Drying Shrinkage of Alkali Activated Fly Ash: Effect of Activator Composition and Ambient Relative Humidity

Abstract

Alkali activated fly ash (AAFA) is an alternative concrete binder that could serve as a substitute for ordinary Portland cement (OPC) for certain engineering applications. While this material has significant environmental and durability benefits, its potential volume instability and propensity to shrinkage and cracking could be of great concern. The subject of the present paper is evaluating the magnitude of drying shrinkage in AAFA binders of different compositions and at various ambient relative humidities (RH). Four AAFA paste mixtures were prepared using class F fly ash and liquid sodium silicate activators with various alkalinity (pH) and silicate modulus (n=(SiO2/Na2O)molar). The results were also compared with an OPC paste with a similar initial porosity. All AAFA pastes were steam cured at 60°C for 24 hours, followed by 6 days of moist curing at 23 o C, while the OPC specimens were moist cured at 23 o C for 7 days. The shrinkage specimens were dried under nitrogen purge at different relative humidities. The results show that activators with intermediate pH and modulus led to AAFA binders with higher compressive strength, denser pore structure, and larger drying shrinkage. Regardless of the activating solution, AAFA pastes dried quicker, and reached equilibrium faster than OPC paste. AAFA pastes also lost more moisture, but generally exhibited a lower drying shrinkage than OPC at similar RH. Steam curing of AAFA for a longer period (7-day vs. 1 day) stabilized the structure of AAFA binders and lowered the drying shrinkage.

A Study on Initial Setting and Modulus of Elasticity of AAM Mortar Mixed with CSA Expansive Additive Using Ultrasonic Pulse Velocity

“…The acceleration of the mortar setting has a significant impact on the physical properties of mortar at early ages [5][6][7][8][9], especially the initial shrinkage stress of the AAM mortar. According to the studies of many researchers, the shrinkage of mortar and concrete is evaluated after the occurrence of the final setting [10][11][12][13][14]. This is because the final setting means that the AAM matrix has been hardened and the deformation that occurs after the hardening of the AAM matrix acts as stress.…”

New Insights into Pore Structure and Hydraulic Conductivity of Sodium Hydroxide Alkali-Activated Slag through Advanced Modelling

“…Nevertheless, it allowed us to use all the reliable experimental data and to have a good agreement between the BET surface area and the area of the pores obtained with the BJH method. Going back to the results, the difference in behaviour takes place mostly in the micropores range, that is, for pores smaller than 20 Å [ 11 ]: Note that 0.5 M compositions present a small number of micropores compared to the other compositions. The results are in line with what was observed for the porosity and the BET surface area: even though S08M05 presents the highest porosity, its surface area is relatively small due to a coarser pore structure, as visible in Figure 10 .…”

“…Even though the effects of carbonation on the pore structure are not clear, all compositions present a much finer pore structure than PC [ 50 ], even though the open porosity can be considered similar to PC [ 11 ].…”

“…Combining Equations ( 7)- (11), it is possible to obtain the following equation to be solved for the moisture transport:…”

“…Specifically, AAS presents promising mechanical properties and chemical durability [ 5 , 6 , 7 , 8 ]. The main issues hindering the wide adoption of said materials, except for the still high costs, are doubts concerning their long-term durability [ 7 , 9 , 10 ], specifically the high drying shrinkage observed when compared to PC [ 7 , 11 ]. The main mechanisms of drying shrinkage in porous materials are capillary pressure in capillary pores, disjoining pressure, and surface free tension [ 12 , 13 , 14 , 15 ].…”