Objective With the increasing use of radio， television， mobile phones， computers， artificial intelligence， and 5G technology，our lives have become more convenient， but it has also inevitably resulted in indoor electromagnetic radiation pollution. The increase in electromagnetic density has changed our indoor electromagnetic environment， potentially leading to serious electromagnetic interference， damage to buildings， harm to electrical equipment， and even health risks posed to humans. Therefore， it is necessary to develop building materials to reduce electromagnetic radiation and improve indoor electromagnetic environment.Generally， electromagnetic shielding and absorbing materials are used. Shielding materials only reflect electromagnetic waves without weakening or eliminate them， while electromagnetic wave-absorbing materials convert the energy from electromagnetic waves into other forms such as heat during internal transmission， thereby attenuating the waves and improving building safety.For civil constructions， the requirements for effective electromagnetic wave-absorbing materials should be inexpensive and easy for construction. Graphite， abundant in nature， inexpensive， and exhibiting good electrical and thermal properties， emerges as the main candidate as a wave absorbing agent in construction. Plaster， which is neither load-bearing like concrete nor decorative like architectural paint， is particularly well-suited as a substrate due to its thick application， large surface area coverage，and ease of construction.

Methods To prepare cost-effective wave-absorbing materials for civil buildings， different types of graphite powders， including natural flake graphite， graphite microchips， and homemade expanded crushed graphite， were mixed into gypsum plaster. Their effects on physical properties， strength， and wave absorption were compared. The structure of the graphite-gypsum plaster composites was characterized using X-ray diffraction （XRD）， scanning electron microscopy （SEM）， and vector network analysis.

Results and Discussion XRD analysis showed that the characteristic diffraction peaks of the graphite powder were at 26. 5° and 54. 7°， corresponding to the （002） and （004） crystal planes， respectively， but with variations in peak strengths and widths.Natural flake graphite exhibited strong and sharp diffraction peaks， indicating large grain size and complete crystallization. SEM images confirmed its morphology of thick flakes around 100 um in size， made up of closely stacked layers resembling fish scales. XDR patterns of graphite microchips showed irregular， thin， curled， and fragmented particles with weak， broad diffraction peaks， indicating incomplete crystallization. Expanded crushed graphite， consisting of granular particles with fragmented lamellae， demonstrated intermediate properties between the other two types of graphite. Incorporating graphite powder increased the standard water consumption for gypsum plaster diffusivity. However， the extent of the increase varied depending on the type of graphite powder used. The difference was caused by variations in the specific surface area， particle size， and morphology.Also， their hydrophilicity differences caused by the varying number of surface oxides on the particles contributed to this variation. Natural flake graphite， with its large grain size and compact structure， showed a slow increase in water consumption， while expanded crushed graphite， with smaller particles and a loose lamellae structure， showed a rapid increase due to higher specific surface area and surface oxides after chemical treatment and physical expansion. Graphite powder also accelerated the setting time of gypsum plaster， shortening both the initial and final setting times， which would be disadvantageous for mortar construction. To mitigate this， a retarder must be added to extend the setting time to meet the required standards. For instance， gypsum plaster mixed with 8% expanded crushed graphite powder had an initial setting time of only 20 minutes. However， after adding 0. 1% wt citrate retarder， the initial setting time was extended to 115 minutes， and the final setting time to 240. 5 minutes， meeting the GB/T 28627 standards. The addition of graphite powder reduced the bulk density of gypsum plaster. When 10% natural flake graphite，10% microchips with particle sizes of 30-50 μm， or 5% expanded crushed graphite was added， the bulk density of the gypsum plaster fell below 1000 kg/m3， meeting the density requirements for lightweight base-layer gypsum plaster.Expanded crushed graphite demonstrated advantages as a lightweight filler due to its loose graphite lamella structure and low grain density， which was a result the high-temperature expansion. However， the addition of graphite powder negatively impacted the mechanical properties of the gypsum plaster. The reduction in strength was attributed to increased porosity from higher water consumption and the poor bonding between graphite particles and calcium sulfate crystals. When less than 8% natural flake graphite or expanded crushed graphite was added， the resulting gypsum plasterboards met the GB/T28627 standards for base-layer gypsum plaster， with compressive strengths above 4 MPa and flexural strengths above 2 MPa. At 10% graphite content， the plaster met the GB/T28627 standards for lightweight base-layer gypsum plaster， achieving more than 2 MPa in compressive strengths and more than 1 MPa in flexural strengths. When 8% expanded crushed graphite was added， the tensile adhesive strength reached 0. 45 MPa， complying with the GB/T28627 requirements for both the base-layer and the lightweight base layer gypsum plaster. Graphite powder significantly improved the wave-absorbing properties of gypsum plasterboards. Compared to natural flake graphite and graphite microchips， adding 8% expanded crushed graphite to the plaster yielded the best waveabsorbing performance， achieving a bandwidth of 10. 8 GHz with RL<-5 dB in 2-18 GHz frequency range， with at least 68. 38% of electromagnetic waves absorbed. Moreover， the setting time， bulk density， and mechanical properties all met the GB/T28627 national standards for lightweight base-layer gypsum plaster.

Conclusion Homemade expanded crushed graphite powder is the most suitable， cost-effective wave absorber among the studied

graphite powder for civil construction materials.

Keywords：graphite powder； gypsum plaster； wave absorption； mechanical properties

JING Min， WEI Qin， CHEN Xi， et al. Effect of graphite powder absorbing agent on gypsum plaster properties［J］. China Powder

Science and Technology，2024，30（6）：1−9.