Compressive Strength and Microstructural Development of Cementitious Mixtures Incorporating Ultrafine Granulated Blast Furnace Slag

Utilization of ultrafine supplementary cementitious materials (SCMs) instead of ordinary ones has shown great potential to enhance the strength development of concrete mixtures. Replacing large amounts of portland cement with ultrafine SCMs, however, may not be feasible due to their negative effect on the water demand of concrete, and extra cost and energy consumption associated with the ultrafine grinding of SCMs. It is therefore important to explore practical methods for efficient use of ultrafine SCMs in producing low portland cement content concretes. To achieve this goal, the current study focused on the use of ultrafine granulated blast furnace slag (GBFS) in combination with a commercial one to replace 30%, 40%, and 50% by weight of portland cement in preparing mortar samples. The compressive strength of the mortars was measured at different ages ranging from 1 to 91 days. Cement paste samples with modified (combination of ultrafine and commercial) and commercial GBFS were also prepared and tested by the isothermal calorimetry, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDS) techniques. The results showed that the samples with 30% and 40% by weight modified GBFS as portland cement replacement had higher compressive strength compared with those made with the same amount of commercial GBFS or 100% by weight portland cement at all the testing ages. The microstructural analyses indicated increased calcium hydroxide consumption and higher reaction degree of clinker phases after 1 day of hydration for the sample incorporating 40% by weight modified GBFS compared with that with the same amount of commercial one, resulting in the superior compressive strength of this sample at such an early age of hydration.