Durability of alkali-activated fly ash and slag concrete

Alkali-activated materials (AAM) have received increasing interest during the past few decades due to their environmental and technological advantages. However the durability of these binders is still considered as an unproven issue that needs to be addressed before their commercial adoption and large-scale production. The present work is focussed on the investigation of durability performances of concretes manufactured through the alkaline activation of fly ash and slag blends using a low-concentrated activating solution (formulated by blending commercial sodium silicate and sodium hydroxide). The aim of this study is to provide new insights on durability properties of AAM and to assess their resistance under severe conditions. For that purpose, concrete specimens have been exposed to accelerated carbonation (varying the curing and the exposure times from 7 to 28 days). After 28 days curing, their chloride resistance has been assessed using the non-steady-state chloride migration experiments following the NordTest method (NT Built 492). Prior to durability test, the compressive strength of investigated mixtures was determined at early ages (1, 7 days) and after 28, 56, 90 days curing. The slag-rich mixtures have shown high compressive strength values (~45MPa after 1 day and ~80MPa after 28 days curing). Excellent mechanical properties have been also found for the fly ash-rich mixtures at early and late curing ages (15MPa after only 1 day and ~50MPa after 28 days curing). From carbonation test, it has been found that the carbonation depth increases as increasing fly ash/slag ratio or the exposure time. However when curing time is increased, a decrease on carbonation depth was observed. Slag-rich concretes were practically not carbonated after 7, 14 days exposure and showed 3-7mm carbonation depth after 28 days exposure. A higher carbonation depth (~18mm) was observed in fly ash-rich mixture. When the concrete specimens were exposed to chloride ingress, a high chloride permeability associated with higher concrete porosity was found for fly ash-rich mixtures with a chloride penetration depth of 18mm and chloride migration coefficient near 15-17×10-12m2s-1. These parameters decreased considerably as increasing slag content reaching values near 5mm and 2×10-12m2s-1, respectively.