Thixotropic behavior of cement-based materials: effect of clay and cement types

摘要

In this thesis, effects of clay addition, clay replacement, and cement types on thixotropic behavior of cement-based materials are investigated.Thixotropy is the property of certain fluid materials that are thick or viscous under normal conditions but flow or become less viscous over time when shaken, agitated, or otherwise stressed. Freshly-mixed cement pastes are thixotropic materials, which become fluid when agitated but restore its structural form at rest. This is because cement pastes experience microstructure change with time due to the particles flocculation and cement hydration.The thixotropic behavior of cement-based materials is important in the modern concrete construction. Shape stability of concrete mixtures is often required for shotcrete and slip form construction so that the concrete mixture can adhere to the substrates or hold the shape right after casting and without support from formwork. Quick structural restore, or high thixotropy, of concrete can reduce formwork pressure in construction. Clay additions or replacement for cement often enhance concrete thixotropy and increase concrete shape stability.In the present study, the typical hysteresis loop rheology test method is employed to evaluate thixotropy of various cement pastes. The pastes were made with different types of cement and with /without clay addition/replacement. The types of cement used are Ashgrove Type I, Lafarge Type I/II, Type IV, and High Alkali Type I cement. The clay materials studied are Actigel, High Reactivity Metakaolin, and Ground Clay Brick. The rheology tests were performed at 0, 15, 30, 45, 60, and 75 minutes right after each paste was mixed. The rates of thixotropy change with time were analyzed for each mixture. The standard flow table tests (ASTM C230) were also conducted and the results were correlated with the rheological results. The rate of heat evolution of each paste studied is measured. The results indicate that clay addition and/or replacement for cement accelerates the rate of thixotropy change. The pastes made with different types of cement have different thixotropy changing rate. High Alkali cement has the highest thixotropy changing rate while both of Type IV and Ashgrove Type I cement have a lower thixotropy changing rate. There is a clear relationship between thixotropy and flow test results. The faster the increasing rate of thixotropy, the faster the decreasing rate of flow can be expected. The rate of heat generation can be correlated with thixotropy increasing rate.