Mechanisms on Accelerating Hydration of Alite Mixed with Inorganic Salts in Seawater and Characteristics of Hydration Products

Abstract

Interest in exploring the use of seawater as the mixing water for preparing concrete is increasing due to the lack of freshwater in some coastal regions and remote islands, where seawater is more accessible. However, up to now, the mechanism of the accelerating effect of seawater on the hydration of portland cement (PC) remains unclear. In this study, alite, a major clinker phase in PC, was hydrated with common salt solutions (NaCl, Na2SO4, and MgCl2) in seawater to explore the mechanism of acceleration. The heat release peaks of the salt-added systems shifted to an earlier hydration time with a higher peak value, which indicated the faster hydration rate of alite pastes compared to the deionized (DI) water system. The addition of the single salts was found to increase the concentration of Ca species in solutions, contributing to the increased formation of calcium–silicate–hydrates (C–S–H) and portlandite at early ages. In the Na2SO4 system, gypsum was the new hydration product, while brucite was formed in MgCl2 systems, which caused the sharp decrease of Mg species in the solution. The morphology of the early formed C–S–H was changed with the addition of the salts, and the C–S–H were characterized as thinner and longer fibers. At later ages, the incorporation of the single salts lowered the polymerization degree of C–S–H, but no noticeable morphological change was observed.

Influence of the availability of calcium on the hydration of tricalcium aluminate (C3A) in seawater‐mixed C3A–gypsum system

“…This could be due to the precipitation of gypsum from SW in the initial stage. This phenomenon has been reported in the tricalcium silicate (C 3 S)—SW paste 45 . Besides, as for the later evolution of SO 4 2− concentration in C 3 A–gypsum–DI paste with a W/S ratio of 10, its concentration approached to zero before the beginning of the acceleration period of C 3 A hydration as reported by previous studies 46–50 .…”

“…This phenomenon has been reported in the tricalcium silicate (C 3 S)-SW paste. 45 Besides, as for the later evolution of SO 4 2− concentration in C 3 A-gypsum-DI paste with a W/S ratio of 10, its concentration approached to zero before the beginning of the acceleration period of C 3 A hydration as reported by previous studies. [46][47][48][49][50] By contrast, SO 4 2− was still detectable in the pore solution of the SW paste (Figure 3C,F) before the occurrence of main exothermic peak, and its concentration was still approximately 0.5 g/L at 120 h of hydration for the SW paste prepared with W/S ratios of 1 and 10.…”

A review of recent advances in the science and technology of seawater-mixed concrete

“…The acceleration in hydration has been attributed to the presence of various ions in seawater, which supposedly accelerate the hydration of tricalcium silicate (C3S). Direct studies of C3S hydration in the presence of seawater confirm the acceleration both in terms of time and peak heights that is seen in cements with results broadly similar to those shown in Figure 1 [ 19,25]. The mechanism of hydration acceleration in in C3S pastes is suggested to be as follows [19]: calcium hydroxide reacts with the soluble ions in seawater, which leads to greater pH, and increased formation of greater amounts of gypsum (Eq.…”

“…Multiple studies have shown that seawater accelerates the early-age hydration of cement [11,[16][17][18][19][20][21][22][23][24][25][26]. Some studies show that the induction period is unaffected, while others show a shortening in the duration of the induction period.…”

“…To further simplify the effects of seawater on hydration processes, hydration of alite in the presence of three salts (sodium chloride NaCl, magnesium chloride MgCl2, and sodium sulfate Na2SO4) was studied [25]. Unlike in cement pastes, all salts reduced the induction period considerably.…”