Katarzyna Łaskawiec, PhD Eng, is employed at the Łukasiewicz - Institute of Ceramics and Building Materials, leading the Autoclaved Aerated Concrete (AAC) and Precast Research Group. She has authored and co-authored a number of technical publications in trade journals and conference proceedings. She specializes and engages in research related to the technology and application of AAC.

 

Piotr Gębarowski, PhD in 2008, works at the Łukasiewicz - Institute of Ceramics and Building Materials in the Autoclaved Aerated Concrete and Precast Research Group. He is author of more than 50 publications on the broad subject of building materials in domestic and foreign journals and presentations at scientific and industry conferences. He is Secretary in KT 193 and Representative in KT 308 with the Polish Committee for Standardization. 

Piotr Zając, MA in Sociology and Civil Engineering. He has been working at Łukasiewicz Research Network for more than 10 years as chief engineer and technical manager.

 

Jaroslaw Stankiewicz graduated from the Warsaw University of Technology, Faculty of Transportation, and has worked at the Łukasiewicz - Warsaw Institute of Technology for more than 35 years. He is professionally engaged in waste management technologies and technologies applied in the area of rock mining. He is co-author of the implementation of technology for the production of artificial aggregates from waste materials.

Modularity-based technologies are used to build both large buildings in city centers and small buildings in the provinces. In city centers, carrying out construction works is often burdensome for residents and delivering materials is expensive and logistically complicated. In provincial locations, especially in locations without previous development in the vicinity, it makes it possible to limit the time and scope of construction work and not imprint on local flora and fauna. Public utility facilities, hotels or large-format stores are being built, and single-family houses, consisting of single or groups of segments joined together, are popular.

Modular construction system technologies use adapted techniques for belt fabrication of industrial products. As a rule, more than 80% of the construction work is done off-site. In the production plant, windows and doors, floors partition walls, roof with covering, as well as installations (electrical, plumbing, heating, including, for example, underfloor heating systems) are assembled on-site. The finish and equipment of the modules depends on the ordered standard. Modular technology is characterized by much better material management, reduction and better use of waste materials, and also reduces CO₂ emissions. As a rule, a higher quality of workmanship and a lower unit cost are achieved.

An important disadvantage of houses made of large-size modules is the need to assemble the building on the construction site using a crane with a high load capacity and reach.  While spatial modules made of wood or on a steel skeleton are justified both economically and technologically, concrete modules are extremely rarely. Their scope of use is mainly limited to installation in buildings in the centers of large cities. This is due to the cumbersome nature of manufacturing and the significant advantage in the manufacture, transportation and installation of flat prefabricated elements.

Autoclaved aerated concrete (AAC) as a lightweight material seems to be advantageous for the purpose of modular prefabrication, but it has significant limitations resulting from relatively low strength and production technology by growing in forms. AAC in applications for the construction of single-family houses and low-rise buildings has sufficient strength and is characterized by thermal insulation limiting the formation and minimizing the impact of thermal bridges.

In the field of prefabrication for small construction sites, large-size prefabrication of AAC should use blocks with dimensions and weight that allow transport with not very specialized trucks and assembly using cranes with a small reach and load capacity. Currently, similar means of transportation are used to transport pallets with AAC blocks. It is unreasonable to project spatial modules or large-size prefabricated elements - both production and transportation would be incompatible with the economically justified production process, as well as the properties of the material. Spatial modules, due to their large size and weight, would require specialized cranes, the availability of which is limited. The target solution should be modularity of elements and geometry of buildings enabling repeatable realizations with the possibility of expansion. A very good solution is the elements that can be installed using HDS trucks, which are popular in construction transport. Small objects, due to their geometry, allow installation at small overhangs, placing transport vehicles right next to the erected building.

The article presents the possibility of using artificial aggregates for the above-mentioned AAC elements. The technology presented is a process for obtaining a product with the expected properties within a narrow range of values. The result is mainly knowledge that allows further use of the technology in the processes of disposal of various groups of waste. The development of an effective AAC production technology based on aggregates from waste raw materials will bring benefits of both economic nature and those related to the protection of the natural environment.

Currently developed concrete-based construction in a large-size modular system focuses on a material that should be characterized above all by high strength, suitable for the intended transport and method of installation. As well as high thermal insulation.

