The expansion of cement paste can occur as a result of secondary ettringite formation due to attack by sulfates. It has been proposed that this expansion is affected by coexisting hydration products such as calcium-silicate-hydrates (C-S-H), although the mechanism has not yet been clarified. Drying conditions may also influence secondary ettringite formation because the structure of the monosulfate is changed. The present study experimentally examined the effects of monosulfate pre-drying conditions and coexisting materials on secondary ettringite formation. Synthesized monosulfate samples dried under varying conditions and with coexisting materials such as C-S-H were mixed with a sodium sulfate solution, and the amount of secondary ettringite was found to increase following drying under 0% or 11% relative humidity. The secondary ettringite proportion was also markedly increased by the presence of C-S-H, regardless of the pre-drying conditions, and calcium-aluminate-silicate-hydrates (C-A-S-H) containing four-coordinate Al were formed in these samples. It is evident that ettringite is likely to form from four-coordinate Al, and that C-S-H promotes secondary ettringite formation because it transitions six-coordinate Al derived from the monosulfate to four-coordinate Al by substituting Al in the Si chains.
This paper presents an experimental and numerical study on a new hybrid structural system with RC frame and stiffened masonry wall. Summarily, four different connection types between frames and reinforced masonry walls were divided in this investigation. Firstly, construction for different connecting modes were described in detail and then tests of four specimens under reciprocating load were carried out to study the influence of connection modes on failure modes and hysteretic response. Results reveal that all these specimens have higher ductility and lower stiffness degradation than RC frame structure and frame-shear wall structure. Furthermore, the specimen of type Ⅳ shows lower ductility and higher ultimate bearing capacity and energy dissipation than the other three because of the more effective connection mode to avoid weak load-bearing point and unsatisfied failure mode. Finally, FE models were established to simulate the seismic behavior of testing specimens. Numerical results can accurately predict the failure modes and cyclic response except for failing to fully replicate the pinching response due to ignorance of bond slip effect. Study results will provide solid support and reference for the further nonlinear seismic response analysis and for the application in practical engineering for this new proposed hybrid structural system.