Untersuchung des Einflusses von Gips- und Fasereigenschaften gipshaltiger Bauabfälle auf die Produktfestigkeit von Doppelbodenplatten nach DIN EN 12825 aus dem Taktpressverfahren

RWTH Publications (RWTH Aachen)

Abstract

This dissertation investigates the material recycling of gypsum-containing construction waste for the production of gypsum fiberboards (GFB) against the backdrop of increasing resource scarcity in the gypsum sector. In Germany, approximately 10 million tonnes of gypsum are used annually, currently sourced in roughly equal parts from natural gypsum and flue gas desulfurization gypsum (FGD gypsum). With the planned Coal Phase-Out by 2038, a significant decline in FGD gypsum availability is expected, thereby increasing the importance of recycling strategies. Although gypsum is inherently recyclable multiple times, only about 13 % of the approximately 700,000 Mg of gypsum-containing construction waste generated annually is currently recycled. Existing recycling processes primarily focus on pure, non-impregnated gypsum plasterboards (GPB, Type A), while or impregnated GPB (type H) and fiber-rich gypsum products such as gypsum fiberboards (GFB) are hardly recycled due to their material composition. The aim of this work is to systematically investigate the relationships between material properties of heterogeneous gypsum-containing construction waste, relevant process parameters, and the mechanical performance of gypsum fiberboards. The focus lies on evaluating the suitability of various waste fractions, particularly different GPB types (A, H, DF) and production residues from GFB manufacturing (reject boards, sanding dusts, gypsum slurries), for the production of raised floor panels in accordance with DIN EN 12825. A three-stage experimental and model-based approach was applied to analyze the influence of gypsum and fiber properties on processing behavior and mechanical strength. In the first step, model boards based on pure stucco gypsum with varying fiber materials were produced. A multivariate regression analysis was used to evaluate the reinforcing effect of fiber properties on bending strength. The lignin content was identified as the dominant influencing factor, followed by fiber length, fines content, and fiber content. Together, these parameters explain approximately 83% of the observed variance. While longer fibers increase strength, higher lignin and fines contents as well as increasing fiber content have a negative effect on mechanical performance. In the second step, gypsum-containing construction wastes were wet-calcined, processed into α-hemihydrate suspensions, and combined in a 1:1 ratio with stucco gypsum and a high-strength recycled paper to produce raised floor panels. Panels containing 50 wt.% secondary raw materials achieved breaking loads of 7.7 to 10.7 kN, meeting the requirements of classes 2 to 4 according to DIN EN 12825. These values significantly exceed those of reference panels made from 100 wt.% stucco gypsum. Mineral impurities and hydrophobic additives showed no significant influence on mechanical strength within the investigated range but affected processing conditions. Higher amounts of mineral impurities led to prolonged conversion and pressing times. Materials with increased short fiber content, such as grinding dusts, resulted in reduced panel strength. In the third step, the developed regression model was applied to systems containing α-hemihydrate. The results demonstrate that α-hemihydrate suspensions significantly improve gypsum properties and increase breaking load by 1 to 2 kN (16–23%) at a substitution rate of 50 wt.%. The strength development is strongly governed by interactions between the gypsum matrix and fiber quality. In particular, high-quality fibers exhibit synergistic effects, leading to a substantial increase in panel strength and enabling the full utilization of the strengthening potential of α-hemihydrate. In contrast, this potential remains largely unused when low-quality fibers are applied, resulting in significantly lower strength improvements. Overall, the study shows that the mechanical performance of gypsum fiberboards results from complex and partly nonlinear interactions between fiber and gypsum properties. A purely model-based prediction of breaking load is only reliable for boards made from 100% stucco gypsum. For boards containing 50% recycled gypsum, accurate prediction is not feasible without adapting the model parameters due to the increased material heterogeneity. At the same time, the study demonstrates that previously underutilized waste fractions, including impregnated GPB and fiber-rich GFB waste, can be successfully used to produce high-performance gypsum fiberboards. The findings provide a solid basis for the development of new recycling strategies, optimization of industrial processes, and targeted control of product quality. Thus, this work contributes significantly to establishing a resource-efficient circular economy in the gypsum sector and reducing the use of primary raw materials.

Authors 1

1. Wojciech Walica corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen