Enhanced Concrete with Natural Pozzolan


Introduction:

Natural pozzolans are available at substantially lower cost than portland cement. Generally they are finer than cement and posses pozzolanic and sometimes cementitious properties. In the past, natural pozzolans such as volcanic earths, tuffs, trass, clays and shales in raw or calcined form, have been successfully used in various type of structures. Their use as partial replacement for cement can lead to considerable cost saving in addition to the improved performance. Improved workability, reduced bleeding and heat of hydration, enhanced ultimate strength, impermeability, chemical durability and improved resistance to thermal cracking. This study is limited to one of natural pozzolan namely pumice and its performance. Pumice is white foam of volcanic glass, interlocking fibers filled with tiny air bubbles. These frothy vesicles (abundant small bubbles) give pumice its unique and infinitely useful qualities. Hess crushes and processes it for use in a wide variety of products and industries. Being chemically inert and crystalline silica-free, pumice is both safe to use and safe for the environment. Hess pumice is the largest producer of finely ground processed pumice which was founded in 1958. Site is located in Southeastern Idaho near the community of Malad City. Enhanced concrete can be produced from pumice blended cements.

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Sample of Pumice Powder



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Mining field (Source: hesspumice.com)


Pumice – Identified as Pozzolanic Material:

Pumice can be classified as pozzolanic material, according to ASTM C 618 pozzolans are the siliceous and aluminous material in finely divided form and in the presence of moisture, at ordinary temperature chemically react with calcium hydroxide to form compounds possessing cementitious properties. Pozzolanic activity test shows the suitability of pumice as pozzolanic material (K.Hossain). In this test, samples were mixed with cement and water and kept 1-2 weeks. The total alkalinity and lime concentration is then measured. The material is considered pozzolanically active if the level of concentration falls below lime solubility isotherm.


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Figure 1: Pozzolanic activity test

Reason for pozzolanic property and supporting evidences:

During volcanic eruption, the quick cooling of magma composed mainly of aluminosilicates results in the formation of glass or vitreous phases with disordered structure. These aluminosilicates having disordered structure will not remain stable when it is exposed to saturated lime which is the basis for pozzolanic property of volcanic glasses. Mineralogical composition is found from X-ray diffraction (XRD) test. It was evident that pumice is more than 99% amorphous from XRD test.

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Figure 2: X-ray diffraction analysis of pumice

Scanning electron micrographs of pumice shows the nature of volcanic particles and the vesicles formed by the expansion of trapped gases while molten lava rapidly cools.

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Figure 3: Scanning electron micrograph


According to ASTM C 618, pumice can be classified as Class N mineral admixture (for raw or calcined natural pozzolan). Class N admixture should have a minimum of SiO2+Al2O3+Fe2O3 content of 70%, pumice has around 85% of the content which is found from XRF test. Presence of siliceous and aluminous compounds is evident from chemical analysis results of pumice.
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Figure 4: Chemical analysis result from XRF test


Effect of Pumice on Properties of Concrete:
By adding a natural pozzolan, many properties of concrete can be influenced, by physical effects associated with small particles, finer particle size distribution than Portland cement and others by pozzolanic and cementitious reactions. Addition of pozzolan can influence concrete mix proportions, degree of hydration of Portland cement, strength and permeability of hardened concrete, resistance to thermal cracking, alkali-silica expansion and sulfate attack. Some of the above mentioned influenced properties are discussed in detail below.

Compressive strength development
Compressive strength of concrete is one of the primary considerations in concrete mixture design. Following ASTM C109, compressive strength of 4x8 cylinders were tested with different grades of pumice. Mix contains 20% percentage of different grade pumice and 80% cement with w/c ratio of 0.485. The mixes contain pumice reaches the strength slower than control mix. But still, minimum strength at age 7 is greater than 3000 psi and at age 28 is greater than 4500 psiHence results show the pozzolanic contribution of pumice to the strength of concrete. Due to pore structure refinement, pozzolanic mix concrete will have higher ultimate strength.

