Abstract
The presented paper aims to describe the influence of accelerating admixtures on the properties and microstructure of cement pastes and mortars. Blended slag cement CEM II/B-S containing two different clinkers (differing amounts of siliceous and aluminous phases) and four types of accelerators (calcium nitrate, sodium hydroxide, cement kiln dust, and crystal seeds) were used in research. Compressive strength tests (after 12, 24, 48 h of curing), Scanning Electron Microscope (SEM) observations together with an Energy Dispersive Spectroscopy (EDS) analysis, Mercury Intrusion Porosimetry (MIP) tests, and X-ray diffraction (XRD) analysis were conducted. Results have shown that SEM and EDS examination of the microstructure of cement pastes modified with accelerating admixtures at the observed points did not reveal differences that would be sufficient to explain the changes in compressive strength. Still, the increase in amorphous phase content indicates a faster hydration reaction rate for all pastes modified with accelerating admixture. It is backed up also by lower non-hydrated compounds content. All admixtures accelerate the hydration reaction of calcium silicate phases of cement, but only NaOH and cement kiln dust (CKD) influence the aluminate phase reaction rate. The pore volume is independent of the clinker type, while the pore size distribution is not.
Highlights
The presented paper describes the influence of accelerating admixtures on the properties and microstructure of cement pastes and mortars
Higher content of C-S-H was found compared to the observed points of the reference sample, which corresponds to the results presented in the previous research [56]
Results obtained for calcium nitrate (CN) and cement kiln dust (CKD) modifications of C1 clinker based cement are in the middle (30% and 31%, respectively)
Summary
The presented paper describes the influence of accelerating admixtures on the properties and microstructure of cement pastes and mortars. The publications [1,2] describe the microstructure of cement pastes and mortars in the range of high C3 A cement [3]. Accelerators involved in research were: modern accelerator providing heterogeneous nucleation seeds; cement kiln dust (CKD), used because of its great fineness and similarity in composition to Portland cement (and possibility to act as nucleating seeds as well); sodium hydroxide, because of its proven activating effect on alkali activated slag (AAS); and calcium nitrate as a reference, well-known, and commonly used accelerator. The most relevant aspect of the conducted research is the observation of the effects of CKD and modern accelerator on microstructure of cement pastes
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