Role of sodium hexametaphosphate in MgO/SiO2 cement pastes

Abstract

The extent of reaction between magnesium oxide (MgO) and silica fume (SiO2) is normally limited and mixes require high water contents to give suitable rheology. The use of considerably lower water contents and the formation of magnesium silicate hydrate (M-S-H) gel as a binding phase is made possible by adding sodium hexametaphosphate (Na-HMP) to the mix water prior to the addition of MgO and SiO2. This results in the formation of extensive reaction products and cured samples with high compressive strength and low porosity. In this work, the effect of Na-HMP on the hydration of MgO/SiO2 mixes is investigated using high water to solids ratio samples to allow monitoring of pH and the solution chemistry during hydration. It is shown that a relatively small amount of Na-HMP inhibits the formation of Mg(OH)2 when MgO is hydrolyzed. It is proposed that this is due to adsorption of phosphate species on the MgO which inhibits the nucleation of the Mg(OH)2.. This gives rise to high Mg 2+ species in solution and elevated pH (>12) conditions relative to when Mg(OH)2 forms. In contrast, the phosphate does not suppress formation of M-S-H gel. In combination with the enhanced dissolution rate of SiO2 at high pH, M-S-H gel can form quickly without competition for Mg 2+ ions by Mg(OH)2 precipitation. Incorporating the optimum concentration of Na-HMP into the mix water therefore transforms the properties of cement paste and mortar samples formed by reacting MgO and SiO2.

Advances in the hydration of reactive MgO cement blends incorporating different magnesium carbonates

“…These relatively high w/b ratios were in line with those used in the preparation of RMC-H blends in a previous study [27]. The use of any superplasticizers that could potentially reduce the w/b ratio was omitted to avoid any alterations in the hydration of RMC [35], which could interfere with the evaluation of the influence of MA and H on the hydration of RMC-H blends. Alternatively, samples 20 M, 40 M and 60 M, involving the replacement of 20%, 40% and 60% of RMC with M, included lower w/b ratios of 0.6, 0.55 and 0.5, respectively.…”

Characterization of Magnesium Silicate Hydrate (MSH) Gel Formed by Reacting MgO and Silica Fume

“…SF dissolution accelerates with time and gradually transforms nesosilicate and dimeric-tetrahedron. Meantime, MSH gel formation rate is limited by magnesium hydroxide precipitation and silicate concentration in solution [ 47 , 48 , 49 ]. MSH gel forms on the surface of SF particles.…”

Performance of MgO and MgO–SiO2 systems containing seeds under different curing conditions

“…M-S-H formation takes place via the reaction between a magnesium source (e.g. Mg 2+ , Mg(OH)2) and the dissolved silica ions (SO4 -), in which the commonly used superplasticizer, sodium hexametaphosphate (SHMP), plays a crucial role by leading to the formation of hydration products at lower water contents [8]. MgO-SiO2 based binders can be effectively utilized in refractory castables [9] and other applications, where relatively lower pH values (i.e.…”

“…~10. [5][6][7][8][9][10][11].0) are required, such as waste encapsulation [10,11] and cementitious materials incorporating biodegradable fibers [12,13]. Furthermore, MgO-SiO2 based binders have the potential to be used as a cementitious binder in building applications due to their satisfactory fresh and hardened properties [14,15].…”