Study On The Hydration-Hardening Mechanism And Reinforced Modification Of Recycled Building Gypsum

Gypsum is an international recommended green building material,due to its low-carbon,recycling,and multi-functional characteristics.However,a large amount of waste gypsum will be discharged every year,which results in serious waste of resources and environmental pollutions.Fortunately,the interconversion of dihydrate and hemihydrate phases in gypsum can be achieved under certain circumstances,which further provides the theoretical basis of infinite recycling of gypsum.Therefore,understanding the internal mechanism of the mechanical deterioration and reinforced modification of recycled building gypsum will have great theoretical and practical values for realizing the large-scale and high-efficiency recycling of waste gypsum.This thesis systematically studied effects of recycling times on powder physical properties,mechanical performance,hydration process and microstructure of building gypsum,which aimed to reveal the strength deterioration mechanism.Based on this,multi methods,such as reducing the water requirement of normal consistency by particle gradation optimization and superplasticizers,improving the amount of hydration products through inorganic active materials,increasing the toughness with fibers were applied in order to reinforce the recycled building gypsum.Meanwhile,the enhancement mechanism was explained by combining the hydration process and microstructural changes of hardened matrix.The strength of recycled building gypsum was significantly degraded,which was47%-58%less than the original gypsum.However,the recycle times did not show notable difference on gypsum strength.The recycled hemihydrate gypsum had a refined particle size,poor particle graduation,and more cracks,resulting a higher specific surface area and voidage of powders,which further increased the water requirement of normal consistency from original 0.62 to 081⁰.85.The main factor of decreasing in recycled gypsum strength was due to the increase in water to gypsum ratio,which resulted the increase in porosity and the coarsen of pores in hardened gypsum.In addition,with the recycling time increase,the degree of supersaturation of ions concentration in initial solution decreased,resulting a larger critical radius of crystallization and a delay in hydration process,which further increased the defects in dihydrate gypsum.Meanwhile,the morphology of gypsum gradually changed from original needle bar to short column of thrice-recycled gypsum and the degree of overlap between crystals became worse.However,the defects and morphology changed in hardened recycle gypsum only resulted a tiny decrease in flexural strength,which could be regarded as a secondary factor.Furthermore,during the interconversion of dihydrate and hemihydrate,the defects,size,and length to diameter ratio in dihydrate determined the surface area,voidage,and degree of sphericity of next recycled hemihydrate powders,which further changed the water requirement of normal consistency and the strength of recycled building gypsum.With the increase of the particle size of recycled hemihydrate,on one hand,the surface area and voidage of powders decreased,which further decreased the water requirement of normal consistency and increased the density and strength of hardened recycled gypsum.On the other hand,the decreasing of supersaturation degree of ions in liquid phase increased the critical radius of crystallization and delayed the hydration process,which resulted gypsum with thick layered shape and was negative for strength.The optimum particle size of recycled hemihydrate for strength was between 80 and 145μm,whilst decreasing strength when the particle size was over 200μm.The particle size distribution of recycled hemihydrate was optimized by using the Fuller distribution model.The results showed that the maximum strength was achieved when distribution index m equalled to 0.6,which was 70%higher than the samples without optimization.The order of reinforcement effect of superplasticizers on recycled building gypsum was:SMF>ASF>ASP>FDN>PC-1641F>PC-S08.Thereinto,polycarboxylic acid-based PC-1641F and PC-S08 showed significant enhancement efficiency,which had a critical amount of 0.5%and resulted an 117%and 89%increase in compressive strength,respectively.Within a higher adding amount,SMF performed the best,which gave an increase of 186%in compressive strength at an amount of 1.2%.The main reason was that SMF could also refine the pore size,leading to the best enhancement under the same water reduction rate;while PC-S08 increased the number of macropores,coarsened the pore size,and the crystal morphology was thick and short,weakening the enhancement effect.The water-reducing rate,air-entraining,and hydration process were three key factors,which determined the porosity,pore size distribution,and the morphology of crystals and further affected the enhancement effect of water-reducer in recycled gypsum.FDN,SMF,ASF,and ASP showed an acceleration effect on the hydration of recycled gypsum,which increased the slurry fluidity loss over time and shortened the setting times.Whilst the polycarboxylic acid-based PC-S08,PC-1641F showed a retarding effect on the hydration of recycled gypsum,which significantly reduced the slurry fluidity loss over time and lengthened the setting times.The order of reinforcement effect of cements on recycled building gypsum was:SAC>AC>OPC.The 28-day compressive strength increase rates were respectively119%,83%,and 50%when the dosage was 20%,compared to blank samples.In addition,the incorporation of OPC and SAC shortened the induction period of hydration,which accelerated the hydration process.However,AC increased the induction period of hydration,which delayed the hydration process.Abundant amount of C-S-H gel and AH₃ were absorbed and wrapped on the surface of dihydrate gypsum,which increased the cohesion of crystal and improved the thermostability,meanwhile,the pores of hardened recycled gypsum was filled by these hydration products,resulting a very dense microstructure.The activity of slag was improved by using alkali as the activator,which resulted a higher hydration degree and larger amount of hydration products,producing a denser hardened microstructure.The aforementioned reasons resulted a 54%increase of 28-day compressive strength at a dosage of 20%,which were twice higher than using CaSO₄ as the activator.The recycled building gypsum reinforced through PVA fibers had obvious strain-hardening phenomenon and the deflection,flexural strength and toughness were significantly improved.The results showed that PVA fibers had an obvious better reinforcing effect on recycled gypsum than PP fibers.Especially with the addition of 1.2%dosage of PVA fibers with 12 mm length,the matrix flexural strength increased by up to47%,and the toughness index TI was as high as 53.9.In addition,compared with hydrophobic PP fibers,hydrophilic PVA fibers significantly accelerated the hydration process of recycled gypsum slurry,resulting in a decrease in the setting times and a significant negative impact on the fluidity of slurry.In the hardened matrix,the interface transition zone cracks of PVA fiber-gypsum was smaller and the structure was denser,resulting in a stronger adhesion force.However,there was obvious cracks in the interface transition zone of PP fiber-gypsum and the structure was loose,which weakened the adhesion force.