Fabrication of slaked lime compacts (plasters) with high compressive strength using a warm press method

• A new warm press technique was employed to form dense slaked lime compacts.
• A compact fabricated 150 °C and 240 MPa under uniaxial pressure for 90 min reached an average compressive strength of 57 MPa.
• Warm press treatment led to mass transport occurred through a sintering-like process during warm pressing.

So far dense calcium hydroxide body (slaked lime plaster) with high mechanical strength has not been fabricated yet. In this study, novel method was tried in order to fabricate the dense calcium hydroxide body. As a result, a calcium hydroxide body with a dense and high mechanical strength was obtained. Pure calcium hydroxide powder was placed in a cylindrical steel mold 15 mm in diameter, and then simultaneously pressed uniaxially at 60–240 MPa and heated to 100–250 °C for various lengths of time. This compacting process is referred to as a warm press. After warm pressing, hardened compacts composed of pure calcium hydroxide were obtained. Increasing both the applied uniaxial pressure and the heating temperature increased the compressive strength of the resulting hardened compacts. Furthermore, increasing the time spent in warm press conditions also increased the compressive strength of the hardened compacts. However, these increases diminished over time, and the compressive strength reached a nearly constant value after approximately 90 min. When the uniaxial pressure, heating temperature, and duration were 240 MPa, 250 °C and 90 min, respectively, the compressive strength of the resulting hardened compact was 57 MPa. Finally, in this study, the phase stability relationship between calcium hydroxide and calcium carbonate is discussed on the basis of chemical thermodynamics. This warm press technique for the fabrication of dense hydroxide ceramics should help clarify the formation mechanism of natural hydroxide minerals.