External Sulfate Attack Of Cement-Based Materials Under Complex Environmental Actions:Experiments And Modelling

External sulfate attack(ESA)is one of the major concerns for the durability of cement-based material(CBM)subjected to the sulfate-bearing conditions.The ESA is usually accompanied by other aggressive actions,such as chlorides and leaching,in natural environment,resulting in a complicated damage mechanism than that of a single ESA.This work aims to study the mechanisms of the damage of CBM under ESA with and without the combined actions,and to propose scientific models for engineering applications.In this study,powder samples prepared from cement paste were immersed into different sulfate solutions,and the ettringite(AFt)content was then quantified at given exposure time to investigate the kinetics of ettringite formation.Then,2mm thickness slice specimens were exposed to different sulfate solutions,and the evolutions of AFt content,pore structure,vapour diffusivity and material expansion were measured to investigate the pore AFt crystallization process and its effect on transport property and material expansion.Finally,disk specimens were immersed into different sulfate solutions,and the sulfates profiles,the mass transfer and cracking pattern were examined at different exposure time to investigate the damage pattern of CBM under ESA.Based on the results of these experimental research,a chemo-poromechanical model to predict the expansion damage of CBM under the combined ESA and chlorides actions,and a spalling rate model to predict the spalling damage of CBM under the combined ESA and leaching actions were proposed.The key findings based on the above studies include:(1)For the AFt formation,high sulfate concentration and DEF accelerate it but the chlorides decelerate it.An exponential relation between the AFt formation rate and the AFt supersaturation was adopted and the exponential order was fitted as 1.90.(2)The AFt crystals formed in micropores(5-40nm)close and then reopen the neck-part of ”ink-bottle” pores,resulting in a reduction and a subsequent increase in the vapour diffusivity,respectively.The chlorides mitigate the pore crystallization process but DEF accelerate it.(3)The damage pattern of CBM under the combined ESA and leaching actions was found to be a spalling pattern and gypsum is found to be a special product in this combined actions.The chlorides and DEF respectively mitigate and accelerate the sulfate ingress,material expansion and cracking.(4)The simulated ESA expansions through the chemo-poromechanical model show good agreement with the experimental measurements in this study and literature.This model interprets the effects of chlorides and sulfate concentration on ESA expansion rate as the variation of the AFt supersaturation and the crystallization pressure.(5)In the spalling rate model,a proportional relation between the spalling depth and the square root of the exposure time is predicted and validated through the measurements of cracking depth in this study and literature.A parametric analysis shows the foremost impact of the aluminate content and the sulfate diffusivity on the damage rate.The main assumptions in models lead to conservative predictions,but yet the model results are meaningful for engineering use.