Heat-Induced Acceleration of Pozzolanic Reaction Under Restrained Conditions and Consequent Structural Modification

Abstract

<jats:p>This study investigated the heat-induced acceleration of cement hydration and pozzolanic reaction focusing on mechanical performance and structural modification at the meso- and micro-scale. The pozzolanic reaction was implemented by substituting 20 wt.% of cement with silica fume, considered the typical dosage of silica fume in ultra-high performance concrete. By actively consuming a limited amount of water and outer-formed portlandite on the unreacted cement grains, it was confirmed that high-temperature curing greatly enhances the pozzolanic reaction when compared with cement hydration under the same environment. The rate of strength development from the dual reactions of cement hydration and pozzolanic reaction was increased. After the high-temperature curing, further strength development was negligible because of the limited space availability and preconsumption of water under a low water-to-cement environment. Since the pozzolanic reaction does not directly require the anhydrous cement, the reaction can be more easily accelerated under restrained conditions because it does not heavily rely on the diffusion of the limited amount of water. Therefore, it significantly increases the mean chain length of the C–S–H, the size of C–S–H globules with a higher surface fractal dimension. This finding will be helpful in understanding the complicated hydration mechanism of high-strength concrete or ultra-high performance concrete, which has a very low water-to-cement ratio.</jats:p>

Elucidation of the Hydration Reaction of UHPC Using the PONKCS Method

“…The results of the TGA are shown in Figure 4 . Regardless of curing time, calcium hydroxide (decomposition range between 400 and 500 °C) was hardly detected; this finding was similar to the authors’ previous hydration studies on UHPC [ 13 , 31 , 32 ]. The main weight loss occurred between 600 and 800 °C, which indicated the decomposition of calcium carbonate [ 33 ].…”

“…During the heat treatment, partial decomposition of C-S-H may have occurred (as determined by the reduced CBW in Figure 4 ); this could be associated with the water loss in the newly formed C-S-H at the perimeter of pre-existing C-S-H. Thus, the MCL and the fractal dimension of the C-S-H were significantly increased [ 13 ].…”

“…The estimated Al/Si ratio had a similar tendency to that of the MCL, as determined by Equation (4) [ 25 ]. However, more rigorous investigation must be performed to refine the equation for estimating the nanostructure of C-(A)-S-H, especially in restrained environments or for UHPC [ 13 , 40 ].…”

“…Generally, the Rietveld refinement method (using an internal or external standard method) can be applied to quantify an amorphous phase using XRD [ 12 , 13 ]. Additionally, if the partial or no known crystal structure (PONKCS) method, which can simulate the intensity of the virtually generated amorphous pattern, is used, quantitative analysis of the amorphous phases is possible [ 14 , 15 , 16 ].…”

“…Since both the raw material and resulting main hydration product are in non-crystalline phases, the XRD-based characterization of the hydration mechanism of UHPC (e.g., the degree of consumption of the silica fume or production of C-S-H) is difficult [ 23 ]. Additionally, the lower degree of hydration of UHPC also makes accurate quantitative phase identification more difficult [ 13 , 24 , 25 ].…”