The role of metakaolin in concrete

(1) Improving the strength of cement slurry and mortar, high strength is one of the indicators of high-performance concrete. One of the main purposes of adding metakaolin is to improve the strength of cement mortar and concrete.

Poon et al. (2001) conducted compressive strength tests on cement slurries with a water cement ratio of 0.3, prepared by replacing Portland cement with 0-20% (mass fraction) kaolin and silica powder. The results showed that the compressive strength of cement slurries containing 5% to 20% kaolin was higher than that of the reference cement at all ages, with cement containing 10% kaolin showing a 20% increase in strength at 28 and 90 days compared to the reference cement. Cement containing 5% to 10% silica powder also showed a 20% increase in strength at 28 and 90 days compared to the reference cement, Its strength at 28d and 90d is equivalent to that of kaolin cement, but its early strength is lower than the benchmark cement. Analysis suggests that this may be related to the severe agglomeration of the silicon powder used and insufficient dispersion in the cement slurry.

(2) Li Keliang et al. (2005) studied the effects of calcination temperature, calcination time, and SiO2 and A12O3 content in kaolin on the activity of metakaolin to improve the strength of cement concrete. High strength concrete and soil polymers were prepared using metakaolin. The results show that when the content of kaolin is 15% and the water cement ratio is 0.4, the 28 day compressive strength is 71.9 MPa. When the content of kaolin is 10% and the water cement ratio is 0.375, the 28 day compressive strength is 73.9 MPa. Moreover, when the content of metakaolin is 10%, its activity index reaches 114, which is 11.8% higher than the same amount of silicon powder. Therefore, it is believed that metakaolin can be used to prepare high-strength concrete.

Qian Xiaoqian et al. (2001) studied the axial tensile stress-strain relationship of concrete with kaolin content of 0, 0.5%, 10%, and 15%. They found that with the increase of kaolin content, the peak strain of the axial tensile strength of concrete significantly increased, and the tensile elastic modulus remained basically unchanged. However, the compressive strength of concrete significantly increased, and the compressive strength ratio correspondingly decreased. When the content of kaolin is 15%, the tensile strength and compressive strength of the concrete are 128% and 184% of the reference concrete, respectively.

Cao Zhengliang et al. (2004) found in their study on the strengthening effect of ultrafine powder of metakaolin on concrete that, under the same fluidity, mortar containing 10% metakaolin increased its compressive strength and flexural strength by 6% to 8% after 28 days. The early strength development of concrete mixed with metakaolin was significantly faster than that of standard concrete. Compared with the benchmark concrete, the concrete containing 15% metakaolin has an 84% increase in 3D axial compressive strength and an 80% increase in 28d axial compressive strength, while the static elastic modulus has an 9% increase in 3D and an 8% increase in 28d.

Huang Zhan et al. (2008) studied the effect of different blending ratios of metakaolin and slag on the strength and durability of concrete. The results show that the addition of metakaolin to slag concrete improves both the strength and durability of the concrete. The optimal ratio of slag to cement is around 3:7, resulting in ideal concrete strength. The arch difference of composite concrete is slightly higher than that of single slag concrete, due to the volcanic ash effect of metakaolin. Its splitting tensile strength is higher than that of the benchmark concrete.

Yang Fengling et al. (2011) used equal amounts of metakaolin, fly ash, and slag to replace cement, and separately mixed metakaolin with fly ash and slag to prepare concrete. The workability, compressive strength, and durability of the concrete were studied. The results showed that when kaolin was used to replace 5% to 25% of cement in equal amounts, the compressive strength of concrete at all ages was improved; When kaolin is used in equal quantities to replace cement by 20%, the compressive strength at each age is ideal. The strength at 3d, 7d, and 28d is 26.0%, 14.3%, and 8.9% higher than that of concrete without kaolin added, respectively. This indicates that for Type II Portland cement, the addition of metakaolin can improve the strength of the prepared concrete.

Zhang Chengbo et al. (2012) used steel slag, metakaolin, and other materials as the main raw materials to prepare geopolymer cement to replace traditional Portland cement, achieving the goal of energy conservation, consumption reduction, and turning waste into treasure. The results showed that when the steel content and fly ash content were both 20%, the strength of the test block at 28 days reached a very high level (95.5MPa). As the amount of steel slag added increases, it can also play a certain role in reducing the shrinkage of geopolymer cement.

Chen Guocan (2010) adopted the technical route of “Portland cement+active mineral admixture+high-efficiency water reducing agent”, magnetized water concrete technology and conventional preparation process, and conducted preparation experiments on low-carbon ultra-high strength stone slag concrete using locally sourced raw materials such as stones and slag. The results indicate that the appropriate dosage of metakaolin is 10%. The mass to strength ratio of cement contribution per unit mass of ultra-high strength stone slag concrete is about 4.17 times that of ordinary concrete, 2.49 times that of high-strength concrete (HSC), and 2.02 times that of reactive powder concrete (RPC). Therefore, ultra-high strength stone slag concrete prepared with low dosage cement is the direction of concrete development in the low-carbon economy era.

(3) After adding kaolin with frost resistance to concrete, the pore size of the concrete is greatly reduced, improving the freeze-thaw cycle of the concrete. Feng Naiqian (2002) found that under a certain number of freeze-thaw cycles, the elastic modulus of the concrete sample with a 15% kaolin content at 28 days of age is significantly higher than that of the reference concrete at 28 days of age. The composite application of metakaolin and other mineral ultrafine powders in concrete can greatly improve the durability performance of concrete.