Hydration Kinetics and Fire Resistance of Recycled Low Grade Alumino-Silicate Refractory Bricks Waste-Metakaolin Composite Cement Pastes
Abstract
The aim of the present work is to study the effect of low grade aluminosilicate refractory bricks (ASRBs) with metakaolin (MK) on the hydration properties and the fire resistance of composite cement pastes at elevated temperature up to 800 oC. The composite cement are composed of constant proportion of ordinary Portland cement (OPC) (80 wt%) with variable amounts of ASRBs and MK. The substitution of 20% ASRBs with MK increases the water of consistency and retarded the initial as well as the final setting times, whereas the free lime contents and the bulk density decreases with MK. The compressive strength increases with MK content which enhanced the early and long term strength. It acts also as microfiller that accelerates initial cement hydration. The fire resistance of composite cement pastes was evaluated up to 800 oC with rate of firing 5 oC/min. and soaking time for 2 h. The physico-mechanical properties such as, weight loss, bulk density and compressive strength were determined at each firing temperature. Moreover, some selected samples were investigated by using XRD, DTA/TG and SEM techniques analyses. Cement pastes containing 20 wt% ASRBs (PC-A) is the optimum composite cement paste which gives high bulk density and compressive strength. It can be concluded the cement paste PC-A has a good fire resistance paste.
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Introduction
Pozzolanas are naturally or artificial silicieous and aluminous materials, which possesses little or non-cementitious properties. But, in a finally divided form in the presence of water, they react with lime (CH) at room temperature to form calcium silicate and calcium aluminosilicate hydrates (C-S-H and C-A-S-H, respectively) [1,2]. Presently, the use of pozzolan incorporated in mortar and concrete has become much common throughout the world. The application of such material is increasing due to their superior structural performance, environmental friendliness and energy conservation implications [3]. Pozzolanas such as Fly ash, silica fume, Rice husk ash, ground granulated blast furnace slag and Metakaolin are commonly used for partial replacement of cement. Each of the pozzolanic materials induces various characteristics which have been explored by the researchers throughout the world [4].
Metakaolin, (MK) Al2Si2O7, is a largely amorphous product of kaolinite Al2(OH)4Si2O5, after calcination at a temperatures between 500 and 850 ℃. It contains silica and alumina in an active form which will react with CH, in the presence of water, forming cementitious C-S-H gel, together with crystalline products, which include calcium aluminate and aluminosilicate hydrate (C3AH6, C4AH13 and C2ASH8) [5,6]. Many researchers have a lot of interest in MK as it has been found to possess both pozzolanic and microfiller characteristics [7-10].
Fired low grad aluminosilicate refractory bricks (ASRBs) are commonly found in furnace as well as fire places. ASRBs are usually composed of a combination of clay minerals with silica, alumina or kaolin. There are few tries recorded for use aluminosilicate refractory bricks fired at temperature above 1500 ℃ as supplementary cementitious materials (SCM's). In a previous work [11], the pozzolanic activity of ASRBs with CH up to 30 wt% has somewhat potential to be used as a pozzolanic and filling material in cement based system. The substitution of 5-10 wt% of OPC by ASRBs gives higher compressive strength than that of OPC up to 90 days.
Conclusion
When MK is substituted instead of ASRBs the water of consistency increases with MK, the initial and final setting times are retarded. The combined water increases due to the high water of consistency of cement pastes. The free lime content of cement pastes increases up to 7 days then decreases up to 90 days. The substitution of ASRBs to 20 wt% MK decreases the free lime up to 90 days due to its high pozzolanic activity than ASRBs. PC-M cement paste gives compressive strength nearly equal to the neat cement paste. The effect of fire on the mechanical properties of cement pastes was carried out by the determination of weight loss, bulk density and compressive strength of heat treated hardened cement pastes treated up to 800℃. Cement pastes containing 20 wt, % ASRBs (PC-A) is the optimum composite cement paste which gives high bulk density and compressive strength. PC-A mix has a good firs resistance paste.