Regulation Of Hardening,Drying Shrinkage Characteristics Of Alkali-Activated Materials And Their Karst Water Attack Performance

Alkali-activated materials are the kind of inorganic cementitious materials derived from the reaction of solid aluminosilicate precursor with alkali activator.They have excellent mechanical properties,low CO₂ emissions as well as chemical corrosion resistance.Nevertheless,there is a lack of systematic research on hardening,shrinkage as well as durability evaluation of alkali-activated materials,limiting their engineering application and large-scale production.Aiming at these problems,the slag was simply modified via CO₂carbonization based on the modification of raw materials.The carbonation mechanism of slag,as well as the gelation reaction kinetics and retarding mechanism of alkali-activated carbonation slag have been investigated.Based on Fuller gradation theory,AAS and alkali-activated metakaolin（AAM）materials containing Fuller gradation micro-aggregate were prepared.Moreover,the shrinkage regulation mechanism was investigated.On the basis of above works,in terms of the durability of bicarbonate（HCO₃-）corrosion of infrastructure in karst areas of China,the resistance to bicarbonate corrosion of cementitious materials with different calcium content is systematically studied.Moreover,the mechanism of carbonation corrosion in karst water was investigated.The main conclusions of this thesis are as follows:（1）Slag was modified by carbonation under natural conditions.The effect of carbonated slag on the setting time,rheological behavior as well as reaction products of AAS was studied.The results show that non-carbonated slag showed high reactivity,the initial setting time of AAS is usually less than 30 min,and the rapid loss of workability is also observed.During the natural carbonation process,the highly reactive calcium species（e.g.,free-Ca O,calcium silicate）in slag reacts with CO₂ to the generation of carbonate nanoparticles（e.g.,calcite,dolomite）on the surface of the slag particles.The amount of carbonate increases with the extension of carbonization periods.After carbonation,the reactivity of slag significantly reduces.The gel formation rate of alkali-activated carbonated slag is slowed down.Moreover,the setting time is significantly prolonged.The initial setting time of pastes prepared from carbonated slag with Na OH or sodium silicate are greater than 45 min.The 28 days compressive strength is greater than 32.5 MPa.These properties meet the technical requirements specified in the national standard（GB 175-2007）.The rheological properties of AAS prepared using carbonated slag are significantly improved.At a constant shear rate,the shear stress of AAS with carbonated slag remain at a lower equilibrium state for a longer time.Moreover,carbonated slag can prolong the setting time and inhibit the loss of fluidity without changing the compressive strength,product composition as well as microstructure of AAS.（2）The modification time of slag was shortened using the method of CO₂accelerated carbonation.The accelerated carbonation mechanism of slag as well as the reaction process and retarding mechanism of carbonated slag were studied.The results show that the accelerated carbonation can significantly shorten the carbonation time of slag and prolong setting time of AAS.In the process of accelerated carbonation,the CO₂ was adsorbed on the slag surface by chemical bonding to generate carbonate and amorphous silicon-aluminum structure.The amount of CO₂ fixation by slag increases with the extension of carbonation time.The maximum amount of CO₂ fixation by slag within 3 h is 11.57 mg/g.The formation of carbonates and amorphous silicon-aluminum structure on the surface of carbonated slag significant effects the early reaction kinetics of AAS.Accelerated carbonation inhibits the dissolution rate of slag as well as the formation rate of gels in AAS.The exothermic curve of gelling reaction of alkali-activated carbonated slag shown an obviously low exothermic additional peak before the induction period.And the duration of the additional peak increases with the increase of carbonization time of slag.The setting time and28 d compressive strength of AAS prepared using 3-168 h accelerated carbonation slag with Na OH or sodium silicate meet the technical requirements of the national standard（GB 175-2007）.Considering the strength,setting time,etc.,the recommended carbonation time of slag is 6-12 h for the Na OH system and 3-6 h for the sodium silicate（Ms=2.0）system.（3）Based on Fuller gradation theory,binary composite AAS and AAM materials containing Fuller gradation micro-aggregate（Fuller-FS）were designed and prepared.The shrinkage regulation mechanism of binary composite binders was studied.The results show that Fuller-FS mainly plays a role of physical filling,which can refine the pore structure as well as improve the drying shrinkage of alkali-activated materials.For AAS systems,incorporating Fuller-FS can improve the setting time and drying shrinkage of AAS.When the content reached 50%,the initial setting time of AAS with Na OH,Ms=1.0 and Ms=2.0 of sodium silicate is prolonged to 135,77 and 248 min,respectively.Compared with the control sample,the incorporation of Fuller-FS can reduce the BET surface area and pore volume in the range of 2-50 nm of AAS,decreasing in capillary pressure during drying,which significantly reduces the drying shrinkage of AAS.For AAM systems,the incorporation of Fuller-FS reduces the water requirement of pastes and improved the rheological properties of AAM.Due to its physical filling effect,the yield stress and dry shrinkage of AAM decrease by about 80%and 65%when 30%Fuller-FS is added.The compressive strength decreases when Fuller-FS content reaches 40%.Considering the strength factor,the content of Fuller-FS should not exceed 50%for AAS and 40%for AAM.（4）High-calcium AAS mortars were prepared by using carbonated slag.The compressive strength,microstructure of products and degradation mechanism of AAS after karst water attack were studied with comparison to Portland cement（PC）mortar.The results show that there are adverse effects on p H value of pore solution and compressive strength of mortars under attack by karst water.Moreover,the AAS samples prepared from carbonated slag have no specific effects on karst water attack.The sodium silicate-activated slag is propitious to karst water attack resistant in terms of p H of pore solution and mass change,due to the dense C-A-S-H gel structure in sodium silicate system.The formation of calcite particles in PC during karst water attack process is mainly related to the chemical reaction of Ca（OH）₂ with HCO₃-.After consuming Ca（OH）₂,C-S-H will be decomposed to form calcium carbonate and silica gel with high a degree of polymerization,resulting in deterioration of pore structure,p H value as well as compressive strength.Due to the absence of Ca（OH）₂,the decalcification of C-A-S-H with low Ca/Si ratio is to formation of Ca CO₃ and silica gels with high degree of polymerization in the AAS mortars after karst water attack.The decomposition of gels leads to the increase of gel pores and reduction of compressive strength.（5）Using metakaolin as raw material,the effect of the activator silicate modulus and slag content on the karst water attack of AAM was investigate.The results show that the karst water attack has an adverse effect on the mechanical properties of AAM.The metakaolin activated with Ms=2.0 sodium silicate shows excellent resistance to karst water attack.However,the compressive strength of AAM decreases more obviously after the karst water attack with the increase of slag content.With the rise of slag content of AAM system,the reaction products are illustrative of phase transformation of N-A-S-H gel to N-A-S-H/C-A-S-H composite gel.Moreover,the formation of C-A-S-H accelerates the decalcification of gels and the formation of calcium carbonate.The decalcification of gels results in a decrease in strength.Furthermore,the more calcium content of AAM system,the more obvious the reduction in compressive strength.The pure AAM exhibits more resistance to karst water attack owing to its stable three-dimensional zeolite-like gel structure,only the capillary pores in the surface layer（0-5 mm）are corroded.With the increase of slag content of AAM samples,both capillary and gel pores of AAM are corroded,and the attack depth increases.