CO2 consists of individual molecules with one central carbon atom double bonded to two oxygen atoms. 
Silicon does not form double bonds with oxygen easily because of differences in the atomic radii. Because of this SiO2 does not form individual molecules but instead forms a network solid, with each silicon atom single bonded to four oxygen atoms and each oxygen atom single bonded to two silicon atoms. As a result, the particles in SiO2 are not free to move as they are in CO2.