Formation of Nano-Sized Ettringite Crystals Identified as Root Cause for Cement Incompatibility of PCE Superplasticizers

Abstract

Recently, nano materials including nanoclays, nano-TiO₂ or nano-sized CSH-PCE seed crystals have gained considerable attention in building materials. It is however frequently overlooked that particularly nano-sized crystals can also form during cement hydration. This study investigates on the early ettringite formation in cement, and on the interaction of polycarboxylate-based superplasticizers with such ettringite. Two different cements and five different polycarboxylate-based superplasticizers were analyzed. It was found that the formation of nano-sized ettringite instigated by common PCE superplasticizers presents the root cause for cement-PCE incompatibility which is frequently observed in concrete exhibiting low water-to-cement ratio. Experiments revealed that PCE superplasticizers act as morphological catalyst and transform common micro-meter sized ettringite into nano-sized crystals (l–200 nm). This effect can cause incompatibility between PCE and cements possessing elevated C₃A content (>7 wt.-%) and high amounts of immediately soluble sulfates. The nano-scale ettringite was identified via X-ray diffraction, elemental analysis and thermogravimetry. It can be separated from a cement paste by centrifugation where it appears as a gel-like, viscous top layer. Almost all PCE polymers, independent of their chemical nature, produce particularly small, nano-sized ettringite and thus require exceptionally high dosages with these cements. Such phenomenon is commonly referred to as “cement incompatibility” of PCE. Only one specific molecular structure of PCE was identified which produces micro-meter sized ettringite, therefore requires low dosages.