Quartz dissolution associated with magnesium silicate hydrate cement precipitation

Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high-pH (∼ 10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation start at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as a result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100–200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution–precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high-pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation and for environmentally friendly building cement, but it has not yet been developed for commercial use. This study provides new insights that could potentially contribute to the further development of M-S-H cement.

XRD investigation of cement pastes incorporating concrete floor polishing waste

The X-ray diffraction (XRD) technique has been widely used in order to investigate anhydrous and hydrated cement phases. In this study, XRD was used in order to analyze the concrete floor polishing waste (CFPW) and cementitious paste containing CFPW. The diffractograms obtained were compared with literature data in order to identify the phases of analyzed materials. Rietveld refinement of XRD pattern of paste containing 12% of CFPW addition was also carried out, in order to analyze calcite and aragonite structures, as these phases are calcium carbonate polymorphs that contribute to matrix filling. XRD pattern of CFPW showed a high concentration of carbonate phases, indicating that the concrete waste was carbonated. The CFPW addition in the cementitious matrix changed the hydrate cement products, as it induced the formation of carboaluminate phases, such as hemicarbonate. Calcite, which is a stable phase, contributed better to the filler effect, as its particles have higher volume than aragonite.

Quartz dissolution associated with magnesium silicate hydrate cement precipitation

Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high pH (~10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation starts at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100–200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution-precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation or for environmentally friendly building cement, but not yet developed for commercial use. This study provides new insights that could potentially contribute in the further development of M-S-H cement.

Stabilization/Solidification of Strontium Using Magnesium Silicate Hydrate Cement

Magnesium silicate hydrate (M–S–H) cement, formed by reacting MgO, SiO2, and H2O, was used to encapsulate strontium (Sr) radionuclide. Samples were prepared using light-burned magnesium oxide and silica fume, with sodium hexametaphosphate added to the mix water as a dispersant. The performance of the materials formed was evaluated by leach testing and the microstructure of the samples was also characterized. The stabilizing/solidifying effect on Sr radionuclide in the MgO–SiO2–H2O system with low alkalinity is demonstrated in the study. The leaching rate in a standard 42-day test was 2.53 × 10−4 cm/d, and the cumulative 42-day leaching fraction was 0.06 cm. This meets the relevant national standard performance for leaching requirements. Sr2+ was effectively incorporated into the M–S–H hydration products and new phase formation resulted in low Sr leaching being observed.

Immobilization of Radionuclide 133Cs by Magnesium Silicate Hydrate Cement

The radionuclide cesium (Cs) was solidified using magnesium silicate hydrate (M–S–H) cement. The influence of Cs+ on the reaction of the M–S–H gel system was evaluated by measuring the compressive strength and microscopic properties of the solidified body. By testing the impact resistance, leaching resistance and freeze–thaw resistance of the solidified body, the immobilizing ability of Cs+ by the M–S–H cement was analyzed. Results indicate that Cs+ only slightly affects the reaction process of the M–S–H gel system, and only slows down the transformation rate of Mg(OH)2 into the M–S–H gel to a certain extent. The M–S–H cement exhibits superior performance in solidifying Cs+. Both the leaching rate and cumulative leach fraction at 42 d were considerably lower than the national requirements and better than the ordinary Portland cement-solidified body. The curing effect of the M–S–H cement on Cs+ is mainly physical encapsulation and chemisorption of hydration products.

Internal Curing Efficiency of Pre-Wetted Lightweight Fine Aggregates on Strength Parameters of Concrete

Internal cured concrete (ICC) using pre-wetted lightweight aggregates is to replace conventional type of curing. Normally curing for conventional type is done by external to internal, which requires a large membrane and a large amount of water is required to do this type of procedure and the water which we use for curing may get runoff or get evaporated. But in ICC type of curing it cure inside to outside, the pre-wetted lightweight aggregates provides sufficient moisture to hydrate cement inside the concrete. The pre-wetted light weight aggregates, which stores water inside and acts as a reservoir. It release water when hydration process is done. Lightweight aggregates such as expanded clay or shale, vermiculite, pumice, slate, perlite having high water absorbing capacity are generally used for internal cured concrete. In this study, vermiculite and expanded clay had been used as replacements of fine aggregate, 2.5%, 5%, 7.5%, 10% and 12.5% are replaced for vermiculite and 5%, 10%, 15%, 20% and 25% for expanded clay.

ЦЕМЕНТИЙН ГИДРАТЖИЛТЫН БҮТЭЭГДЭХҮҮНД НАНОЦАХИУРЫН ОКСИД НЭМЭЛТИЙН ҮЗҮҮЛЭХ НӨЛӨӨ

Addition of nano-silica effected to increase the content of calcium hydrasilicate and to decrease the content of calcium oxide for resulting the hydrate cement. Additional mass percent of nanosilica in all three cements of 1.0 % was important to increase compressive strength because the formation of calcium hydrasilicate is increased and the content of calcium oxide is decreased which were become from the results of X-ray diffraction analysis. Therefore, the addition of nanosilica of 1.0 % is optimal dosage. This is related to the dissolution of calcium hydroxide

Hempcrete - Cement Composite with Natural Fibres

The paper describes use of hemp boon as a natural organic filler for building materials, especially concrete designed as heat - insulating filler material around the load-bearing structure of wooden buildings. In constructions, hemp has been used in the form of mats made of hemp fiber, with the addition of bonding bicomponent fibers and soda solution for protection against burning and rots. Mats are formed by pneumatic fleece, they are subsequently thermally treated and then cut to the desired dimensions. Calcium-hemp building material is a revolutionary construction and thermal insulating material which can be used for building the entire building, bricks or other insulation are not necessary. The trend is spreading across Europe from France, where the mixture of boon and lime was used in the 16thand 17thcenturies for the construction of timber-framed houses. Although there are hundreds of buildings made from hempcrete in Europe, its use in our country develops very slowly. Concrete is a mixture of hemp boon, lime hydrate, cement and water. It is a recyclable material that offers high thermal and sound insulation. The biggest advantage is undoubtedly the speed of construction, namely hemp concrete hardens very quickly.