Effect of high SCM content and limestone addition on early-age strength and hydration kinetics of low-carbon binders

A slow development of early strength remains a major issue associated with low-carbon binders. This study investigated the effect of high supplementary cementitious material (SCM) content and limestone addition on the early-age (3 days) hydration kinetics and strength development using isothermal calorimetry, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and compressive strength tests. The properties of volcanic pozzolan from Iceland (VPI), a newly emerged natural SCM in the Nordics, were compared with those of fly ash and slag. The results indicated the fineness of SCMs grains as an important factor affecting the early strength development of corresponding binders, along with the intrinsic chemical structure of SCMs. The highest strength and heat release was exhibited by the binder with 40% VPI (the finest SCM) for the first 3 days, which was surpassed after 3 days by the binder with 40% slag. Binders with fly ash showed slow strength gain due to coarse fly ash particles with non-reactive crystalline phases. The addition of limestone increased the rate of clinker hydration. However, this was not sufficient to compensate for the strength loss by dilution in binders with slag and VPI, while the strength of the binder with fly ash remained intact with limestone addition. Different types of SCMs were observed to affect the slope of linear correlation between the heat of hydration and compressive strength at early age.