Effects Of Molecular Structure Of Polycarboxylate Superplasticizer And Retarders On The Adsorption And Hydration Behavior Of C₃A-gypsum System

This article relies on the sub-topic " the microstructure formation mechanism ofcementitious paste in modern concrete"of the national "973" project(2009CB623201)， by measuring adsorption amount of superplasticizers ontoC3A-CaSO42H2O particles and the hydration heat of binary system，combined XRD，SEM and other testing methods，a systematic study of the influencing rule ofpolycarboxylate molecular structure and the three kinds of retarders or accelerators（STPP，BS，Na2SO4）on the adsorption capacity and hydration process ofC3A-CaSO42H2O system was made，and relevant adsorption model was established.The specific details are as follows:1The influencing rule of polycarboxylate molecular structure on the adsorptioncapacity and hydration process of C3A-CaSO42H2O system was investigated.Polycarboxylate superplasticizers regulation for C3A-CaSO42H2O systemhydration was related to its molecular structure. With the increase of carboxyl group(-COOH) and the reduce of ester group (-COOR) in the polycarboxylatesuperplasticizer molecular structure, the adsorption amount of polycarboxylatesuperplasticizer gradually increases, the first and second exothermic peak delay andwaken effects gradually strengthened，the delay effects of Hydration reaction is alsogradually enhanced. With the increase of concentration of polycarboxylatesuperplasticizer, the adsorption of system gradually increase. When the concentrationreaches about1.0%, the system gradually to reach adsorption saturation. As theliquidus temperature rises, the adsorption amount of the system also increases：whenthe liquidus temperature rises from5℃to45℃, the adsorption amount of systemsuccessively increased49.6%,37.8%,26.9%,37.5%,19.2%,30.3%.2The coordinating performance of polycarboxylate superplasticizer andcondensate transfer component on the adsorption capacity and hydration controlmechanism of C3A-CaSO42H2O system was studied.There is an adsorption competition between polycarboxylate superplasticizer and retarders. The presence of retarder will reduce the adsorption amount polycarboxylatesuperplasticizer in C3A-CaSO42H2O System. When the dosage of retarder reaches0.05%，the adsorption amount of the system will sharply decline：When the dosage ofSTPP reaches0.05%，the adsorption amount of system successively reduced39.6%、54.3%、55.0%、47.0%、46.6%、50.4%；When the dosage of BS reaches0.05%，the adsorption amount of system successively reduced33.6%、28.7%、48.5%、114.0%、43.4%、156.4%；When the dosage of Na2SO4reaches0.05%，the adsorptionamount of system successively reduced56.1%、55.0%、44.6%、37.7%、40.9%、31.9%.The influence of three kinds of retard components on adsorption amount ofPolycarboxylate superplasticizer were：Na2SO4＞STPP＞BS；the delay effects ofretard components on C3A-CaSO4·2H2O system hydration heat curve were：STPP＞BS＞Na2SO4. In the early period of hydration，retarding component has little effecton the hydration process，but the adding of Na2SO4will promote the formation ofettringite at early stage.3The adsorption models were established and adsorption mechanism ofsuperplasticizers was revealed:The mechanism that three retard constituents influencing the adsorption behaviorwere different. Adsorption competition will occur between polycarboxylatesuperplasticizer and retarders in the surface of C3A particles. Sodiumtripolyphosphate and Ca2+in the solution generate the complex with a strong negativecharge，which was easily adsorbed to the C3A particle surface；Boric acid moleculereact with Ca2+in solution generate calcium Boric acid complexes, and one clad layerwas formed on the particle surface；SO42-has a stronger electronegative thancarboxyl，preferentially adsorbed on the C3A particle surface，then rapid response togenerate ettringite which was coated in particle surface. And the adsorption modelsof these three retard constituents were established.