Design,Synthesis And Performance Strengthening Mechanism Of Polyacrylamide Internal Curing Microgel

As a new type of internal curing material,superabsorbent polymer（SAP）can regulate the relative humidity in cementitious materials by its water absorption and release characteristics.Therefore,it is one of the commonly used methods to reduce shrinkage and early cracking of cementitious materials.However,due to the irregular shape of SAP,which is mainly composed of ionic monomers,its water absorption rate is low,and water release in pore solutions is uncontrollable.A large number of pores remaining after water release,ultimately affecting the mechanical properties and durability of the cementitious material,leading to low internal curing efficiency.Therefore,how to control the micro morphology,water absorption and release characteristics of SAP by molecular structure design,composition and other factors,improving the interaction between internal curing materials and cementitious materials is the key to solve the above problems.In this paper,the spherical microgel and composite microgel suitable for internal curing of cementitious materials were designed and synthesized by adjusting the content of monomers and cross-linking agent by using nonionic monomer and ionic monomer as the copolymerization composition.Secondly,the effects of anionic content and cross-linking agent on the absorption and release behavior of the above materials were studied.The influence of microgel chemical structure and composite microgel on the macro and micro properties of cementitious materials was systematically studied.Further,the composite microgel internal curing technology was applied to the low humidity curing environment to evaluate its influence on the relevant properties of mortar.Finally,the internal curing mechanism of the composite microgel was discussed in detail from the aspects of hydration characteristics,pore structure and affected zone simulation.The conclusions are as follows:（1）Strongly alkali-resistant P（AM-co-MAA）microgels with different anion contents were synthesized using acrylamide（AM）and methacrylic acid（MAA）as raw materials.When the MAA content is 20%,the electrostatic repulsion between-COO^-can not only provide the driving force for microgel to absorb water in the cement filtrate,but also prevent the microgel from excessively complexing with Ca²⁺,which led to the reduction of water retention rate.The absorptivity and water retention rate of P（AM-co-20%MAA）in cement filtrate were 28 g/g and 76.2%,respectively.20%MAA content microgel with spherical morphology and flexible network exert better flowability of the cement paste.As the microgel exert high water retention in mortar,it can prevent the water evaporation on the surface of mortar and reduce the dry shrinkage rate by 36%when the dosage is 0.1%.The introduction of microgel into cement paste can promote the cement hydration process,and there are no irregular residual pores in the microstructure.Compared with the reference group,the arch hydration products in the modified mortar could improve the compressive strength of mortar by 18%.（2）With N-isopropylacrylamide（NIPAM）and MAA as monomers,P（NIPAM-co-MAA）microgel with a pH sensitive range of 12-13 was synthesized by adjusting the content of anions and crosslinking agents,so that it can release water in time within the above pH range.The chemical structure of 20%anion content microgels were stable in cement filtrate and did not undergo significant desorption in the pore solution,thus facilitated the water provision in the late stage of cement hydration.The water released from the microgels was accelerated as the amount of cross-linking agent decreased,and the water release rate of the microgels was accelerated as the pH of the cementitious material increased.Compared with the reference group,the autogenous shrinkage and drying shrinkage of the microgel modified mortar with2.3%crosslinker was reduced by 14.6%and 13.0%,the hydration degree of mortar was increased by 9.39%.The compressive strength of mortar can be increased by 13.7%when the anion content of microgel was 20%.（3）A series of P（NIPAM-co-MAA）/SiO₂ composite microgels with core-shell structure were synthesized by loading highly volcanically active nano silicon oxide（NS）onto the microgel surface.The load of SiO₂ could be regulated by the reaction time.Compared with P（NIPAM-co-MAA）microgel,the introduction of SiO₂ not only has no significant impact on the absorption rate of the composite microgel,but also reduced the water desorption in the osmotic pressure stage,and can release water in time when the relative humidity drops.The composite microgel with 2.3%cross-linking agent content can avoid large harmful pores when the content is lower than 0.5%,and SiO₂ increased the compressive strength of the modified cement paste by 8.1%.Compared with the reference group,the composite microgel reduced the autogenous shrinkage of cement paste by 310μm/m,which was decreased by 30%.（4）The effects of P（NIPAM-co-MAA）/SiO₂,P（NIPAM-co-MAA）and commercially available SAP on related properties of mortar under low humidity conditions（50RH%）were investigated comparatively.The composite microgels with high water retention and controlled water release were able to maintain the internal relative humidity of the mortar at a high level,and the total shrinkage was decreased by 126.22μm/m compared to the reference group in the low humidity conditions.Correlation analysis showed that the composite microgels reduced the autogenous shrinkage in the early hydration stage and significantly alleviated the drying shrinkage when the internal relative humidity was low.The internal curing effect of P（NIPAM-co-MAA）/SiO₂ compacted the microstructure of the mortar,and the compressive strength of the mortar was higher than that of the microgel modified mortar.Meanwhile,the pore structure refining action of composite microgel prevented the water penetration inside the mortar,and the introduction of additional pores improved the freeze-thaw resistance of mortar.（5）The effects of composite microgels on hydration,pore structure,and affected zone were investigated by nitrogen adsorption and desorption（NAD）and backscattered electron（BSE）methods to reveal its internal curing mechanism.The SiO₂ in the composite microgel induced the volcanic ash reaction and accelerated the hydration process of cement paste,which improved the hydration degree.The filling effect of the composite microgel refined the pore structure of the cement matrix,and some large pores were gradually transformed into fine pores.The pore-filling effect of composite microgel reduced the pore area fraction in the cement paste by 13.8%.The introduction of SiO₂ increased the chemically bound water content in affected zone,and the dense arch hydration products increased the microhardness of affected zone by 145%compared to cement matrix,showing significant affected zone enhancement.The hydration kinetic model was in agreement with the measured hydration curves.The introduction of SiO₂ in the composite microgel and the increase of its dosage promoted the nucleation,crystal growth and phase boundary reaction processes.Meantime,the dense microstructure of the cement paste paste slowed down the diffusion process.