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The Influence of Metakaolin from Saaba (Burkina Faso)

Burkina Faso

The paper evaluates the feasibility of reducing clinker in the Portland Cement production using local metakaolin in Burkina Faso. Standardized testing methods Burkina Faso have been used for this purpose, and experiments were performed on mortar prisms containing different amounts of metakaolin. Important results about the physical, mechanical and durability characterization of blended mortars were carried out in this study. The obtained results are discussed based on available literature data. These results have shown increased physical and durability properties for blended mortars. Although the mechanical strengths remained relatively low for higher MK incorporations, the latter grow to surpass these of PC mortars (the reference) at 28, 56 and 90 days of curing. The results in the paper, have confirmed the possibility of using metakaolin to partially substitute cement, a possibility to reduce the CO2 production by the cement industry in Burkina Faso.

The progressive increase of the energy prices over the past decades in Africa, has restrained the availability of the conventional building binder “cement” to the large majority of poor and middle-income countries populations. These same countries, display the highest demographic trends and hence, high decent housing demands. In addition to this cost-related hindrance, the recent environmental considerations of cement production industry (emission of one ton of CO2 per ton of cement) motivated the search of “alternatives”. The cement industry contributes for up to 5% – 8% of global CO2 emissions in 2014 [1] .

The potential of Burkina Faso in terms of clayey materials has recently drawn the attention of researchers from varied fields of engineering and technology. These include road construction materials stabilization [2] [3] , general building materials (bricks) [4] and water treatment and sanitation applications [5] . Although varied are these areas of research, none has addressed the exploitation of pozzolanic activity of thermally activated clays in hydraulic binders.

In fact, clays with high kaolinite content (kaolin clays) are potential pozzolanas and can be used as supplementary cementitious materials when activated (thermally or chemically). In the geological context of Burkina Faso, most of the kaolin clay quarries are found nearby the tuff-hillsides and are sought to have evolved from them [6] . These kaolin clays, when sintered at temperatures around 500˚C – 800˚C produce metakaolin [7] . This material with high content of amorphous silica (SiO2) and alumina (Al2O3) reacts with the cement hydration Portlandite (Ca(OH)2) to create secondary calcium silicate hydrates (C-S-H) and alumina containing C-A-H and/or C-A-S-H phases [8] . In this study, higher proportions were attempted for the optimization of CH consumption. In fact, Kostuch et al. [9] have found 20% MK incorporations necessary for full removal of CH in concrete at 28 days of curing.

The synthesis of MK by the dihydroxylation/amorphization of kaolin clay is a key step for optimum pozzolanic activity of the resulting material [10] . The influence of heat treatment parameters over the reactivity of MK have been explored extensively [11] [12] [13] [14] . Ambroise et al. [15] have suggested temperatures above 700˚C. Nevertheless, higher temperatures can lead to the recrystallization and formation of mullite and cristobalite non-reactive phases [16] [17] [18] . Raw clays differing in terms of purity, the most accurate heat treatment program should be deduced from individual clay thermal analyses using TG/DTA.

The incorporation of MK in cement production would have a substantial economic impact and environmental relief in Burkina Faso. In fact, there are no local industries that produce Portland Clinker for its production requires high technology and considerable amount of fuel (energy). Hence, the clinker is imported from the West and Asia, increasing its price. On the environmental point of view, for instance the production of one ton of flash metakaolin only produces 0.098 ton of equivalent CO2 versus 0.913 tons per one ton of PC of type CEM I [19] .

Considering the two motivational aspects, this study aims at evaluating the possibility of using the local calcined clay (metakaolin) at high proportions as a partial substitute of clinker in West Africa. Throughout this study, the effect an increasing substitution rate of cement by MK over the physical, mechanical and durability properties of the MK blended cement was explored. The used materials were characterized using the ICP-OES for chemical composition, the x-rays diffraction for mineralogy and TG/DTA for thermal analyses.