Determination of the Suitability of Cassava Starch as an Admixture in Cement Paste and Concrete

Determination of the Suitability of Cassava Starch as an Admixture in Cement Paste and Concrete

Chapter One

AIM AND OBJECTIVES

The aim of this research is to determine the suitability of cassava starch as an admixture in concrete and cement paste mortar.

The objectives of this research work are:

1. To investigate whether locally available starch can be used as organic admixture. Two water soluble starches was selected to be used in this study, they are laboratory starch and arrowroot. 
2. To study the characteristics of starch like particle size distribution, fineness, biodegradability.  
3. To study the effect of starch inclusion on various characteristics of fresh, hardened and mature cement paste, cement mortar and concrete. 
4. To study the durability parameters of concrete with starch admixture. Durability was assessed in terms of permeability of mature concrete by rapid chloride penetration test (RCPT), rapid migration test (RMT) and sorptivity test.  
5. To explore the effects of starch admixture on micro structure level of mortar specimens by scanning electron microscopy (SEM) imaging  
6. To study the changes in the test parameters with variation of starch content and determine optimum dose of starch admixture for desired results. 

Chapter Two

LITERATURE REVIEW 

General 

A good quality concrete is desirable for any construction. A good quality concrete is strong, durable and ensures economy as well. Factors effecting concrete quality are the use of admixtures, availability of moisture for curing, mix proportions, curing temperatures (Tattersall, 2007) and quality of raw materials (Kurdowski, 2014). A number of ways are ventured to attain good quality concrete. Among them using supplementary material is quite a popular one. Problems arise regarding the quality of concrete and subsequently of concrete structure due to poor quality of materials, incorrect specifications, faults in design, poor workmanship during construction, exposure to extreme environmental conditions etc. (Shash, 2005). 

Hydration Mechanism in Concrete 

Hydration products of cement paste primarily denote hydrated silicates of calcium (CS-H), calcium hydroxide (portlandite) and forms of calcium aluminate (ettringite). CS-H phase has similar chemical compositions as the mineral tobermorite (5CaO·6SiO2·5H2O). Largely the C-S-H phase is responsible for the strength of the hardening mass and also covers a significant volume in hydration product compared to other phases. The degree of hydration is dependent on the density of C-S-H gel. The pores in the hardened cement paste reduce with crystallization of ettringite and calcium hydroxide with time (Franus et al., 2001). Locher et al. (1974) described the stages of hydration process and the development of cement paste structure (Figure, 2.1) relating age, porosity and phases in hydration product.  

Admixture 

Admixtures are used in concrete and cementitious material to attain a wide range of effects. There are set controlling, viscosity altering and water reducing admixtures among many. More detailed categories are accelerating admixtures, water reducing admixtures, retarding admixtures, air entraining admixtures etc. According to functions the classification is as follows: air-entraining admixtures, accelerating admixtures, hydration-control, corrosion inhibitors, shrinkage reducers, retarding admixtures, water- reducing admixtures, colouring admixtures, plasticizers, alkalisilica reactivity inhibitors, and miscellaneous admixtures such as workability and bonding admixtures, foaming, grouting, damp proofing and permeability reducing admixtures, gas-forming, anti-washout and pumping admixtures (Joseph et al., 2016). Different admixtures are blended together to achieve specific rheology requirements (Bessaies-Bey, 2016). 

A high performance concrete cannot be imagined without the usage of good set of admixtures. These can alter one or more characteristic features of concrete and cement mortar. The effectiveness of admixtures depends on factors such as functional groups, chemical configuration and molecular weight of the added elements (Ouyang et al., 2009).  

Chemical admixtures 

In recent times, if more than ever, admixtures are used to enhance the workability, mechanical properties and durability of concrete. These admixtures can be of an impressive variety of molecules with very different chemical structures and physical properties. Superplasticizers are as high-range water reducer used to improved workability for concrete to a great extent at relatively low water to cement ratio, which leads to improvement of strength and permeability. It infused in the host material and reduces the yield stress and viscosity (Hot, 2014). Another commonly used admixture is air-entraining admixtures to impart resistance of freezing and thawing. Admixtures lead to reduction in water evaporation to mitigate shrinkage, and generate expansion to compensate shrinkage and prevent cracking. Bleeding and segregation can be reduced by introducing viscosity agents or stabilizers (Khayat & Mikanovic, 2012) or by enhanced of water retention capacity (Bulichen, 2012; Brumaud, 2013). Retarders delays the setting and hydration process (Plank, 2009) by increasing flowability retention (Perez, 2007; Li et al., 2012). 

Plasticizers and superplasticizers 

Plasticizers and superplasticizers are often used for the following reasons (also depicted in Figure 2.10 ): 

-to increase workability while keeping the mix proportion unchanged 

-to reduce the water requirement (w/c ratio) while keeping the workability unchanged to attain higher strength and better durability 

-to reduce both cement and mixing water while keeping the workability unchanged to attain better performance in terms of creep, shrinkage and thermal strains as heat of cement hydration is reduced with reduced cement content. 

