Mechanical and physical enhancement of gypsum composites through a synergic work of polypropylene fiber and recycled isostatic graphite filler Nelson Flores Medina a,b,⇑ , M. Mar Barbero-Barrera a a Departamento de Construcción y Tecnología Arquitectónicas, E.T.S. Arquitectura, Universidad Politécnica de Madrid, Avda. Juan de Herrera 4, 28040 Madrid, Spain b Escuela de Arquitectura, Ingeniería y Diseño, Universidad Europea de Canarias, C/ Inocencio García 1, 38300 La Orotava, Canarias, Spain highlights  Synergy between PPF and IGF has been studied in gypsum-based composites.  Energy of fracture and flexural strength are increased with IGF in composites with and without fibers, especially in those with fibers achieving an optimization of the fiber.  Thermal conductivities are increased with IGF.  IGF as addition in gypsum-based panels and drywalls reinforced with PPF increases the mechanical strength and the durability of the products which depend on these mixtures. article info Article history: Received 4 June 2016 Received in revised form 16 September 2016 Accepted 14 November 2016 Keywords: Gypsum-based composites Reinforced gypsum Polypropylene fiber Synthetic graphite Isostatic graphite filler abstract This paper deals with the enhancement of gypsum composites through the synergic work of polypropy- lene fibers (PPF) and isostatic graphite filler (IGF) additions. IGF is a waste by-product that has shown a good performance in plain gypsum, and in this research it is demonstrated that it can also be revalorized as gypsum matrix activator in reinforced gypsum-based composites. Hence, several series of gypsum composites with a 0–0.6% volume of PPF and with 0–5–10–15–20–25% of graphite addition by weight substitution of gypsum were developed and the mechanical and physical properties tested. The microstructure of the composite and the contact surface with fiber are also analyzed by Scanning Electron Microscopy (SEM). The progressive increase of graphite addition modifies the composite prop- erties, increasing mechanical resistances (compressive strength, flexural strength, Young’s modulus, toughness), density and thermal conductivity and on the other hand, reducing their porosity and their water absorption. The mechanical performance obtained with PPF and IGF is similar to the increase of the fiber length or the volume fraction of the fiber added in gypsum-based composites, which can favor the manufacture of reinforced gypsum composites. This improvement is achieved through the activation of the gypsum matrix, through the nucleation of the hydrated calcium sulfate in contact with IGF, and the increase of fiber/matrix bond when IGF is added. Gypsum composites with IGF and PPF addition can be used in boards and renders in the construction industry, obtaining more durable and sustainable materials. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Gypsum-based composites are widely used in building con- struction because of their low cost, their easy manufacture and their many benefits for house living conditions, since gypsum is a good hygro-thermal regulator [1]. Gypsum raw material can be obtained from quarries, from construction debris or from waste by-product from the processing of phosphate for fertilizers (ie. phosphogypsum), and it can easily be recycled [2–4]. As gypsum is one of the weakest and softest binders used in building, it can be blended with cement or slags to obtain a better mechanical per- formance, or to increase its water resistance since it can be dam- aged by water hydrolysis [5]. To avoid the degradation of http://dx.doi.org/10.1016/j.conbuildmat.2016.11.073 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Departamento de Construcción y Tecnología Arqui- tectónicas, E.T.S. Arquitectura, Universidad Politécnica de Madrid, Avda. Juan de Herrera 4, 28040 Madrid, Spain. E-mail address: nelson@arquingenieros.com (N. Flores Medina). Construction and Building Materials 131 (2017) 165–177 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat