# Porcelain Tile Polishing Residue in Concrete as an Additive or Replacement for Portland Cement
## 1 Introduction  
The extensive production of Portland cement, which amounts to about 4.2 billion tons per year, leads to a significant carbon footprint that raises concerns in the cement industry. The main reason for this is the high levels of CO\({}_{2}\) emissions produced during the manufacturing process, which result from the use of high temperatures (around 1450 \({}^{\circ}\)C) and the chemical decomposition of calcium carbonate used as raw material [1]. In this scenario, the use of alternative materials in cement replacement is continuously growing [2]. Porcelain tile polishing residue (PPR) is generated in the final polishing stage of porcelain tile manufacturing; PPR is a fine, white, and moist powder. It is estimated that Brazil alone generates about 60,000 tons of PPR every year [3, 4]. The proper destination of this residue becomes an issue especially in the largest ceramic tiles producers in the world, such as China, Brazil, and Italy [5, 6]. A medium-sized company in Brazil's northeast region produces approximately 200 tons of waste per month [7]; a large company in the southern region generates approximately 1000 tons per week [2]. Thus, the PPR can also become a local issue. Currently, the main destination of PPR is disposal in landfills [5, 8]. PPR is composed of silica (60-70%), alumina (15-20%), as well as magnesium (1-6%) from the abrasive used in the polishing process [7, 8, 9, 10]. Although PPR comprises crystallinequartz, the presence of amorphous quartz has also been reported; this indicates its possible pozzolanic reactivity [8; 9; 11; 12; 13; 14; 15; 16; 17; 18; 19]. Further, PPR is used in concrete because no additional high energy is required for its comminution and sieving because its granulometry is suitable for use as a supplementary material in cementitious systems, with an average diameter of approximately 10 \(\upmu\)m [7]. Previous publications reported the positive influence of PPR in concrete including: a filler effect [8; 9; 11; 19; 20]; a reduction of permeability and porosity [7; 8; 9; 11; 20] and, consequently, increased durability [8; 9]; the discontinuous granulometry between PPR and cement which contributes to the reduction of porosity [9; 12; 17; 19; 21]; PPR enabling of a better bond between the paste and aggregate, thereby acting as an agent that can increase mortar cohesion [9]; pozzolanic activity, where lower amounts of calcium hydroxide and higher C-S-H were found in cementitious materials with PPR in relation to the reference (material without PPR) [17; 19]; an increase in mechanical compressive strength at advanced ages [7; 8; 9; 11; 12; 19; 20]; reduction of autogenous shrinkage caused by slow reactions of pozzolanic activity [8; 12]. In turn, increasing the residue content reduces the workability of the mixture in the fresh state [7; 11; 22]. The high surface area of PPR requires more water than cement or other cementitious mixture compounds to achieve a desirable workability [19]. Furthermore, irregular and angular shapes of PPR particles hinder the effect of rolling motion in concrete [19]. However, this loss of workability can be compensated for by increasing the superplasticizer (SP) content [3; 6]. For cement hydration in the presence of PPR, the greater the cement replacement by PPR, the lower is the heat of hydration released [9; 19] and the faster are the hydration kinetics [19; 20] at early ages (i.e., 24-48 h). The presence of PPR promotes heterogeneous nucleation during cement hydration [8; 9; 19; 20], which accelerates the cement hydration caused by fine particles of the waste [9]. However, the agglomerative (i.e., pozzolanic) activity of the PPR is slower than that of Portland cement, and it results in a lower heat release in the early ages. As durability indicators, cementitious materials incorporating porcelain tile waste showed lower oxygen permeability [12] and lower chloride ion diffusion coefficients [12; 23] in addition to a lower loss of mechanical strength when exposed to accelerated aging tests [12]. [PERSON] et al.