Concrete which undergoes a thermal treatment before (pre-casted concrete blocks) and during (concrete embedded in-situ) its life-service can be applied in plants operating at high temperature and as thermal insulation. Sintering is a process which occurs within a concrete structure in such conditions. Progression of sintering process can be monitored by the change of the porosity parameters determined with a nondestructive test method -ultrasonic pulse velocity and computer program for image analysis. The experiment has been performed on the samples of corundum and bauxite concrete composites. The apparent porosity of the samples thermally treated at 110, 800, 1000, 1300 and 1500 °C was primary investigated with a standard laboratory procedure. Sintering parameters were calculated from the creep testing. The loss of strength and material degradation occurred in concrete when it was subjected to the increased temperature and a compressive load. Mechanical properties indicate and monitor changes within microstructure. The level of surface deterioration after the thermal treatment was determined using Image Pro Plus program. Mechanical strength was estimated using ultrasonic pulse velocity testing. Nondestructive ultrasonic measurement was used as a qualitative description of the porosity change in specimens which is the result of the sintering process. The ultrasonic pulse velocity technique and image analysis proved to be reliable methods for monitoring of micro-structural change during the thermal treatment and service life of refractory concrete.Key words: ultrasonic pulse velocity; sintering; porosity; concrete for high temperatures; image analysis; mechanical compressive strength.High-temperature concretes are commonly used as constructive elements and linings of metallurgical furnaces and other plants operating at high temperatures (linings for oil refinery plants, thermal insulation in plants, linings in nuclear power plants, linings in chemical and petrochemical industries, etc.). Benefits from the application of concrete instead of common refractory materials are as follows: simplified building of refractory linings, an economic aspect, i.e., a cheaper process of manufacturing and the possibility of damaged lining reparation [1].

ZAGORKA М. RADOJEVIĆ, Institut za ispitivanje materijala, Beograd Mineralni dodaci se ekstenzivno koriste kao zamena za cement u građevinskim betonima i malterima. Leteći pepeo je jedan od najčešće primenjivanih dodataka koji unapređuje reološka, mehanička i termička svojstva materijala. Međutim, ovaj industrijski nusprodukat sadrži teške metale zbog čega je neophodna optimizacija njegove dozaže pri dizajnu maltera. Glineni materiali kao što su zeolit i bentonit svojim sorpcionim mehanizmima mogu da preduprede migraciju toksičnih elemenata iz pepela imobilišući ih unutar strukture. U ovom radu eksperimentalno su ispitana tri različita maltera pripremljena na bazi Portland cementa, rečnog peska i kombinacije mineralnih dodataka-letećeg pepela, zeolita i bentonita. Malter M1 sadržao je sva tri dodatka u odnosu 1:1:1, malter M2 je sadržao zeolit i bentonit u odnosu 1:2, dok je odnos zeolita i bentonita u malteru M3 bio 2:1. Cilj istraživanja je bio da se ispita uticaj mineralnih dodataka na fizičko-hemijska, termička i mehanička svojstva maltera. Primenjene su instrumentalne metode-X-ray difrakcija, diferencijalno termijska i dilatometrijska analiza i FTIR spektroskopija. Rezultati istraživanja su ukazali da se primenom ovih mineralnih dodataka mogu dobiti ekonomični malteri sa dovoljno dobrim performansama.