Experimental Study On Clay Solidification By Carbonated Reactive Magnesia

Soft clay is prevalent in some water-abundant area such as the southeast coastal areas of China and its engineering characteristics are very poor,which seriously affect the engineering safety.In addition,dredging is of great significance to maintain the navigability of waterways and improve the water environment.However,a large amount of soft clay generated by this process cannot be used directly,which may lead to the problem of large storing yard.In the past,traditional cementitious agents such as lime and cement were widely used to improve the soft clay to meet the needs of engineering utilization.Whereas,with the continuously rising requirements for environmental protection,it is significant for engineering and environment to develop more efficient,energy-saving and environmentally friendly solidifying materials and methods that are applicable for soft clay solidification.In recent years,reactive magnesia has provoked rising attentions due to its much lower calcination temperature than cement.Especially,the carbonated reactive magnesia method,which can sequester CO₂ and apparently improve the soil strength in a few hours,has a good application prospect in the field of soft soil solidification.However,the carbonated reactive magnesia method in current researches were mainly used to solidify silt,while studies focus on clay were deficient.And studies have demonstrated that expansion phenomena existed in the carbonated reactive magnesia solidified soils,while researches focusing on this expansion characteristics were insufficient.Furthermore,the direct carbonating process may cause inconvenience in practice,so it is necessary to optimize the treating method.In this paper,the carbonated reactive magnesia method was used to solidify the soft clay from Wuxi.The effects of initial moisture content and reactive Mg O content on the direct carbonated reactive Mg O solidified soil were studied,the strength of solidified specimens was measured,and the expansion characteristics of carbonated specimens were monitored and discussed in depth.In order to simplify the process of carbonization,microbial technology was used to induce the carbonization of reactive Mg O,and sodium hexametaphosphate was used to modify this process,and ideal solidification results were obtained.As the directly carbonated reactive Mg O method was able to yield high expansion stress and consolidation strength,it was used in the model test of an expansive pile conducted in this paper.The main achievements of this paper are as follows:（1）Triaxial specimens with different active magnesia content and different initial water content were prepared by a compaction method.The specimens were carbonized or hermetically cured for 12 hours,respectively.The unconfined compressive strength,mass and volume changes,and water content changes of these specimens were measured,the pores and microstructure were tested.The effects of reactive magnesia content and initial water content on the apparent changes,physical and mechanical properties and microstructure of solidified specimens were systematically discussed.The results showed that the carbonization process can lead to apparent volume expansion in the solidified soil.When the reactive Mg O or the initial water content is less than 25%,the expansion process may destroy the bond between soil particles,resulting in cracking or even loose state of the specimens,and serious decline in strength.When both active Mg O and initial water content are higher than 25%,the crystallizing products generated from the carbonization process fill the pores between soil particles and rapidly form cementations in soil,which ultimately significantly increases the strength of the solidified soil.In the studied scope of this study,the strength of the specimens gradually increased with the increase of active Mg O or initial water content.（2）A monitoring device was installed and the expansion stress and volumetric deformation of the carbonated specimens during carbonization process were successfully monitored.The unconfined compressive strength,mass variation,porosity and microstructure of the carbonated specimens were measured.The effects of active Mg O content and confining pressure on the expansion characteristics,mechanical properties and microstructure of the sample were discussed.The specimen carbonated under 200 k Pa carbonating pressure began to show obvious volume expansion and expansion stress after carbonating for 2 hours,and the expansion stress and volume reached relatively stable after carbonating for about 8 hours,and the expansion stress generated during the carbonization process was large,always higher than1MPa.In addition,with the increase of reactive Mg O content or confining pressure,the expansion stress and cementation strength increased while the volume deformation decreased,and a certain confining pressure can significantly reduce the soil expansion.（3）The biocarbonated reactive Mg O by Bacillus pasteurelli method was further modified by sodium hexametaphosphate to solidify clay.The effects of sodium hexametaphosphate and reactive Mg O content on initial rheological properties,early strength,mechanical properties and microstructure of solidified samples were revealed by falling cone tests and unconfined compressive strength tests.The microstructure of the solidified specimens was observed by SEM and EDS tests,and the solidified mechanism of was further discussed based on XRD and TG tests.The results showed that sodium hexametaphosphate can significantly improve the initial rheological properties of the mixture,and lead to a dispersion effect on the initial soil.Then,as the curing process went on,the crystalline products such as magnesium hydrogen phosphate hydrate and magnesium carbonate hydrate were gradually formed in the specimens through the biological and chemical reactions and generated ideal cementations to the soil particles,which significantly improves the strength of the specimens.The strength of the specimens with the highest strength can exceed 2.5MPa in three days.（4）The scale model test of carbonated reactive Mg O expansion pile was carried out to reveal the squeezing effect of pile expansion on the surrounding soil and the bearing capacity of solidified pile.The results showed that the expansive pile obtained high cementation strength after carbonization for 12h.The pile has an obvious expansion and extrusion effect on the surrounding soils.The expansion stress near the pile body reaches 168k Pa,and high expansion stress could be detected at the position 1 time of the pile diameter away.In addition,the expansion process caused the surrounding soils to be gradually compacted,resulting in a good connection between the pile and the soils,thus causing a high bearing capacity of the pile,which still does not appear obvious subsidence phenomenon when the load reached 560kPa.