Synthesis of tobermorite 11 Å during the formation of autoclaved aerated concrete with the addition of diatomite

The fundamental structure of an autoclaved aerated concrete, is closely linked with the formation of tobermorite, which is called the heart of concrete and directly affects its mechanical properties. However the presence of only tobermorite 11 Å (instead that of 9 or 14 Å), is more desirable. For this reason, it is important to study what are the conditions during the synthesis of concrete, that can favor the formation of this phase. In this paper, the effect of natural diatomite addition in conjunction with the evaluation of two different calcium-silicon oxides ratio during the synthesis of an autoclaved aerated concrete, were studied. The main objective was to promote the tobermorite 11 Å formation, which can improve the compressive strength of that concrete. The synthesis was carried out testing two CaO/SiO2 ratios (0.5 and 0.8) with different percentages of diatomite (0, 0.5, 0.10, 0.15, 0.20 and 0.25 %). It was found that with an oxide ratio of 0.5 and 0.15 % of diatomite, the presence of tobermorite 9 Å increased from 29 to 33 % and the concrete reached a maximum value compressive strength of 4.66 MPa. On the other hand, for an oxide ratio of 0.8 and 0.20 % of diatomite, it was found the presence of only tobermorite 11 Å (which corresponds to 38 %), getting so a maximum value of 4.73 MPa. In both previous cases, it was found an increase of 16.5 and 18.25 % respectively, in comparison with the 4 MPa necessary for this type of materials, to be considered for structural applications. According to the above, the oxide ratio of 0.8, in conjunction with addition of diatomite, can promote the formation of tobermorite 11 Å mainly, which also improved the compressive strength values.