The plan to develop an aggregate that can contribute to obtaining a product with appropriate functional properties will guarantee greater production efficiency and competitiveness of products on the market.

Lightweight artificial aggregates for AAC products

In the production technology of AAC, aggregates with a high content of silica are used, which are the main material forming the structure of the product. Due to the convergence of the main building material of the aggregate structure used for the production of AAC products and lightweight artificial aggregates, steps have been taken to use these aggregates for AAC modular elements.

Determination of the composition of mixtures of mineral powder with sludge as a material for obtaining artificial aggregates

The base recipe forming the basis for the modification contained 50% hydrated sludge, 40% mineral material containing more than 95% silica and 10% flux, which was glass waste mainly from recycled packaging. The artificial aggregate was characterized by a homogeneous structure with a strength 2x that of expanded clay and environmental safety, despite sewage sludge being the largest mass component of aggregates containing dangerous substances. Flux (glass) was an additive that allowed the aggregate structure to form at temperatures lower than the melting point of its main building block silica.

The concept of designing the composition of the modified aggregate was based on the assumption of using traditional mineral components in such an amount that the chemical composition of the new mixture would correspond to the chemical composition of the base aggregates. Hence, it was necessary to analyze the chemical composition of alternative natural raw materials in order to develop aggregate formulations containing components other than the base aggregate, but with a similar percentage of silica and components performing flux functions.

The main premise of the maximum utilization of raw materials, in particular waste raw materials, recipes with different percentages of raw materials were proposed. The basis for the production of artificial aggregates were:

·       Sludge

·       Glass from municipal waste

·       Silica Osiecznica

·       Granite waste

·       Ash from coal power industry

Table 1: List of samples and composition of the mixture for the production of lightweight aggregates using waste raw materials

For each designed formula (Table 1), to ensure the reliability of the results and eliminate random errors due to the scattering of raw material properties, the effectiveness of mixing methods and the thermal process, 3 samples each were prepared, which is particularly important when using waste materials.

Qualitative determination of the process parameters for the preparation of granules for the thermal process

An important step in preparing the mass for firing is the mixing and granulating process. The first has the task of homogenizing the mixture for the production of granules, in order to obtain a minimum dispersion between the properties of lightweight aggregates obtained from a single earn.

The mixing process is carried out in two stages:

·       Mixing of dry ingredients

·       Mixing of dry ingredients and sludge

The purpose of the granulation process is to form the mass before firing in such a form that after the thermal process, aggregate in the cubic form with the desired grain size can be obtained. In the case of aggregate intended for AAC products, the preferred range is 0-2 mm.

An important process parameter before granulation is the humidity of the mixture indirectly related to its stickiness, which is a common in the case of mixtures containing sludge.

The method of making granules was implemented in stages:

·       Weighing the individual ingredients for the production of granules

·       Moving the dry mixing process

·       Moving the process of mixing dry ingredients and sludge

·       Forming of granules

Each of the individual processes was carried out in three repetitions, for testing the properties of the aggregates, samples were taken after averaging three batches, so that the properties of the semi-finished products were representative.

Table 2: Test results of the granulation process in a roller granulator

Based on the research results, it was concluded that a necessary condition for obtaining granules is to reduce the humidity content of the mixture.

Based on the above work, the following assumptions were made:

·       Mineral components should be characterized by minimum humidity content, so that free transport is possible

·       The technological line should have a system monitoring the total humidity content of the substrates, so that it is possible to adjust the process parameters for the mixing and granulating operations

·       The pelletizing system should be connected to the heat input from the tube furnace

·       The properties of individual components and the quantitative structure of fractions for the production of lightweight aggregates affect all parameters of the granulation process for firing. In each case of changing the recipe and properties of substrates, the parameters of preparation of aggregates for firing should be verified.

·       From the point of view of the possibility of producing lightweight aggregates, at the stage of preparing the firing process, it is necessary to define two stages of the process - mixing and granulation. The first is to obtain products with repeatable properties, the second is decisive for the formation of granules that meet the condition of repeatable shape and non-adhesive structure. The obtained results indicate that the decisive parameter of the process is the humidity of the components and the mixture at each stage of the production process.

Determination of the parameters of the sintering process

The parameters of the sintering process on the aggregate structure are sintering temperature, annealing time and total process time. The sintering temperature for base formulations is 1150°C. In the case of using a new raw material, it is advisable to verify the entire process both in terms of sintering temperature and total process time in order to determine the possibility of reducing the energy intensity, thus ultimately reducing production costs.

