Study On The Preparation Of Lime-metakaolin Cementations Material And The Modification Of Them With Natural Mineral Fibers

Historical building is an important part of human cultural heritage,it witnesses the development of human history and the progress of science and technology.Now in China,there are a large number of existing masonry and geotechnical ancient buildings such as ancient walls,tombs,stone statues,grottoes and rock paintings,the critical point of site protection of these kind heritages lies in the restoration materials used is proper or not,therefore,research and development advanced and suitable cementatious material has always been the research focus in related field.As a new kind of restoration mortar,lime-metakaolin(L-MK)cementitious material become a research hotspot in recent years,it has the hydraulical property like cement as well as can compatible with historic masonries like traditional lime paste or mortar.In this paper,based on the research status at home and abroad,L-MK paste and mortar were prepared by using calcium hydroxide and metakaolin as raw materials,and with modification and harmless for health as starting points,natural non-asbestos mineral fibre modified L-MK mortars were prepared with palygorskite,wollastonit,brucite and sepiolite fibres.The main research contents and results are as follows:(1)Severa L-MK pastes with different substitution rates of lime by metakaolin were prepared and curing mechanism and fundamental performance of them were studied.The obtained result shows that hydration reaction between calcium hydroxide and metakaolin is a slow process,higher metakaolin content in L-MK paste favors the hydration reaction and Al doping is common existed in C-A-S-H.When metakaolin content is 40%~50% in paste,C4AH13 and C2ASH8 is the main product in early and final stage,respectively.When metakaolin content is 10%~20%,C4AH13 is the main product in all 28 days and a lot of unreacted Ca(OH)2 is existed.In metakaolin content 30% paste,C4AH13 is the main product in early stage,C4AH13 and C2ASH8 have similar content in final stage and some unreacted Ca(OH)2 is existed.In all pastes,C4A?H11 is only an intermediate hydration products existed in early stage.Besides,the fundamental performance of L-MK paste is increasing with the metakaolin content increased.(2)The influence of metakaolin content,water-binder ratio,binder-sand ratio and curing humidity on L-MK mortar were tested.the result shows that the existence of sand delays the progress of hydration reaction.The performance such as shrinkage property,mass stability,flexural strength,compressive strength,water stability,sulfate resistance of L-MK mortar is increasing with the metakaolin content increased.Curing condition also effects the carbonation and hydration progress,which further effect the performance of mortar,in addition,the water-binder ratio and binder-sand ratio also significant affect the fluidity of mortar and the performance after setting.(3)The influence of different size and dosage of natural palygorskite and wollastonite fiber power on L-MK mortar were investigated.the results shows that The greater mesh count leads to more significant modification effect,but the functional mechanisms are different,because of high adsorbability,palygorskite can prompt hydration products growth along fiber surface and form a vein structure,wollastonite can participate in the hydration reaction and increase the structure dense consequently.Both palygorskite and wollastonite can improve the flexibility of L-MK mortar,besides,palygorskite can decrease the mortar fluidity and mechanical strength,increase the drying shrinkage and water absorbability but wollastonite can increase the mortar fluidity and mechanical strength,decrease the drying shrinkage and water absorbability.(4)After the chemical dispersionin,most of the brucite and sepiolite fibre bundles become single fibres,wet-mixing can help fibres better dispersed in the L-MK mortar than dry-mixing.On this basis,both brucite and sepiolite fibres can decrease the mortar fluidity,increase the drying shrinkage and water absorbability,and the modification effect is particularly obvious in sepiolite-modification L-MK mortar.Besides,two kinds fibres can enhance the mechanical properties of the mortar attributed to inducing a network structure within the L-MK mortar matrix.(5)Based on the integral geometry concept and previous studies about fiber-reinforced cementitious composites,the quantitative relationship between randomly dispersed fibers and cracks is investigated.On the assumption that the size of microcracks are constant value,the quantitative relationship is determined both by fibre volume content and draw ratio of fibres.The brucite fibres' draw ratio is smaller than sepiolite fibres,this is benefit to brucite fibres for its modification function at lower volume content.