Modification on the chloride binding capacity of cementitious materials by aluminum compound addition

Abstract

This study investigates the effects of aluminum compound on the binding of admixed chloride in cementitious materials. Three commonly available aluminum compounds, aluminum oxide, aluminum hydroxide and colloidal nano-alumina are applied. The bound chloride content, hydration products and calcium hydroxide content are analyzed. The results show that aluminum compounds transform to AFm phases such as carboaluminate, monosulfoaluminate if there is no available chloride. When the mixing water includes chloride, the above AFms transform to chloroaluminate, e.g. Kuzel’s or Friedel’s salt, depending on the sodium chloride and aluminum compound content. The bound chloride content is significantly improved with aluminum compound addition during the whole curing age, even though in an early age of 3 days. Since aluminum compounds react slower than alumina-containing clinker, the bound chloride content of aluminum compound blended pastes experience a considerable increase after 7 days, whereas that for plain paste is very minor. Generally, admixed chloride promotes the formation of delayed ettringite and this effect can be suppressed by aluminum compound addition due to higher amount of formed AFms. A higher dosage of admixed chloride increases the bound chloride content and reduces the calcium hydroxide as well as free water content in the system.