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Chapter Three

MATERIALS AND METHODS 

General 

Specifications and properties of materials used are focused in this chapter. The properties primary ingredients of mortar and concrete i.e. cement, fine and coarse aggregate as well as the supplementary material starch are discussed for better understanding of the behavior of the final product. 

Cement 

Cement was used as binding material for both mortar and concrete casting in this study. From the array of different types of cement, CEM I (ASTM C 150) or ordinary Portland cement (OPC) was used. It is said to be the general purpose cement for common use. However it is not prescribed to be used if the concrete is in contact with soil or ground water. Thus the concrete or mortar produced with OPC has some durability issues. 

The composition of the cement is presented in Table 3.1. The relative amounts of the components have effects on the properties of cement and the products as mortar and concrete. X-ray fluorescence (XRF) analysis gives relative amount of elements in the cement which was converted to the weight fraction of each element in oxide form assuming that there is sufficient amount of oxygen charge balance the other elements all the time. The value of loss on ignition (LOI) was found to be 1.8%. The relative percentage of components obtained from X-ray fluorescence (XRF) data was normalized according to LOI in Table 3.1. Cement was heated to 900 °C -1000 °C and the loss of weight was measured after a constant weight was obtained. At this elevated range of temperature, any water or CO2 present in the cement specimen is driven off.  Presence of water and CO2 indicates prehydration and carbonation respectively. ASTM C 150 limits the maximum loss on ignition (LOI) to a value of  3.0%. 

Chapter Four

EXPERIMENTAL PROGRAM 

General 

This chapter specifies the details of laboratory procedures carried out to assess properties of fresh concrete, strength of mortar and concrete mixes with starch admixture and their durability parameters. 

The experiments consist of test on cement, cement mortar, fresh concrete and hardened concrete. Additionally, scanning electron microscopy (SEM) was sought to examine the microstructure of cement mortar. Much emphasis was given on durability tests of concrete. To assess the durability properties of concrete compressive strength test, rapid chloride penetration test (RCPT), rapid migration test (RMT) and sorptivity test were employed. An image capturing device was used to take photographs of concrete specimens from RMT. The experiment procedures were carried out mostly as per ASTM (American Society for Testing and Materials) and AASHTO (American Association of State Highway and Transportation Officials) standards. 

Scanning Electron Microscopy (SEM) 

Scanning electron microscopy was used to examine the topography and morphology of starch particles and the microstructure of mortar. A Jeol JSM-7600F Schottky Field 

Emission Scanning Electron Microscopy (FE-SEM) was used (Figure 4.1, Figure 4.2).  

The accelerating voltage was kept at 5.00 kV while the working distance was held at 8-10 mm at various magnifications. As the process involves vacuum condition and high energy beam of electrons, the samples for SEM were prepared accordingly. 

Chapter Five

RESULTS AND DISCUSSIONS 

General 

In this chapter, results of various tests performed in the study for evaluating the effect of starch on properties of cement mortar and concrete are presented. Characteristics of cement were evaluated by X-ray diffraction (XRD) analysis and initial and final setting time tests for cement infused with starch. Furthermore, the effect of starch addition on setting time of cement also investigated. The morphology of starch granules were observed by scanning electron microscopy (SEM).  Finally, strength and durability characteristics of concrete, with and without starch admixture, were examined by conducting compressive strength test, sorptivity test, rapid chloride penetration test (RCPT) and rapid migration test (RMT). All three methods of durability tests i.e. sorptivity, RCPT and RMT validated that the inclusion of starch has compelling impacts on durability of concrete. The results are presented in the form of tables, graphs and images.   

Determination of Composition of Cement 

X-ray fluorescence (XRF) analysis 

X-ray fluorescence (XRF) analysis gives relative amount of elements in the cement which was converted to the weight fraction of each element in oxide form assuming that there is sufficient amount of oxygen charge balance the other elements all the time. The value of loss on ignition (LOI) was found to be 1.8%. The relative percentage of components obtained from X-ray fluorescence (XRF) data was normalized according to LOI in Table 5.1. Cement was heated to 900 °C -1000 °C and the loss of weight was measured after a constant weight was obtained. At this elevated range of temperature, any water or CO2 present in the cement specimen is driven off.  Presence of water and CO2 indicates prehydration and carbonation respectively. ASTM C 150 limits the maximum loss on ignition (LOI) to a value of  3.0%.

