Research On The Controlling Of Setting Time Of Slag-metakaolin Geopolymer

Geopolymer is a new type of green cementitious material,which is produced by the reaction of metakaolin,slag and fly ash with alkaline activator.The reaction mechanism and reaction products are completely different from that of Ordinary Portland Cement.Due to the different types of raw materials and alkali activators,the diffrrences in proportion and temperature and so on.The setting time of geopolymer can hardly meet the construction requirements.Useing metakaolin and slag as the main silica-aluminum material,chooseing the mixture of sodium silicate and sodium hydroxide as alkaline activator to form geopolymer.Adding CaCl₂,Ca?NO₃?₂ and clinker as accelerators and BaCl₂,Ba?NO₃?₂,ZnCl₂ and Zn?NO₃?₂ as retarders into the paste to find out the admixtures that can effectively regulate the setting time of the geopolymer.The setting and hardening properties of the geopolymer with admixtures were studied.Noncontact electrical resistivity measurement was used to monitor the electrical resistivity of the geopolymerization,micro-calorimetry was used to measured the reaction heat,combined with XRD,SEM and FTIR analyzed the effects of the admixtures on the geopolymerization.The result shows that:In the processof preparing geopolymer,CaCl₂,Ca?NO₃?₂ and clinker were used as accelerators,the setting time of the paste is shortened to varying degrees,and the clinker had the best accelerating effect.2.0%clinker can shorten the initial setting time of the geopolymer by 66.7%and the final setting time by 62.8%.With the addition of clinker the 3 d compressive strength of geopolymer increased and the 28 d compressive strength had no obvious change.After the clinker was incorporated into the system,the maximum rate of heat release of geopolymerization was significantly higher than the blank group.The main reason is that C₃S and C₂S in the clinker react with the excess waterin the system to generate C-S-H gel and Ca?OH?₂,promote the progress of geopolymerization,shorten the setting time of the geopolymer paste and increase the compressive strength of the specimens.The XRD results showed that there was anorthite in the reaction product with the addition of clinker.In the processof preparing geopolymer,BaCl₂,Ba?NO₃?₂,ZnCl₂ and Zn?NO₃?₂were used as retarders,the setting time of the paste is delayed to varying degrees,and BaCl₂ had the best accelerating effect.2.0%BaCl₂ can prolong the initial setting time from 240 min to 646 min,and the final time from 264 min to 705 min.With the addition of BaCl₂ the 3 d compressive strength of geopolymer increased slightly,but the 28 d compressive strength decreased with the increase of BaCl₂ content.When adding 2.5%BaCl₂,the 28 d compressive strength of the specimen was decreased from 53.6 MPa in the blank group to 39.5 MPa.After 1.0%BaCl₂ was incorporated into the system,the rate of heat release of geopolymerization was lower than that of the blank group.It may be due to Ba²⁺react with[SiO₄]^4-in the system form insoluble substances and precipitated on the surfaced of metakaolin and slag,which hinders direct contact of the solid particles and the alkali activator and prolong the setting time.The XRD results showed that there was afwillite and gonnardite in the reaction product with the addition of BaCl₂.Geopolymerization was devided into dissolution reaction period,deceleration period and stabilization period by the change of the reaction heat.According to the kinetics parameters,dissolution reaction period was controlled by nucleation ratio,the deceleration period and stabilization period were controlled by the diffusion reaction.The Heat-Electric Model of the geopolymerization was established combine the curve of change of renction and the electrical resistivity.This model revealed the thermal effect,described the formation and development of the geopolymerization.Through the Heat-Electric Model,the reaction was devided into structure rapid forming period,structure developing period and structure stable forming period.The results of XRD,SEM and FTIR indicated that there were C-S-H,C-A-S-H and N-A-S-H gels in the reaction product.And the connectivity of C-A-S-H gel decreased with the addition of admixtureds.The setting time of geopolymer paste was regulated by adding admixtures,it provided a certain theoretical basis and reference for the application of geopolymer.