Researchers have gained new insights regarding the use of supplementary cementitious materials commonly used in concrete mixtures. A popular practice since the ’70s, adding SCMs to cement mixtures has become the industry standard. Depending on the desired effect, concrete developers choose SCMs such as fly ash, ground granulated blast furnace slag, silica fume, calcined clay, calcined shale and natural pozzolans based on the materials’ various properties.
Typically, SCMs make up only a small percentage of the total concrete recipe. Although more economical than other additives, SCMs can disappoint in terms of overall performance during a structure’s early stages when compared to concrete with a lower percentage of SCMs. Additionally, for concrete with more SCMs than typically used, it is difficult to predict the mechanical properties along with durability in these structures. However, most in the industry understand SCMs increase concrete’s long-term strength even if early-age strength is weaker.
The combined findings of research from 2011 and 2013 explain that calcining kaolinite at temperatures higher than 750 degrees Celsius can lead to reduced metakaolin reactivity. Instead, aim for lower than 750 degrees Celsius but higher than 600 degrees Celsius to effectively dehydoxylate MK and lead to SCMs with good pozzolanicity.
Concrete made of 5 to 15 percent MK, clay mineral kaolinite in a dehydoxylated form, improves the pore microstructure of concrete and results in exterior absorption being reduced at first, according to a 2013 study. Another study from 2011 found that MK has the lowest chloride penetration when compared with other self-compacting concrete mixes.
Silica fume and other fine SCMs are difficult to effectively mix into cement batches, which leads to the underutilization of the material; however, researchers discovered in 2012 using ultrasonic vibration to blend silica fume with cement pastes created improved mixtures.
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