| Gypsum-based self-leveling mortar has the advantages of easy construction,high early strength,fire and sound insulation,etc.It can not only effectively alleviate the environmental pollution caused by desulfurized gypsum,but also provide a new type of building material.However,the gypsum-based self-leveling mortar has the problems of poor water resistance and low strength,which greatly restricts its application range.In this paper,FGD gypsum is used as cementitious material,cement,Aluminum sulfate cement,slag and water reducer are used as modified materials,and clean river sand is used as aggregate to prepare FGD gypsum-based self-leveling mortar.The single-factor variable method and orthogonal test method were used to study the mix ratio optimization of G20 and G25 products that meet the performance requirements of"Gypsum-based Self-Leveling Mortar" JC/T1023-2021.Using the principle of slag excitation by the weak alkalinity of Aluminum sulfate cement,and gradually increasing the content of slag to replace sand,the water resistance of gypsum-based self-leveling mortar material was studied.Through microscopic analysis(XRD,SEM)and mortar pore solution Chemical analysis,the water resistance mechanism of gypsum-based selfleveling mortar was discussed.The main research contents and conclusions are as follows:(1)The single factor test method was used to study the influencing factors and laws of the properties of desulfurized gypsum-based mortar.The test results show that the setting time of desulfurization gypsum is appropriate when 0.15%of retarder is added.When the content of desulfurized gypsum in the mortar is more than 40%and the water-binder ratio is 0.5,the mechanical properties meet the strength requirements of G20 self-leveling mortar,but the 30min fluidity does not meet the requirements.When the mortar mixed with 45%gypsum is mixed with 5%cement(composed of 90%slag and 10%OPC)and the water-binder ratio is 0.5,the mechanical properties meet the strength requirements of G20 and G25 self-leveling mortars,but the fluidity of 30min does not meet the requirements.The mortar mixed with 45%gypsum and the mortar mixed with 5%cement increases the water-binder ratio(0.50-0.56),slightly increases the fluidity of the mortar for 30 minutes,but reduces various mechanical properties;adding a water reducer(0.1%-0.4%),the fluidity of mortar increased significantly at 30min,but the compressive strength and flexural strength at 1d were significantly reduced;the addition of Aluminum sulfate cement(1%-5%)reduced the fluidity of mortar at 30min.But it can increase the 1 d compressive and flexural strength.The conclusion is that it is difficult to prepare G20 and G25 products that meet the performance requirements of the "Gypsum-based Self-Leveling Mortar" JC/T10232021 specification only by adjusting a single factor.(2)Using the orthogonal test method,the optimization of the mix proportion of desulfurized gypsum-based mortar that meets the performance requirements of the current specification is studied.The experiment selected 4 factors and 3 levels to carry out the orthogonal test:the content of Aluminum sulfate cement instead of sand(1%,3%,5%),the content of desulfurization gypsum(40%,45%,50%),the water reducing agent(0.2%,0.25%,0.3%),water-to-binder ratio(0.52,0.54,0.56),the range analysis,factor effect analysis and supplementary tests were carried out on the test results,and the requirements of "Gypsum-based self-leveling mortar" JC/T1023-2021 were obtained.The mixing ratio of G20 and G25 products required by the standard performance,in which the mixing ratio of G20 is 45%of desulfurized gypsum,3%of Aluminum sulfate cement,5%of cement,47%of sand,0.3%of water reducing agent,water-binder ratio 0.56 and G25 is 45%desulfurization gypsum,5%Aluminum sulfate cement,5%cement,45%sand,0.3%water reducing agent,and the water-binder ratio is 0.52.However,the compressive softening coefficients of these two types of mixing ratio samples are still relatively low,and they belong to air-hardening cementitious material products.(3)Using the principle of stimulating slag by the weak alkalinity of Aluminum sulfate cement,and gradually increasing the content of slag to replace sand,the absolute dry flexural and compressive strength,water-absorbing saturated flexural and compressive strength and softening coefficient of the mortar are all significantly increased.The strength of the specimen with 20%slag content(GLM-S20)in the desulfurized gypsum-based self-leveling mortar reached the highest,the compressive softening coefficient reached 0.81,and the material had the best water resistance.At this time,the density of the mortar specimen is the highest,the internal structure of the specimen is relatively dense,and the dimensional shrinkage rate is 0.04%,which can meet the requirements of the specification.(4)When the desulfurized gypsum mortar is mixed with an appropriate amount of Aluminum sulfate cement and slag powder,the water-resistant amorphous hydration product(C-A-S-H)in the hydration product covers and encapsulates the highly soluble dihydrate gypsum crystals(CaSO4ยท2H2O)(in which acicular ettringite with water resistance is distributed),thereby improving the water resistance and flexural and compressive strength of the desulfurized gypsum-based self-leveling mortar.Since the pH value of the pore solution of the desulfurized gypsum-based mortar mixed with Aluminum sulfate cement and slag is close to neutral after curing for 28 days,the further participation of the slag in the chemical reaction slows down or stops,so the long-term(140d)immerse into the water and in the Intensity increases slightly in moist air.This result indicates that the desulfurized gypsum-based self-leveling mortar mixed with appropriate amount of Aluminum sulfate cement and slag powder can be used for civil engineering in wet environment and water. |
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