Estudo de um caulim calcinado do Estado da Paraíba como material de substituição parcial do cimento Portland.

Abstract

The use of burnt clays as pozolanic material in mortars and concretes is increasing in the last years due not only to the improvement of mechanical properties but also due environmental benefits limited to the reduction of clinker utilization and to the durability increase. Paraiba State disposes of several natural deposits of kaolin, but its use as cement replacement material was never investigated. This paper has as objective to verify the potentialities of the local calcined kaolin as partial cement replacement material for use in concretes and mortars. A benificied kaolin from a company of the interior of the State was used. Two fineness were considered: one that passes in the sieve ABNT n°. 200 and another in the sieve ABNT n°325. The raw material was characterized by chemical and mineralogical tests. Differential thermal analysis showed that the calcination temperatures would be between 700°C and 980°C, so, three temperatures have been tested: 700°C, 800°C and 900°C. Replacement percentages by weight of the Portland cement were 10%, 20%, 30% and 40%, besides the reference mortars without substitution and a mortar with 10% of silica fume. The binder: sand ratio was 1: 1.5 by weight, and the water/binder ratio was fixed in 0,4 for all mixes. The pozzolanic activity index with lime and ordinary Portland cement was established. Compressive strength was obtained at the ages of 7, 28 and 90 days. The speed of propagation of ultrasonic pulse was also studied in the mortars at 28 days and also the water absorption and the dry and saturated specific mass were obtained, always as function of the percentages of replacement of the Portland cement. In the studies, 5 x 1 0 cylindrical samples were used and the cure was always by immersion in water. The results showed that a calcination for 700°C is already enough to produce a pozzolan of excellent properties; the finer material (passing in the sieve number 325) presents better behavior and the percentages of the replacement of the cement by 20% and 30% of calcined kaolin give mortars with excellent performance, being comparable its strength to the one of mortars with 10% of replacement of a consecrated pozzolan as silica fume. The ultrasonic pulse velocity decreased with the increase of the percentage of replacement, showing that the calcined kaolin modifies the pore structure of the mortars. The local kaolin has a big potential to be exploited in the field of civil construction.