Mechanical and microstructural properties of Alkali Pozzolan Cement (APC)

Alkali Pozzolan cement (APC) comprises a high volume of fly ash, lime, ordinary Portland cement (OPC), and a dry Na2SO4 activator. APC can be stored in dry form and only requires water to create a cementitious binder. Mechanical and microstructural properties of APC were investigated to determine the compressive strength and hydration products of different APC pastes. Air-cured APC pastes (w/b = 0.3) were found to gain 65–77% of the strength of similar OPC pastes at 28 days, while being 75% greater than that of corresponding high-volume fly ash ones. Microstructural properties studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thermal analysis were able to explain the observed mechanical properties. The attacking of the smooth fly ash spheres (observed via SEM) by Ca(OH)2, evidenced by a reduction in the latter (observed through thermogravimetry) over a period of 112 days, could be linked to the strength gain of APC over that period. This strength gain could also be associated with the formation of ‘broad humps’, at the relevant 2θ values (around 29° and 32°) in XRD curves, which became more pronounced with time. Strength could also be correlated to the percentage of hydrate bound water, with the differing amounts of lime consumption by fly ash partly responsible for scatter.