Study On Hydration Behaviour And Early Strength Of Alkali-Activated Slag Binder At Low Temperature

In the vast northern area of China,the winter is long and the weather is cold,which puts forward higher requirements for the cementitious materials used in low-temperature construction in cold areas:rapid setting,high early strength,continuous hydration in low-temperature environment,etc.The setting and hardening of alkali-activated slag cement（AAS）are fast at normal temperature and has high strength at early stage,which has important development potential and research value in low temperature environment.The influence of basic parameters such as the type and dosage of activator on the development of compressive strength of AAS is not clear,the reasons for the reduction of compressive strength caused by low temperature curing are not clear,and there is a lack of literature on improving the early strength at low temperature.It is an important project to solve the above problems by clarifying its low temperature hydration and hardening behavior,improving its early strength by effective technical measures and developing the high early strength of AAS in low temperature.The influence of the type of activator,alkali equivalent,water glass modulus and other factors on the compressive strength of alkali slag cementation material under low temperature environment was studied,the hydration behavior was analyzed by using resistivity and hydration degree,and the early microstructure changes were analyzed by mercury injection and backscattering images.Basis on this,By changing the fineness of slag or introducing a variety of admixtures to study its influence on the early strength,the reasons for the early strength enhancement were analyzed by combining the internal temperature change,XRD,TG,MIP and backscatter images.Finally,the relationship between environmental temperature and compressive strength was studied by usig maturity method,and the early strength development at low temperature was predicted.The main conclusions are summarized as follows:（1）Under low temperature environment（5 ^oC）,the slag particles and hydration products were not closely overlapped in early stage,the hydration products layer was weak,the pore size distribution was coarser and the total porosity was increased,which results in the early compressive strength of AAS mortar decreased.The 1 d compressive strength of mortar（<8 MPa）was only 1/3 of that cured at 20 ^oC in the same age.In the meantime,the hydration degree of slag activated by Na OH was higher than that of sodium silicate,but the AAS mortar prepared by sodium silicate has higher compressive strength and lower porosity.In the same alkali equivalent conditions,water glass with modulus of2.0 can obtain higher compressive strength in early stage.（2）Increasing the fineness of slag,adding a suitable amount of PC clinker,calcium sulphoaluminate cement,Ca（OH）₂,CaO,nano-SiO₂etc.,these admixtures can improve the early strength of alkali slag mortar to different degrees,and the efficiency is between8%and 43%.Among the admixtures,the addition of 15%PC clinker showed the best effect,and the 6 h and 24 h compressive strength of mortar can be increased by 35%and43%to 15.8 MPa and 22.1 MPa,respectively.In addition,the addition of PC clinker or CaO can also promote the development of compressive strength at 28 d.（3）Adding PC clinker,CSA,CaO and CaCl₂ can reduce the total porosity,increase the thickness of hydration product layer,improve the microstructure of AAS,and new phases were produced in the early stage,but these new phases were related to the admixture,the admixtures will not change the composition of the 28 d hydration product.The addition of 15%PC clinker also can obviously accelerate the exothermic rate,increase the amount of C-S-H gel product at the early stage.These admixtures play a role in increasing the internal temperature of the slurry,increasing the amount of hydration products,reducing the porosity or optimizing the microstructure,etc.,and improving the early strength at low temperature.（4）In a certain range of error,the maturity theory can be used to predict the early compressive strength of AAS cured at low temperature,the predicted early compressive strength results were more accurate.