The Preparation And Performance Of Super-retarding Water Reducers Based On Corn Starch

A new type of super-retarding and high range water reducer, basedon the deoxidization of glucose units structure of the natural cornstarch through multi-modification processes, such as oxidation,acidification and etherification etc., resulting to carboxyl group andether group being grafted into the deoxyglucose chain according to thebasic principles of macromolecular structure design, is prepared. Itseffect on the performances of cementitious materials, such as fluidity,setting time, and mechanical properties etc., was investigated. And thenthe mechanism on super-retarding and water reducing effect to freshcement paste system was also discussed. A modified naphthalenesulfonate superplasticizer with the feature of lower slump loss made ofblended PNS with modified corn starch is successfully prepared andnational invention patent has also been granted. Gypsum-free Portlandcement is also prepared by using the modified corn starch as settingcontrolling agent and its performances, especially, high temperaturemechanical properties is also investigated in this paper. The researchresults as following:The modified process on natural corn starch consist of thefollowing three distinct steps:1. The optimal parameters of oxidizationprocess are as following: the starch concentration is30%, pH value is9,the dosage of NaClO is10ml,the reaction time is4h, the reactiontemperature is40℃;2. The optimal parameters of acidation process areas following: the oxidized starch concentration is40%, the dosage ofHCl is1.4%, the reaction time is2h, the reaction temperature is30℃;3.The optimal parameters of etherification process are as following: theacidated-oxidized starch concentration is10%, pH value is12, thedosage of ethylene oxide is3ml, the reaction time is22h, the reactiontemperature is45℃.The optimal dosage of modified starch in the cement system isroughly0.3%of cement by weight. Under that dosage, the loss of thefluidity of fresh cement paste after120min is zero, the setting time is longer than24h, and the mortar has higher mechanical strength. It canbe demonstrated that in the early hydration of cement paste withmodified the hydration exothermic peak appeared after dozens of hours.The dispersion effect of the modified starch on cement paste may beattributed to the steric hindrance effect of modified starch due to thesmaller amount of modified starch adsorption on cement particle andnearly the same Zeta potential between the paste with modified starchand the control. The super retarding capacity of modified starch on thecement paste may be attributed to its stronger complexing capacity withdispersing into the liquid resulting to less calcium ion concentration inthe liquid, consequently, longer setting time. The influencing factors,such as amout of oxidized starch, pH value and temperature oncomplexing capacity of modified starch were studied.Gypsum-free Portland cement has different hydration productsfrom Portland cement and then has better high-temperature strengththan Portland cement. Form600℃to800℃, the strength decrease ofPortland cement is37%, while the gypsum-free Portland cement is onlyabout10%.The modified naphthalene, prepared by compatible modified starchwith naphthalene sulphonate superplasticizers (FDN), are obviouslyimprove the slump retaining of FDN. When the content of the modifiedstarch is0%~13%in modified naphthalene, the slump loss of cementpaste is89%~17%after2hours. Compared to FDN, the compression ofmortar with modified naphthalene is enhanced by above10%. Thedosage of the modified naphthalene is more than FDN in order to getthe same slump of concrete, but the slump loss of concrete with themodified naphthalene is1.9%after2hours, while the one with FDN is9.6%.