Table 3: Results of the aggregate sintering process for a variable firing curve

The basic conclusion is that annealing time has a significant effect on the mechanical properties of the aggregate. Increasing the annealing temperature is less expedient and, as a result, has no economic benefit.

Testing the properties of the obtained lightweight aggregates

An objective indicator for evaluating the design and production process of lightweight aggregates is property tests performed according to relevant standards. For lightweight aggregates, the main indicator tests concern density, absorbability and crushing strength.

Table 4: Results of basic properties of lightweight aggregates

At the stage of the granule preparation process, a cycle of adjustment of process parameters was carried out in order to obtain a maximum fraction of 0-2mm, which is particularly useful in the technology of aggregates for AAC. The fly ash sample made expands the possibilities of using various materials including UPS to be used as a substitute in the production of aggregates according to the proposed technology.

The basic conclusion is that the use of other wastes, e.g. granite powder in the mix, increases the density and mechanical strength of the aggregate obtained from it and reduces its absorbability in relation to aggregates obtained without its use.  

Research has shown the possibility of completely replacing the glass flux and lime powder with other wastes. Technological tests of making modular wall elements with the use of lightweight artificial aggregates.

Technological tests of modular elements made of AAC using lightweight artificial aggregates

In the technological tests of AAC production, it was assumed that the base recipe would be based on the modified universal SW technology. It was decided to successively replace part of the original aggregate (mixed sand and water) with Gransil aggregate in weight amounts: 10, 20, 25, 30 to even 50%.

It was assumed that:

·       the initial temperature of the concrete mass after pouring will be about 35-40°C;

·       spillage of the concrete mass about 120-135 mm;

·       the temperature of initial maturation and binding of the mass will be around 50-60°C;

·       growth of concrete mass should guarantee appropriate density of the product due to its height, there should be no scratches and cracks in the top layer. The mass after rising should not settle. An important factor at the moment of rising is compatibility: the rate of hydrogen release with the initial rising of the mass, the appropriate heat balance, properly selected raw materials and the moment of hardening of the mixture.

The setting of the concrete mixture occurs in stages and in the first period up to about 30 minutes is achieved by crystallization of portlandite and ettringite. The resulting crystalline products complete the setting process and give the aerated concrete a low strength.

The next technological stage is the autoclaving process. The autoclaving process consists of four phases. The time of the first phase - autoclave blowing was set at 5-7 minutes. The period of pressurization to maximum pressure (phase II) and depressurization (phase IV) to atmospheric depends on the maximum pressure in a given cycle and will be approximately 120 - 180 minutes. The time of the third phase - maintaining a constant pressure of 1.1 MPa of saturated steam will be approximately 480 minutes.

Hydrothermal treatment is to ensure optimal thermo-humidity conditions for the course of chemical reactions between the binder and aggregate in the concrete mass and the formation of low-basic hydrated calcium silicates of the CSH(I) type as a result of these reactions, which determine the final properties of the finished product. Considering the conditions for the formation of new chemical compounds and changes in the phase composition of the cake during autoclaving, it can be said that the intensification of these reactions occurs at temperatures above 170°C.

Based on the analysis of the results of the performance properties of the obtained AAC elements on a semi-technical scale, two recipes were selected according to the assumptions:

·       aggregate - ground sand in a ball mill in a water environment 60% and Gransil aggregate (fraction with grain sizes: 0-0.1; or 0.1-2) accounts for about 10%. The remaining 30% is binder - blended cement and lime (balance).

·       aggregate - ground sand in a ball mill in a water environment 50% and Gransil aggregate (fraction with grain sizes: 0-0.1; or 0-2) accounts for about 20%. The remaining 30% is binder - blended cement and lime (balance).

