Preparation Of Modified Starch/Bentonite Composite Powder And Its Application As Dry Mortar Additive

Hydroxypropyl methyl cellulose (HPMC) has been widely used in dry mortar as thickening and water retention additive, but the workability of the mortar containing HPMC is weakened by its poor thixotropy. Therefore, the development of a new type of additive which can replace or partially replace HPMC become an important issue. By analysis of the Elements Construction Additive Product BENAQUA 4000, we found that esterified starch/clay composites can also mediate the viscosity and thixotropy of cement mortar and improve its performance.In this paper, vinyl acetate, succinic anhydride, common corn starch and waxy corn starch were used to prepare corn starch acetate and waxy maize starch succinate, and the influence of the preparation parameters on the degree of substitution (DS) of the modified starch was discussed. Bentonite was then added to the modified starch to prepare modified starch/bentonite composite additive. The water retention, viscosity and thixotropy of the cement was tested when using starch acetate, starch succinate, bentonite and modified starch/bentonite composite additive alone or combined use of them with HPMC together.The DS of the prepared starch acetate is from 0.02 to 1.14 and increasing the amount of esterifying agent or shortening the reaction time can increase the DS of the final products. The DS of starch succinate range from 0.19 to 0.45, and after the heating process, starch succinate with low DS gelatinized completely, but those with high DS only gelatinized partially. Succinic anhydride content and reaction temperature are the key factors to control the DS of the starch succinate. The use of starch acetate in dry mortar can slightly increase its water retentivity (WRV), while significantly increase the thixotropic index of the mortar from 2.22 when no additive was used to the maximum of 12.51. Combined use of starch acetate with HPMC will give a high WRV of 99%, and the thixotropic index also increased from 4.06 when using HPMC alone to 6.83. The starch acetate samples with the DS about 0.5 provide the best results on increasing the thixotropic index. The performance of gelatinized starch succinate is better than the corresponding no-gelatinized one. With the increasing of the bentonite proportion in mortar containing HPMC, its thixotropic index rose from 3.61 to 4.06. Both viscosity and thixotropic index of the mortar using starch acetate/bentonite composite powder decrease with the increase of the betonite proportion in the additives. When combined use of starch acetate/bentonite with HPMC, the WRV of mortar increase to 98%, and thixotropic index increased from 4.06 to 5.30. No-gelatinized starch succinate/bentonite composite additive has little effect on increasing the viscosity and thixotropic index of the mortar. However, gelatinized starch succinate/bentonite can not only raise the mortar viscosity, but also increase its thixotropic index from 4.06 to about 4.25. As the thixotropic index increased, the corresponding area of the thixotropic loop also enlarged when the thixotropic index was in the range from 3.5 to 5.0. The prepared modified starch/bentonite composite additives are cost-effective products.