Enhancing the crystallization phenomena and strength of porcelain stoneware: the role of CaO

Limestone was used to modify the fluxing action of two potash feldspars (a pure potash feldspar and a soda-potash feldspar) labeled P and C, respectively, in the formulation of porcelain stoneware based on Cameroonian raw materials. The effect of limestone addition (0–10 mass%) was investigated in the range of sintering temperature between 1125 and 1300 °C. Characterization of sintered samples including thermal behavior (DTA, TG and dilatometry test), phase evolution, densification parameters, flexural strength, morphology as well as pores structure was investigated in details. The maximum flexural strength (138 MPa) was obtained at 1175 °C with P series (7 mass% addition of limestone) and at 1200 °C (122 MPa) for C series. The maximum density (≈ 3.1 g cm−3) and lower water absorption (≈ 0%) were obtained at 1200 °C. Mullite and anorthite were identified as main crystalline phases. Starting from 1175 °C and mostly at 1200 °C, the two series compositions (with 7 mass% addition) presented a self-glazing phenomenon which gave them significant brightness and high aesthetic quality; these properties were accomplished only at 1300 °C for reference samples with no lime addition (P0 and C0). A proper addition of limestone (3–7 mass%) with potash feldspar significantly reduced the sintering temperature (~ 150 °C) and permits the production of high strength (122–138 MPa), low energy and sustainable porcelain stoneware.