Research On The Deterioration Regularity And Damage Mechanism Of Geopolymer Concrete Under Freeze-thaw Cycles

Geopolymer is a new type of cementitious materials,which is the reaction product of aluminium-silicon material and high alkaline solutions.Geopolymer exhibits more excellent performance,and it is also an environmental-friendly and low-carbon material during its preparation and application process.Thus,geopolymer will have great application prospect.However,there are still less researches about the anti-freeze property and damage mechanism of geopolymer concrete.It is important to systematically understand the mechanism of structural damage under freeze-thaw cycles and further improve the durability of the geopolymer concrete.This work was supported by a project from the Natural Science Foundation of China,i.e.,Research on the deterioration regularity and damage mechanism of geopolymer concrete under freeze-thaw cycles.In this thesis,the mixing proportion of geopolymer concrete was first designed based on the on the aggregate dense-packing theory and paste thickness coated on aggregate.Then,the pore structure of geopolymer concrete was characterized,analyzing its formation mechanism.Besides,the changes of mechanical properties and water saturation degree of geopolymer concrete were also studied.Based on above research,the mathematics model and evaluation parameters were established to describe this deterioration process.The results showed that the pore structure and water saturation degree were the main factors for anti-freeze property of geopolymer concrete.The pore structure was depended on the parameters of alkali activators,mix proportion and curing regime of geopolymer concrete.The cooling rate,lowest temperature and solution types of freeze-thaw process were the external factors for the geopolymer concrete under freeze-thaw damage.For geopolymer concrete,the size distribution of pore was mainly range from0.01?m to 0.1?m.During the process of free-thaw cycles,there was a decrease in the amount of pore from 0.01?m to 0.3?m and an increase in the amount of pore from 0.3?m to 10?m.In addition,it was found that the porosity and the number of freeze-thaw cycles of geopolymer concrete followed the equation of the N=78.5+1.17exp(-x/1.28).Moreover,the pore structure of geopolymer concrete has fractal characteristic,and the fractal dimension of pore structure decreased with the increase of numbers of freeze-thaw cycles.Additionally,the fractal parameters of pore axis and specific surface area could characterize the anti-freezing property of geopolymer.With the aggravation of freeze-thaw damage,the fractal parameters of pore axis would increase,and the fractal parameters of specific surface area would decrease.When geopolymer concrete was totally immersed into water,its water saturation degree was about 0.85-0.90.When one face of geopolymer concrete was in contact with water,its water saturation degree was just about 0.4-0.6.The critical saturation degree was 0.6,thus the relatively higher water saturation degree could be the main reason for the poor anti-freeze property of geopolymer.The specific reasons of a high-water saturation degree for geopolymer concrete are:1)more small pores(0.01?m to 0.1?m)could adsorb more water;2)the Na⁺in the N-A-S-H gel was easily leached into water due to the weak bond,in order to balance the charge,the position of Na⁺in N-A-S-H gel would be replaced by H₃O⁺,so more water would be adsorbed in the N-A-S-H gel;3)the residual alkali ions in the pore solution would absorb more crystal water.The anti-freeze property and pore structure of geopolymer concrete can be improved by decreasing the water amount used in mixing.Besides,the higher curing temperature of geopolymer concrete would also refine its pore structure,and its anti-freezing property at 14d was improved.Additionally,when the lowest freezing-temperature was below-10?,the damage of freeze-thaw of geopolymer concrete can be accelerated.During the process of the freeze-thaw cycles,there was a significant mass loss compared with the change of relative dynamic elastic modulus.Thus,the main failure form of geopolymer concrete under freeze-thaw damage was the surface spalling,and the damage of internal structure is slower.Salt freezing can accelerate the rate of strength loss of geopolymer concrete.After freezing-thawing damage of geopolymer concrete,the residual flexural strength and damage degree had a good linear relationship,i.e.(?),and there was a linear relationship between compressive strength and material parameter(?),i.e.,f=k?+b.During the experiments of water freezing and salt freezing,the damage law of compressive strength and residual flexural strength are similar.This illustrates that,during the process of water freezing,the residual alkaline ions can be leached from the pore structure of geopolymer concrete into the external water solution,thus the water freezing was gradually switched to salt freezing.According to the attenuation law of relative dynamic elastic modulus,the freeze-thaw factor(w)can be calculated,which can be used to evaluate the anti-freezing property of geopolymer concrete.The higher compressive strength of geopolymer concrete,the smaller freeze-thaw factor,showing a better anti-freezing property.Based on the relationship between the freeze-thaw factor and compressive strength of geopolymer concrete,the anti-freezing property can be forecasted.The initial water saturation degree has greatly influence on the strain of geopolymer concrete during the freeze-thaw cycles.According to the curve of temperature and strain,the mechanism of effect of water saturation degree on the concrete deformation is as followed:when the water saturation degree was less than 40%,the strain of specimen tended to 0??due to a continuous shrinkage.In this case,the freezing damage was very limit.When the water saturation degree was more than 80%,the expansion strain of geopolymer concrete was about 500??-700??and 50??-100??in the cooling and heating stages respectively.In this case,the freezing damage was easily occurred.This work set up a bridge between the influence factors and macroscopic properties of geopolymer concrete under freeze-thaw cycles,revealing the damage mechanism of freeze-thaw failure.These findings in this thesis could give theoretical and scientific supports for the future application of geopolymer concretes in the engineering.