Crystal growth and transition of calcium carbonate depositing by homogeneous precipitating reaction in CaCl₂- (NH₂) ₂CO-urease system were studied. The synthesis process of calcium carbonate in the present work characterized as homogeneous reaction among calcium ion, NH₃ and CO₂ happened by hydrolyzing (NH₂) ₂ CO solution using urease for biological catalyst under room temperature. Characterization of the synthetic calcium carbonate was determined by means of X-ray diffraction, scanning electron microscopy and chemical analysis.
The different modifications of calcium carbonate were prepared respectively by adding urea into the mixed solution of 0.20mol/dm³ CaCl₂ solution and 0.02-0.20g/dm³urease solution at 20-50°C as made 1-30 in (NH₂) ₂ CO/CaCl₂ mole ratio. The urea solution was easily hydrolyzed to CO₂ and NH₃ by adding urease in CaCl₂- (NH₂) ₂CO system and then calcium carbonate was precipitated slowly as vaterite or calcite but aragonite was not found.
The crystal growth and transition of calcium carbonate were affected remarkably by synthetic conditions such as (NH₂) ₂CO/CaCl₂ mole ratio, concentration of urease and reaction temperature. For example, under the concentration of urease less than 0.03g/dm³ and the mole ratio of (NH₂) ₂ CO/CaCl₂ 1, the spherical vaterite with 4 μm diameter as secondary particles formed independently and then transformed to rhombohedral-like calcite with 5-8 μm diameter in mother liquor. The crystals of rhombohedral-like calcite was developed correspondingly by increasing transition rate of vaterite to calcite. Moreover the primary particles of spherical vaterite had a tendency to change from needle-like to plate-like crystal with rising reaction temperature until 50°C.

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Formation and coagulation processes of vaterite in the reaction of the system Ca (OH) ₂-CH₃ OH-H₂O-CO₂ were investigated by measuring the electrical conductivity, pH and Ca²⁺ ion concentration in the slurries and by means of X-ray diffraction, thermal analysis (TG-DTA), electron microscopic observation, measurements of IR spectra and specific surface area of the products. When CO₂ gas was introduced into the slurry at a constant flow rate, ellipsoidal coagula composed of vaterite crystallites were formed. The longitudinal and lateral diameters of ellipsoidal coagulum were 30 and 20 nm, respectively. By the electron microscopic observation of the products during the reaction it was suggested that these coagula were formed by the crystallization of amrphous CaCO₃ into pure vaterite crystal and the successive coagulation of vaterite crystallites. The c-axes of vaterite crystallites oriented parallel to the longitudinal direction of ellipsoidal coagulum. When pH value in the slurry was kept at 10 and 11 by control of flow rate of CO₂ gas, on the other hand, spherical and marble-like coagula were formed. The specific surface area and the particle size of vaterite coagula were controlled by particlate growth used spherical coagula as seed. Coagulation model of vaterite crystallites depending on pH value in the slurry was proposed.

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In this report, we discuss the reaction of various types of hydrated products and sulfuric acid by means of SEM, . X-RD, FT-IR and N₂ gas adsorption. And the acid resistance of expansive concrete was reported compared with the ordinary concrete. Ca (OH) ₂ used was reagent grade and AFt, AFm, C-S-H, Tobermorite and Xonotlite were synthesized. Reaction products of AFt or AFm and H₂ SO₄ are CaSO₄·2H₂O and Al₂ (SO₄) ₃·nH₂O. In the case of C-S-H with H₂ SO₄, CaSO₄·2H₂O and SiO₂ gel are produced. And the reaction products of Ca (OH) ₂ or Xonotlite or Tobermorite and H₂ SO₄ are detected CaSO₄·2H₂O. In the case of Xonotlite or Tobermorite, the form of SiO₂ are not cleared. When AFt is reacted with sulfuric acid, CaSO₄·2H₂O and AL₂ (SO₄) ₃·nH₂O are produced. When NaOH is added to these mixtures, AFt is reproduced.
Though the phase composition of hydrated products of expansive concrete is different from that of ordinary concrete, the same tendency is indicated for acid resistance. Compressive strength of hardened expansive concrete in sulfuric acid solution is decreased in analogy with the case of ordinary concrete. But compressive strength of concrete cured in dil-H₂SO₄ solution from early age is increased. This may be caused on the neutralization of anhydrated materials and sulfuric acid on the surface of concrete or in the pore of hardened body.

