Research On The Preparation And Performance Of Natural Hydraulic Lime

Natural hydraulic limes?NHL?is a kind of natural inorganic cementitious material,which is energy-saving,environmental friendly,green and sustainable,because of its strong adhesion,good permeability,strong self-healing ability,good frost resistance and alkali resistance.At present,the domestic knowledge of NHL is not enough,and the research on its preparation process and application has not been carried out.In this paper,low-grade and high silicon limestone tailings are selected as raw materials to study the preparation process and performance of NHL.In addition,in order to improve the early mechanical properties and durability of NHL,nano-SiO₂ was introduced into the NHL system,and the effect of Nano-SiO₂?NS?on the hydration hardening,gel properties and hardening structure of NHL was studied.According to the binary phase diagram of CaO-SiO₂,in order to ensure the complete decomposition of CaCO₃,the NHL calcination temperature range is 900¹250?.By adjusting the content of CaO in raw materials,different grades of NHL can be designed.The best calcination condition is 1200?,10?/min,1h.At room temperature,the optimum digestion humidity was 70±5%RH,and the digestion time was 8 days.The 7-day compressive strength of NHL mortar prepared with N1 ratio is 2.2 MPa,and the 28 day compressive strength is 5.4 MPa,which meets the requirements of the standard forNHL5compressive strength.The 7-day compressive strength of NHL mortar prepared by the mixture of N2 and N3 is 1.8 MPa and 1.4 MPa respectively,and the 28 day compressive strength is 4.2 MPa and 3.5 MPa respectively,all of which meet the requirements of NHL3.5in the standard.The 7-day compressive strength of NHL mortar prepared by N4 and N5 is1.3 MPa and 0.6 MPa respectively,and the 28 day compressive strength is 2.6 MPa and1.6Mpa respectively,N4 meets the requirements for NHL2 compressive strength in the standard,and N5 does not meet the requirements for NHL minimum strength in the standard.The main mineral phases of NHL2 and NHL5 are CaCO₃,C₂S,SiO₂,Ca?OH?₂ and C₄AF,the main mineral phases of NHL2?N4?and NHL5?N1?prepared from limestone tailings are C₂S,C₃A,Ca?OH?₂ and C₄AF.Compared with the commercial NHL2,the color of the self-made NHL2?N4?is slightly yellow,but the difference is not big.Compared with the commercial NHL5,the color of the self-made NHL5?N1?is earthy yellow,the color is darker.The compressive strength of self-made natural hydraulic lime NHL2?N4?and NHL5?N1?at different ages is higher than that of commercial NHL2 and NHL5.In terms of durability,after 28 days of carbonization,four kinds of NHL have different degree of carbonization reaction,but the overall carbonization depth is similar.It is found that the growth rate of compressive strength of commercial NHL mortar after carbonization is higher than that of self-made NHL mortar.After 20 freeze-thaw cycles,the mass loss rate of NHL2 mortar block is 14.9%higher than that of N4 mortar,and the mass loss rate of NHL5 mortar block is 23.2%higher than that of N1 mortar.The frost resistance of NHL2?N4?and NHL5?N1?made of limestone tailings is better than that of NHL2 and NHL5,respectively.After 30 cycles of sulfate drying and wetting,all four kinds of mortar blocks expanded and cracked,resulting in serious damage.When NS is introduced into NHL,in the early hydration process of NHL slurry,the initial hydration rate of C₂S will slow down due to the adsorption of NS on C₂S particles and the competitive adsorption of NS on water,but the later reaction of NS and Ca?OH?₂ will increase the hydration heat flow and accumulated heat release.The carbonation depth and carbonation rate of NHL mortar increase with the increase of NS content?0%³%?,and the addition of NS reduces the porosity and large porosity of NHL mortar,increases the proportion of medium porosity and improves the compactness of NHL mortar.NHL2 mortar with NS can capture and utilize more CO₂,especially under accelerated carbonation condition.