Delayed ettringite formation (DEF) is a significant deterioration process in concrete which involves the growth of ettringite [Ca6Al2(SO4)3(OH)12 ·26H2O] crystals leading to cracking and reduction of compressive strength. Conditions leading to DEF are well known and include among others cement chemistry, presence of humidity, heat curing of concrete structures, and the presence of cracks. The mechanisms and kinetics by which deterioration occur is still not well understood despite numerous investigations. Understanding the mechanism and kinetics of concrete deterioration due to DEF is important in order to prevent such costly deterioration and to improve concrete durability.

In this research, concrete specimens were prepared with type III Portland cement and under different conditions that were designed to either promote or inhibit DEF. These consisted of a control set, a set subjected to a heat cycle and a third set made with elevated potassium content of 1.72% and also thermally cycled. They were tested periodically up to 380 days by conventional methods such as expansion and weight change measurements and compressive strength testing. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDAX) confirmed the presence and the morphology of ettringite in voids at different ages. Simultaneous neutron and X-ray tomography, a new nondestructive microscopic method was used to scan the specimens at regular intervals in order to assess the feasibility of the method in monitoring the progress and characterizing DEF induced damages.

The linear regression analysis of the correlation of expansion with weight change data revealed that expansion and deterioration process occurred in three distinct successive stages. In the first stage, the ettringite fills the pores with little or no expansion; in the second, the expansion appears to be creep due to expansive stresses in the filled pores and in the third stage, crack propagation leads to significant expansion and loss of compressive strength. The results of the linear regression also revealed that the mechanism of DEF is the replacement of pre-existing calcium hydroxide crystals. Through non-linear curve fitting, the kinetic of deterioration was modeled using the Kolmogorov-Avrami-Johnson-Miehl model. The simultaneous neutron and X-ray tomography allowed visualization of the interior of the specimen due to enhance phase segmentation. MATLAB routines were developed to allow for correction for beam hardening and to enhance phase segmentation. The study showed that with improved resolution, proper sample sizing, the method can be effectively used to characterize concrete damage due to expansive phases.