Effect Of Ultra-fine Powder (CaCO₃ And SiO₂) On Performance Of Sulphoaluminate Cement

Green high-performance is the tendency of development of cement-based materials, and the modification technology of ultra-fine powder is one of the main ways to attain high-performance. Sulfoaluminate cement is a well-known type of energy-saving eco-cement. In this paper, effects of ultra-fine powder (SiO2 and CaCO3) on sulfoaluminate cement and its hydrates are studied and good experiment relults were gotten. During the application of ultra-fine powder, the first step is to resolve the dispersing problem. The results show that mechanical method and super plasticizer are effective on improving the dispersibility of ultra-fine powder. After dispersing, the strength of samples increases significantly. Ultra-fine powder have high specific area and big surface energy, and it strongly absorb external impurities and form new surface structure, which leads to fall the fluidity of cement paste and to shorten the setting time of cement. Compairing with ultra-fine silicon powder (Nano-SiO2 and silica fume), the fluidity of cement paste with suitable super plasticizer and longer setting time are attainable by controlling the quantity of ultra-fine calcium powder (ultra-fine CaCO3 and ordinary limestone powder). Ultra-fine powder not only dramaticly increases compressive strength of sulfoaluminate cement, but also obviously improved its bending strength, and that later strength of the blending doesn't fall down but increased stably. SEM showed that pure cement hardened pastehave mainly made up of large scale crystalloid and hydrate gels. But there are little better network structure formed via the third chemical bonds between them. While adding ultra-fine powder into cement, average pore diameter, specific surface area, total pore volume of cement hardened paste became smaller, and microstructure is densest and homogeneous. In sulfoaluminate cement, ultra-fine powder plays twofold effect, one is particle effect, another is chemical activity. Particle effect includes packing effect, particle size distribution and nucleation. Moreover, when the curing temperature is above 90oC, the performance of sulfoaluminate cement is not quite steady, and especially when high temperature cured at later ages, the sample is likely to lose strength. This is similar to the specmen with ultra-fine powder. But comparing with ultra-fine calcium powder (ultra-fine CaCO3 and ordinary limestone powder), ultra-fine silicon powder (Nano-SiO2 and silica fume) slowers the disabled speed of strength of sulfoaluminate cement, which is due to the secondary reaction between ultra-fine silicon powder and hydrate.