Making High Concrete Strength From Granulated Calcined Clay
CHAPTER ONE
OBJECTIVES OF STUDY AND AIMS
This study is conducted to accomplish some predefined objectives. These objectives are:
- To study the performance of concrete containing different percentages of calcined clay and to identify the optimum replacement percentage.
- To investigate the effect of calcination temperatures to the strength performance of calcined clay-concrete.
- To compare the performance of calcined clay with other cement replacement materials (CRMs).
CHAPTER TWO
LITERATURE REVIEW
INTRODUCTION
This chapter presents the review of related literature on making high concrete strength from granulated calcined clay. Views and opinions of other authors will be presented as follows.
CALCINED CLAY
The main source of such material is gravel which is obtained from river beds. Moreover, the extraction of gravel from river beds has caused siltation of rivers, a negative environmental impact. In order to find a more economically viable and environmental friendly material for coarse aggregate in concrete, it is worthwhile to study the viability of synthetic aggregates such as, calcined clay.
The term synthetic aggregate or Calcined Clay, refers to a material obtained from the processing of a soil or clayey material with satisfactory mechanical strength for a particular purpose. These characteristics are usually obtained by heating the ceramic body at high temperatures, above 760°C. The quality depends crucially on the ceramic raw material, firing temperature, and process of ceramic mass conformation. In some cases, certain properties of ceramic products can be improved by using soils with a higher percentage of flux elements.
Calcined clays consisting of single clay minerals e.g. kaolinite, montmorillonite and illite have been in focus of many investigations as they exhibit pozzolanic reactivity and may serve as type II additions to concrete. Most researchers used powder samples of more or less pure clays for their studies. Normally, the powder samples were calcined carefully in a laboratory furnace at well-defined temperature levels, which were maintained for a sufficient time span in order to ensure completion of the calcination. Subsequently the samples were allowed to cool down naturally to ambient temperature. The investigations reveal a clear ranking with respect to pozzolanic reactivity starting with Ca-montmorillonite and metakaolin followed by mixed-layer mica/smectite, Na-montmorillonite, sepiolite and finally illite. This ranking holds for the individual optimum calcination temperature. Thus, the second most important parameter influencing pozzolanic reactivity of calcined clay is its calcination temperature that is somewhere in the range from 760 °C [1400 °F] for 23 kaolinite and 930 °C [1706 °F] for illite and mixed-layer mica/smectite.
The efficiency of calcined clay depends on physical and chemical properties of the final product. Particle size distribution is an important physical effect since fine dispersed calcined clay may serve as nucleus thus enhancing early hydration of cementitious systems. Additionally, silica and alumina ions dissolve from the clay layers into the pore solution the easier the smaller the particles are. These ions are the source of the pozzolanic reactivity of calcined clays. The rate of the chemical interaction between the calcined clay and the pore solution is indicated by the change in the concentration of calcium hydroxide (CH) in the 6 pore solution. The reactivity of the calcined clays was studied either in combination with ordinary Portland cement (CEM I) or Ca(OH)2. Usually, other cements were not considered.
CHAPTER THREE
MATERIALS AND METHODS
SIEVE ANALYSIS
A sieve analysis (or gradation test) is a practice or procedure used (commonly used in civil engineering) to assess the particle size distribution of a granular material by allowing the material to pass through a series of sieves of progressively smaller mesh size and weighing the amount of material that is stopped by each sieve as a fraction of the whole mass.
The size distribution is often of critical importance to the way the material performs in use. A sieve analysis can be performed on any type of non-organic or organic granular materials including sands, crushed rock, clays, granite, feldspars, coal, soil, a wide range of manufactured powders, grain and seeds, down to a minimum size depending on the exact method. Being such a simple technique of particle sizing, it is probably the most common.
CHAPTER FOUR
FINDINGS AND RESULTS
Standard Consistency and Setting Time
The effect of clay pozzolana on standard consistency and setting times of Portland cement is as presented in the tables below. The table shows that the amount of water required for the desired consistency for Type I and Type II pastes increased ranging from 3.7%-29.6% and 3.7%-37% respectively as compared to the control paste. The reason could be due to the lower specific gravity of clay pozzolana compared to the ordinary Portland cement, hence the requirement of larger volumes of clay pozzolana to replace the same mass of ordinary Portland cement. It was observed from the table that relatively, same weight replacement values between 5% and 25% of Type I clay pozzolana required more water than its Type II pozzolana counterpart in the cement paste. This shows that Type I pozzolana may have a relatively high pore structure than Type II sample.
From table 4.1, the initial and final setting times of both types of clay pozzolana cements (Type I and Type II clay pozzolana samples) were greater than the control paste. Both initial and final setting times of the pozzolanic paste increased with the percentages of clay pozzolana. The reason for this trend was that the clay pozzolana reduced the cement content which serves as the stiffening agent. This consequently reduced the rate of hydration and led to slow stiffening and hardening of the paste thereby prolonging the hydration process.
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
Conclusion
Based on the results obtained from the investigations, the following conclusions are drawn:
Chemically, the pozzolana samples are siliceous. The main oxide (SiO2 + Al2O3 + Fe2O3) content exceeded the minimum of 70% set by ASTM: C618standard specification for calcined natural pozzolana for use in concrete.
The addition of calcined clay to Portland cement increases the normal consistency of the blended Portland cement mixtures.
The concrete slump as well as their respective densities decreased as the clay pozzolana content increased. The addition of clay pozzolana retarded both initial and final setting times. This is of particular importance in ready mixed concrete as there is extra time to ensure fresh concrete delivery to site.
The compressive strengths of blended pozzolana-cement concrete were lower than that of the control plain concrete at early curing age of 7days. At 28days, the shortfall in compressive strength on the grades of concrete averaged 6% for Tanoso (Type II) and Mankranso (Type I) samples on 20% replacement.
Beyond 28days, the compressive strength improved significantly. Partial substitutions of up to 20% for both samples surpassed the 28days strength of plain concrete, varying from 1% to 15% at 56days and 5% to 31% at 90days of curing. The flexural strength improvement was significant. At most 15% blended concretes for both samples surpassed their respective grades tensile strengths at age 28 days. Incorporation of up to 30% of Tanoso samples equaled or exceeded the 28days strength at 56days testing.
Similarly, Portland cement-mankranso pozzolana concretes exhibited same for replacement up to 25%.
Recommendation
It recommended that concretes prepared with pozzolana did not show any tendencies of bleeding and segregation.
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