Mechanism of expansion of mortars immersed in sodium sulfate solutions

The mechanisms behind the expansion of samples immersed in solutions containing sulfate ions have been controversial. Expansion is believed to originate in the formation of ettringite, but it is not possible to find any direct correlation between these two processes or to relate expansion purely to the increase in solid volume. Crystallization pressure is considered to be the most likely mechanism. A key aspect of this mechanism is the existence of supersaturation in the solution with respect to the growing crystal and it is not possible to directly measure the composition of the pore solution at different points within the specimen. In this paper we present a refinement of the crystallization pressure hypothesis to explain sulfate related expansion. We provide evidence that expansion is related to the transformation of monosulfate crystals embedded in the C-S-H to ettringite. Initially incoming sulfate reacts with aluminate containing hydrates in large pores to give ettringite without expansion. During this process the composition of the solution is buffered by the precipitation of solids so the rate of ingress does not depend strongly on the solution composition. After all this "freely available" aluminate has reacted, the concentration of sulfate ions in the pore solution increases, as indicated by the rise of sulfate absorbed on C-S-H. When the solution concentration reaches a critical level it provides the driving force for the precipitation of ettringite crystals in small pores within the C-S-H. Due to the confinement and the supersaturation of the solution this formation of ettringite leads to expansion of the paste. (C) 2012 Elsevier Ltd. All rights reserved.

On the relevance of volume increase for the length changes of mortar bars in sulfate solutions

Wolfgang Kunther, Barbara Lothenbach, Karen Scrivener

The ingress of sulfate ions into cementitious materials leads to the formation of ettringite, gypsum and other phases. The increase in solid volume through the formation of these phases is often assumed to be the only reason for expansion. In this paper we systematically compare the volume increase predicted by thermodynamic modeling to macroscopic expansion for mortars made with CEM I in different sulfate solutions and for mortars made with a range of blended cements in sodium sulfate solution. It is shown that the length changes cannot be explained by simple volume increase alone. A more plausible explanation of expansion lies in the theory of crystallization pressure, in which crystals forming from a supersaturated solution may exert pressure on their surroundings. It is observed that expansion occurs in systems where thermodynamic modeling predicts the co-existence of ettringite with gypsum. In such a case, if monosulfate and gypsum are both present locally, the solution can be highly supersaturated with respect to ettringite, whose formation in confined conditions (such as within C-S-H) can then exert expansive forces. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2013

Influence of bicarbonate ions on the deterioration of mortar bars in sulfate solutions

Wolfgang Kunther, Barbara Lothenbach, Karen Scrivener

This work investigates the influence of bicarbonate ions on the deterioration of cementitious material exposed to sulfate ions. Mortars based on a CEM I and on a CEM III/B cement were investigated. Experimental investigations were compared to thermodynamic modeling and phase characterization to understand the differences in deterioration. The presence of bicarbonate ions significantly reduced the expansion of the CEM I mortars. Thermodynamic modeling showed that at high concentrations of bicarbonate ettringite and gypsum become unstable. Microstructural characterization combined with information from thermodynamic modeling suggests that conditions of high supersaturation with respect to ettringite are unlikely in the samples exposed in solutions containing bicarbonate. Consequently, expansive forces are not generated by the crystallization pressure of ettringite. There was little expansion of the CEM III/B sample even in the sodium sulfate solution. In the bicarbonate solution this mortar showed a highly leached zone at the surface in which calcite was observed. (c) 2012 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2013

Degradation mechanism of slag blended mortars immersed in sodium sulfate solution

Karen Scrivener, Wei Sun, Cheng Yu

Additions of slag are usually considered to give cementitious materials which perform well in sulfate bearing environments. This paper compares the deterioration of slag cement blends to those of plain Portland cement and demonstrates that this occurs more through loss of surface than macroscopic expansion. When slag blended mortar is immersed in sodium sulfate solution, the sulfate ions penetrating into samples are mostly fixed by aluminate phases in a relatively narrow region close to surface, due to the refinement of pore size and buffering effect by slag addition. After all available aluminate phases have been reacted, the concentration of sulfate ion in pore solution will increase. Once the solution concentration reaches a critical level, fine AFm crystals confined within the C-S-H can react to form ettringite, exerting expansion force. If the system contains sufficient confined AFm phases, this process can cause spalling of the surface layer. Then, sulfate ions can penetrate into the sound area, manifesting another expansive area. Furthermore, the penetration depth of slag sample does not depend strongly on the concentration of sulfate solution, but higher concentration increases the crystallization pressure of ettringite, thereby causing more damage. (C) 2015 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015