Reducing Cement Content in Masonry with Rice Husk Ash, a Promising Supplementary Cementitious Material.
Increasing awareness surrounding the negative environmental impacts associated with widespread use of cement, namely in the form of inordinate carbon dioxide emissions, has lead to great strides in the development of technologies and materials aimed at reducing reliance on this carbon-intensive material in the manufacture of concrete and masonry building materials. Ordinary Portland cement (OPC) is the key component in concrete, the most voluminous manmade product on earth, and studies have shown that production of cement causes 6-7% of global greenhouse gas emissions. 1, 2
The use of supplemental cementitious materials (SCMs) represents an important modern means of offsetting the use of OPC. SCMs are fine particulates--either pozzolanic or cementitious in nature--that react when hydrated to form cementitious compounds. SCMs can be used to replace a portion of the OPC in concrete and cement-stabilized masonry materials while maintaining or enhancing performance characteristics (like compressive strength and durability). Examples of SCMs commonly used include: fly ash--a by-product of coal-fired power production, ground-granulated blast-furnace slag--a by-product of iron processing, silica fume--a by-product of silicon metal manufacturing, and natural pozzolans--metakaolin and various other calcined clays are the most commonly used, though volcanic ashes and calcined shales also fall under this category. Pozzolanic materials are unique among SCMs in that they have long been recognized for their ability to react with calcium hydroxide (typically in the form of hydrated lime), in the absence of OPC, to contribute to the durability of earthen and masonry building materials. Pozzolans have been utilized to this end for thousands of years, dating back to ancient Rome and Egypt.
Despite their great potential to reduce the need for OPC and render real environmental benefits, the production of most SCMs is regionally limited, and some of those most commonly used in concrete (fly ash, for example) are not amply produced in certain parts of the US, reducing the practicality of their application in such regions and lessening the potential environmental gains these materials present. For example, in the Northeast region and on the West coast, insufficient amounts of fly ash are produced to keep up with demand. By contrast, in the North and South East Central regions and the West North Central region of the US, fly ash production exceeds cement demand. Although produced in abundant amounts in some parts of the country, fly ash and other combustion co-products must be produced in proximity to cement production sites to ensure their economic and environmental viability as sustainable cement substitutes. Life cycle analyses shows that transporting fly ash more than 50 miles from its origin dramatically increases its environmental impacts, and reduces its economic viability as a cement replacement. 3
Finally, another concern is that the widespread utilization of fly ash as a OPC substitute could effectively subsidize coal-fired electricity generation, which is currently responsible for 20% of the world’s total GHG emissions, as fly ash is transformed from a liability to a co-product that can be sold at a profit. 4
There is, however, an SCM produced in many regions where traditional SCMs are not widely available that is gaining much attention internationally for its potential to positively impact the carbon footprint of cement-stabilized materials: rice husk ash (RHA). The harvest of paddy-grown rice yields around 78% rice and bran, with the remaining 22% of the harvested material being rice husk. This rice husk is then used as a fuel source in steam generators that drive a partial boiling process (known as “parboiling”) that is a necessary stage in the processing of the harvested rice. After combustion, around 25% of the combusted rice husk remains as RHA. Traditionally treated as a waste material, popularization of the use of RHA as an SCM has illuminated the potential value of this abundant and renewable resource. 5 California produces more than two-million tons of rice annually, making it the second-largest rice growing state in the United States. 95% of California's rice is grown within 100 miles of Watershed Materials' pilot manufacturing facility in Napa, California, representing a vast resource with the potential to significantly reduce the carbon footprint of masonry and concrete products produced in the region. 6
Naturally high in amorphous silica, RHA is a highly-reactive pozzolanic material, and can be used to replace upwards of 30% of the OPC in a cementitious mix design while enhancing mechanical properties. Supplementing a portion of OPC with RHA has been shown to encourage the formation of a highly dense, minutely porous calcium-silica-hydrate (C-S-H) gel around cement particles, and to improve the microstructure of the interfacial transition zone between the cement paste and the aggregate in cement-stabilized systems. 7 These characteristics of OPC-RHA concretes have been shown to contribute to superior early- and ultimate-compressive strengths, improved impermeability, exceptional resistance to chloride ion penetration and improved freeze-thaw durability. 8, 9, 10 At Watershed Materials, the extreme compaction undergone by masonry units in the company's proprietary manufacturing process induces exceedingly-intimate contact between constituent particles, encouraging a highly-efficient progression of the hydration reaction undergone by cementitious and pozzolanic components of the mix design and rendering binders with exceptional mechanical properties.
The incorporation of locally-produced RHA into the production of Watershed Materials' low-OPC, low-carbon masonry units not only further reduces the product's carbon footprint, but also yields an elegant and singular aesthetic. Basaltic aggregates emphasize the dark tones induced by the ash, which are strikingly juxtaposed by the white of crushed recycled concrete and mineral aggregates in the mix design.
Integrating hydrated lime and RHA in this mix design promotes pozzolanic reactivity and long-term strength development beyond that which is characteristic of typical OPC-SCM blended cements; non-traditional cementitious gels that result from interactions between highly-active pozzolanic materials and calcium hydroxide in the form of hydrated lime will contribute to strength and durability development for upwards of one year (though design strengths in excess of 1900 psi will be developed before 28 days). Non-traditional binders like those described above, consisting of lime and pozzolanic ash, were used by ancient romans in the construction of concrete-like materials of prodigious durability, with the lifetimes of modern concretes being diminutive by comparison (anecdotally, these novel roman mix designs were also thoroughly compacted in-place to improve their performance, further mirroring Watershed Materials' manufacturing processes). 11, 12 The development of non-traditional cementitious binders in Watershed Materials' masonry materials is further encouraged by the inclusion of locally-recycled concrete aggregates (rich in calcium and containing un-hydrated cement paste). 13
Utilizing regionally-appropriate SCMs is one of a wide range of efforts being pursued by Watershed Materials research and development division in its quest to alleviate the problems associated with the overuse of OPC and drastically shrink the carbon footprint associated with high-performance masonry and concrete products. Completely eliminating reliance on OPC and regionally-limited SCMs as stabilizers in the production of carbon negative, highly-durable masonry products is a primary goal of our dedicated laboratory staff, whose National Science Foundation-funded research has yielded radical advances in alternative stabilization and masonry production technologies and methods. These discoveries have great potential to significantly expand the range of materials used in the production of masonry products and to greatly decrease the environmental costs associated with producing these important building materials. To be discussed in future posts…
3. Babbitt, Callie W., and Angela S. Lindner. "A life cycle comparison of disposal and beneficial use of coal combustion products in Florida." The International Journal of Life Cycle Assessment 13.3 (2008): 202-211.
10. Nehdi, Moncef, J. Duquette, and A. El Damatty. "Performance of rice husk ash produced using a new technology as a mineral admixture in concrete." Cement and concrete research 33.8 (2003): 1203-1210.
11. Cordeiro, Guilherme Chagas, Romildo Dias Toledo Filho, and Eduardo de Moraes Rego Fairbairn. "Use of ultrafine rice husk ash with high-carbon content as pozzolan in high performance concrete." Materials and Structures 42.7 (2009): 983-992.
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