By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Aug 12 2024In an article published in Materials, a team of researchers investigated the effect of tapioca starch (TP) on the quality and performance of cement mortar. After incorporating TP, experimental studies were conducted to evaluate the mortar's flow characteristics, mechanical properties, and durability.
Relationship between the TP content and carbonation depth according to the TP carbonation age. Image Credit: https://www.mdpi.com/1996-1944/17/16/3889
Background
The concepts of repair and reinforcement have gained popularity as effective strategies to extend the service life of structures while minimizing environmental impact. However, only a limited range of repair materials have proven effective for use in aging concrete structures. Among these, inorganic cement mortar, which closely resembles the properties of the original concrete, is most commonly employed.
For successful repair of older structures, it is crucial to use materials that exhibit adhesion, contraction, and thermal expansion characteristics similar to the existing materials, ensuring effective integration. Moreover, these repair materials must offer high durability to significantly extend the lifespan of the structures.
TP, a powdered starch derived from the roots of cassava—a tropical plant commonly found in Central America, Southeast Asia, and Africa—has been recognized for its potential beyond the food industry. When processed and used as an additive in repair mortar, TP is known to enhance the performance of the mortar. However, its application as a repair mortar additive has largely been confined to Southeast Asia and surrounding regions. This study, therefore, aimed to develop a domestic repair mortar with improved flow, mechanical properties, and durability by incorporating TP as an additive.
Methods
As part of this study, a repair mortar was formulated using ordinary Portland cement (OPC) with three varying TP content levels (0.025 %, 0.050 %, and 0.075 % of OPC mass). The impact of TP content on the mechanical properties of the mortar was then systematically analyzed through standard testing methods.
The flow of the mortar was measured to assess its workability and consistency. The setting time of the mortar was determined using the Vicat needle method. Additionally, the mortar's rheology was examined by comparing results from previous studies utilizing a mortar viscometer with a modified chamber size. The resulting plastic viscosity and yield stress were characterized using the Bingham model.
Beyond compressive and flexural strength assessments, the bond strength of the mortar was determined through the pull-off method. Length change measurements were also conducted to evaluate the drying shrinkage, which provides insights into the mortar's potential for cracking and its durability under dry conditions.
To further investigate the mortar's durability and long-term performance in challenging environments, its resistance to carbonation was assessed. Moreover, the mortar's ability to resist chloride ion penetration was tested in a 3 % NaCl solution, providing an understanding of its durability in environments prone to chloride exposure.
Results and Discussion
TP had a significant impact on several properties of the mortar. For example, as TP content increased, the plastic viscosity rose while the yield stress decreased, suggesting a reduction in workability. However, this lower yield stress could be advantageous in applications where initial flow is essential. This trade-off between viscosity and yield stress due to TP requires careful optimization to achieve the desired workability.
Interestingly, TP also delayed the mortar’s initial and final setting times by 10 to 15 minutes and 12 to 23 minutes, respectively. This extended setting time could be beneficial during construction by providing a longer working window.
At optimal TP content, the mortar showed improved strength at early ages of three and seven days while maintaining the required flow. Although the compressive strength of the mortar decreased slightly at a 0.075 % TP content, it still matched that of plain mortar.
Moreover, the compressive strength at 28 days remained unaffected by TP content. This suggests that TP enhances early strength by filling internal pores through a gelation effect, but does not impact long-term strength after 28 days. A similar pattern was observed for both flexural and bond strength. Overall, an optimal TP content improved the interfacial bond strength and absorption performance of the mortar by increasing viscosity.
The mortar containing TP also exhibited approximately 5% less shrinkage compared to plain mortar. The high swelling and absorption capacity of TP counteracted the shrinkage, with the expansion effect nearly doubling within the first three days and stabilizing after seven days.
Additionally, the fine TP particles enhanced the mortar’s carbonation resistance by effectively filling its pores. TP also reduced the chloride ion diffusion coefficient in the mortar by increasing the density of the internal matrix through its gelation properties.
Conclusion
In conclusion, the researchers thoroughly investigated the impact of TP on the quality, mechanical behavior, and fundamental properties of repair mortar. The results were noteworthy: the mortar's early compressive strength increased by up to 20 %, bond strength improved by approximately 60 %, and final shrinkage was reduced by 5 % at a 0.050 % TP content.
The acceleration of early strength development and the enhancement of bond strength were directly linked to the unique properties of TP. The reduction in shrinkage was attributed to TP's expandability, which effectively compensated for shrinkage stresses. These findings suggest that TP has the potential to significantly enhance mortar quality without sacrificing performance.
However, the high sensitivity of mortar properties to even small amounts of TP indicates the need for further research to determine the optimal TP content range for consistent mortar quality control. The data provided in this study offer valuable insights for the future application of TP in repair mortars and other construction materials, including concrete.
Journal Reference
Jang, C.-H., Kim, Y.-J., & Oh, S.-R. (2024). Experimental Study on the Effects of Tapioca Starch on Cement Mortar Quality Improvement. Materials, 17(16), 3889. DOI: 10.3390/ma17163889, https://www.mdpi.com/1996-1944/17/16/3889
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