Properties of magnesium potassium phosphate cement pastes exposed to water curing: A comparison study on the influences of fly ash and metakaolin

Highlights

• Water curing yielded slow development of compressive strength of MKPC paste.

• Incorporation of MK/FA improved the water resistance of MKPC paste.

• MK contributed more to compressive strength and water resistance of MKPC paste than FA.

• Pre-curing in air improved the water resistance of MKPC paste.

• Al and Si were enriched in the reaction product matrix of MKPC paste containing MK.

Abstract

Under moist condition the magnesium phosphate cement (MPC) exhibits poor water resistance. In this study, the effects of metakaolin (MK) and fly ash (FA) on the properties of magnesium potassium phosphate cement (MKPC) pastes exposed to water curing were investigated. The compressive strengths, water resistance and microstructures of the MKPC pastes containing 30 wt% and 50 wt% of MK or FA as replacements of MKPC under various water curing regimes for up to 180 d were examined. Results showed that the water curing yielded much slower compressive strength development and lower ultimate compressive strengths of all the MKPC pastes during the curing age in comparison to air curing. Incorporation of MK or FA improved the water resistance of MKPC pastes and the MK contributed more considerably than the FA. This may be attributed to the reaction of MK in the MKPC system, which led to the formation of an intermixed phase of struvite-K with Al and Si and/or other amorphous aluminosilicate-bearing phase and thus was beneficial for the microstructure densification, compressive strength increase, and water stability improvement of the reaction products in the MKPC pastes. In comparison, fewer contents of Al and Si were incorporated in the hydration product of MKPC pastes containing FA probably owing to the lower reactivity of FA in the MKPC pastes.

Introduction

Magnesium phosphate cements (MPCs), a type of chemically bonded ceramic [1], are typically produced via acid-base reaction between dead burnt magnesia and phosphate salts in terms of ammonium dihydrogen phosphate (NH₄H₂PO₄) or potassium dihydrogen phosphate (KH₂PO₄) through solution as shown in Eqs. (1), (2) [2], [3].

$$\mathrm{N}{H}_4{H}_2\mathrm{P}{O}_4+\mathrm{M}\mathrm{g}\mathrm{O}+5H_2\mathrm{O}\to \mathrm{M}\mathrm{g}\mathrm{N}{H}_4\mathrm{P}{O}_4\bullet 6H_2\mathrm{O}\left(\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{e}\right)$$

$$K{H}_2\mathrm{P}{O}_4+MgO+5H_2\mathrm{O}\to \mathrm{M}\mathrm{g}\mathrm{K}\mathrm{P}{O}_4\bullet 6H_2\mathrm{O}\left(\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{e}-\mathrm{K}\right)$$

Owing to its advanced properties of rapid setting and strength development and high bonding performance, MPC has a great potential to be used as repair materials for deteriorated concrete structures [4], [5]. However, the mechanical strength of MPC would decrease when it is immersed in water or exposed to moist environment due to its poor water stability [6], [7], [8]. Li et al. [8] investigated the influences of various curing regimes including air curing, moist curing and water curing on the compressive strengths of magnesium potassium phosphate cement (MKPC) pastes, and found that a decrease of up to 40% in the compressive strength of MKPC paste was caused after water curing for 28 d. Sarkar et al. [9] transferred the magnesium ammonium phosphate cement (MAPC) mortar into water curing after 28 d of air curing, and found that the compressive strength of the MAPC mortar was decreased by 20% at the age of 90 d due to the followed water curing. Similarly Seehra [10] reported that the residual strength of MAPC mortars was only 83% after the curing of 2 d in air and 30 d in water.

The leaching of unreacted phosphate salts in the MPCs and the dissolution of struvite were considered as the two main reasons accounting for the poor water resistance of MPC when exposed to water [8], [11], [12]. Li et al. [8] believed that the leaching of unreacted phosphate salts in the MKPC paste led to a decrease in pH of solution and thus facilitated the dissolution of struvite-K when cured in water. Instead Chong et al. [11] reported that the increase in pH as a result of increasing addition of limestone powder accelerated the dissolution of MgKPO₄·6H₂O. Li et al. [12] proved that the dissolution of struvite-K and the correspondingly induced loose microstructure were the main reasons causing the loss on compressive strength of MPC after being immersed in water.

