Effect Of Polycarboxylate Superplasticizer Mixed With Retarder Or Early Strength Agent On The Strength Of Steam-cured Concrete

Superplasticizer and composite admixtures,which are made up of superplasticizer and other admixtures,are used to improve the strength of steam-cured concrete,especially early strength.It is not only conducive to save energy and protect environment,but also beneficial to the turnover rate of the mold and productivity.At present,the application of polycarboxylate superplasticizer in steam-cured concrete is more and more extensive,and the enhanced effect of the superplasticizer on the strength of steam-cured concrete is the topical subject of precast concrete research.There is less research focus on the use of appropriate amount of inorganic salts and small molecules of organic compounds mixed with polycarboxylate superplasticizer.Thus,in this paper,the effect of polycarboxylate plasticizer mixed with microdosage??0.05%?retarder and early strength agent on the strength of steam-cured concrete was studied systematically,and the mechanism was researched.?NaPO₃?₆,Na₅P₃O₁₀,Na3PO₄,C₆H₁₁NaO₇ and C₁₂H₂₂O₁₁ are usually used as common retarders,merely used in steam-cured concrete.But the results of this paper manifest that the phosphate could delay cement hydration at high concentration,but the phosphate is beneficial to the early strength and the late strength of steam-cured concrete at microdosage of phosphate?about 0.01%?,the significance of effect is?NaPO3?6> Na₅P₃O₁₀ > Na₃PO₄.The addition of?NaPO₃?₆ could improve the early strength of steam-cured concrete by 14.7%,and improve the late strength by 15.3%.The addition of C₆H₁₁NaO₇ has no effect on the strength of steam-curing concrete when the dosage of C₆H₁₁NaO₇ less than 0.05%,but the compressive strength of steam-cured concrete will decrease with the dosage increasing.When the dosage of C₁₂H₂₂O₁₁ less than 0.03%,the impact on the early strength is slight,but beneficial to the late strength,and it will maintain the retardation with the dosage increasing.CaCl₂,Na₂SO₄,Na₂SiO₃,Na₂S₂O₃ and?HOCH₂CH₂?₃N are commonly used as early strength agents for concrete.The results show that when compared with?NaPO₃?₆,the dosage of the early strength agents to improve the early strength of steam-cured is often higher,and the effect on the growth of the late strength is weak.CaCl₂ mixed with polycarboxylate plasticizer to improve the early strength is optimal,the enhanced effect on the early strength is stronger than?NaPO₃?₆,but the late enhanced effect is weaker;the early strength effects of Na₂SO₄,Na₂SiO₃ and Na₂S₂O₃ decrease,Na₂SO₄ and Na₂S₂O₃ have no effect on the late strength,Na₂SiO₃ could slightly improve the late strength,but the effect is not as good as phosphate;microdosage of?HOCH₂CH₂?₃N has a little effect on the early and late strength of steam-cured concrete.The results of X-ray diffraction?XRD?,Scanning electron microscopy?SEM?and Ca?OH?₂ concentration test show that the reason for the enhanced effect on the strength of steam-cured concrete is the microdosage phosphate can reduce the content of Ca?OH?₂ in pore solution to promote cement hydration,at the same time the crystal growth of cement hydration products become more regular and well-distributed.But if the dosage of phosphate is too high,the reaction products between phosphate and calcium will cover the cement surface and delay the cement hydration,which is adverse to the development of steam-cured strength.