General construction assumptions:

o   assumed exposure class according to EC2 - XC1; corrosion source - carbonation; reliability class RC2;

o   AAC 500 class (to be finally agreed during the implementation phase at the production plant);

o   reinforcement in the form of mesh reinforcement made of non-rusting X2CrNi12 steel or standard concrete reinforcing bars with a characteristic strength of min. 450 MPa (anti-corrosion coated), or non-metallic fiberglass rods;

o   it is expected that the element in the wall will be loaded mainly vertically, the eccentricity will be small, the resultant force will most often not extend beyond the core of the cross-section;

o   standard assumptions provide for the compression operation of the element as non-structurally reinforced; meshes ensure the integrity of the element during transport and installation, in the embedded element act as reinforcement to support the integrity of the slender wall;

o   reinforcement with welded meshes should be characterized by a weld load capacity of min. 25% of the load capacity of the thicker of the combined bars;

o   the adopted minimum thickness of the lagging of 3 cm provides a minimum fire resistance class REI 30, assumed in the calculations lagging deviation of 0.5cm;

o   the width of modular elements from AAC is assumed equal to 60 cm; it results from the maximum useful height of elements in the forms, this direction facilitates the cutting of AAC from forms into elements during production.

Physical properties for ACC were tested after the autoclaving process (Table 5).

Physical properties were tested according to the following standards: density according to PN-EN 772-13, load capacity under dominant longitudinal load according to PN-EN 1740, shrinkage according to PN-EN 680, thermal conductivity according to PN-ISO 8301.

Table 5: Functional properties AAC

Conclusions

The results of tests of autoclaved aerated concretes (AAC), produced according to the developed technology, showed that using Gransil aggregate, AAC is obtained with strength parameters as when using only sand. The shrinkage of ABK with the use of Gransil aggregate is lower than in the reference concrete, and the thermal conductivity coefficient λ AAC is favorably lower.

References

[1]    fib Bulletin 60, “Prefabrication for affordable housing”

[2]    Jatymowicz H., Siejko J., Zapotoczna-Sytek G.: Technologia autoklawizowanego betonu komórkowego. Arkady, Warszawa, 1980.

[3]    Zapotoczna-Sytek G., Balkovic S.: Autoklawizowany beton komórkowy. Technologia. Właściwości. Zastosowanie. PWN, Warszawa, 2013.

[4]    Kotynia R., Wymiarowanie i kształtowanie wybranych konstrukcji betonowych ze zbrojeniem niemetalicznym, XXXIII Ogólnopolskie Warsztaty Pracy Projektanta Konstrukcji, Szczyrk, 6-9.03.2018 r.

[5]    EN 12602:2016-11 Prefabricated reinforced components of autoclaved aerated concrete

[6]    EN 1990:2004 Eurocode. Basis of structural design

PN-EN 1990:2004/NA:2010, PN-EN 1990:2004/A1:2008, PN-EN 1990:2004/AC:2010, PN-EN 1990:2004/Ap1:2004, PN-EN 1990:2004/Ap2:2010

[7]    EN 1991-1-1:2004 Eurocode 1:: Actions on structures - General actions - Densities, self-weight, imposed loads for buildings

EN 1991-1-1:2004/NA:2010, PN-EN 1991-1-1:2004/AC:2009, PN-EN 1991-1-1:2004/Ap1:2010

[8]    EN 1991-1-2:2006 Eurocode 1: Actions on structures. General actions - Actions on structures exposed to fire

EN 1991-1-2:2006/NA:2010, PN-EN 1991-1-2:2006/AC:2009, PN-EN 1991-1-2:2006/Ap1:2010

[9]    EN 1991-1-3:2005 Eurocode 1: Actions on structures - General actions. Snow loads

EN 1991-1-3:2005/NA:2010, PN-EN 1991-1-3:2005/AC:2009, PN-EN 1991-1-3:2005/Ap1:2010

[10] EN1991-1-4:2008 Eurocode 1: Actions on structures - Part 1-4: General actions -Wind actions

EN 1991-1-4:2008/NA:2010, PN-EN 1991-1-4:2008/A1:2010, PN-EN 1991-1-4:2008/AC:2009, PN-EN 1991-1-4:2008/Ap1:2010, PN-EN 1991-1-4:2008/Ap2:2010

[11] EN 1992-1-1:2008 Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings

PN-EN 1992-1-1:2008/NA:2010, PN-EN 1992-1-1:2008/Ap1:2010

[12] EN 1992-1-2:2008 Eurocode 2: Design of concrete structures - Part 1-2: General rules - Structural fire design

PN-EN 1992-1-2:2008/NA:2010, PN-EN 1992-1-2:2008/AC:2008, PN-EN 1992-1-2:2008/Ap1:2010