Rice Husk Ash: is it a good substitute for cement in concrete?

Italy alone produces 460,000 tons of rice husk per year, from which, in addition to exploiting the calorific value of combustion for centralized water heating, ash could be used in the production of concrete. In this way, it would be possible to exploit a material that otherwise has no further uses: not only to produce heat, but also as a substitute for cement. This is possible because the obtained ash is made up of over 85% silicon dioxide, that develops a strong pozzolanic activity. In this study the results obtained from laboratory tests on concrete samples, made with an increasing percentage of cement replacement with rice husk ash are analyzed. Two fundamental parameters are taken into consideration: the compressive strength at different maturation periods of the material and its workability, with the use of different water/cement ratios and fluidifiers. It is observed that the concrete resistance reaches a peak for RHA replacement between 5 and 15%, causing only a relative loss of workability, which can easily be recovered with the use of plasticizers. A coefficient has been calculated that can be adopted as a reference for the "environmental" performance of concrete, defined as the ratio between resistance and emissions, and its behavior has been studied by varying the percentage of RHA replacement.