Pozzolanic activity of kaolinite material rich in gibbsite calcined at low temperature and its effect on physical and mechanical properties of Portland cement mortars

Gibbsite in tropical kaolinite material greatly influences its reactivity as pozzolanic material. The aim of this research is to study the effect of gibbsite, Al(OH)3, (15.9 mass%) on the pozzolanic activity of a kaolinite material calcined at low temperature. To this effect, the as-received raw kaolinite material was calcined at 600°C and the output product was used to partially replace Portland cement by 0, 10 and 20 mass% to produce mortars. The calcined kaolinite material was firstly subjected to the modified Chapelle test and strength activity index in order to evaluate its pozzolanic activity and physical and mechanical properties of the as-produced specimens were assessed. The results show that the calcined kaolinite material presents a high pozzolanic activity (Chappelle test = 1665 mg of Ca(OH)2/g) compared to the reference un-calcined kaolinite material (535.0 mg of Ca(OH)2/g). After 28 days of curing, mortars obtained from partial replacement with 10 mass% of the calcined kaolinite material show higher compressive strength (54.5 MPa) compared to those obtained with 20 mass% (51.3 MPa). Indeed, these values are higher compared to those of mortars produced without replacement. In fact, aluminium compound promotes the formation of metastable hydrated phases (CAH10/C2AH8) at early age which temporally hinder cement hydration. Conversely, these phases are progressively transformed into stable hydrated phases of C3AH6 with time, thereby favoring the hardening of specimens. Thus, partial replacement of Portland Cement by 10 mass% of the calcined kaolinite material is suitable to obtain mortars endowed with enhanced compressive strength for construction purpose