# Porcelain Tile Polishing Residue in Concrete as an Additive or Replacement for Portland Cement
## 3 Results and Discussion
### Hardened State Properties  
#### 3.2.1 Compressive Strength  
Compressive strength results are shown in Figure 5. No improvements in compressive strength at 7 days were achieved when PPR replaced cement; however, better results were obtained at both 28 and 91 days. There were better resistances than the reference sample with the addition of PPR at all evaluated levels. Figure 4: Workability values of all mixes by slump test. Figure 5: Compressive strength of the concretes. At 7 days, samples with cement replacement by PPR showed a reduction in compressive strength as the waste content increased. The reactivity of PPR is useful at advanced ages, usually for greater than 28 days of hydration. [PERSON] and [PERSON] [9] noted that higher strengths were achieved at more advanced ages, where this increase was proportional to the amount of PPR incorporated into the mixture; this shows the potential pozzolanic effect of the material. Accordingly, [PERSON] et al. [4] observed that replacing 10% cement with PPR resulted in strengths relative to the reference mixture (i.e., without waste) of 90%, 95%, and 98%, respectively, at 28, 91, and 365 days. The compressive strength at 7 days decreased by 2.29% when PPR replaced 10% of cement, 3.98% for S20, 11.61% for S30, 29.80% for S40, and 40.12% for S50. The values of compressive strength for S10, S20, and S30 are relatively close to the reference, probably because of the lower substitution rates being associated with a better packing action [9; 11; 19; 20]. Substitution contents higher than 30% did not present a beneficial effect, which results in the lower resistance rates at 7 days because of the removal of the cement, which is the main reactant. An inverse behavior was observed in the mixtures containing PPR compared to the mixtures when PPR replaced cement. The compressive strength increased with the addition of PPR. This inverse behavior occurred because of better compactness caused by the high ratio of powder material consumption and aggregate consumption, which enhanced the filler effect of PPR. The addition of PPR to concrete contributed to an increase in the average compressive strength at 7 days of 24.52% for 10% PPR (A10), 37.59% for A20, 43.85% for A30, 50.98% for A40, and 54.95% for A50. As can be observed, it reached strengths above 50 MPa for A40 and A50. [PERSON] [7] confirmed this fact, wherein the compressive strength at 7 days increased proportionally with the addition of PPR. For the mechanical strength at 7 days when PPR is used as an additive and as partially replacement of the cement, the filler effect occurs quickly, and this is essential for concrete elements that require rapid demolding. Higher average compressive strengths were obtained at 28 d for mixtures with partial replacement of cement by PPR. Increases of 7.45%, 16.16%, and 13.18% were achieved for concrete S10, S20, and S30, respectively, compared to that for the reference. Concrete S40 showed strength behavior similar that of the reference concrete considering the standard deviation bars. Further, there was a decrease of approximately 15.15% when the PPR replaced 50% of the cement. Significant increases in compressive strength at 28 d were reported in several studies [8; 12; 19]. Depending on the percentage of cement replacement by PPR, [PERSON] et al. [8] observed improvements in the compressive strength of the reference concrete for the proportions of 10%, 20%, and 30%; however, there was a tendency to reduce this strength for cement replacement levels by PPR higher than 30%. Concrete with the addition of PPR exhibited behavior similar to that presented at 7 days. The average compressive strength increased with the addition of PPR. An increase of 37.69% was obtained for A10; 53.79% for A20; 65.69% for A30; 77.89% for A40; and 92.22% for A50. [PERSON] [7] found an increase in the compressive strength of concrete with PPR when incorporated as an additive.