AbstractThe present study investigates in detail the effects of the direct application of power ultrasound (PUS) at several frequencies and power densities to Portland cement (PC) pastes and mortars. The ultrasound frequencies of 26, 67 and 132 kHz that respectively correspond to power densities of 0.011, 0.007 and 0.003 W/cm3 were deployed to treatments of fresh cement pastes and mortars. These various ultrasound intensities were generated using a large bespoke, multi-frequency ultrasonic bath. A fixed ultrasonic bath operating at 40 kHz with a power density of 0.014 W/cm3 was also used to study the effects of PUS on the particle size distribution (PSD) of PC.
In the first part of the study, changes in the PSD parameters of PC based on wet dispersion in both isopropanol (IPA) and deionised water (DI water) were determined using laser diffraction method. Next, ultrasound at various frequencies was directly applied to the fresh PC pastes at water-to-cement (w/c) ratios of 0.50 and 0.80. The chemical composition of the pore solution extracted from the fresh and hardened cement pastes were examined using the inductively coupled plasma optical emission spectrometry (ICP-OES) technique after applying ultrasound for various periods. The work presents a robust ICP-OES method developed to quantify both Ca, K, Na and S as the high-concentration analytes, and Al, Fe, Mg and Si as the low-concentration analytes in the PC paste pore solution. Additionally, hydrate assemblages were identified by X-ray diffraction (XRD). A semi-quantification of important components was performed using data obtained by thermogravimetric analyses (TGA/DTG). The changes in the setting times of cement pastes subjected to PUS were also reported.
The PSD data shows that applying PUS for up to 5 minutes marginally deagglomerates the coarse size fraction of PC particles in aqueous dispersion. However, ultrasound may not be able to effectively disaggregate the PC particles in normal pastes, even at longer exposure; particularly the finer size fractions (< 10 μm). Direct application of PUS to the PC paste does not alter the main types of PC hydration products but appreciably changes the pore solution composition and slightly affects the main hydrate proportions and kinetics. The study reveals for the first time, that applying PUS can potentially disturb the early formation of ettringite, whilst appreciably promoting the formation of amorphous aluminium hydroxide hydrate by considerably releasing aluminium ions into pore solutions of cement pastes at both w/c ratios of 0.50 and 0.80. Upon ultrasound exposure (particularly at longer times), noticeable changes were observed in Ca and Si concentrations denoting the influence of PUS on the dissolution of alite and/or aluminate phases due to the cavitation. Moreover, PUS was shown for the first time to be able to increase carbonates by intensifying the carbonation (ultrasound-assisted carbonation) in the PC pastes, particularly in the system with a higher water content.
In the next part of the study, the development of hydration under the exposure of ultrasound irradiation at various frequencies/power densities and durations was assessed. First, the effect of PUS on the extent of PC hydration was investigated by precisely measuring the chemical shrinkage, using a self-developed automated dilatometry method, to 14 days of hydration. To measure the evolution of chemical shrinkage, an automated vision inspection system was developed. Additionally, the mechanical performances of the PC mortars were evaluated at testing ages of 1, 3, 7, 28 and 91 days. Two different sonication procedures were adopted for the treatment of the fresh mortar mixes. This was to investigate the effect of applying ultrasound at various stages on the mechanical properties of mortars and the properties of the interface between the cement paste and the aggregates (interfacial transition zone, ITZ). In the first approach, ultrasound was applied to all the components of the freshly mixed mortar, i.e. water, PC and fine aggregates. In the second approach, PC slurry was first prepared using water and cement and ultrasonication was subsequently applied to the cement slurry. Finally, the fine aggregates were added during the final stages of mixing to obtain mortars with sonicated paste.
The results indicated an enhancement in the evolution of chemical shrinkage when both 26 and 132 kHz frequencies were applied to the cement paste for a duration of 2 minutes. Cement pastes sonicated at 26 kHz for 2 minutes showed a slightly higher rate of chemical shrinkage than those treated at 132 kHz for the same irradiation time. Similarly, the corresponding mortars yielded a higher rate of development for both compressive and flexural strengths when subjected to a higher acoustic intensity at 26 kHz over 91 days curing in water, than those treated by a gentler insonation at 132 kHz. The application of PUS to fresh mortar mixes reduced the strengths of hardened cement mortars at 1 day compared to the non-sonicated mortar. In this regard, the effect of ultrasound with a higher intensity at 26 kHz is more profound than PUS at 132 kHz. However, at later ages, the mortars treated at 26 kHz acquired a higher compressive and flexural strength than either those treated at 132 kHz or the control samples. Longer exposure of PUS at 26 kHz considerably enhanced 1-day strength that could be related to the deaeration effect of ultrasound on the partial removal of entrapped air in fresh mortar. The addition of aggregates to the ultrasonicated paste was found to lead to a considerably significant improvement in the flexural strength of the mortars, potentially indicating the modification of the bulk cement paste and the ITZ. These effects were supported by microstructural analyses of the hardened PC mortars fracture surfaces, using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques.
The present research concludes that the application of PUS in general has positive effects on the extent of cement paste hydration and then mechanical properties of mortar when a higher power density is used. Furthermore, the observations made for the first time in this work strengthen the position that ultrasound could be a promising technique to enhance ternesite hydration in ternesite-rich belitic calcium sulphoaluminate cements, as a novel low-carbon binder and an alternative to PC. This will be further investigated in a post-doctoral programme.
|Date of Award||May 2021|
|Supervisor||Eshmaiel Ganjian (Supervisor), Timothy Mason (Supervisor) & Mark Tyrer (Supervisor)|