Sulfate, phosphate, maleate, and oxide derivatives from native corn and potato starch were produced at the laboratory level. Their structural characterization was carried out using infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The structure of the granule remained almost intact after the starch modifications. The effect of derivatives as chemical admixtures in cement pastes was evaluated by initial flow, setting time, and compressive strength at seven days tests. All the derivatives assessed were identified as viscosity modifying agents. Besides, corn starch sulfate, phosphate, and maleate also showed a slight setting retarding effect. The addition of corn starch phosphate, maleate, and oxide to pastes reduced the development of resistance at 7 days compared to the reference without admixtures, whereas potato starch derivatives even increased it.

Keywords: Starch; Polysaccharide; Admixture; Concrete.

1.INTRODUCTION

Concrete is used around the World as a construction material and is the second most-consumed substance [1]. It contains cement, mineral additives, water, coarse (stone), fine aggregates (sand), and chemical admixtures [2,3]. The cement paste is the active constituent of concrete and substantially determines its performance. Initial flow and setting time are traditional parameters that can be used to describe the behavior of the fresh pastes [3].

The paste properties can be modified using different types of chemical admixtures [3]. These confer beneficial physical and chemical characteristics to the paste, such as flowability, stability, delayed setting, among others [4]. Viscosity modifying agents (VMA) are a type of chemical admixtures, which can improve both concrete viscosity and stability, and help to reduce the tendency for segregation [5]. Biopolymer-based VMAs, such as starch, can be obtained from abundant, low-cost sources and with relatively simple production methods. Usually, they are less expensive than some synthetic admixtures, reason why their use as renewable raw material is promising [6].

Starch is a widely studied biopolymer very suitable for chemical modification due to the presence of hydroxyl groups in its main chain [7]. Starches with different properties can be produced using diverse modification methods. Thus, the inclusion of side groups significantly affects properties such as solubility, viscosity, and gelatinization [5]. Moreover, chemically modified starches has better properties, for example, increased moisture resistance, lower thermal degradation, and improved compatibility [8].

The objective of this work was to produce four types of anionic corn and potato starch derivatives, which were sulfate, phosphate, maleate, and oxide. The effect of the derivatives on the performance of fresh and hardened cement pastes was also determined experimentally. Based on the results obtained, their potential use as chemical admixtures for concrete was identified.

2.MATERIALS AND METHODS

Unmodified corn starch and potato starch were used as raw materials to produce the derivatives. Corn starch was food grade (Ingredion, Colombia). Potato starch was analytical grade (Sigma-Aldrich, USA). To produce the cement pastes previously homogenized structural type Portland cement was used (Cementos Argos, Sogamoso Facility, Colombia).

2.1.Production and characterization of starch derivatives.

Starch sulfates were produced according to the method described by Shaabani et al. [9]. The degree of substitution (DS) with sulfate groups was determined by elemental analysis. To produce starch phosphates was carried out the procedure established by Passauer et al. [10]. The method presented by Zhang et al. [11] was used to determine the degree of substitution with phosphate groups. To produce starch maleate the methodology of Crépy et al. [12] was employed. The degree of substitution with maleate groups was determined using the procedure described by Joly et al. [13]. Starch oxides were prepared according to the method developed by Chong et al. [14]. The methodology of Kuakpetoon and Wang [15] was used to determine the carboxyl content. All the compounds were analyzed by FTIR, ¹H NMR, SEM, XRD, and DSC in an aqueous medium.

2.2.Performance tests of pastes.

The preparation of pastes was carried out according to ASTM C305 (2014). The initial flow was evaluated by the mini-slump test (DIN EN-1015, 2007). The setting time was determined based on ASTM C403 (2016), and the seven-day compressive strength was evaluated according to ASTM C39 (2018).

3.RESULTS AND DISCUSSION

3.1.Production and characterization of starch derivatives.

The DS of corn and potato starch sulfate, phosphate, maleate, and oxide are presented in Table 1. The highest DS was obtained for corn maleate (0.48) and potato maleate (0.50), while the DS for corn starch oxide (0.14 COOH/100 AGU) and potato starch oxide (0.67 COOH/100 AGU) were the lowest.

Figure 1 includes the FTIR spectra of corn (a) and potato starch (b) with their derivatives. All the spectra show a signal at 3311 cm^-1, corresponding to the stress vibration of the hydroxyl groups of the anhydroglucose units (AGU) of starch. The signal at 2925 cm^-1 corresponds to the methyl and methylene groups.

The results in Table 2 are based on ¹H NMR spectra of corn and potato starch and their derivatives. It includes the degree of branching (DB) of the starch molecules. The DB of corn starch (3.8) decreased with chemical modification, and corn starch phosphate (2.0) and maleate (2.0) showed the greatest decrease. Likewise, in the case of potato starch (5.7), phosphate (2.9) and sulfate (2.8) derivatives presented a further reduction in the DB.

Figure 2 includes SEM images of corn and potato starch and their derivatives. In most of the cases, the structure of the granule remains almost intact after chemical modification. To complement this information, data on particle size and distribution of corn and potato starch and derivatives, calculated based on the results of SEM analysis, are included in Table 3.

Table 4 presents the degree of crystallinity (DC), in terms of the ratio of crystalline phase to the amorphous phase, for corn and potato starch and their derivatives calculated from XRD. Corn starch has an α crystalline structure (DC=26.4), and potato starch has a β crystalline structure (DC=27.0). Both types of structures were not observed after modification with sulfate and phosphate groups.

Figure 3 shows the temperature of gelatinization in aqueous media of (a) corn starch (69.4 °C), sulfate (74.1 °C), and oxide (69.4 °C), and of (b) potato starch (58.9 °C), sulfate (57.8 °C), and oxide (54.3 °C). As corn and potato starch maleate and phosphate were immediately gelatinized when dissolved in water, the gelatinization temperature was not observed by DSC.

3.2.Performance tests of pastes.

Figures 4, 5, and 6 include the initial flow, setting time, and the 7-days compressive strength of pastes with water to cement ratio of 0.45 with starch-based admixtures (0.5% concentration per cement mass) and without them. Figure 4 shows a decrease in the flow of the pastes, mainly those containing corn starch phosphate (104 mm) and maleate (92 mm) compared to the reference (147 mm). Potato starch phosphate (119 mm) and maleate (94 mm) also significantly reduced initial flow. In Figure 5, an increase in paste setting time with the addition of corn (13.3 h) and potato starch maleate (19.1 h) compared to the reference without admixtures (9.5 h) can be observed. The other starch derivatives did not significantly change the initial setting time. Finally, Figure 6 shows, in general, a decrease in the 7-day strength of the paste containing corn starch derivatives. In the potato starch derivatives, only the oxide lowered resistance (31 MPa) in comparison with the reference (36 MPa), and the phosphate even increased it (49 MPa).

4.CONCLUSIONS

Corn and potato starch sulfate, phosphate, maleate, and oxide were successfully produced by simple reactions, keeping the structure of the starch granule with low degradation. The addition of anionic groups in the starches decreased their degree of branching and crystallinity. Corn starch anion derivatives are viscosity modifying agents in the cement pastes.

5.ACKNOWLEDGEMENTS

The authors are grateful to Sika Colombia S. A., Colciencias, and the Universidad Nacional de Colombia by funding this research through the contract FP44842-351-2017.

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