Non-destructive evaluation of internal sulphate attack in cement-based materials applying non-linear ultrasonic techniques

One of the most aggressive attacks to which cement-based materials can be exposed is that produced by sulphate. During this attack expansive products are formed, causing volumetric strains in hardened materials, which brings microcracking and the reduction of their strength and durability.

The use of non-destructive techniques for characterizing the microstructure and service properties of cement-based materials has become an important research field. Particularly, they can also be useful for following the development of deleterious processes which can affect those materials. Among them, non-linear ultrasonic (NLU) techniques have shown to be useful for evaluating the material degradation. In recent studies, they have been used for detecting cracks due to steel corrosion in concrete structures. NLU technique is based on the fact that the non-linear interaction of ultrasound with cracked materials generates higher acoustic harmonics.

The aim of this work is to study the possibility of using the NLU technique for the non-destructive evaluation of damage in cement-based materials due to sulphate attack. Cement pastes were prepared using ordinary Portland cement to which an appropriate amount of calcium sulphate 2-hydrate was added during the setting for producing an internal attack in the samples. Furthermore, its effects have been followed with mercury intrusion porosimetry, scanning electron microscopy, linear ultrasonic technique, X-ray diffraction and X-ray fluorescence. The expansion and the mechanical strengths of the samples were also determined. The preliminary results indicate that the NLU technique could be useful for studying the development of sulphate attack, complementing the information provided by other techniques.