Introduction
Moderate heat Portland cement (MHC) enhances early concrete strength, reduces hydration heat, and increases crack resistance, making it widely used for dam construction. Most dams located underwater therefore having to face to leaching issues. Prolonged concrete exposure to water leads to the dissolution of Ca(OH)2 from hydration products, thus decreasing internal alkalinity so as to cause the breakdown of ettringite, C-(A)-S-H, and clinker. The phenomena result in deterioration of mechanical properties and impermeability. Current research on the leaching of cementitious material mainly focuses on ordinary Portland cement (OPC) and its blends with mineral admixtures, while little attention is paid to MHC and moderate heat Portland cement-fly ash (MHC-FA) systems. The latter two MHCs are commonly employed in dam construction. In this work, leaching tests were conducted on MHC pastes and MHC-FA pastes. Comparison with OPC pastes system was further carried out, focusing on the evolution of leaching depth, deterioration of compressive strength, and the impact on phase composition and C-(A)-S-H structure.
Methods
Specimens with a water-to-cement ratio of 0.5 were prepared from MHC pastes (M0), MHC with 30% FA pastes (M30), and OPC pastes (P0). The size of the formed specimens was 40 mm× 40 mm×40 mm. After curing in a standard concrete curing room for two years, five surfaces of the specimens were sealed with epoxy resin, leaving one surface exposed for accelerated leaching in a 5 mol/L NH4Cl solution. The specimens were taken out from the solution after 7, 14, 28 d, and 56 d, following a test on compressive strength and leaching depth. The leaching depth is determined by spraying a 1% phenolphthalein solution in alcohol on the cut-open specimens. Samples were taken from areas 0–3 mm and 3–6 mm from the leaching exposure surface and pristine surface at 56 d. The thickness of 0–3 mm is labeled as the external layer of the leaching zone, and the thickness of 3–6 mm is labeled as the internal layer of the leaching zone. Characterization techniques included X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), and Solid-State 29Si Nuclear Magnetic Resonance (29Si NMR) spectroscopy. In addition, the porosity of samples was evaluated using the hydrostatic weighing method.
Results and discussion
The Leaching depth of M0 develops slower than OPC, while the M30 is the fastest. Initially, the compressive strength of M0 is higher than P0, but adding 30% fly ash to M0 lowers its compressive strength. Before leaching, the compressive strength of M0 is higher than that of P0, and the M30 has the lowest compressive strength. The compressive strength of the specimens decreases as the leaching time increases. Among them, the compressive strength of M0 is always the highest, and M30 is always the lowest. Compared with OPC, MHC has higher initial compressive strength and a lower strength decline rate, benefiting its long-term corrosion resistance. Although M30 has the lowest initial intensity, its intensity decreases more slowly. Additionally, before leaching, M30 has the highest porosity, P0 is in the middle, and M0 the lowest. Leaching increases the porosity of all samples evenly. Despite M30 has the highest initial porosity, the increasing rate of porosity is the lowest. Similar phenomena are observed for M0 and P0, while P0 shows the highest increasing rate of porosity. There is no significant difference in hydration products among the P0, M0 and M30 in the unleaching regions. The prominent XRD diffraction peaks of the sample before dissolution include Ca(OH)2, sjoegrenite, ettringite, CaCO3 and gypsum, and C3S, C2S and C4AF. M30 also contains quartz peaks. The long-time curing leads to carbonation in the external layers. Thus, after leaching, a significant CaCO3 peak appears. For the leached external layers of P0 and M0, the main phases include CaCO3, sjoegrenite, ettringite and gypsum, as well as C3S, C2S and C4AF. All ettringite dissolves in the leached external layers of M30, showing weakened peaks of C3S and C2S, yet ettringite remains in the internal layer. TGA reveals the Ca(OH)2 contents of 26.0% for P0, 22.1% for M0, and 5.3% for M30, showing that P0 has the strongest leaching buffering effect. Based on the former result that M0 has the best leaching resistance, the experiments reveales no guaranteed relationship between strong buffering and optimal leaching resistance. This can be understood as that Ca(OH)2 is the most soluble, and is therefore easier to create more pathways for leaching, leading to significant microstructural damage. The 29Si NMR results reveal that a significant increase in MCL is observed in all the samples due to the decalcification of C-(A)-S-H. Both the internal and external layers of M30 exhibit the highest growth proportions, accompanied by the formation of Si-Al gel. Although the MCL of M0 exceedes that of P0 in both the internal and external layers after leaching, the growth of silicate chain is still lower than that of P0. However, M30 experiences the lowest compressive strength reduction rate, suggesting that microstructural changes from soluble substance leaching may show greater impact on declining mechanical properties than silica gel formation due to C-(A)-S-H decalcification. During the leaching, the stability of AlVI is greater than AlIV.
Conclusions
The main conclusions are summarized as follows: Compared to OPC pastes, MHC pastes experience a slower leaching depth development, leading to enhanced residual compressive strength, decreased pore degradation, mitigated declines in compressive strength and changes in C-(A)-S-H gel structure. Before leaching, OPC pastes has the highest Ca(OH)2 content, MHC pastes the middle and MHC-FA pastes the lowest. Leaching leads to a significant increase in the MCL of C-(A)-S-H in the paste. After leaching, both the internal and external layers MHC-FA pastes exhibit the largest increase in MCL, accompanied by the production of silica-alumina gel. The average increase of silicate chain length is observed greater in C-(A)-S-H of OPC pastes than that of MHC pastes. During leaching, the stability of AlVI exceeds that of AlIV.