Effect of phosphate additives on the hydration process of magnesium silicate cements

Abstract

The role of phosphate additives on the hydration process of magnesium silicate cement pastes was investigated through a multi-technique approach. A MgO/SiO 2 mixture was hydrated for 28 days either in the absence or in the presence of sodium hexametaphosphate, trimetaphosphate or orthophosphate. Information on the kinetics of the hydration reaction was acquired by monitoring the free water index by means of differential scanning calorimetry, while the hydration products were thoroughly investigated by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and 29 Si solid-state nuclear magnetic resonance spectroscopy. The overall results provide new insight into the effect of phosphates on the hydration reaction and on the structure of magnesium silicate hydrate cements. All additives showed a plasticizing effect and promoted the formation of the binding phase magnesium silicate hydrate (M-S-H), without significantly altering its structure. Sodium orthophosphate was found to be by far the bestperforming additive, even better than sodium hexametaphosphate, which is commonly used in these cementitious formulations. For the first time, 31 P solid-state NMR investigation allowed orthophosphate ion to be identified as the effective species. Keywords Magnesium silicate hydrate Á Orthophosphate Á Plasticizer Á Hydration kinetics Á Thermal analysis Á MAS NMR A1 Monica Tonelli and Francesca Martini have contributed A2 equally to this work.

Research progress on magnesium silicate hydrate phases and future opportunities

“…Also the formation of M-S-H in hydrated cement paste in the presence of phosphate was studied in [22,23,30,70],…”

“…Phosphates can used as superplasticizer reducing the amount of water needed to achieve a good workability [22]. Additionally, the phosphates act as accelerator to the reaction resulting in faster M-S-H formation [22,23] and help to obtain good compressive strength to the M-S-H matrix after 28 days (~50-80 MPa) [22,24]. The interest in M-S-H as the main component of cementitious materials grew in the last decades due to the low pH of the cement [25,26].…”

Chemical and Microstructural Properties of Designed Cohesive M-S-H Pastes

“…) [49,50] and minor changes in the structure of M-S-H gels have been observed [48,50]. NaHMP could thus cause chemical and mineralogical differences between in situ precipitated M-S-H and synthetized M-S-H. At high w/b ratios, the structure could be modified.…”

“…Concerning the first aspect, two protocols for manufacturing M-S-H pastes have been developed in the literature to meet the high need of water of MgO powder [45], either using a high water/binder ratio (w/b = 2) [44,46] or sodium hexametaphosphate (NaHMP), a superplasticizer (0.025-0.3 g in 10 g of water, with 0.4 < w/b < 0.8) [45,47]. The influence of NaHMP on the structure of M-S-H and other hydrated phases has been studied by several authors [48][49][50]. Although it led to a better rheological behavior and the formation of M-S-H over brucite (due to the adsorption of phosphate species on MgO, which inhibits the nucleation of the Mg(OH) 2 ), the formation of secondary phases (such as…”

Use of magnesium-silicate-hydrate (M-S-H) cement mixes in 3D printing applications

“…Previous studies have looked into the effect of different raw materials [20,21], role of superplasticizers [22,23], durability and mechanical properties [17,24] and microstructural characterization [25,26] of the final products in the MgO-SiO2 binders. With high initial compressive strengths reaching up to 70 MPa after 28 days of curing [27], MgO-SiO2 binders have the potential to be used as structural or non-structural building components.…”