NATURAL ZEOLITES AS POZZOLANS IN CONCRETE
The unique porous qualities of zeolite make it an important mineral in the blends of concrete, cement & mortar.
Zeolites have been widely used across the planet for thousands of years
The use of natural zeolites as pozzolanic materials in construction dates back 3000 years, to the Greek and Roman periods when zeolites, occurring as altered volcanic ash, tuff, and trass, were used with lime in mortars and concretes for construction. The Romans used Neapolitan Yellow Tuff zeolites near Pozzuoli. Italy, used it in construction of aqueducts, public buildings, and highways. The lime and zeolite combination showed excellent cementitious properties. Natural zeolite tuffs have been used for many years as cement pozzolans in Serbia, Germany, Italy, Bulgaria, China, and Russia.
Despite their widespread applications in pollution control, energy conservation, agricultural, mining, and metallurgical applications during the last few decades: construction industry appears to be the largest natural zeolite end user. Worldwide the estimated total natural zeolite production of 3.6 metric tons per year in 2004, construction industry appears to consume about 2.4 metric tons per year, where the bulk of the production is in dimension stones used in building construction and lesser amounts in lightweight aggregate concrete and as pozzolans in concrete.
The use of zeolites in the US concrete industry still lags behind the rest of the world
Approximately 50,000 tons per year of zeolitic tuff is being used for the production of portland pozzolan cements in Germany. Italy consumes yearly about 3 million tons and China about 5 million tones of zeolitic tuffs as a component of blended portland cements. The use of natural zeolite as a pozzolan in the US cement and concrete industry, though not recent, however, is still limited. As early as in 1912, the Monolith Portland Cement Company used zeolitic ash flow tuffs from Tehachapi, California for use as a pozzolan at 25% portland cement replacement level to build the 400-km Los Angeles aqueduct.
Despite their inherent crystalline nature, the fine size and higher surface area of the natural sedimentary zeolites plus their strong cation exchange capacity make them a suitable pozzolan in cements. Zeolites consume the calcium hydroxide component of portland cement hydration by the base exchange process and form cementitious products. Pozzolanic characteristics of many natural fine crystalline zeolites are easily comparable to or, in some instances, even better than their aluminosilicate “glassy” rivals such as fly ash, or ground granulated blast furnace slag.
Since ancient times, zeolite tuffs have been a widely used additive in construction materials. In the cement industry, natural zeolite is a popular pozzolan (reacts with calcium ions or calcium hydroxide in the presence of water) that is used as a cement blending material.
Additionally, zeolite tuffs contain small pours and channels with large internal and external surface areas, a feature that allows them to absorb up to 30 percent of their weight in water (Markiv et al., 2014). Together, these properties provide zeolite additives with the ability to increase the compressive strength of concrete and enhance durability by reducing concrete permeability (Markiv et al., 2014).
A number of studies have examined how to reduce the required amount of cement in concrete production, both from an economic and environmental standpoint. Natural zeolite is one raw material that has proven effective as a partial substitute for Portland cement.
A trial by Sedlmajer et al. (2015) examined the properties of concrete containing natural zeolite as an active admixture in concrete. The researchers composed individual concrete mixes based on gradual increases in the substitution of Portland cement with natural zeolite.
All admixtures (containing zeolite) were compared to a reference concrete where only Portland cement was used (Sedlmajer et al., 2015). The dosage of zeolite, which contained 45 percent clinoptilolite and 35 percent of amorphous phase, ranged from 7.5 – 30 percent of the mass of the cement. The results indicated that zeolite is an active admixture that contributes to the formation of microstructure and to the improvement of required properties of hardened concrete (Sedlmajer et al., 2015).
Researchers concluded that a “finely milled natural zeolite appears to be a suitable raw material in the production of concrete with which it is possible to partially substitute Portland cement” (Sedlmajer et al., 2015, p. 528).
A paper by Jana (2007) examined clinoptilolite zeolite as a pozzolan and replacement to Portland cement in concrete mixtures. Findings indicated that the most significant effects of zeolite are in reduction in chloride permeability, a significant reduction in expansion due to alkali-aggregate reaction, and an improved resistance to acid and sulfate attacks (Jana, 2007).
Ahmadi and Shekarchi (2009) conducted an experiment that tested the effectiveness of zeolite and silica fume in enhancing mechanical and durability properties of concrete. They measured both pozzolanic reactivity and ability to substitute cement in different proportions in concrete mixtures. Experimental tests included slump, compressions strength, water absorption, oxygen performance, chloride diffusion, and electrical resistivity of concrete (Ahmadi & Shekarchi, 2009).
Results indicated that concretes containing zeolite improved and were comparable to or better than concretes prepared with silica fume Ahmadi and Shekarchi (2009).
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