The reaction between rice husk ash and Ca(OH)2 solution and the nature of its product

In this study it was confirmed that, at temperatures around 40°C and in the presence of water, the amorphous silica contained in rice husk ash (RHA) can react with Ca(OH)2 to form one kind of C-S-H gel (Ca1.5SiO3.5· xH2O). The C-S-H gel looks like flocs in morphology, with a porous structure and large specific surface. The average particle diameter of the reaction product, ranging from 4.8 to 7.9 μm, varies slightly with the condition under which the reaction occurs. When the product is heated, it gradually loses the water that exists in it, but it maintains an amorphous form up to 750°C. Above 780°C it begins to transform to crystalline CaSiO3. One of the main reasons for the improvement of concrete properties upon addition of RHA possibly may be attributed to the formation of more C-S-H gel and less portlandite in concrete due to the reaction occurring between RHA and the Ca2+, OH− ions, or Ca(OH)2 in hydrating cement.