The objective of this technical paper was to propose the optimum partial replacement of cement (PRC) by rice husk ash (RHA) and fly ash (FA) based on the complete consumption (CC) of calcium hydroxide (CH) from hydration and the strength activity index (SAT) based on equivalent calcium silicate hydrate (C-S-H) as well as the propagation of uncertainty due to randomness inherent in main chemical compositions in cement, RHA and FA. Firstly the hydration and pozzolanic reactions as well as stoichiometry were reviewed. Then the optimum PRC by RHA and FA based on CC of CH was formulated. The long-term SAI would be proposed based on equivalent C-S-H. Contribution to strength by calcium aluminate hydrate and calcium ferrite hydrate would be transformed to equivalent C-S-H. Based on the reasons mentioned above and the given values of strength for the age of 360 days, the long-term SAI could be defined. This concept could be extended for the SAI at any age based on Bogue's curve and nano-structures of C-H-S gel. After that the propagation of uncertainty due to main chemical compositions in cement, RHA and FA was discussed. The linear characteristic of the gradient of a function was derived for an estimate. Monte-Carlo simulations together with Goodness-Of-Fit tests were proposed for the better estimates. The reliability analyses for applying the suitable PRC were reviewed. Finally an applicability of the concepts mentioned above based on statistical data of materials available was demonstrated. The results from analyses were consistent with the tested results by the authors and other researchers. The results of this study provided guidelines of suitable utilization of RHA and FA for PRC. It was interesting to note that these concepts could be extended to optimize PRC by combination of other types of pozzolan which were described in the other papers of the authors.
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