Effect Of SRA/MEA On Microstructure,Mechnical Property And Shrinkage Of HDCC

High ductile fiber-reinforced cementitious composites（HDCC）has been widely studied and applied on many fields such as construction,road and bridge due to its strain-hardening and multi-crack properties.However,imported PVA fiber is expensive,and HDCC has a large shrinkage deformation,which seriously affects its application in the field of repair and reinforcement of existing buildings/structures.In this paper,HDCC was prepared using domestically produced PVA fibers,cement,fly ash,shrinkage-reducing admixture（SRA）,and magnesium oxide expansive agent（MEA）.The effects of different dosages of fly ash,SRA,MEA,and the combined use of MEA and SRA on the microstructure,mechanical properties,and shrinkage performance of HDCC were systematically studied.The main conclusions are as follows:As the fly ash content increases,the amount of cement in HDCC decreases,and the effective water-cement ratio increases,resulting in an earlier onset of heat release during hydration,while the induction period,acceleration period,deceleration period,and stable period become slower.The un-hydrated fly ash particles inside act as fillers,significantly reducing the chemical bond energy G_d and interfacial friction forceτ₀.In addition,increasing the fly ash content reduces the total percentage of capillary pores,while increasing the percentages of gel pores and harmful pores,which adversely affect the mechanical properties of HDCC.However,high relative humidity inside HDCC and a reduction in the number of small capillary pores significantly reduce the shrinkage deformation of HDCC.The addition of SRA reduces the alkaline ion concentration in the pore solution,leading to an increase in the concentration of Ca²⁺and limiting the dissolution of C₃A and C₃S.This delays the chemical reaction process of the slurry,reducing E_m and fracture toughness J_tip and allowing the fibers to better perform bridging functions with the matrix.In addition,the addition of SRA reduces the surface tension of the pore solution,shifts the maximum accessible pore size of small capillary pores to the right,and increases the internal relative humidity,significantly reducing shrinkage deformation.However,increasing the SRA dosage will reduce the mechanical properties of HDCC.Therefore,it is necessary to choose the appropriate dosage of SRA according to engineering requirements.The addition of MEA affects the hydration reaction process and mechanical properties of HDCC.Under the effects of crystal nucleation and ion-ion interaction,the addition of MEA advances the initial and induction periods of heat release while delaying the hydration rate during the acceleration,deceleration and stable periods.G_dandτ₀ slightly increase with increasing MEA content.As MEA content increases,HDCC exhibits improved multi-cracking and strain-hardening characteristics,mainly due to the lower strength of brucite,a hydrate formed from MEA,compared to C-S-H,which facilitates bridging between fibers and matrix.Furthermore,the addition of MEA causes the total proportion of capillary pores to decrease while increasing the proportion of gel pores and harmful pores,which can affect the mechanical properties of HDCC.The expansion force generated by the successive hydration reaction of brucite can offset some shrinkage deformation.Therefore,it is necessary to adjust the dosage of MEA reasonably according to engineering requirements to avoid detachment or cracking at existing structural interfaces.The combined addition of MEA and SRA reduces the alkaline ion concentration in the pore solution and limits the dissolution of C₃A,C₃S,and MEA particles,delaying the chemical reaction process of the slurry.This leads to a significant reduction in E_m,J_tip,G_d,andτ₀ of HDCC,improving its ductility and toughness.In addition,the combination of MEA and SRA has a synergistic effect,not only compensating for the adverse effects of SRA on mechanical properties but also significantly reducing the shrinkage deformation of HDCC.This paper prepared HDCC with a tensile strain greater than 3%using domestically produced PVA fibers and other materials.Therefore,low-cost HDCC has important research significance and application value in repairing and strengthening reinforced concrete structures as a repair material.