Improving the frost resistance of roof tiles beyond current prediction schemes

Frost resistance is an important requirement for clay roof tiles and the most difficult to keep under control during industrial production. Durability under freeze/thaw cycles is related with the capillary pore system, and current models predict the lower the open porosity and the coarser the pore size, the better the frost resistance. This implies, however, high firing temperature and rather refractory clay bodies, which are in contrast with energy and resource efficiency. Therefore, a different approach is needed to improve frost resistance of roof tiles without stressing the concept of low porosity and coarse pore size. A case-study is presented to demonstrate how excellent frost resistance can be achieved by adjusting raw materials formulation and firing schedule. Raw materials and bodies were characterized by XRF, ASTM C958 and technological properties. Both laboratory tests were performed: the products were characterized by water absorption (ASTM C67), open porosity and pore size distribution (Hg porosimetry), phase composition (XRD-Rietveld), mechanical strength (EN 538) and frost resistance (EN 539-2) up to 400 freeze/thaw cycles. Durability of roof tiles was drastically improved acting on both pore size distribution (eliminating pores <0.1 ?m) and increasing mechanical strength (>13 MPa). Phase composition plays a complex role with opposite effects of new formed phases versus amorphous phase and residual clay minerals. A satisfactory target (no damages after 250 freeze/thaw cycles) was achieved firing at 900 °C (7% pores <0.1 ?m) while the best performance (no damages after 400 cycles) required firing at 950 °C (<1% pores <0.1 ?m). Excellent frost resistance (well beyond the standard 50 cycles) can be obtained through proper design of firing schedule and body composition, even for temperatures below 950 °C.