Thermal stability of cements resulting from alkaline activation of kaolin, metakaolin and volcanic scoria with partial replacement of aluminium hydroxide

Abstract

This work concerns the study of the thermal stability of cements resulting from alkaline activation of kaolin (K), metakaolin (MK) and volcanic scoria (Z) with partial replacement of aluminium hydroxide. To that end, two replacements (amorphous aluminium hydroxide (A) and aluminium oxy-hydroxide (B)) were used to substitute the aluminosilicate precursors (0, 10, 20 and 30 % by mass) and the resulting mixtures were activated in alkaline medium. Depending on the starting materials and the synthesised products, the thermal behaviour, the physical and mechanical properties as well as the products obtained by thermal treatment (300-1200 °C) were studied. The synthesised products with replacement showed low compressive strength as compared to those without aluminium hydroxide. This is due to the fact that the replacements behave as fillers. Contrarily, the synthesised products initially containing the replacement and heated at elevated temperatures exhibited improved compressive strength and volume stability. In fact, without replacement, the synthesised products from K or MK showed weak volume retention (weak swelling) and cracks whereas those from Z showed swellings at 1100 °C. Also, without replacement, the synthesised products from K or MK were morphologically damaged whereas those from Z melted completely at 1150 °C. Inversely, the synthesised products obtained by partial replacement of 10 % of A or 30 % of B exhibited good volume stability up to 1200 °C, with the absence of cracks and swellings. In addition, the synthesised products from K with 30 % of B and heated at 1100 °C showed compressive strength of 29.1 MPa whereas those replaced with 10 % of A showed compressive strength of 60.2 MPa at 1150 °C. Heated at 1150 °C, the synthesised products from MK and from Z replaced with 30 % of B presented compressive strength of 76.7 and 35.0 MPa respectively. This is due to their densification and the formation of new crystalline phases (mullite, corundum, nepheline and carnegieite). Unfortunately, heated at 1200 °C, these products lead to partial dissolution of nepheline and carnegieite, which generates closed pores, thereby lessening their compressive strength. Hence, during their alkaline activation, the partial replacement of aluminosilicates (K, MK or Z) with 10 % of A or 30 % of B improves compressive strength and thermal stability of heated products.