Factors affecting pore system and setting time of sustainable mortar with alkali activated GGBS/Fly ash binder

Abstract

It is essential to reduce the environmental footprint of the construction industry. One approach to achieving this goal is to replace ordinary Portland cement (OPC) by alkali-activated cementitious materials such as ground granulated blast furnace slag (GGBS) and fly ash. This article presents the findings of an experimental study that evaluated water absorption characteristics, and initial/final setting times of mortar that uses alkali activated blend of GGBS and fly ash. It was found that the relationship between water absorption and square-root of time (t0.5) of mortar pore system for up to 6 hours of standard base immersion, is linear with high coefficient of correlation, for binders including alkali activated GGBS and fly ash. This is similar to the relationship that characterizes mortar with OPC binders. Sorptivity of mortar prepared with alkali activated GGBS binder decreased with increase in solution molarity as well as ratio of sodium silicate/sodium hydroxide at the age of 7 days. At the age of 28 days, GGBS-based mortar exhibited a decrease in strength of correlation between water absorption t0.5 in several mixes, when the molarity of NaOH activator is 12M or higher. The pattern persisted for GGBS-based mortar at the age of 90 days, but with more mixes experiencing decrease in strength of correlation (R2<0.98). Loss of linearity in GGBS-based mortar was attributed to shrinkage induced cracking. At the ages of 7 days and 28 days, mortar based on alkali activated 50% fly ash + 50% GGBS exhibited linear absorption relationship, except for high NaOH molarity of 16M, which indicates coarser mesh at higher solution alkalinity, along with shrinkage cracking. At the age of 90 days, mortars with 50% fly ash + 50% GGBS exhibited linear absorption relationship with high R2 value, indicating finer mesh, which was consistent in all NaOH molarities. Initial and final setting times were found to be strongly corrected to initial ratio of (Ca + Si)/Al established at mixing time. Setting time of mortar with high GGBS content (greater than 50%) was too short for many practical applications and a suitable retarder is necessary.

Keywords; Materials, Cement, Industry, Covariance and correlation .