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Monosulphate to be generally known as a hexagonal plate-like crystal is expected to be utilized as new inorganic filler in place of kaolinite clay.
In the present work is the synthesis of monosulphate in the aqueous solution under the atmospheric pressure.
The synthetic method of this sample was that an original raw material was heated in the aqueous solution with continuous stirring. The effects of the reaction temperature, time and the added substances on the product were investigated. The unusual specifically of the obtained product was observed by XRD and SEM. These results were summarized as follows :
(1) Monosulphate was synthesized by heating the starting raw substance having the mole ratio of 12 : 4 : 1 to Ca (OH) ₂ : Al (OH) ₃ : Al₂O₃ (SO₄) ₃·17H₂O at 60°C for 168 hr. The following synthetic equation of monosulphate was derived :
12Ca (OH) ₂ + 4Al (OH) ₃ + Al₂ (SO₄) ₃·17H₂ O + aq.→3 (3CaO·Al₂ O₃·CaSO₄·12H₂ O)
(2) The width of the hexagonal plate crystal of monosulphate was controlled by the reaction temperature and thickness of them by the concentration of calcium acetate in the aqueous solution.

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Pure calcium ferrite hydrate was synthesized at various hydration temperatures : 5°C, 25°C, 40°C, 60°C and 80°C. Hydration products were studied by means of XRD, electron micro diffraction, scanning electron microscope-energy dispersive X-ray analyser (SEM-EDX) and thermal analysis. It was confirmed by XRD that a tetra calcium alminate hydrate-like layered compound formed at various hydration temperatures and it was stable at lower hydration temperature. Microscopic examination of products hydrated for 90 days at 5°C showed a thin plate-like hexagonal crystal, maximum up to 3μm across, and single crystal pattern of sharp hexagonal reciprocal lattice with a₀=0.58nm. SEM-EDX investigation of these hexagonal plates showed that the average of their composition ratio CaO : Fe₂O₃ is equal to 4.0±0.3 : 1.0. Thermal gravity analysis of products hydrated for 90 days at 5°C showed that composition ratio Fe₂O₃ : H₂ O in C₇ FH_x crystal is equal to 1.0 : 12.0, and the composition formula of unit cell may be written as [Ca₂Fe (OH) ₆] OH·2.5H₂ O. A well aged sample such as C₇FH₁₂ could mostly be indexed assuming hexagonal cells giving (001) and (110) refractions, the lattice constants were a₀=0.590nm and c₀=0.795nm (for C₄FH₁₂). It was confirmed by thermal differential analysis that all the intercalated waters of C₄ FH₁₂ crystal were lost at 150°C.

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Some magnesium phosphates were prepared by adding a Mg (NO₃) ₂ solution into mixed solutions of KOH and H₃PO₄ at temperatures of 25-95°C, and investigated with respect to their formation processes and chemical compositions.
The formation regions of the resulting magnesium phosphates were mainly affected by synthetic temperature and pH. Namely, MgHPO₄·3H₂O (DMP3) was formed in an acidic pH region at 25-29°C, while in an alkaline pH region, KMgPO₄·6H₂O (KMP6) and KMgPO₄·H₂ O (KMP1) were formed at 25-60°C and 70-95°C, respectively. Further, a double salt of Mg₃ (PO₄) ₂ hydrate and potassium phosphate was formed under a limited conditions of temperature (50-95°C) and pH (near 7).
KMP6 changed to an amorphous phase at 100°C by dehydration of 6 moles of water corresponding to the lattice water, and crystallized to KMgPO₄ (KMP) at 380°C. On the other hand, KMP1 changed to an amorphous phase by dehydrating the lattice water at 200°C, and crystallized to KMP at 470°C.
By suspending KMP6 in water, KMP6 varied to Mg₃ (PO₄) ₂·22H₂O (TMP22) at 40°C, and to Mg₃ (PO₄) ₂·8H₂O (TMP8) at temperatures between 60°C and 80°C.

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