Effects of Increasing Dosages of an Alkali-Free Accelerator on the Physical and Chemical Properties of a Hydrating Cement Paste

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Title: Effects of Increasing Dosages of an Alkali-Free Accelerator on the Physical and Chemical Properties of a Hydrating Cement Paste

Author(s): A. Bravo, T. Cerulli, C. Maltese, C. Pistolesi, and D. Salvioni

Publication: Special Publication

Volume: 217

Issue:

Appears on pages(s): 211-226

Keywords: accelerator; alkali free; ettringite; setting time

Date: 9/1/2003

Abstract:
Flash setting accelerators are commonly used for shotcrete which is basically a sprayed concrete for underground constructions. Such admixtures cause a very rapid hardening of cementitious systems thus allowing overhead and vertical applications. Two main classes of products are currently available in the market: alkali-free (non alkaline aluminium salts) and alkali rich (sodium or potassium silicates or aluminates) accelerators. The first ones were commercially available in the last few years but only recently they have started to be appreciated by the market. The wide literature on the subject shows that the use of alkali-free accelerators gives many advantages: reduced chemical reactivity of the admixture toward human skin; less rebound, high water impermeability, increased long term compressive strength of sprayed concrete. The action mechanism of alkali-free flash setting admixtures is still not clear; therefore, a chemical morphological analysis was carried out in order to study the effects of this new class of accelerators on the physical/chemical properties of cement pastes. XRD and temperature profile studies were performed on cement systems added with different amounts of accelerator. An ESEM study was also carried out in a time interval from a few minutes to 8 hours. Specific surface area was determined according to BET method on mortar samples mixed with the accelerator (2% and 6% by cement mass) after 24 hours and 28 days of curing. Furthermore soluble ions concentration (Al3+, S042-) was also determined by ICP spectroscopy. Setting time and compressive strength were measured to compare the effects of the admixture on the mechanical properties of modified mortars with respect to its influence on the chemistry and morphology of hardening cement.