Influence of cement and aggregate type on thaumasite formation in concrete

In this study the influence of binder type on the formation of thaumasite in mortars made with expanded clay lightweight aggregates (LWA) was examined. For this purpose mortar prisms (20x20x160mm) were made with different binders and expanded clay lightweight aggregate or quartz sand, which after 28 days of curing in water were exposed to a 2.37 % Na2SO4 solution. The length and weight change of the prisms was recorded in triplicate as a function of time of exposure to dry-wet cycles in the sulphate solution or deionised water, at a temperature of 5 ± 1°C.

The influence of the binder type on the expansion of the prisms in the 2.37 % Na2SO4 solution with dry and wet cycling can be ordered from strong to weak as follows: (1) CEM I + limestone filler, (2) CEM I, (3) CEM I + fly ash, (4) CEM III. The use of specific cements or cement blends can clearly inhibit the formation of thaumasite (and/or ettringite). The influence of the aggregate type (quartz versus LWA) on the sulphate reaction is more difficult to determine because the porosity of LWA can accommodate the growing sulphate crystals.

All the mortar prisms made with quartz aggregate were bent or broken after 21 to 24 weeks exposure to the sulphate solution, while most mortar prisms made with expanded clay aggregate were still intact. The only exception being the mortar prisms containing limestone filler. The latter disintegrated between 24 and 34 weeks of exposure to the sulphate solution. The expansion of the prisms made with CEM I and CEM I + fly ash levelled off between 50 and 100 weeks of exposure to the sulphate solution. The prisms made with CEM III showed minimal expansion (≤ 0.1 %) after 3 years of exposure. The prisms exposed to demineralised water showed no or minimal expansion.

The presence and mineralogy of secondary sulphate phases in the mortars was checked by XRD analysis and SEM-EDS analysis of (degraded) mortar prisms. In most cases a solid solution of ettringite-thaumasite was observed. Based on the composition, treatment and expansion of the different mortar prisms, key factors controlling the formation of thaumasite or a solid solution of thaumasite-ettringite are discussed.