In order to improve the water resistance of MPC, many strategies have been developed, which include prolonging the air curing time [13], [14], reducing the content of phosphate salts by optimizing the formulation of raw materials [8], [15], incorporating minerals or other additives, etc. Gai et al. [16] found that the incorporation of silicasol and cellulose into the MAPC system for generating a gel-like substance enhanced the density and the water resistance. However, the cellulose reduced the compressive strengths of MPC pastes. Ma et al. [17] employed silane-based redispersible polymer powder (RPP) to modify MKPC. A small amount of RPP could refine the pore structure and transform the hydrophilic surface of some minerals into hydrophobic surface. Nonetheless increasing dosage of RPP increased pore volumes in the MKPC and hence caused poor water resistance. Chitosan was used to protect the reaction product or other soluble phases in MPC from contacting with water and hence improve the water resistance of MPC [18].

Mineral admixtures, in terms of fly ash (FA), silica fume, slag, etc., were used in the MPC to improve its mechanical strength and water resistance [19], [20], [21], [22]. The combination of fly ash and silica fume optimized the pore structure, increased density, and improved the strength of MKPC at late age [19]. Slag was also reported to densify the microstructure of MKPC and therefore reduce the dissolution of reaction product and improve the water resistance of MKPC [20]. According to the study performed by Zhang et al. [21], partial replacements of MgO with sulphoaluminate cement contributed to improvement in compressive strength and water resistance of MKPC. This is believed to result from the densification of matrix owing to the filling effect of amorphous calcium sulphoaluminate hydrates in the blended MKPC. Besides the filling effect, as reported in a recent research, the improvement of strength and water resistance was also related to the synergy between the MKPC and CSA, which led to new substances generation and besides changed the morphology of reaction or hydration products [23]. Incorporation of waterglass was also reported to be beneficial for the improvement of water stability of MKPC at early age [24]. This is due to the formation of magnesium silicate hydrate gels as a result of reaction between waterglass and the Mg²⁺ in phosphate hydrates, which filled up pores in hardened MKPC pastes and prevented external water from diffusing into the pastes [24].

FA and blast-furnace slag were regarded as inert fillers in MPC systems to cut the cost and reduce exothermic output [25], [26], [27]. FA was reported to fill in the pores of hardened MPC paste and thus enhance the water resistance of the MPC paste [6], [12], [13]. Moreover, Chen et al. [28], [29], [30] partially substituted MgO with Al₂O₃ or metakaolin (MK) to improve the compressive strength and water resistance of MAPC mortar, and this might be due to the possible formation of aluminum phosphate gel owing to the reaction of alumina or the aluminum introduced from the MK in the MAPC mortar. According to Gardner et al. [31], when FA or blast-furnace slag were combined with MKPC there was a chemical interaction taking place rather than a simple dilution process due to the potential formation of potassium aluminosilicate phosphate gels. The reaction of FA in the FA-MKPC mortars was reported by Xu et al. [32], but the secondary reaction products could not be directly determined by XRD or TGA analysis. Recent studies shown that aluminosilicate material such as MK could be activated not only by highly alkaline silicate solution [33] but also by phosphoric acid to obtain a new type of geopolymer known as the phosphoric acid-based geopolymers, which possesses high mechanical properties [34], [35], [36].

Although previous studies indicated that the strength and water resistance of MKPC could be improved by properly adding mineral admixtures to some extent, the intrinsic mechanism with respect to the influence of water curing on the properties of MKPC cement pastes has not been well understood. As the MK and alumina were reported to be beneficial to improve the compressive strength and water resistance of MAPC, whether the MK plays the same role in the water resistance of MKPC has not been investigated. Both the FA and MK contain aluminosilicate phases, which have the potential to be activated by phosphate salts, but they might exhibit different effects on the properties of MKPC under water curing. This is of significance and interest to be investigated. The influence of MK and FA on the properties of MKPC pastes exposed to water immersion was studied in comparison. The reaction products and microstructures of the MKPC pastes containing MK or FA were examined with XRD and SEM/EDS further to understand the relevant mechanism.