The Study On Hydration And TSA Towards Cement-Based Material Cantaining Ultra-fine Limestone

Most project in order to attain the purpose of energy saving and waste recycling, and economic and ecological benefits,join the ground limestone in cement concrete, due to the special factor CO₃^2-,to achieve scientific and rational use of ground limestone,the characteristics of Cement-Based Material Containing Ultra-fine Limestone are studied in this thesis.At the same time,A new type of sulfate attack on cement concrete material have been found recently when exposed to high humidity, low temperature,sulfate and carbonate,and the main corrosion product thaumasite is different from ettringite and gypsum.Being different from the traditional sulfate attack due to ettringite formation,the CSH gel of the cement paste is directly decomposed by the thaumasite form of sulfate attack,and the concrete is eventually transformed into a pulpy mixture without strength.So the thaumasite form of sulfate attack is more aggressive than traditional sulfate attack.This special corrosion progress is defined as the thaumasite form of sulfate attack(TSA).Crystal structure of thaumasite and ettringite are so similar that for a long time ettringite and gypsum form of sulfate attack have been responsible for deterioration occurring in constructions.The first international symposium about thaumasite and the thaumasite form of sulfate attack has been held in England in June,2002,which has been a sign for TSA to be of a great concern to the concrete technologists,based on the reported field cases of thaumasite word widely,incorporating with sorts and concentration of aggressive medium in different type of soils,underground water,climate of China, it can be derived that underground construction and foundation have been probably suffering risks of TSA in most of China,such as northwest,and coast area of west especially.Financial supported by Program of National Natural Science Found (NO.50378075),Hi-tech Research and Development Program of China (NO.2005AA332010),the research of the sulfate attack on Cement-based material cantaining Ultra-fine limestone powder.The main research work and compliments are listed as follow.1.Ratio of water demand and mechanical properties of cement-based material containing ultra-fine limestone powder were studied,and mechanism of structure formation during hydration at early age was analyzed by means of electronic resistivity and X-ray diffraction of paste.Results show that ratio of water demand of cement-based material with the increase of different mesh 35%limestone powder first decreases and then increases compared with cement-based material without any limestone powder.The influence to mechanical properties of cement-based material at early performance with low content increase,and decrease with high content. Furthermore ultra-fine limestone powder can improve the brittleness of cement-based material.Ratio of compressive strength to flexible strength of cement-based material without any of limestone powder at 28d is more then those adding 5%,15%,35% limestone powder to cement-based material.During hydration in 600 min,ultra-fine limestone powder can accelerate the hydration rate,so electronic resistivity of cement paste is higher than that without limestone powder.When hydrating after 600 min, electronic resistivities of cement pastes with ultra-fine limestone powder increase far slowly than that without limestone powder.Additional,ultra-fine limestone powder in cement paste can react with aluminates to form calcium aluminates monocarbonate which is stabler then calcium aluminates.2.XRD,DSC and calorimetric analysis present almost a same law between the cementitious material system and single-mine.With the increase of ground limestone amount,Ground limestone defers the formation of ettringite and improve the stability of ettringite.The calorimetric analysis prove that the hydration process of cement is changed by the ground limestone.The addition of 10%ground limestone advances heat-releasing of cement hydration,increases heat-releasing speed and amount, shortens the induction period,advances the entering into the acceleration period ahead about 40minutes and increases 8.5%heat of cement of per unit mass in 24h.3.It has been found that thaumasite forms in the cement mortar containing 35% limestone filler after 7 months of immersion into sulfate solution at a lower temperature.The surface cracking and spallation occurs in the pure cement mortar specimen after 1 year of immersion into a 2%MgSO₄ solution at 5±1℃,and a lot of ettringite and gypsum form on the prism surface due to sulfate attack.On the other hand,a soft and mushy mixture formation follows the surface expansion and cracking in the 35%limestone filler mortar,both thaumasite and gypsum are detected in the deteriorated mortar,therefore the mortar undergoes the typical thaumasite form of sulfate attack.4.Fly ash and slag can restrain or retard formation of thaumasite and improve TSA resistance of Portland cement-based material sufficiently.Increasing content of fly ash and slag of binder,appearance of specimen is kept more intact,ratio of strength loss decreases much,and total contents of which decreases when exposed to thaumasite form of sulfate attack conditions, erossion phases changes from three phases compounds of ettringite,gypsum and thaumasite to two phases compounds of ettringite and gypsum.