Durability of blended cements containing illitic calcined clays
Illitic clays have pozzolanic activity after thermal treatment at 950 ºC causing dehydroxilation and structure collapse to form metastable aluminosilicate associated with large proportion of iron oxides (~7.0%) and alkalis (~5.0%Na2Oeq). In this paper, the performance against sulfate attack, chloride penetration and alkali silica reaction of different blended cements containing illitic calcined clays were studied. Sulfate attack was studied using the ASTM C1012 expansion test in blended cements containing low and very high C3A portland cements. After one year, calcined clay completely controls the expansion of very high C3A-cement and improves the performance of low C3A-cement. Chloride resistance of concrete mixtures was determined by natural chloride diffusion (ASTM C1556). Addition of calcined illitic clay (25%) refines the pore structure as indicate the MIP analyses improving the resistance to chloride diffusion and increasing the fraction of bound chlorides some part as chloroaluminate. Alkali-silica reaction was studied using the ASTM C441 procedure with 25% calcined clay added to low, medium and high alkalis Portland cements. The illitic calcined clay reduces the expansion of Portland cements due to the consumption of alkali by the pozzolanic reaction as indicate the determination of available alkalis. Also, the alkalis provide from pozzolan does not impairs significantly the expansion of low alkali Portland cement. Illitic calcined clay used as appropriate portland cement replacement improves the performance of cementitious materials due to the pozzolanic reaction that refines the porosity and consumes the calcium hydroxide and the alkalis in the system.