The utilization of seawater and sea sand for concrete production can effectively relieve the environmental pressure caused by the consumption of freshwater and river sand resources. Previous studies demonstrate that the mechanical properties of seawater sea sand concrete are consistent with those of normal concrete, which can meet the construction needs. However, the multiple element ions in seawater alternate the components and microstructure of hydration products via participating in the hydration reaction, affecting its durability performance. Mineral admixtures can be used to mitigate the possible adverse effects of multiple element ions on cementitious materials. The existing research on the interaction between inorganic ions and the internal microenvironment of cementitious materials mainly focus on the effect of single ions, and the influence mechanism of multiple element ions lacks systematic investigations. Meanwhile, the influence mechanism of mineral admixtures on the performances of seawater sea sand concrete Is still unclear. In this paper, the pore solution composition and the mineral phases of the hydration products were characterized to reveal the evolution mechanism of the internal microenvironment under the coupling effect of multiple element ions in seawater and mineral admixtures.

A cementitious system was designed via the centroid simplex optimization of Portland cement, fly ash and silica fume. The artificial seawater was prepared according to the standard (ASTM D1141-98) at a pH value of 8.29. Sea sand from Jiaozhou Bay, China, was used as fine aggregates with a fineness modulus of 2.65. The polycarboxylic acid high-performance water reducer with a water reduction rate of 30% was used to adjust the flowability of fresh stage cement paste, (Shanxi Feike New Material Science and Technology Co. Ltd., China). The specimens with a water/binder ratio of 0.35 and a sand/binder ratio of 1.0 were then prepared for characterization, and the water-reducing agent dosage was adjusted to maintain the same fluidity among different sample groups.

After curing for 3, 7 d and 28 d, the ion concentration in pore solution obtained from a pressure filtration method was characterized by inductively coupled plasma emission spectrometry, and its pH value was measured by a pH meter. The phase composition of powder samples after soaking in isopropanol and further drying was characterized by a model MiniFlex 600 X-ray diffractometer. In addition, the thermal behavior of the powdered samples aged at 28 d was determined by thermogravimetric analyzer/derivative thermogravimeter. The compressive strength of cubic mortar specimens with the sizes of 70.7 mm×70.7 mm×70.7 mm aged for 3, 7 d and 28 d was measured.

The compressive strength of seawater sea sand mortar specimens is higher than that of freshwater river sand mortar specimens aged at 28 d. This can be attributed to the pore filling effect by hydration products and the formation of more compact C-S-H gels in the presence of the multiple element ions.

The multiple ions in seawater also increase the pH value of the pore solution. The pore solution alkalinity can be further regulated via mixing mineral admixtures. At the same dosage, silica fume significantly outperforms that of fly ash in reducing the alkalinity of the seawater cement paste pore solution. Silica fume can nearly consume the calcium hydroxide component at a dosage of 30%, and the pH value of the pore solution decreases to 11.69 at 28 d. Also, silica fume can generate more hydrated calcium silicate gels with a low Ca/Si ratio to enhance its binding ability with sodium and potassium ions, and the enhancement effect is better than that of fly ash. The active aluminum phase in fly ash can promote the generation of aluminum phase products (i.e., AFt and Friedel's salt), and enhance the effective solidification of sulfate and chloride ions. The chloride ion solidification effect of fly ash is better than that of silica fume, and the chloride ion concentration of pore solution can reduce to 0.27 mol/L at the dosage of fly ash of 30%.

The results showed that the multiple element ions in seawater promoted the hydration of tricalcium silicate and dicalcium silicate in cement, increased the alkalinity of seawater cement pate pore solution and accelerated the pozzolanic reaction between mineral admixtures and calcium hydroxide. Silica fume significantly outperformed fly ash in reducing the alkalinity of the pore solution. The effective solidification of sodium and potassium ions was achieved through the generation of more hydrated calcium silicate gel products with silica fume. Fly ash could promote the generation of aluminum phase products (i.e., AFt and Friedel's salt) and enhanced the solidification effect of chloride ions, and its enhancement effect was better than that of silica fume. This study could serve as a solid base for the alkalinity regulation and performance design of seawater sea sand concrete.