Research On Properties And Micro-structure Of Metakaolin Geopolymer Modified With Silane Coupling Agent

Geopolymer is a new green gel material with energy saving and has been generally accepted.In this thesis,metakaolin and cement were used as the original cementitious materials and alkali activator was to activate.Two parts were tested and analyzed to prepare metakaolin based geopolymer specimens which meet the mechanical strength of the national standard.Then two different types of silane coupling agents（SCAs,including KH-550 and KH-560）and different SCAs dosages（1%,2%,3%,4%）were used as admixtures to prepare modified metakaolin based geopolymer.The purpose is to prepare self-waterproof geopolymer materials and study the mechanism function of SCAs on geopolymer systems.Macroscopic characterization are including mechanical properties,fluidity and consistency,hydration degree,capillary water absorption,capillary porosity,water resistance,filter loss and WRV,wettability and contact angle test.Moreover,sulfate corrosion resistance test and freeze-thaw cycle test are conducted as durability experiments.Finally,microscopic experiments are including SEM,FTIR,XRD and BET tests to reveal the reaction mechanism of SCAs on geopolymer systems produced by macroscopic phenomena.Main experimental results are as follows:（1）In accordance with national mechanical strength specifications,metakaolin geopolymer was prepared as a control group by previous studies and design tests.The modified geopolymer specimens were prepared with different types of SCAs and different dosages as experimental groups.Working properties experiments for analysis included mechanical strength at 3^rddays,7^thdays and 28^th days,as well as fluidity,consistency and capillary porosity.The results showed that KH-560 modified metakaolin geopolymer was better than KH-550 modified metakaolin geopolymer under the same dosage.The mechanical properties of the specimens showed a trend of increase and then decrease when the dosages increased from 1 wt%to 4 wt%.Excessive SCAs addition reduced mechanical properties.The fluidity and consistency showed a trend of increase and then decrease with the increase of SCA dosages.The capillary porosity experiments showed that the capillary porosity decreased significantly when suitable dosage SCA added,butexcess SCA would increase capillary porosity.（2）Effect of self-waterproof geopolymerexperiments were investigated by capillary water absorption,water resistance,filter loss and WRV test,wettability and contact angle test.The capillary water absorption tests results showed that the incorporation ofboth SCAs had an suppression effect on the water absorption capacityof the geopolymer system.In the case of the same dosage,the modified specimens water absorption rate inc-reased when SCAs increased,but the overall water absorption rate was smaller than that of the geopolymer specimens without the SCA modification.When KH-550（A）was chose to modify,the variation of water absorption in stable stage was 1 wt%A<2 wt%A≈3 wt%A<4 wt%A<control specimen.When KH-560（B）was chose to modify,the variation of water absorption in stable stage was:1 wt%B<2 wt%B<3 wt%B<4 wt%B<control group.The water resistance experiments were best at 1 wt%of the two SCAs,excessive SCA would lead to lower water resistance than control group.The filter loss and WRV tests corresponded to the water resistance experiments.The wettability test and contact angle test showed that the incorporation of a small amount of SCA improved the geopolymer specimens hydrophobic ability but reduced the hydrophobic ability when excessive.The phenomenon of self-waterproof geopolymer was mainly related to the water repellency of the SCA itself and the oxygen bridge formed by the reaction.（3）The modified metakaolin geopolymer specimens including sulfate wet-dry cycle and freeze-thaw cycle were studied to investigate the effect of SCAs on geopolymer durability.The experiments showed that two SCA had a significant effect on durability of the metakaolin geopolymer.For the same SCA in sulfate wet-dry cycle test,the weight loss rate of modified geopolymer specimens showed a decrease and then increase trend at28^th days and 56^th days with increase dosage,and the corrosion resistance coefficient showed the opposite trend.For different types of SCA,the durability of KH-560geopolymer was better than KH-550 geopolymer at the same dosage.Appropriate SCA improved geopolymer specimens compactness,which was related to early hydration reaction enhancement of the geopolymer.While excessive incorporation leaded to spherical micelle effect to reduce performance.The freeze-thaw cycle test also confirmed that the appropriate amount of SCA improved the freeze-thaw resistance.Excessive alkaline silicone had bad impact on the geopolymer structure due to the"alkali overload".In addition,the damage to the geopolymer system from the freeze-thaw cycle test was more obvious than that from the sulfate wet-dry cycle.（4）Microscopic characterizations were performed in combination with hydration degree,SEM,FTIR,XRD and BET experiments to reveal the mechanism effects of SCAs on metakaolin geopolymer system.The results of hydration degree experiments showed that a moderate amount of silicone promoted the early hydration reaction of the geopolymer,resulting in a denser structure of specimens.SEM confirmed that the structure of geopolymer formed by a small amount of SCA was dense,while in excess the rod-like structure of SCA wasinterspersed in the pores of the specimens andthe structure wasn’t sufficiently dense.FTIR and XRD experiments showed that the chemical composition of formed modified geopolymer specimens didn‘t change significantly,regardless of whichever SCA or SCA dosages.BET tests were performed by testing the changes of N₂ adsorption and desorption curves,BJH adsorption and desorption.The BET specific surface area,maximum adsorption capacity,total pore volume,average pore size and average mesopore pore size of the control and modified geopolymer pastes were also tested.The BJH adsorption and desorption curves showed that the pore diameter of the specimens increased slightly after incorporation of silicone modification,but the appropriate amount of silicone effectively reduced the d V/dlog（D）value.Also according to the pore capacity and the mesopore test,they confirmed the macroscopic experimental conclusion.