Applications of Silica Derived from Rice Hull
Synthesis of Cements and Zeolites from Rice Hull Ash
Portland cement is one of the most consumed materials in the world. The traditional method used for the production of cement is based reactions at temperatures around 1450 °C.
 Portland cement is a complex material, composed basically by calcium silicates, calcium aluminates and calcium aluminoferrites, among others. The calcium silicates, Ca3SiO5 and P- Ca2SiO4, determine most of the adhesive properties of concrete. The cement production releases huge amounts of CO2, about 6% of total CO2 emission caused by human activities.
Rodrigues and coworkers [26, 27] developed a synthetic method for the synthesis of P- Ca2SiO4 using silica from rice hull. Silica was obtained from rice hull by heating this material at 600oC in an open furnace.  The silica was mixed with other solids (calcium oxide and barium chloride) in stoichiometric proportions. It was found that addition of small amounts of barium chloride (2%, molar ratio) facilitates the synthesis of desired material, having the P- structure. After hand-mix, water was added to solids and a suspension having water: solid ratio of about 20:1 was obtained. The suspension was submitted to an ultrasonic cleaner bath (Thornton, 25 kHz) for 60 minutes. After this procedure a mixture of solids was obtained: an intermediate silicate and calcium hydroxide. Heating of intermediate calcium silicate renders P-Ca2SiO4. This material shows potential to be used as a cement.
Many papaers deal with the synthesis of zeolites from rice hull ash. Cheng and coworkers  synthesized MCM-22 by hydrothermal method and studied the adsorption of red 5GN, a dye used in textile industry. The preparation of MVM-22 was carried put after 3 days of hydrothermal processing. NaA zeolite , ZSM5.