Number



fly ash, strength activity index, statistical modeling, physical and chemical characteristics, SPR-3020

SPR Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

Date of this Version