An AFM-SEM investigation of the effect of silica fume and fly ash on cement paste microstructure

Atomic force microscopy (AFM) was used to observe particle shape and surface texture details of normal portland cement and supplementary cementing materials (silica fume, low-calcium fly ash, and high-calcium fly ash). The latter materials mixed with cement were examined after prolonged hydration. Significant innovative information on particle shape and hydrated paste microstructure was obtained. Conventional microscopy techniques, such as scanning electron microscopy (SEM), cannot provide such detailed images and surface texture characteristics of the fine materials (especially silica fume) and of the product microstructure. AFM showed, for the first time, that silica fume particles are primarily composed of two complimentary parts (hemispheres or semicylinders). Nano-size particles were found in all materials. A relatively smooth product surface was observed in the hydrated cement paste. The hydrated surface of the addition-cement pastes presented small spheroid bulges, giving an additional roughness as was measured by AFM. A sufficient correlation of this microscopical quantitative information with macroscopical engineering and durability properties of cement products is also presented.