The autoclaved concrete industry: An easy-to-follow method for optimization and testing

This work describes a simple test method that is used to optimize the amount of ground crystalline sand of a given fineness that reacts equivalently with hydrated lime (CH) under autoclave conditions. The stimulated pozzolanic reactivity of the inertness of crystalline silica under autoclave curing leads to the formation of various amorphous and crystalline C–S–H structures. A series of 10 mixtures incorporating different proportions of ground crystalline quartz and calcium hydroxide with a water-to-binder ratio of 0.5 were prepared, cured under autoclave conditions and investigated using XRD, SEM-EDS and FT-IR analyses. A stoichiometric ratio of 1.32 was found between approximately 57% ground crystalline silica and 43% calcium hydroxide. Applying this ratio as a conversion factor, the optimum content of ground crystalline sand required to entirely consume the generated calcium hydroxide in hydrated cement was calculated to produce an optimized autoclaved concrete. Similarly, the amount of the same ground crystalline sand that should be added to cement was successfully obtained. A verification test was conducted using Portland cement paste to assess the validity of the conversion factor. The microstructural analysis of the optimized autoclaved cement paste with 30% ground crystalline sand reveals the formation of different nano-sized grains of C–S–H with a low Ca/Si ratio of 1.09. The average compressive strength of the 2.5-h autoclaved cement paste with 30% ground sand displays a value equivalent to that of the 28-day normally cured cement paste without ground sand. This result confirmed the validity of the current approach, which was supported by the good correlation among the XRD, SEM-EDS and FT-IR results. Consequently, mass production of optimized autoclaved concrete can be easily and economically achieved.