One-part alkali activated slag cement for conservation of existing structures

Since replacement of portland cement by other cementations materials is one of the main strategies to reduce the environmental impact of cementitious mixture, several innovative portland-free binders have been investigated. This paper is aimed to study a ground granulated blast furnace slag (precursor) activated with a mixture in powder form (activator) of sodium metasilicate pentahydrate, potassium hydroxide and sodium carbonate to manufacture portland-free mortars for conservation, restoration and retrofitting of existing masonry buildings and concrete structures. Several activator/precursor combinations (2%-32% by mass) were used to investigate the effect of alkali activation on the rheological, elastic and physical performances of repair mortars. The experimental data show that by changing the activator/precursor combination it is possible to “tailor” the 28-day compressive strength of the mortar. The activator dosage represents the key parameter influencing not only mechanical performance but also the hydraulic shrinkage: the higher the activator dosage, the more pronounced the mortar shrinkage. Shrinkage values for alkali-activated mortars (AAM) are significantly higher (2000 – 4000 ∙ 10-6) compared with those of cement-based mortars with the same compressive strength. Consequently, a reduction of shrinkage by means of shrinkage reducing (SRA) and/or water retention admixtures is necessary. However, although shrinkage is very high, the modulus of elasticity is about 40% lower than that of a portland cement mortar of the same strength level. On the basis of the experimental data AAMs seem to be more promising for a sustainable future in construction since the GER (Gross Energy Requirement) and GWP (Global Warming Potential) are dramatically reduced by 80 - 90% and 70 - 80%, respectively compared with traditional portland cement mortars with the same compressive strength.