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Figure 6: Mortar Compressive Srength (Sample graph - to show the trend)

Acceleration of cement hydration:
When the cement is hydrated, the compounds react with the water to acquire a lower energy state from higher energy state which requires the release of energy in the form of heat, also known as an endothermic reaction. The heat of hydration can help with cold weather placement or it may be a hindrance with mass concrete structures
Water adhered to the saturated porous fine aggregate particle have been shown to maintain a higher relative humidity within the concrete, which creates the environment conductive to cement hydration. Flocculated structure of particles of Portland cement is dispersed more efficiently by fine particles of any material, which increases early hydration. Pores reduce the heat of hydration during early periods which helps in reducing the cracks in mass concrete structures. The formation of C-S-H is one of the major heat generators of the hydration process. When portland cement is hydrated the C3S + H is a fast reaction forming C-S-H + CH. Hence C3S content is reduced by replacing portion of cement with pozzolanic material, which helps in reducing the heat produced during hydration process.

Sulfate Resistance:

Following ASTM C1012, five mixture designs were tested for sulfate resistance. One with 100% cement and other mixes with 20% replacement of different grades of pumice. The specimens were tested through 15 weeks and then again at 6 months. Test values below 0.05% at 6 months indicate high sulfate resistance and test values below 0.10% at 6 months indicate moderate sulfate resistance. Figure 6 shows their percent length change through 6 months. All the pozzolanic mixes are within the limit of 0.1%, hence qualified to be MS (Moderate Sulfate resistance). Out of 5 mixes tested, 4 mixes can be classified as HS (High sulfate resistant cement) since the length change is less than 0.05% after 26 weeks. Improved performance is seen in the mixes which has pumice as part of cementitious particles. The sulfate expansions of the mixtures containing the pozzolan were well below the limit for blended cement considered highly resistant to sulfate attack. Strong correlation was reported between glassy composition and sulfate resistance of concrete in the literature. Due to large amount of calcium hydroxide in the hydrated cement paste, Portland cement concrete is not durable to acidic environments. Pumice has the ability to reduce the calcium hydroxide content of cement paste, can improve the chemical resistance of concrete.

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Figure 6: Bar reactivity to Sulfate(Sample graph - to show the trend)

Corrosion resistance:
When pozzolans are incorporated in concrete, it cause pore structure refinement by transforming large pores into smaller ones. The continued pozzolanic reaction makes the resultant product more dense and compact. Pozzolanic reactions are time dependent and continue to occur over very long period, much after the hydration products in cement concrete are fully formed. Both strength and impermeability increases with time in cement concrete containing pozzolans. This impermeability resists the movement of ions which act as a basis for corrosion resistance. Resistivity increases over time for the mix with pozzolans whereas it remains constant for the mix with 100% cement.

Conclusion:
The resultant concrete uses relatively lesser amount of cement and also helps in reducing the carbon footprint of building materials by its capability of less carbon dioxide emission property. Its white color makes it easier to produce desired color concrete. Greener concrete can be produced in the future with a desired aesthetic appearance using less cement. Thus environmental friendly product can be produced in ready mix and precast industry with improved performance.

References:
  1. http://hesspumice.com/
  2. Concrete Admixtures Handbook, Properties, Science, and Technology by V.S. Ramachandran
  3. Volcanic ash and pumice as cement additives: pozzolanic, ASR and autoclave expansion characteristics, K. M. Anwar Hossain, Cement and Concrete Research.
  4. Pumice based blended cement concretes exposed to marine environment: Effects of mix composition and curing conditions, K. M. Anwar Hossain, Cement and Concrete Composites.
  5. Properties of volcanic pumice based cement and lightweight concrete, K. M. Anwar Hossain, Cement and Concrete Research.
  6. Robertson’s Concrete - Rhyolite Evaluation Report, September 2009, By Shannon Hanson, Tim Garfield, Pratanu Ghosh, and Paul Tikalsky.