Synthesis,Properties Of Superabsorbent Polymers And Mechanism For Internal Curing In Cementitious Materials

With the continuous improvement of the requirements of modern concrete in our country,high strength concrete（HSC）has been widely used due to its excellent mechanical properties.However,HSC usually has a large autogenous shrinkage due to its high content of cementitious materials and low water-to-binder ratio,which increases the cracking risk of concrete at early ages.Besides,since the microstructure of HSC is very dense even at early ages,it limits sufficient rapid transport of curing water into the interior of concrete members.Therefore,conventional methods for curing of concrete cannot contribute substantially for mitigating the autogenous shrinkage.SAP can release its absorbed water into the concrete matrix during self-desiccation,then the autogenous shrinkage can be mitigated.Therefore,it is of great significance to study the relationship between the structure and performance of SAPs in cement-based materials.According to Flory’s swelling theory,SAPs with different absorption and desorption properties are designed and synthesized.The chemical structure of SAPs has been characterized,then the absorption and desorption performance of SAPs is studied in both deionized water and cement filtrate.Afterwards,the effect of SAPs with different chemical structures on the cementitious macroscopic properties is revealed.Finally,internal curing mechanism of SAPs with different chemical structure is studied based on the in-situ characterization technology of internal curing water.The main innovative results are as follows:（1）The influence of chemical structure of SAP on its absorption and desorption was investigated.SAP with different performance are prepared by adjusting the monomer type,monomer ratio and crosslinking density.The results show that osmotic pressure is the main absorption driving force for ionic SAP（anionic and zwitterionic）.The osmotic pressure increases with increasing the anionic density of ionic SAP.The main absorption driving force of non-ionic SAP is the affinity of functional groups,and its water absorption is lower compared to ionic SAP.A large number of ions dissolved during cement hydration will inhibit the swelling of ionic SAP.Reducing the anionic density or introducing cationic functional groups can mitigate this inhibition.In addition,the complexation between carboxyl groups of anionic SAP and Ca²⁺in cement filtrate further constrain the swelling behavior and make it exhibit self-releasing water（self-releasing SAP）.By reducing the sites in the SAP molecular network that can be complexed with Ca2+,the releasing rate can be reduced,and even no self-releasing.Thus,non-ionic SAP and zwitterionic SAP do not show self-releasing（retentive SAP）.The resistance to the swelling of SAP increases with increasing the crosslinking density,leading to lower water absorption.（2）The influence of different SAP types on the fluidity and autogenous shrinkage of cementitious materials are investigated.The absorption and desorption of SAP both contribute to the fluidity of cement pastes.Adding the same content of SAPs,the fluidity of cement pastes decreases as the absorption of SAP increases.The influence of self-releasing and retentive SAP on the fluidity loss of cement pastes is quite different when the extra water is the same.Since the water released by self-releasing SAP will re-enter the pastes during remixing,the fluidity loss decreases with the increase of the water releasing rate.Both self-releasing and retentive SAP can effectively mitigate the autogenous shrinkage.The internal curing performance of SAP mainly depends on the amount of water it introduced instead of its release time.（3）The effects of different SAP types on hydration and mechanical properties of cementitious materials are studied.SAP have influence on the hydration during first day after mixing,and this coincide with the SAP release time.The water continuously released by SAP participates in hydration,which delays the hydration induction period and main hydration peak.The delay increases with the increase of the desorption rate of SAP.The water absorption and desorption process does not change the hydration degree of cement pastes at later age,and total w/c is the main factor influencing the hydration degree.This is also the main reason why cement pastes with different SAP types show similar mechanical performance and close to that of reference group with the same w/c.Besides,it can be concluded that SAP promote the early growth of calcium hydroxide in cement pastes,and the structure of C-S-H in cement pastes with anionic SAP may change due to cation exchange.These may cause the early volume expansion of cement pastes.（4）A method for in-site characterization of SAP internal curing water is established,and the water release process of different SAP types in cementitious materials is explored.The internal curing mechanism of different SAP types in cementitious materials is elucidated.According to the results of X-ray computed tomography and H-nuclear magnetic resonance,it is found that the spherical closed pores formed by both self-releasing and retentive SAP are saturated before 11 h（around final setting time）,followed by significant desorption with a similar release rate.The internal curing mechanism of SAP in the cement paste can be summarized as water absorption-water releasing-water diffuses to matrix for internal curing,which can be abbreviated as“absorption-releasing-diffusion”.For water-retentive SAP,the process of"releasing"and"diffusion"is synchronous,while for self-releasing SAP,"releasing"is completed before"diffusion".However,the key factor that determines the curing performance of SAP is the process of"absorption"and"diffusion".Therefore,retentive SAP is more stable in cementitious materials and more convenient to design mix proportion.In the application of self-releasing SAP,prolong mixing time or remixing should be avoided.Besides,it should be noted that using improper absorption value may cause adverse impact on internal curing performance.