Study On Mechanism And Physical-Mechanical Properties Of Stabilized Expansive Soil By Cementitious Material From Calcium Carbide Residue And Rice Husk Ash

Expansive soil is a typical ?catastrophic soil? and widely distributed in China.Due to expansion,contraction,cracking of expansive soil,it often causes serious damage to various superficial light-duty engineering facilities built in expansive soil areas,resulting in huge loss every year.So expansive soil needs to be effectively stabilized in engineering construction.In addition,large amounts of solid waste such as calcium carbide residue and rice husk ash are formed in industrial production,which occupy land resources and destroy water and soil environment.So these wastes are in urgent need of resource utilization.Based on the background of ecological civilization construction and concept of ?treating waste with waste?,this paper creatively using cementitious material from calcium carbide residue(CCR)and rice husk ash(RHA)to stabilize expansive soil.Through theoretical analysis and a series of experiments such as mortar strength test,scanning electron microscopy,X-ray diffraction,unconfined compression test,direct shear test,CBR,swelling test,shrinkage test,digital photography,CT scanning,mechanical properties of this cementitious material,and mechanism,physical and mechanical properties of stabilized expansive soil have been studied.The main research contents and results are as follows:(1)The strength of cementitious material from calcium carbide residue and rice husk ash is mainly affected by CCR/RHA ratio and setting time.In the initial curing period of CCR-RHA mortar,the higher RHA content,the higher compressive and flexural strength.But in the medium and long-term curing period,the strength did not increase with the increase of RHA content.There is a peak where the RHA blending ratio is 65%.Furthmore,the compressive and flexural strength of CCR-RHA mortar increased with the increase of setting time.The increasing rate was relatively fast within 28 days,and then slowed down,but the growth did not stop within 90 days.It indicated hydration reaction between CCR and RHA last for a long time and was much slower than cement hydration.By comparing the mechanical properties of each group of mortar samples,the optimal ratio of CCR-RHA is adapted 35:65.In addition,compared with P·O32.5 cement mortar,the CCR-RHA mortar with optimal ratio reached half of its compressive strength and one third of flexural strength.The strength of CCR-RHA cementitious material is derived from the hydration reaction of CCR and RHA,including clcium silicate oxide hydrate,clcium aluminum oxide hydrate,gismodine,clcium aluminum oxide carbonate hydrate and so on.Various colloidal and crystalline hydration products are intertwined,overlapped,and gradually hardened,forming a tightly integrated and dense structure in three-dimensional space.From both macro and micro analysis,CCR-RHA cementitious material exhibits good cementation properties.Meanwhile it has superior mechanical properties,significant economic and social benefits as a soil stabilized material.(2)The expansive soil shows as a compact granulation structure,and the pores are very developed.It can be seen that the "face-to-face" superimposed curved montmorillonite and flaky illite.When CCR-RHA cementitious material was added in expansive soil and cured 7 days or 14 days,a certain amount of flocculent gel appeared,and the montmorillonite and illite were reduced greatly,the pore number was decreased significantly,and the structure compactness was enhanced.At 28 th day of curing,the montmorillonite and illite flakes disappeared,the soil aggregates were closely linked,the number and size of pores were significantly decreased,and the compactness was further improved.Some fibrous and long rod-shaped crystals,called as ?calcium zeolite?,were grown between the particles.The gel is interlaced with the crystal to form a three-dimensional network skeleton structure.At the 90 th day of curing,a large amount of crystals were formed in the stabilized expansive soil,and there were various hydration products such as clustered ettringite,fibrous and long rod-shaped calcium zeolite,and plate-like gehlenite,and so on.The crystals were intertwined to form a solid spatial network structure.In addition,with the increase CCR-RHA content,the agglomeration of expansive soil particles was increased,the degree of pore development was obviously weakened,the soil structure was more compact,and the overall stability was better.The microstructure and mineral composition reflect the mechanism of CCR-RHA cementitious material stabilizing expansive soil,mainly including: hydrated reaction of calcium carbide residue-rice husk ash,ion exchange and agglomeration,coagulation reaction and carbonation reaction.