The Effects And Mechanism Of Limestone Powder On Properties Of Cementitious Composites

Due to the increasingly shortage of high quality mineral admixtures such as fly ash and slag,limestone powder as a new cement-based composite material has attracted much attention.In this paper,the physical and mechanical properties of cement with adding single limestone powder and admixture of limestone powder and fly ash,such as water consumption of standard consistency,setting time,fluidity,strength of mortar etc.were discussed firstly.Then the above properties of cement were compared with limestone powder,fly ash and slag powder respectively.Secondly,the effects of limestone powder on the shrinkage carbonization resistance and resistance to sulfate resistance of cement-based composites were studied.At last the effects of limestone powder on the hydration process,hydration products and interface between cement and aggregate were analyzed by means of XRD,SEM,specific surface area tester and AC impedance tester.The experimental results of adding single limestone powder in the cement show that the water consumption of cement standard consistency and the setting time of cement was reduced effectively with the increase of the content of limestone powder when the content of limestone powder is less than 15%.In the meantime the flexural and compressive strength of cement mortar at 3 d age was improved.However,it is not useful for the improvement of cement mortar strength at 28 d age.When the amount of limestone powder to replace cement is more than 15%,it is unfavorable for the above properties of cement.The test results of double mixing limestone powder and fly ash in cement with 1:1 ratio are that when the total amount of double mixing is 30%,the water consumption of the cement was reduced,the setting time was shortened,and the strength of cement mortar at 3 d was improved.In addition,the strength of cement mortar at 28 d was higher than that of cement with 30%limestone powder,slightly lower than that of pure cement mortar.Comparing the effects of limestone powder,fly ash and slag powder on the properties of cement-based composites,it is found that the water consumption of cement with standard consistency can be reduced by adding single above three kinds of admixtures as replacing the cement with 20%.Among them,the standard water consumption of cement by adding fly ash was the least,followed by the addition of slag powder,and it was the most with limestone powder.The setting time of cement were delayed by adding both fly ash and mineral powder except for adding limestone powder.The strength of the cement mortar with limestone powder at 3 d was increased and the strength of the mortar at 28 d was reduce.While the 28 d strength of the mortar was increased and its 3 d strength was reduced with the fly ash and slag powder.The test results of the durability of limestone powder on cement mortar show that the carbonation resistance of cement mortar was the strongest with 10%limestone powder instead of cement.While the carbonization resistance of cement mortar was decreased as the content of limestone powder was more than 10%.The shrinkage value of cement mortar was decreased with the increase of the amount of limestone powder which was more than 10%.The resistance to sulfate attack of cement mortar was decreased linearly by adding limestone powder.The effect of limestone powder on the hydration heat of cement is that the hydration heat of cement was increased gradually with the content of limestone powder increased from 0%to 10%,while it was reduced with the increase of the amount of limestone powder which was more than 10%.The results of AC impedance reveal that the mortar resistance of mortar with 10%limestone powder was higher than that of pure cement mortar.Therefore it reduced the porosity of the cement mortar.The results of XRD test of cement hydration products at 3 d and 28 d indicate that a few of 3 CaO.Al₂O₃.CaCO₃.11 H₂O were prepared by limestone powder reacting with C₃A in cement.SEM results show that the density of the interface between the cement and the aggregate was increased as the addition of limestone powder and it may inhibit the formation of cracks in the interface transition zone.On the other hand the amount of the Ca?OH?₂ with sheet shape in the hydration products was reduced,which improved the interface structure and strength.In addition,the pore size distribution of the cement stone was concentrated at about2.5¹0 nm after adding 10%limestone powder in the cement,thus the pore size was refined,which was beneficial to the enhancement of the strength of the cementitious composite.