The Occurrence Of Magnesium In Portland Clinker And Cement And Its Effect On Volume Stability Of Cement Mortar

The efficient use of magnesium-rich limestone is a practical requirement in research on Portland cement clinker chemistry,and it is a significant challenge in the cement industry that should be solved at the earliest.In this study,the relationship between the crystallization state and the distribution of dolomite in limestone,and the occurrence of periclase in clinkers was analyzed.The crystal growth process,crystal size distributions,and periclase nest formation mechanisms were analyzed.The effects of MgO on the crystal polymorph and the contents of aluminate and ferrite phases were investigated.Moreover,the formation process of magnesium silicate hydrate?M-S-H?in hydrate products was determined,and the structures of the M-S-H and Mg-Al layered double hydroxides?Mg-Al LDHs?prepared using low-activity MgO were characterized.This study discusses the occurrence states of Mg²⁺in cement and provides a theoretical foundation for the efficient utilization of magnesium-rich limestone.The main research results and conclusions are as follows:?1?The backscattered electron?BSE?images of limestone with different MgO contents for producing cement clinkers were analyzed.The results show that dolomite is dispersed and aggregated in limestone,and the particle size of dolomite ranges from micrite to megacryst.The dispersed dolomite particles are mostly planar-e floating dolomite rhombs.?2?The X-ray diffraction?XRD?,optical micrograph?OM?,scanning electron microscopy?SEM?,and BSE images were used to analyze 11 industrial clinkers.The results indicate that the periclase content changes slightly in the case of the clinkerscontaining less than 2.0%MgO.When the MgO content exceeds 2.0%,the periclase content increases with the MgO content.In industrial clinkers,periclase first forms a sheet-like or plate-like crystal.Subsequently,an octahedron structure is formed when ample space is available,and a subhedral or anhedral crystal is formed within limitedspace.The crystal size of the periclase is mainly ranges between 1 and 6.5?m;most crystal sizes range between 2.5 and 3.0?m,and the maximum particle size is as large as 40?m.The nest of the periclase is mostly elliptical,and large-particle dolomite withhigh purity is a significant reason for the aggregated distribution.?3?The laboratory clinker with two typical compositions was synthesized under different calcined regimes,and 15 aluminate and ferrite phases with different MgO contents were prepared.The phase and phase content were characterized via XRD combined with Rietveld refinement,and the morphology was characterized using the BSE image.The results indicate that prolonging the holding time and increasing the calcination temperature enhance the growth of the periclase crystal.After the burning temperature is increased and the holding time is prolonged,the crystal integralities of periclase gradually increase from anhedral or amorphous to euhedral or subhedral,and the most probable size of periclase also increases.The periclase dissolved in the ferrite and aluminate phase aggregate,integrate,and grow.The C₁₂A₇ content in the synthetic aluminum phase increases with the MgO content,and the C₄AF diffraction peak height in the Fe-rich aluminum phase increases gradually.Increasing the calcination temperature improves the MgO dissolution in the glass phase of the clinker,and the periclase content in the clinker is reduced significantly.?4?The differential thermogravimetry?DTG?and SEM images of cement prepared with the 11 industrial clinkers were analyzed.The results indicate that the Mg?OH?₂ content in samples cured at 20?is approximately 40%of that in samples cured at 80?at 7 and 28 d.Moreover,the hydration rate of the periclase at early age is higher than that at later age under the same hydration conditions.At later stages,periclase hydration is mainly controlled by ion diffusion,and the hydration rate decreases.The shape of Mg?OH?₂ formed at the early hydration age is hexagonal flaky or irregularly flaky.The Mg?OH?₂ particle thickness increases owing to the higher crystal growth rate along the c-axis at later age,and multi-layered,block-like,or plate-like Mg?OH?₂ crystals form.The area around the Ca?OH?₂ is the Mg?OH?₂-rich zone;the Mg?OH?₂ in this zone is readily carbonated together with Ca?OH?₂,and the carbonation product is CaMg?CO₃?₂ crystal.?5?SEM and energy-dispersive analysis of X-ray?EDAX?of magnesium-rich areas in cement paste indicate that Mg²⁺can replace Ca²⁺of calcium silicate hydrate?C-S-H?to form M-S-H and calcium-magnesium silicate hydrate?C?M?-S-H?.Mg²⁺originating from the clinker can produce Mg-Al LDHs with an atomic Mg/Al ratio of 2:1 or 3:1.?6?The structures of M-S-H and LDHs prepared using different activity MgO cured at varying temperatures were investigated via XRD,thermogravimetry analysis?TG?,Fourier transform infrared spectroscopy?FTIR?,and nuclear magnetic resonance?NMR?.The results reveal that the optimum water curing temperature for the formation of M-S-H is approximately 50?,and the formation rate of M-S-H reduces at very high or relatively low curing temperatures.However,increasing the curing temperature improves the degree of silica polymerization,and the Q² site tetrahedron is easily converted to the Q³ site tetrahedron.The increase in MgO activity and curing temperature contribute to the formation of LDHs.?7?In a clinker with more than 3%MgO content,the periclase content shows a good correlation with the volume deformation of the cement paste.Cement with higher periclase content exhibits lower shrinkage and higher expansion in dry and water bath environments,respectively.