Study On The Physical And Chemical Effects Of Mineral Admixtures During Hydration Process At Early Age

Cement hydration is a multiphase chemical react process. With the extensive and deep use of mineral admixtures in cement and concrete, the raw materials of cement-based material has become various, and the hydration circumstance and degree of cement mineral components has been changed, which in turn changed the hydration process and mechanism of cement, especially for early hydration process. The researches on the early hydration of cement-based material aim at understanding the formation law of microstructure. The change of microstructure is important for controlling the early strength development of cement, and affects the long-term properties of cement-based material. Researches on the physical and chemical effects of mineral admixtures usually focus on the long-term properties, such as strength, shrinkage and crack. However, when the studies on such effects are carried out in the early hydration age, they will lead to the effective instructions towards the choice and application of mineral admixtures in practical project.Ordinary portland cement was used as matrix and fly ash, blast furnace slag and metakaolin were used as mineral admixtures in this paper. The meaning and stage division in early hydration of cement-based material is studied by the method of non-contacting electrical resistivity device. The effects of w/c ratio, the types and dosage of mineral admixture on hydration process is also researched.The resistivity method can descript the early hydration process continuously and accurately for cement-based material. The results indicate that the early hydration process can be divided into four stages in according to the resistivity curves. The four stages are dissolution stage, induction stage, setting stage and hardening stage. And in the first two stages, the resistivity of paste is determined by the conductivity of liquid phase, but in the later two stages, the resistivity is a sign of formation and development of microstructure in the paste. The lower w/c ratio leads to higher resistivity of paste, the dissolution stage and the induction stage decreases and the setting stage and hardening stage starts earlier. The mineral admixtures prolong the first three hydration stages. The resistivity curves take a "sudden" change when mixed with blast furnace slag or metakaolin. The starting time of setting stage is identical with initial setting time. However, the ending time of setting stage lags behind the final setting time. The trend of resistivity curves in hardening stage can reflect the pozzolanic activity of mineral admixture more effective than the early age strength, and it keeps closely relation with the strength at 28 days.Fly ash has the dispersing function, packing effect and microfiller effect in earlyhydration process. It can provide depositing surface not only for the secondary hydration products, but for the hydration products of cement. In early hydration process, blast furnace slag and metakaolin can disaggregate in alkaline media, and the grain surface is attacked obviously. They have pozzolanic activity in early hydration process, and they exhibit synthesis effect when they are hybrid, which can accelerate the compactness and development of microstructure in this paste.