Ceramics-Silikáty 51, (1) 45 - 51 (2007)

Undersize fractions occurring during the recycling of concrete waste were used for the preparation of belite clinker. The ratio of the undersizes to limestone containing a high percentage of CaO was 1 to 2.25. The mix was converted into the belite cement and its hydration properties along with those of two types of alite cements prepared from the same recycled undersizes were investigated. The hydration character of Portland cements corresponds to the kinetics of the reaction of the 1^st order and its progress may be assessed from the reduction in the content of unhydrated CaO in the cement body. Quantified data characterizing the hydration products were obtained by means of the thermal analysis after standard reaction soaks of 1, 3, 7 and 28 days. The original CaO content (55 %) in the belite cement dropped to 51 % after 7 days; there was only 44 % of unhydrated CaO after a month. The original CaO content (56 %) in the ternary alite cement dropped consistently to 21 % after the hydration lasting a month; the decrease in the original CaO content (54 %) in the binary alite cement follows a similar pattern in the first 7 days, with an inflection occurring within a month and a decrease to the final value of 12 % of unhydrated CaO. The values of the velocity constants in the kinetic reaction of the 1^st order were expressed statistically from the values of the residual CaO remaining after the hydration. Belite cement is characterized by the value k = 0.0071 whereas the alite cements show the value k' = 0.225 (ternary system), respectively k'= 0.326 (binary system). This means that the belite cement prepared from recycled concrete would require an unacceptably long hydration time.