Formation of magnesium silicate hydrate cement in nature

Abstract

Tillite lithified by magnesium silicate hydrate (M-S-H) cement has been formed at the surface 10 of the Feragen Ultramafic Body in SE Norway. Serpentinization of olivine-rich rocks led to formation of 11 brucite that dissolves during weathering to form a high pH (>9) Mg-rich fluid. This fluid dissolves quartz 12 deposited by glaciers during the Weichselian glaciation. Subsequent evaporation leads to the precipitation 13 of a nanocrystalline magnesium silicate hydrate phase with the approximate composition 14 Mg8Si8O20(OH)8·6H2O. The resulting cemented rock is characterized by disintegrated and partly dissolved 15 quartz grains that are surrounded by the M-S-H cement. This process occurs at surface conditions in a 16 subarctic climate and is known to take place on the timescale of a few decades, as constrained by mining 17 activities. Magnesium-based cement is currently of high interest as the search for environmentally friendly 18 cement is a pressing task considering that the widely used Portland cement accounts for about 7% of the 19 worldwide anthropogenic CO2 emission. As this study provides new key insights into the formation process 20 of natural CO2-neutral magnesium-based cement on the field-and microscale, it could contribute in the 21 development of a new sustainable construction material. 22 23 24 CO2 sequestration (Cipolli et al. 2004;Hansen et al. 2005;Hänchen et al. 2008;Kelemen & Matter 2008; 35 Wilson et al. 2009;Beinlich et al. 2010Hövelmann et al. 2011;Oskierski et al. 2013), and also the 36 reason for earlier research at the FUB (Beinlich & Austrheim 2012). This ultramafic complex is accordingly 37 weathered and displays an extensive fracture network with reaction rims despite its location in a subarctic 38 climate (Ulven et al. 2017). Furthermore, the FUB is partly covered with glacial till deposits that contain 39 quartzofeldspatic material, emplaced during the Weichselian glaciation. The presence of felsic glacial 40 deposits in the weathering zone of ultramafic material creates a unique geochemical system which allows 41 mineral-fluid reactions involving minerals that are usually incompatible in nature. Therefore, the cement 42 lithifying the tillite at Feragen is expected to have a unique composition and contain both ultramafic and 43 felsic clasts. 44 45 Natural magnesium silicate hydrate cement has to our knowledge not been described before, but is well-46 known in the cement industry as M-S-H (magnesium silicate hydrate) cement. The production of Portland-47 based cement, the most common type of building cement which primarily hardens due to the precipitation 48 of C-S-H (calcium silicate hydrate), accounts for 7% of the total anthropogenic CO2 emission and is 49 therefore the second largest source of anthropogenic CO2 (IPCC 2005). The growing environmental 50 concerns have made the search for an environmentally friendly alternative a state of the art research topic 51

Quartz dissolution associated with magnesium silicate hydrate cement precipitation

“…The Feragen ultramafic body consists of dunite and peridotite which is serpentinized to various degrees (Moore and Hultin, 1980). The serpentinized rocks have a weathering rind of about 1-2 cm which is depleted in magnesium due to the dissolution of brucite, while olivine and serpentine appear unaffected by the weathering and are approximately equally abundant inside and outside the rind (Ulven et al, 2017;De Ruiter and Austrheim, 2018). The rocks also contain many fractures providing fluid pathways that enhance the dissolution.…”

“…Furthermore, other minerals might contribute to the Si and Mg concentration in solution. For example, feldspar is also replaced by cement (De Ruiter and Austrheim, 2018).…”

“…Mica is a representative 2:1 phyllosilicate mineral, so it is likely that other phyllosilicate minerals have similar values. The exact amount of pressure needed or involved in the dissolution is nevertheless unclear and it therefore remains speculation whether the magnesium silica hydrate cement, which is a 2:1 phyllosilicate (Roosz et al, 2015;De Ruiter and Austrheim, 2018), will enhance the dissolution of quartz at ambient conditions by precipitating on the quartz surface and having a more negative electrochemical surface potential than quartz.…”

“…The natural magnesium silicate hydrate cement is similar, both compositionally and structurally, to synthetic M-S-H cement (De Ruiter and Austrheim, 2018), and it is therefore reasonable to ask what we can learn from nature in our attempts to develop M-S-H cement for commercial use. M-S-H cement has been suggested as a low CO2 cement that could replace Portland cement (Imbabi et al, 2012), although it is currently mainly of interest as a cement for the encapsulation of nuclear waste due to its pH being lower (9-10) than that of conventional Portland cement (>12) (Zhang et al, 2011;Zhang et al, 2012).…”

Direct Observations of the Coupling between Quartz Dissolution and Mg-Silicate Formation

“…8,9 The experimentally obtained data on quartz dissolution do, however, not correspond to the recently described abnormally fast dissolution of quartz within natural rocks that have been weathered at high pH and Mg-rich conditions at the surface. 10 Controversially, that means that the rates obtained from the field are faster than the rates obtained from laboratory experiments. For other silicate minerals such as feldspar, a discrepancy between laboratory and field results has widely been observed, where the laboratory results almost always indicate higher rates.…”