Science & Innovation
Application of natural zeolites
Promising raw material for AAC production
Natural zeolites are tectoaluminosilicate minerals with specific microporous framework structure, where the present voids are filled by ions and water . They are exploited in several countries, e.g., China , Turkey , Greece , or Slovakia , mainly as a molecular sieves  and catalyst support . Thanks to their high specific surface and moderate pozzolanic reactivity, zeolites as a mixture with lime are commonly used as hydraulic binder . Positive influence of natural zeolite on concrete (compressive strength, sulfate corrosion and durability properties) was investigated by various methods in the past [9-12]. It was noticed, that zeolite has a good potential as supplementary cementitious material in concrete . Effect of zeolite on hydration of Portland cement is mostly visible in early- stage of this process . Studies of hydration heat development and mechanical properties of blended cements containing zeolites, confirm the effectiveness of natural zeolite as a binder in terms of its participation in the hydration process, depending on its amount in the blended binders [15-17]. A relatively new direction of zeolite application is its use in the technology of aerated concrete production . This paper presents the application of natural clinoptilolite (also in a compilation with granulated blast furnace slag), dosed on the various stage of autoclaved aerated concrete production. In this way, some important news about microstructure development, compressive strength and hydration rate, which weren’t reported before, can be explained. Further details on the research can be obtained from .
Materials and methods
In investigations two types of Portland cement: CEM I 42.5 R and 52.5 R, natural clinoptilolite (also as a mixture with slag: 10 % of zeolite + 90 % of slag), 2 types of lime (calcium oxide), aluminium powder and industrial sand slurry (with post-production waste) were used. A part of cement CEM I 42.5 R (10-30 % by mass), part of gypsum (50-100 %) and part of the sand slurry (10-30 %) was replaced by natural zeolite. The research program is shown in figure 1. The AAC block-samples (A - C, A’- C’, A’’- C” and ZG0, ZG50) were prepared for designed bulk density class: 520 kg/m³, by mixing ingredients in accordance with established laboratory procedure and manual moulding in a small system (mold: 15 cm x 15 cm x 15 cm). A specimen D was prepared for designed bulk density class: 540 kg/m³ by mixing ingredients according to the company standards and moulding on existing production line (big mold: 6,30 m x 1,74 m x 0,70 m). Production parameters for all recipes were constant: autoclaving cycle (from closing the autoclave to its opening) – 12 h 20 min, time of the proper autoclaving: 8 h 10 min, pressure 11 atm. All tests were carried out at the AAC plant Solbet Stalowa Wola / Poland. Natural clinoptilolite from the Zeocem Slovakia was used. Chemical and phase composition of this mineral presents table 1. Clinoptilolite contains over 50 % of active SiO2. The highest proportion of fractions in zeolite - approx. 90 % - constitute grains from 0-16 μm and 16-32 μm. The chemical composition of the other materials used was previously presented in .
Phase composition (identified under X-ray powder diffraction - XRD), microscopic observation (in scanning electron microscope – SEM) and thermogravimetric analysis (TG/DTG/DTA) for selected AAC samples: 0 (referential without zeolite), D, B’ and A’’, half year after autoclaving process were done. The measurement of compressive strength for all AAC samples was done by the hydraulic testing device. The test were performed according to polish standard, after autoclaving process and 48 h of drying (in 105 °C).
Results and discussion
From the XRD patterns , it can be founded, that specimens produced with zeolite addition have lower amount of Ca(OH)2 with increasing ettringite content. Furthermore, there is less calcium silicates in referential samples than in samples with zeolite. The microstructure of fractured samples after half year of curing was observed under the scanning electron microscope. The particular phases were identified and analysed based on the EDS (the descriptions under the SEM illustrations). The results are shown in figure 2 and figure 3. The reference sample (without zeolite) shows a typical microstructure with more or less developed C-S-H and common autoclaving product – tobermorite (see figure 2). C-S-H occurs mainly in compact gel (see figure 2a) and as a honeycomb (see figure 2b). Tobermorite adopts the shape of a “desert rose” (see figure 2b) placed mainly in the pore space as well as in the form of ribbons. C-S-H in the gel and fibre form is particularly noticeable in the samples with zeolite addition (see figure 3) Similarly, as in reference sample, presence of tobermorite is observed, but it has the form of large, well-formed ribbons and “desert roses”. In contrast to the reference sample, structure seems to be more compacted in the whole AAC with zeolite additive. Hydrogarnets are also visible in all zeolite samples.
AAC mechanical properties
Results of compressive strength measurements and estimated bulk density class for all samples after 2 days of drying in 105 °C (after autoclaving process) are shown in figure 4 and in table 2. All AAC samples achieved mechanical properties required by the manufacturer. The best results in comparison to the referential sample, are observed in a sample composed with 20 % of zeolite added to the sand slurry. Interested is that replacement of CEM I 42,5 R with a high alite content, by 10 % of zeolite gives the same effect on mechanical properties, which means that zeolite can be efficient and cheaper substitute for Portland cement even in a small amount. Due to the specific framework of zeolite (a system of channels and chambers and empty spaces inside the structure), its addition reduces the density of aerated concrete.
The experimental analysis of properties of autoclaved aerated concrete containing natural zeolite as a partial replacement of Portland cement and sand slurry showed a good potential for clinoptilolite as a supplementary cementitious material. Thanks to the relatively good pozzolanic properties, zeolite - by consuming the rest of lime presented in the sand slurry with AAC waste from production - contributes to waste reduction and improves the production efficiency.
- Microscopic observations confirmed the positive effect of zeolite on the properties of concrete by sealing the structure, thanks to the presence of a well-formed C-S-H phase, in the form of honeycomb and compacted gel, as well as in the form of long fibers and an additional amount of tobermorite. X-ray and thermogravimetric analysis confirmed the presence of preferred phases in AAC.
- The compressive strength of mixes containing zeolite was for a few mixes found to be lower than that of a reference sample (with the exception of 10 and 20 % zeolite addition to the sludge), but could still be considered as acceptable. From a technological point of view zeolite can be treated as a promising raw-materials to the AAC production.
Ewelina Grabowska is a graduate of the AGH University of Science and Technology in Krakow. In 2011, she graduated from master’s studies in specialty of binding materials, obtaining the title of a construction materials technologist. After that time, she completed an internship at the BASF Poland. For the next 4 years she continued her academic career at her alma mater, obtaining in 2016 the title of doctor of chemical sciences in the discipline of chemistry. During her doctoral studies, she started working for Benda-Lutz Skawina, where she currently holds the position of head of Research & Development department. She mainly designs a new generation of aluminum powders and pastes for the aerated concrete production. Other scientific interests include cement and concrete chemistry. firstname.lastname@example.org