Making High Concrete Strength From Granulated Calcined Clay

Project and Seminar Material for Building Technology BT

Making High Concrete Strength From Granulated Calcined Clay

Chapter One


1.1 Background Of The Study

Cement is a significant source of anthropogenic release of carbon dioxide. The CO2 derives mainly from kiln fuel combustion, transport and distribution and decarbonating of limestone. The latter source is fairly constant. Thus one procedure to lower the release of carbon dioxide is reducing the clinker content of the cement by shifting the production from CEM I to CEM II or CEM III cements. Another approach is replacing cement partially in concrete mix design by Type II additions like fly ash, granulated blast furnace slag or silica fume. An alternative to these afore mentioned options provides the use of calcined clay either as reactive part of the cement [1] or as Type II addition in concrete [2].

Metakaolin is known as a very reactive calcined clay and has been in focus of many investigations [e.g. 3, 4, 5, 6, 11]. Its widespread use in concrete is prohibited mostly by its high price compared to other Type II additions. Suitable and less expensive clay qualities consist rather of a mixture of clay minerals, which range between the clays used in the ceramic industry and those required for the cement production than of single type clay minerals. Thus it is worth taking a closer look at mixed clays. The reactivity of any calcined clay depends on both its mineral composition and the calcination temperature [e.g. 1, 3 – 11].

In most cases these investigations used homogenous clay samples that were calcined at constant temperature and for a period of several hours. Furthermore these clays were ground prior to calcination ensuring a complete reaction to take place. If coarse crushed clay is fed into a rotary kiln it is exposed to varying temperatures on its journey through the kiln combined with temperature gradients due to the size of the chunks after crushing and in addition a varying degree of oxidation. This paper focuses on the impact of such calcined clay on various mortar and concrete properties and its inherent ecological potential.

1.2 Objectives of Study and Aims

This study is conducted to accomplish some predefined objectives. These objectives are:

  1. To study the performance of concrete containing different percentages of calcined clay and to identify the optimum replacement percentage.
  2. To investigate the effect of calcination temperatures to the strength performance of calcined clay-concrete.
  3. To compare the performance of calcined clay with other cement replacement materials (CRMs).

1.3 Statement of the Problem

The cost of the material is very high because of the long distance that must be traveled to obtain it, which increases construction costs and the extraction of gravel from river beds has caused siltation of rivers, a negative environmental impact.

1.4 Scope of Study

This study focuses on the strength performance of concrete with calcined clay Strength is the most important property of concrete since the first consideration in structural design is that the structural elements must be capable of carrying the imposed loads. Strength characteristic is also important because it is related to several other important properties which are more difficult to measure directly. With regard to this matter, the development of compression strength of calcined clay concrete is studied. Cement replacements by 5%, 10%, 15%, 20% and 30% with calcined clay are studied. Concrete tests are conducted on the concrete samples at the specific ages. All the strength tests are limited to the ages of 28 days. 5 In the study of the effect of calcination temperatures to the strength performance of calcined clay, the temperatures are set within the range of 600°C-800°C. The temperatures interval used is 50°C. For the performance comparison study, the cement replacement materials used are silica fume and ground granulated blast furnace slag. These two cement replacement materials are chosen as they are the most common replacement materials nowadays and will be good comparisons to calcined clay. The comparison is made on the compressive strength performance of calcined clay, silica fume and slag concrete.

Chapter Five

Conclusion and Recommendations

5.1 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%.

5.2 Recommendation

It recommended that concretes prepared with pozzolana did not show any tendencies of bleeding and segregation.

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