A November 2013 CSHub research profile letter, “Early hydration: A local business,” charts calorimetry (heat measurement) curves from tricalcium silicate paste experiments—with specimens of three particle size distributions (PSD)—to support a proposed model of microstructure-based kinetics. It suggests a “reaction zone” extending a few micrometers in from the cement particle surface, and can reconcile longtime observations on PSD and water/cement ratio. Understanding that zone could lead to new strategies to tailor early cement hydration; explore the relationship of admixtures and supplementary binding agents to cement particle reaction kinetics; and, pinpoint reaction products’ distribution and characteristics throughout cement based materials’ microstructures.