| Coastal areas mostly consist of soft soil with low shear strength.Buildings and infrastructure are prone to uneven settlement due to the low shear strength and high water content of the soft soil layer.In order to ensure the safety of buildings or infrastructure,the ground improvement on the soft soil layer is generally required.Cement is usually used as a soil binder agent for ground improvement.However,cement is a material with high pollution,high energy consumption and large resource consumption.Therefore,there is an urgently need to develop a green soil binder agent for the purpose of environmental protection.Geopolymer is an alkali-activated gelling material formed by the polymerization of amorphous silico-aluminate raw material.The amorphous silico-aluminate raw materials are normally obtained from low-cost industrial solid waste such as slag,fly ash,etc.,and are excited by alkaline activators to generate new economic materials with high early strength and high durability.Geopolymer also performs the merits of excellent engineering performance and environmental protection.Hence,geopolymer can be used as an alternative curing agent to cement in ground improvement.The preparation methods of geopolymer are mainly classified as"two-part"method and"one-part"method.As for the two-part geopolymer,a liquid alkaline activator is firstly prepared by a mixture of sodium hydroxide solution with certain Al/Si molar ratio.The liquid alkaline activator is then added into the silico-aluminate raw material to synthesize geopolymer paste.Finally,the geopolymer paste is used as a binder agent to stabilize the soil.However,adopting alkali solutions limits the commercial application of the two-part technology since it is difficult to handle these viscous,corrosive solutions in the construction site.It is expected that only a dry mixture would be needed in addition to water if the geopolymer was adopted for in-situ ground improvement.The"one-part"method refers to mix the silico-aluminate raw material with the solid alkali activator material in advance,and then the water is added into the solid mixture to synthesize the geopolymer.This preparation method not only effectively simulates the on-site construction technology of the binder agent,but also eliminates the environmental impact of the alkaline activator and reduces the transportation cost of the solution during the on-site construction.Therefore,it is very promising to adopt one-part geopolymer as a novel binder to stabilize soft soil.However,to the best of the author’s knowledge,there are no published studies investigating the feasibility of one-part geopolymer as a soil stabilizer,or the mechanical behavior of the soft soil stabilized by one-part geopolymer.In view of this,this paper intends to use"one-part"method to prepare slag and fly ash-based geopolymer as binder agent to stabilize soft clay.The applicability of slag and fly ash-based geopolymer to stabilize soft clay is studied,and its curing mechanism is revealed.The main research work is listed as follows:(1)Slag(SL)and fly ash(FA),which belong to industrial waste materials,are used as the raw materials(silico-aluminate material),and solid sodium hydroxide(Na OH,NA)is employed as an alkaline activator.The mixture consisting of slag,fly ash and solid particles alkaline activator are first prepared by"one-part"method,and then distilled water is added to prepare slag and fly ash based geopolymer(SL-FA based geopolymer)slurry.The influence of slag to fly ash ratio,water to binder ratio,and curing temperature on the unconfined compressive strength of SL-FA based geopolymer is investigated.(2)The"one-part"SL-FA based geopolymer is used to stabilize soft clay.The effects of slag to fly ash ratio,concentration of,alkaline activator,and water to binder ratio on the strength development of the SL-FA based geopolymer stabilized soft clay is investigated and evaluated though the unconfined compressive strength(UCS)test.Scanning electron microscope(SEM),Energy Dispersive Spectrometer(EDS)and X-ray Diffraction(XRD)analysis were conducted to investigate the development of microstructure of the one-part SL-FA based geopolymer stabilized soil clay.(3)Based on the experimental results,the response surface method is used to analyze the factors such as the ratio of silicon-aluminum raw materials,the ratio of activator to raw materials,and water to binder agent ratio to the compressive strength development of the SL-FA based geopolymer stabilized soft clay.The analytical model with multi-parameters is established to predict the early compressive strength of the SL-FA based geopolymer stabilized soft clay.Based on the experimental and theoretical study,the following conclusions can be drawn:(1)For the SL-FA based geopolymer prepared by the"one-part"method,the inclusion of appropriate amount of fly ash in the raw material promoted the reaction of the geopolymer,thereby improving the compressive strength of the SL-FA based geopolymer.However,when the content of the FA in raw material exceeded certain amount,increasing the FA content would perform negative impact on the UCS of the SL-FA based geopolymer;Lower water to binder agent ratio increased the alkaline concentration,and then enhanced the UCS of the SL-FA based geopolymer.The factor of curing temperature had a great impact on the UCS development of the SL-FA based geopolymer.In current study,for the 85~oC curing temperature,the best mixing proportion of the one-part SL-FA based geopolymer was found to be 90%SL and 10%FA in raw material and water-binder ratio of 0.7,in which the UCS reached 7.7 MPa at 7 days curing time.For curing condition of room temperature,the best mixing proportion of the one-part SL-FA based geopolymer was found to be 90%SL and 10%FA in raw material and water-binder ratio of 0.6,90%slag and 10%fly ash and the water to binder agent ratio is 0.6 in which the UCS achieved 2.94 MPa at 7 days curing time.(2)In the one-part SL-FA based geopolymer stabilized soft clay;the main hydration phase was the amorphous C-S-H gel,which can be effectively activated by the solid NH in one-part geopolymer.The reactive minor phases in one-part geopolymer consisted of calcium aluminate hydrates(C-A-H)and calcium aluminosilicate hydrates(C-A-S-H)gels,which were the reaction products from the FA in raw material.The occurrence of the C-S-H,C-A-H or C-A-S-H gels would overlap each other with the increasing of the curing time,and connect the soil particle to achieve high compactness in the microstructure of stabilized soil.When the content of the FA in raw material exceeded certain amount,increasing the FA content would perform negative impact on the early UCS of the stabilized soil.The main reason was that the Si-O-Si and Al-O-Al bond in FA material were broken and rapidly polycondensated to form an amorphous aluminosilicate material(N-A-S-H).Such reacted structure was very stable,and cannot induce a secondary reaction between the soil particles,and only served to fill the porosity in the soil.At same time,the content of solid NH activator and water-binder ratio would determine the concentration of the NH solution,which influenced the reaction rate in the one-part geopolymer paste.In this study,the best mixing proportion of the one-part SL-FA based geopolymer stabilized soil was found to be 90%SL and 10%FA in raw material,solid NH activator to raw material ratio of 0.15 and water-binder ratio of 0.7,in which the UCS reached 1.5MPa at 14 days curing time.(3)The impact of the SL and FA content,sodium hydroxide content and water to binder ratio on the UCS of the one-part SL-FA based geopolymer stabilized soil were analyzed based on the response surface method.Then a prediction formula of 14D UCS of the one-part SL-FA based geopolymer stabilized soil was established based on multiple linear regression method.The well agreement between the experimental results and predicted values were achieved.Hence,this formula provides a certain theoretical basis for the strength development of the one-part SL-FA based geopolymer stabilized soil.The outcome of current study sheds light on the practical use of one-part alkali-activated geopolymer as a soil binder in ground improvement. |
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