HOT ALKALI CARBONATION OF SODIUM METAPHOSPHATE MODIFIED FLY-ASH CALCIUM ALUMINATE BLEND HYDROTHERMAL CEMENTS

Sodium metaphosphate-modified fly ash/calcium aluminate blend (SFCB) c ements were prepared by autoclaving for 1 day at 300 degrees C and the ir resistance was evaluated in a highly concentrated Na2CO3 solution a t 300 degrees C. The hydroxyapatite and analcime phases formed in the autoclaved SFCB cements prayed an essential role in conferring resista nce to the degradation of cements caused by alkali carbonation. Althou gh the carbonating reaction of the analcime phase led to the formation of cancrinite, this analcime cancrinite transformation did not show a ny influence on the changes in the mechanical and physical properties of the cements. Additionally, there was no formation of the water-solu ble calcium bicarbonate in the cements exposed for 28 days. Contrarily , the conventional class G cement systems were very vulnerable to a ho t alkali carbonation. The major reason for the damage caused by carbon ation of the cements was the fact that the xonotlite phase formed in t he 300 degrees autoclaved cements was converted into two carbonation p roducts, calcite and pectolite. Furthermore, the reaction between calc ite and carbonic acid derived from Na2CO3 led to the formation of wate r-soluble calcium bicarbonate, thereby causing the alteration of dense structures into porous ones and the loss of strength of cements. Copy right (C) 1996 Elsevier Science Ltd