Chapter Six

CONCLUSION AND RECOMMENDATIONS 

Conclusions 

The effects of starch admixture on mortar and concrete were investigated in this thesis. Obtained results were compared for specimens with and without admixture. Following conclusions can be drawn from experiments performed: 

1. Morphology and topography of starch particles were investigated by SEM imaging. Laboratory starch had finer grain and had narrower size range as observed from fineness test and particle size distribution. The morphology had effect on the extent of mixing of starch granules with cement water paste. 
2. The maximum delay in setting time for starch addition occurred between 1% and 1.5% of starch inclusion. The maximum value of setting time in this set of samples increased by 62.50% and 64.71% in initial setting time and final setting time respectively when compared with control sample for laboratory starch admixture. Up to certain amount of starch addition, the retardation continued to increase. When the amount of starch polymers exceeded the optimum level, a competition for adsorption site started and that weakened the barrier, resulting in decrease in retardation. 
3. Presence of starch admixture reduced the surface tension on cement particles and increased the workability of fresh mix for both mortar and concrete. 
4. The concern regarding the effect of biodegradable organic material within cement matrix on compressive strength had been refuted by the findings of this study. The compressive strengths of both mortar and concrete were in acceptable range past the initial effect of retardation up to 14 days of curing. There was not much increase in strength parameters, however none of the mixes fell behind the control specimen at maturation.   
5. Starch addition enhanced the durability parameters of mortar and concrete. Starch increases the workability that leads to a better mixing of the constituents that results into better durability. Permeability parameters reduce  for concrete with arrowroot admixture as indicated by sorptivity, RCPT and RMT results.

1. The mechanism of set retardation and viscosity modification by starch can be attributed to the polymer structure of starch. The polymers having one hydrophobic and one hydrophilic end produce tiny air bubbles. The layer of polymer around cement particles retarded the setting process and the entrained air lowered viscosity of fresh mix. In addition to that the presence of MgO in starch as trace mineral also contributed to the delayed setting time. 
2. A closer look to the microstructure of mortar specimen without and with starch admixture revealed the effect of better mixing of the components in terms of pore size distribution and proportion of hydration products. 
3. Laboratory starch had greater retardation effect compared to arrowroot but arrowroot preformed better in durability aspects. 
4. The optimum dose of arrowroot is between 1% to 1.5% of cement content for use as set retarding and viscosity modifying admixture. 

Recommendations 

This study explored the urgency and validity of focusing attention on bio-admixtures that are environmentally friendly. The results suggested that organic admixtures are viable options and the concrete industry should follow this road for the sake of nature.   

The author stepped into this investigation without absolute knowledge of the probable outcome as everything from a natural source is unique. The behavior of starch depends on the origin of plant. Though starch was examined in some previous studies, their behavior and impact was different on their own.    

So far the results indicate that starch can be an excellent alternative set retarding and viscosity modifying admixture that it has great impact on durability performances. It needs to be substantiated by further experimental evidences. Some recommendations for future research work are described below: 

1. The mechanism of set retardation and viscosity modification by starch can be attributed to the polymer structure of starch as stated earlier. The determination of polymer structure of starch used is needed for better understating of the internal process of hydration and resulting effects. 
2. The water cement ratio had been uniform in this study. A reduced water cement ratio might be used and the resulting workability parameters must assess to harness the full impact of viscosity modification. 
3. The shrinkage parameters of mortar and concrete should be tested for further examination. 
4. Starches from different sources can be used to see the variation effects. Pretreatments such as heating, moistening can be employed on starch as needed. 
5. Though the low biodegradability index (BI) of starches indicated negligible biodegradability, tests should be conducted for longer curing days to assess long term effect of biodegradability of starch inside cement matrix. 

REFERENCES 

• Abalaka, A.E., 2011. Comparative effects of cassava starch and simple sugar in cement mortar and concrete. ATBU Journal of Environmental Technology, 4(1), pp.13-22. 
• Abdalla, K.Z. and Hammam, G., 2014. Correlation between biochemical oxygen demand and chemical oxygen demand for various wastewater treatment plants in Egypt to obtain the biodegradability indices. International Journal of Sciences: Basic and Applied Research, 13(1), pp.42-48. 
• Ahmad, A. and Kumar, A., 2013. Chloride ion migration/diffusion through concrete and test methods. International Journal of Advanced Scientific and Technical 
• Akindahunsi, A.A. and Uzoegbo, H.C., 2015. Strength and durability properties of concrete with starch admixture. International Journal of Concrete Structures and 
• Baki, I., 2017. Impact of Bio-admixture on Cement Mortar. Undergraduate Dissertation, BUET 
• Baroghel-Bouny, V., Nguyen, T.Q. and Dangla, P., 2009. Assessment and prediction of RC structure service life by means of durability indicators and physical/chemical models. Cement and Concrete Composites, 31(8), pp.522-534. 
• Bessaies-Bey, H., Baumann, R., Schmitz, M., Radler, M. and Roussel, N., 2016. Organic admixtures and cement particles: Competitive adsorption and its macroscopic rheological consequences. Cement and Concrete Research, 80, pp.1-9. 

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