Study On Hydrating And Hardening Mechanisms And Influencing Factors Of Recycled Building Gypsum

At present,due to the continuous reduction of gypsum mineral resources,and the consumption of gypsum increased day by day,a huge contradiction has formed between the market’s demand and gypsum mines.In theory,the main phase of gypsum is calcium sulfate dihydrate（CaSO₄·2H₂O）.After calcinated at a certain temperature,it can be reused.It can prevent large-scale accumulation of abandoned gypsum and occupation of land.However,because of lacking of a large amount of research foundation about the basic performance and the influence factors of recycled building gypsum,the waste gypsum is still treated by packing and landfill.It is well known that the performance of recycled building gypsum deteriorates seriously,and there are no specialized optimization guidelines for recycled building gypsum,resulting in a huge waste.In this paper,the hydration and hardening laws of the building gypsum and recycled building gypsum which obtained under the same preparation process are compared.The hydration and hardening mechanism of recycled building gypsum has been systematically studied.The influence of temperature,pH value and particle size distribution on the hydration and hardening of recycled gypsum was discussed.The influence laws of temperature,pH value and particle size on the recycled building gypsum was revealed.The recycled building gypsum powder is poor in performance,more fine particles,high water requirement,a large increase in setting time,a serious decrease in strength,up to 49%-53%,and increase in water absorption.The hydration process of recycled building gypsum was studied with self-made thermal insulation device.The results showed that the peak temperature and the cumulative heat release in the previous period(Q₂₀ and Q₃₀)of recycled building gypsum was reduced,but the final cumulative heat release（Q80）has no significant difference.the heat release rate slightly increased.The early hydration rate was lower than building gypsum,but the final hydration rate has no significant difference,closed to 100%.The micro-morphology,pore structure and particle size distribution of recycled gypsum were studied by SEM,mercury intrusion technique and particle size analysis.The results showed that the microstructure of recycled building deteriorated,the crystal size was coarsened,distributed in clusters,and the length-diameter ratio decreased,lead to large porosity.The pore size,the average pore size,the most probable aperture and the cumulative pore volume increased,which lead to the pore structure deteriorated seriously.The size distribution of recycled building gypsum is uneven and narrow,with more fine particles.The recycled building gypsum was easy to grind.The specific surface area increased,which lead to a great increase in the standard water requirement.The supersaturation of recycled building gypsum decreased,resulting in the decrease of the dissolution and the precipitation rate and the peak temperature and the increase of the setting time.The length-diameter ratio decreased,the crystal lapped strength weakened,crystal contact points decreased,and the average pore size and porosity increased,resulting in a serious reduction in the strength of the hardened body.The effect of temperature on hydration and hardening of recycled building gypsum was studied.The strength,peak temperature,cumulative heat release and the final hydration rate of the recycled building gypsum prepared at 180°C was the highest.However,the setting time exceed the national standard GB/T 9776-2008“Construction Gypsum”,and the standard water requirement was 0.83.The microstructure observed by SEM technology revealed that crystal morphology form at 180°C is needle-like and overlapped closely,which is the typical crystal morphology.The effect of pH on the hydration and hardening of recycled building gypsum was studied.With the change of pH value the induction period of gypsum,extension degree and setting time were shorten,and the peak temperature,accumulated heat release and early hydration rate were improved,but ultimate hydration rate has no obvious difference.When the pH value is greater than 7,the strength increased with the increase of pH value.The microstructure observed by SEM technology found that the crystal form is still mainly need-like.the hexagonal plate-like CH has formed,which increased the number of crystal contact points and the lapped strength.The effect of particle size on the hydration and hardening of recycled building gypsum was studied.The bulk density and specific surface area increased with the decrease of particle size,and the setting time and the degree of expansion decreased with the decrease of particle size.The peak temperature,accumulated heat release,heat release rate,and early hydration rate increased with the decrease of the particle size,but the final hydration rate has no obvious difference.The length-diameter ratio decreased with the increase of the particle size.The crystal morphology was transformed from needle-like to plate and short-column,lead to the serious degradation and the poor porosity.