Polycarboxylate Superplasticizers With β-Cyclodextrin Pendants Possessing Clay Tolerance: Synthesis And Dispersion Properties

The cement dispersing power of polycarboxylate superplasticizers(PCEs)is limited by clay minerals as impurities in aggregates and limestones.Due to the stronger affinity of PCEs to clay than to cement,the amount of PCEs sorbed on cement pariciles significantly decreases,leading to the worse cement dispersing efficiency of PCEs.To enhance clay tolerance of PCEs,the clay sacrificial agents mixed with PCEs can be employed;however,the necessary dosages of clay sacrificial agents are too high to be applied in practice.In addition to their excellent cement dispersing power,the most outstanding character of PCEs is that the properties could be modified by means of molecular structure design.In this work,the molecular structure of PCEs were optimized with β-cyclodextrin(β-CD)pendants in order to enhance clay tolerance.To do this,a novel monovinyl β-CD monomer(MA-β-CD)was designed and synthesized,and then PCEs with β-CD pendants were prepared by employing MA-β-CD.The dispersion robust of PCEs with β-CD pendants toward different types of clays was discussed,and the interaction mechanism between clays and PCEs with β-CD pendants was proposed.Finally,the effects of PCEs with β-CD pendants on the dimensional stability of concrete containing clay were studied.The main issues of this work were as follows:(1)Based on the interation with clay,steric effect and water solubility,molecular structure of β-CD monomers for limiting the clay sorption was designed and monovinyl monomer MA-β-CD was prepared.The reaction conditions,such as the molar ratio of GMA/NH-β-CD and types of catalyst used,were investigated.It was found that the yield for MA-β-CD was highest,up to 84%,when the molar ratio of GMA/NH-β-CD was 1.5 and the catalyst was water.The molecular structure of MA-β-CD was characterized by using ESI-MS,1H NMR,13 C NMR and FTIR,and the structureal characteristic of MA-β-CD with monovinyl group was confirmed.(2)PCEs with β-CD pendants were synthesized by changing the content of MA-β-CD,and the effect of β-CD pendants on cement dispersing effectiveness of PCEs was discussed.The molecular properties of PCEs with β-CD pendants were analyzed by means of GPC,and the molecular structure was characterized by 1H NMR and FTIR.It was demonstrated that the polycarboxylate polymers displayed linear shape with β-CD as pendant groups.The results of slump tests on cement paste indicated that the cement dispersion effectiveness of PCEs was enhanced first and then weakened with increasing the content of β-CD pendants.When the molar ratio between MA-β-CD and TPEG was 0.24/0.76(Poly(MAST)-24),the cement dispersing force was strongest.(3)The dispersion robustness of PCEs with β-CD pendants toward different clays,such as montmorillonite and Kaolin,were evaluated by performing slump tests.The interaction mechanism between PCEs with β-CD pendants and clay was proposed.(a)When the content of montmorillonite was 1.5 wt.%(by weight of cement,same for following part),the conventional PCEs(Poly(MAST)-0)lost its dispersing force.In contrast,the negative effect became weaker for PCEs with increasing the content of β-CD pendants.For example,Poly(MAST)-24 with a higher content of β-CD pendants was still able to disperse cement when the content of montmorillonite was 2.0 wt.%.When the content of Kaolin was 3.0 wt.%,the dosages of PCEs with β-CD pendants required to achieve a fluidity of 22.5±0.5 cm were smaller than that of the conventional PCEs.In particular,the dosage of Poly(MAST)-24 was smallest.These resultes showed that β-CD pendants would enhance the dispersion robustness of PCEs toward clay.(b)PCEs with β-CD pendants interacted with clay(Ca-montmorillonite or Kaolin)via surface adsorption which originated from electrostatic interactions between carboxylate groups of PCEs and the positively charged sites(Ca2+)of clay particles.Absoprtion tests showed that the amount of PCEs absorbed by clays was reduced by β-CD pendants.For example,the amount of Poly(MAST)-24 absorbed on montmorillonite was 118 mg/g,decreased by approximately 50% than that of conventional PCEs(215 mg/g).Therefore,the dispersion robustness of PCEs with β-CD pendants toward clay was attributed to β-CD pendants.(4)The cement dispersing effectiveness of PCEs with β-CD pendants in concrete mixture with different types of clay(montmorillonite,kaolin,dust in sand and dust in artificial sand)was studied in detail.It was found that the required dosages of PCEs with β-CD pendants for concretes with the same flowability was decreased compared with conventional PCEs,especially for Poly(MAST)-24,whose dosage was smallest.The compatibility of PCEs with Portland cement was studied as well,and PCEs with β-CD pendants displayed good compatibility with cement of smaller water requirement for normal consistency.The dosage of Poly(MAST)-24 required to cast concrete from sands and stones with high dust content was lower than that of conventional PCEs.The compressive strength of concretes was not negatively affected by Poly(MAST)-24.These results demonstrated that the cement dispersing force of Poly(MAST)-24 was powerful in concrete containing clay(or dust),and the applicability of Poly(MAST)-24 was good.(5)The effects of PCEs with β-CD pendants on the dimensional stability of concrete containing clay were investigated.The results of cracking test by using flat concrete specimens showed that the appearance of cracks was delayed and the total cracking area per unit area of concrete surface was decreased due to Poly(MAST)-24.Shrinkage of concrete with Poly(MAST)-24 was smaller than that with conventional PCEs.These results indicated that the stability of concrete containing clay was improved by Poly(MAST)-24.In this study,monovinyl β-CD monomer was optimally designed and PCEs with β-CD pendants was synthesized.As a result,the clay tolerace of PCEs was enhanced by β-CD pendants.By doing this,deeper insights are obtained into the relationship between molecular structure and property of PCEs.Moreover,Poly(MAST)-24 has a good capacity to disperse cement in concrete with high content of clay,which would be of great significance to mitigate the negative effect of clay on concrete.