Degradation Of Calcined Clay-Limestone Composites Gelation System Under Sulfate Attack And Acid Corrsion

With the trend of global warming,implementing low carbon cement and reducing carbon emissions of cement industry has been inevitable.As a newly proposed low carbon cement system,calcined clay and limestone cementitious composites can significantly reduce the energy consumption and carbon emission of cement industry without altering the mechanical properties.The reactive alumina in calcined clay can react with limestone in alkaline condition to form calcium carboaluminate,which would refine the pore structure,thereby improving the strength and impermeability of cementitious materials.However,the presence of reactive alumina and calcium carboaluminate may lead to another durability issue when the cementitious composites are exposed to sulfate or acid solutions.In this paper,the degradation of calcined clay and limestone cementitious composites under full immersion,semi-immersion,low temperature sulfate attack and acid corrosion was studied.The appearance,dynamic modulus,expansion rate and ion penetration depth of mortars were monitored,and the composition,content and distribution corrosion products in the mortar were also investigated by multiple micro analysis methods during the erosion process.The results show that:(1)At early age,the reactivity and strength of calcined clay and limestone cementitious composites are lower than that of Portland cement.While the reactive alumina in the calcined clay will react with limestone to form carboaluminate,which can promote the strength development in the later age.(2)In full immersion test,the addition of calcined clay and limestone will refine the pore size of mortars,then inhibit the penetration of the sulfate ions.This could significantly reduce the expansion and damage of mortars under the sulfate attack.(3)In semi-immersion test,the addition of calcined clay and limestone reduce the damage of mortar below the surface of solution due to the refinement of pore size of the mortars.However,the damage of mortar above the solution is increased since reactive alumina that introduced in the cementitious composites can react with sulfate to form more corrosion products.(4)The calcined clay and limestone cementitious composites show an improved sulfate attack resistance under low temperature,without showing the evidence of thaumasite formation.(5)Due to the lower content of portlandite in the system,calcined clay and limestone cementitious composites have a reduced acid attack resistance.This study investigated degradation process and mechanism of calcined clay and limestone cementitious composites under sulfate attack and acid attack.These research findings are beneficial to the performance improvement and further practical application of calcined clay and limestone cementitious composites.