What are the applications of natural zeolite in the field of building materials?(1)

Natural zeolite is a kind of shelf silicate mineral, which is very widely distributed in the earth's crust. At present, zeolite deposits have been found in more than 40 countries in the world, including the United States, Japan and Russia, and the total number of deposits reaches more than 1,000, and more than 40 kinds of zeolites have been found. 

In China, zeolite resources are also quite abundant, and widely distributed and inexpensive, with origins in more than 20 provinces and regions, including Zhejiang, Hebei, Shandong, Henan, Heilongjiang, Liaoning, Jilin, and Inner Mongolia. China has found a large number of zeolite varieties of plagioclase deposits, in addition to square zeolite, grossular zeolite, sheet zeolite, sodium zeolite, pyrochlore and turbidite and many other varieties. 

Since zeolite has the characteristics of light and fragile, low hardness, high void, large internal and external surface area, and good ion exchange, adsorption, catalysis, heat resistance, acid resistance, radiation resistance, it has been widely used in many fields such as building materials industry, petroleum and chemical industry, environmental protection, agriculture and animal husbandry, light industry and cutting-edge technology.

First, Structure and performance of natural zeolite

Natural zeolite is a class of silica-aluminate porous crystal materials, consisting of SiO2, Al2O3, H2O, Na2O, K2O and CaO, etc. The chemical formula is: Ax/n(SiO2)(AlO2)x-mH2O, where A is a cation, x is the number of cations, n is the number of valence, m is the number of water molecules. Silica-oxygen tetrahedra and alumina-oxygen tetrahedra are the most basic structural units that form the zeolite skeleton, where the alumina-oxygen tetrahedra carry a negative charge, giving the zeolite an anionic skeletal structure that requires additional cations to balance. 

However, since the cations are weakly bound to the skeleton, they are highly susceptible to exchange interactions with other cations, which is the reason for the ion-exchange properties of zeolites. In the skeleton structure of zeolite, there are many cavities and pores, which allow molecules of a certain size to move freely, and cations and water molecules are distributed among them. When the water molecules inside the zeolite are removed due to the change of external conditions, the skeleton structure of zeolite is not destroyed, but forms open cavities that can be used to adsorb a large number of molecules, making it have adsorption properties.

Second, the application of natural zeolite in the field of building materials

The main application area of zeolite is the building materials industry, and more than 40% of the world's zeolite resources are used in cement and its products. Initially, zeolites were used in the cement or concrete industry in order to reduce the cost of cement production and improve the strength, permeability and frost resistance of concrete, but with the progress and development of science and technology, the application of zeolites has become more and more extensive, and in the field of building materials, it also involves coatings, ceramics and even rubber and plastic materials.

Application in cement and concrete

Zeolite can be used as cement admixture to reduce cost, increase grade and improve product performance. As early as 1912, the United States used zeolite in cement for construction. 

Since 1978, China has made important progress in the research of using natural zeolite as concrete admixture, and natural zeolite has become a new type of concrete admixture with the widest application in recent years. Using the ion exchange properties of zeolite, concrete alkali-aggregate reaction can be inhibited by adding zeolite. Feng Naiqian of Tsinghua University has done a lot of research in this area and proposed a mechanism for natural zeolite to inhibit alkali-aggregate reaction by ion exchange. 

Feng Xiaoxin et al. studied the effect of the addition of natural zeolite on soluble alkali and effective alkali in cement, and analyzed the mechanism of zeolite inhibition of alkali-aggregate reaction, and found that zeolite reduces the amount of soluble alkali by ion exchange, and reduces the amount of effective alkali by ion exchange and improving the absorption capacity of C-S-H gel for Na+ and K+, while the amount of alkali contained in zeolite itself does not affect its alkali-aggregate reaction inhibition effect. Based on the ion-exchange properties of zeolites, the use of zeolite-doped cement materials for the removal of ammonia nitrogen and heavy metal ions has also been reported.