(3)With the increase of the CCR-RHA content,the maximum dry density of the stabilized expansive soil decreases linearly,and the optimum moisture content increases gradually.Different from strain hardening model of expansive soil,the stabilized expansive soil under axial compression and direct shearing show as strain-softening model.The rising phase of stress-strain curve can be described by the Logistic function,and the falling part can be fitted with rational fractional form.Judged from the shape of stress-strain curve,with the increase of CCR-RHA cementitious material and setting time,the stabilized expansive soil is more and more like a brittle material.However,after the damage,the stabilized expansive soil still retains a certain residual strength whicn mainly provided by the necking core.The addition of stabilized significantly improves unconfined compressive strength,cohesion and CBR value of expansive soil.Under normal circumstances,these mechanical indexes increase rapidly with the increase of CCR-RHA content,reaching a peak at 25% and then decreasing.Meanwhile,unconfined compressive strength,cohesion and CBR increased with the increase of setting time.And there is a good linear correlation between compressive strength,cohesion,CBR and logarithm of setting time.The softening coefficient of stabilized expansive soil is much larger than that of expansive soil,indicating that the CCR-RHA cementitious material significantly improves water stability of expansive soil.(4)The free swelling ratio of stabilized expansive soil is in accordance with the CCR-RHA content as exponential decay model.The addition of CCR-RHA cementitious material greatly reduces the swelling ratio with or without pressure and swelling force of expansive soil.With the increase of setting time,the swelling ratio with or without pressure and swelling force of stabilized expansive soil decrease accordingly.Based on the comprehensive results of swelling ratio and swelling force test,it was found that the anti-expansion performance was best when CCR-RHA content was 25%.In addition,when CCR-RHA content was greater than 20% and cured for 14 days,the original expansive soil with medium expansion potential has been changed to non-expansive soil and can be directly applied in construction.(5)After stabilized by CCR-RHA cementing material,the linear shrinkage and volume shrinkage of expansive soil are greatly limited.The line shrinkage rate and volume shrinkage rate decreased with the increase of CCR-RHA content and setting time.The shrinkage index of stabilized expansive soil is much smaller than that of the original expansive soil,and the CCR-RHA content of 25% can achieve the best shrinkage inhibition effect.(6)Qualitative and quantitative analysis of apparent crack and internal crack revealed that the surface crack ratio of expansive soils decays rapidly with the increase of CCR-RHA content and setting time.At the early stage of curing,there is large reduction of surface crack ratio,but the effect is less and less obvious with setting time growing.Compared with the original expansive soil,the internal crack of the stabilized expansive soil is obviously improved,the number of cracks is reduced,the crack width is decreased,and the connected crack is eliminated.Additionally,the volume crack ratio and CCR-RHA content are approximated accord as an exponential decay model.It can be seen that the CCR-RHA cementitious material plays an important role in inhibiting expansive soil cracking.With the increase of dry-wet cycles number,the region number,crack nodes number,cracks number of original expansive soil decreased and the average crack length increased.And the average crack width and crack ratio increased at first and then decreased,reaching the maximum at the third dry-wet cycle.However,the region number,crack nodes number,cracks number,average crack length and average crack width of stabilized expansive soil increased with the increase of dry-wet cycles number.This represents that degree of crack development was positively correlated with cycles number.The first and second cycles have the most obvious effect on the cracking of the stabilized expansive soil.After the third dry-wet cycles,the surface crack development tends to be stable.Under the action of dry-wet cycle,the cracking of original expansive soil developed significantly.However,after the addition of CCR-RHA cementitious material,the crack ratio of stabilized expansive soil was greatly reduced,and the crack resistance was significantly enhanced.This ability is mainly derived from the cementation of CCR-RHA,ion exchange and agglomeration and hard coagulation.The above results demonstrate the rationality of CCR-RHA as a cementitious material,and confirm the superiority of the cementitious material in stabilizing expansive soil in terms of performance,economy and environment.This research provides scientific basis and theoretical guidance for engineering application of stabilized expansive soi in construction of building cushion,subgrade of road and railway,airfield foundation,dam filler and other earthworks.In future,because of excellent performance,lowered construction and disposal costs,reduced environmental pollution,the expansive soil stabilized by CCR-RHA will be more and more concerned by engineers and technicians.