Development Of Alkali-activated Geopolymer Grout And Its Physical-mechanical Properties

With the rapid development of infrastructural facilities in western mountainous areas and deep mining in eastern China,complex geological conditions which will increase considerably the groundwater-induced risk might be encountered frequently in underground construction activities.During the pre-grouting for surrounding rock masses,cement-based grouts often show poor applicability under some specified geological conditions such as high ground temperature,resulting in unfavorable grouting quality and poor control of the groundwater-related risks.To improve the grouting quality under complex geological conditions,this thesis aims to investigate the feasibility of developing geopolymer-based grouts and thus to design applicable geopolymer grouts for pre-grouting in shallow rock masses with ground temperature of about 25 ℃ and for injection in deep formations with ground temperature of about45 ℃.Considering notable advantages of the geopolymer material,development of geopolymer-based grouts not only can fulfill the requirements of grouting under complicated conditions but also will meet the national strategies on the recycling re-use of the industrial by-products and ecological economy development in China.The temperature is defined as the controlling factor that influences the properties of grouts to be developed in this thesis.25 ℃ and 45 ℃ are defined as the ground temperatures in shallow rock masses and deep formations with 1500 m depth,respectively.All experimental investigations were conducted under the predefined temperatures.Both rheological properties of grout in fresh state and physical-mechanical behaviors of hardened grouts were investigated systematically via fluid rheological measuring,triaxial creep test and permeability testing,whereas microstructural characteristics of the products and reaction mechanism of the geopolymerization were studied using integrated methods including the scanning electron microscope and energy dispersive spectrum analysis(SEM-EDS),X-ray diffractionon(XRD),Fourier transform infrared spectroscopy(FTIR)and thermal analysis techniques(TG-DSC).Some conclusions obtained can be drawn as follows.(1)The feasibility of developing geopolymer-based grouts has been confirmed.The property evolution of the alkali-activated fly ash-slag hybrid geopolymer grout in both fresh and hardened states were obtained,and accordingly the fly ash-slag hybrid geopolymer grout with favorable properties was developed successfully for pregrouting in shallow rock masses with temperature of about 25 ℃.In the proposed fly ash-slag hybrid geopolymer grout,the slag as the additive is employed to improve the reactivity of alkali-activated fly ash geopolymer grout.When the temperature is about 25 ℃,the initial fluidity of geopolymer grout increases considerably with the increasing of water-solid ratio,and the hydration rate of alkali-activated slag will be accelerated with the decrease of water-solid ratio or with the increase of both the slag dosage and concentration of the activator.It is found that the plastic strength of the geopolymer grout at 25 ℃ increases with time expotionally.The hardened geopolymer grout at 25 ℃ consists of many spherical products with diameter of 30-50 μm,in which unreacted fly ash particles and free water are involved.Chemical products mainly include amorphous calcium silicate hydrate and covalently bonded hydrated alkali-aluminosilicate gel.The hardened geopolymer grout can be identified as one kind of porous media.(2)Both effects of elevated temperature on properties of the geopolymer grout and mechanism of borate in retarding the setting of the geopolymer grout were figured out,and the optimized fly ash-slag hybrid geopolymer was proposed for pregrouting in deep formations with ground temperature of about 45 ℃.Elevated temperature will accelerate the reaction of the grout,enhance the initial fluidity,improve the grout stability and promote the development of strength,but will also shorten the setting time.In this case,borate as the retarder can be employed to control the reaction rate of the grout.Compared with the hardened grout at 25 ℃,both micro-structures and products features of hardened geopolymer grout at 45 ℃are the same.Moreover,the Natrolite can be observed in the hardened grout at 45 ℃,owing to the fast reaction of the geopolymer grout at high temperature.(3)The fluid constitutive model for the developed geopolymer grout was established and the estimation method for predicting the grout viscosity and yield stress was also proposed.The rheology of geopolymer grouts in 4 hours after mixing can be described exactly by the Bingham fluid model,and the hydration of the slag has pronounced effects on the time-dependent rheological behavior of the geopolymer grout.The geopolymer grout always behaves as shear-thinning.With the increase of slag content,temperature and activator concentration or if the water-solid ratio is relatively low,the hydration of the slag will be rapid.In this case,rheological parameters of the geopolymer grout will change significantly and thus the rheological time-dependence will become relatively notable.The borate as the retarder will delay the hydration of slag and then decrease the rheological time-dependence of the geopolymer grout.The apparent viscosity of geopolymer grout will first decrease with the increasing of temperature and then increases slightly.The workability of the geoplolymer grout at high temperatures will be more favorable than that at low temperatures.Based on the volume fraction of solid in grout and the flow spread test,models for predicting the apparent viscosity and yield stress of the geopolymer grout at different temperatures are proposed to help understanding the rheological parameters timely.(4)The analytical modelling of creep characteristics of hardened geopolymer grout was proposed appropriately based on the triaxial creep testing.According to the features of creep deformation of the hardened geopolymer grout,the accelerated creep can be described properly using the improved non-linear visco-plastic element,while both the intial creep and the steady-state creep stages can be described using the generalized Kelvin’s model.The good fit indicated that the proposed model can accurately describe the characteristics of the nonlinear creep of the geopolymer grout under triaxial stress states.Addtitionally,it is also found that the porosity and permeability of the hardened geopolymer grout increases with the increase of temperature and the water-solid ratio.Because of the relatively low compressive strength of hardened geopolymer grout,the developed fly ash-slag hybrid geopolymer grout is much more applicable to be the qualified pre-grouting material for water control rather than for ground reinforcement.