Impact of long-term natural weathering on the properties of alkali-activated fly ash mortars modified with silica fume and sodium tripolyphosphate.

• Including SF into the AAFA mixture decreased workability, whilst STPP increased it. • Including 5–15% SF or/and 4% STPP in AAFA increased compressive strength. • Exposing specimens to outdoor natural weathering showed an adverse effect on their strength. • The adverse effect of weathering could be decreased with including up to 15% SF and 4% STPP. As known, exposure of any material to outdoor natural weathering, which has varied changes in temperature, relative humidity, percentage of sunny/cloudy, precipitation, CO 2 and polluted air may adversely affect its properties compared to its unexposed counterpart cured at the lab. This paper aims to evaluate the effect of one year of outdoor natural weathering exposure on the properties of neat alkali-activated fly ash (AAFA) mortars and those modified with silica fume (SF) and sodium tripolyphosphate (STPP). To increase the resistivity of AAFA mortars to weathering exposure, the FA was partially substituted with 5–20 wt% SF. To further improvement, a fixed ratio of 4% STPP was added to the neat FA mortar specimens and those containing SF. After initial curing, half of the specimens were cured inside the lab for one year (unexposed specimens), whilst the other half were exposed to outdoor natural weathering conditions for the same period. The variations in the compressive strength, mineral composition, hydration products and microstructure were compared. The results showed that exposing specimens to natural weathering has a negative effect on their compressive strength. This negative effect can be mitigated by introducing SF up to 15%. To further reduce this negative effect, 4% STPP can be added. [ABSTRACT FROM AUTHOR]