On the hydration process of the slag–anhydrite–lime system: a discussion on the stoichiometry of hydrates and the development of strength

Abstract

To better understand the hydration of slag, various combinations of slag–anhydrite–lime were investigated, mainly by conduction calorimeter. Compressive and bending strength were also measured. The following conclusions were obtained: in the case of existing calcium hydroxide (Ca(OH)2), hydration products were calcium–silicate–hydrate (C–S–H) and aluminium-rich (Al, Si, Ca)OH gel. C–S–H had a calcium/silicate ratio of 1·4 and the aluminium-rich (Al, Si, Ca)OH gel was located on the surface of unreacted slag particles to hinder the hydration of slag. A mixture of slag and anhydrite (without lime) reacted to produce C–S–H and ettringite, but the initiation time of reaction was very long – as much as 400 h. In this case, the calcium oxide/silica (CaO/SiO2) ratio of the calcium-silicate-hydrate was considered as low as 0·6 to 0·8. Monosulfate (AFm(SO4)) was not produced from slag hydration, as the solubility of slag is lower than that of monosulfate. The compressive strength of mortar without gypsum was 25 N/mm2 at 91 d, whereas that containing gypsum of 15% by mass was 70–80 N/mm2 at 91 d. In the case of 2% calcium hydroxide, the compressive strengths with gypsum were all less than 15 N/mm2 at 91 d and they were lower than that without gypsum of 25 N/mm2 at 91 d. Reducing the addition of calcium hydroxide was very effective for mortar strength with gypsum but addition in excess of 2% retarded the hydration. The strength of mortar without formation of ettringite (slag–lime system) had lower bending/compressive strength ratio.