Effect of thermally activated paper sludge on the mechanical properties and porosity of cement pastes

The present article discusses the effect of paper sludge additions, calcined at 700 ºC for two hours, on cement paste pore structure and mechanical strength. Both total and capillary porosity were observed to depend on the percentage of calcined sludge added to the cementitious matrix. While a 10% addition induced values for both slightly higher than the control, adding 20% prompted the opposite result, reducing porosity values with respect to the control. Substantial refinement was observed, with a rise in pores smaller than 0.01 μm (gel pores) when the calcined sludge was added. Such refinement was greater at the higher percentage of sludge. After approximately 15 days, strength was lower in both the additioned pastes compared to the control. A high correlation (R2≥0.939) was found between total porosity and compressive strength for both percentages studied.