Investigating the role played by portlandite and C-A-S-H in the degradation response of pozzolanic and slag cements to external sulphate attack

Abstract

Cements made with pozzolanic materials and/or latent hydraulic materials are developed to replace Portland cement and decrease the impact of cement production on the CO2 emissions. Those cements need to be qualified regarding their performances and durability, knowing they exhibit good behavior when exposed to external sulphate attack. Pozzolanic materials offer a specific composition of hydrated cement phases with less portlandite and more C-A-S-H than Portland cement. Moreover, they contain a reduced proportion of clinker, hence a reduced share of C3A. All these characteristics explain the unique response of pozzolanic cement to external sulphate attack. However, the pozzolanic reaction is more than often incomplete, with some remaining portlandite in the hydrated cement paste. In order to investigate the role played by this remaining portlandite fraction during external sulphate attack, the behavior of two CEM III and two CEM IV cement-based mortar samples were studied and compared to two CEM I- type cements, one SR and one non-SR. The pozzolanic nature of these cements has been investigated by monitoring the portlandite content of cement paste over time by thermogravimetric analysis. In addition, the behavior of synthetic C(-A)-S-H containing portlandite and in suspension in sulphate solutions has been assessed by X-ray diffraction and scanning electron microscopy. Results have confirmed the sulphate resistance of the studied blended cements compared with a sulphate-resistant Portland cement (CEM I SR3), although the cements still containing portlandite did undergo a slight expansion. The test on C(-A)-S-H showed that the presence of portlandite encourages the precipitation of ettringite within the C-A-S-H nano porosity, even in the absence of pH regulation.