The kaolin used in this study was collected in a mine located in Romana (Sassari, Italy). For the chemical composition of kaolin and its mineralogical, morphological and spectroscopic characterization, see Novembre et al. . The kaolin was triturated, and the sandy fraction was separated by retention in a sieve. The fraction below 90 μm was then collected, suspended in distilled water, sonicated, and centrifuged for separation of the silt fraction and collection of the clay fraction. Preliminary calcination of kaolin was carried out using the following procedure: aliquots of kaolin were placed in open porcelain crucibles which were heated in a Gefran Model 1200 furnace (Gefran Spa, Brescia, Italy) to the calcination temperature (650ºC) at a pressure of 1 atm. The heating rate of the sample was 1.5ºC s-1. Once the calcination temperature was reached, the crucibles were left in the furnace for 2 h and then removed and cooled at room temperature. The NaOH used in the synthesis protocol were purchased from Riedel-de Haën (Honeywell Riedel-de Haën, Bucharest, Romania). The purity of the reagent was of 99%. 2 g of metakaolinite have been dissolved in 20ml of a NaOH (8%) solution. The initial mixture had the composition: 6.25 SiO2 – 1.00 Al2O3 – 3.6 Na2O. The mixture was homogenized for two hours with a magnetic stirrer. Then was put inside a stainless-steel hydrothermal reactor and heated at 10 °C/min until the desidered temperature (65-100 and 190°C) and kept for different times. Synthesis products were sampled periodically from the reactor, filtered with distilled water and dried in an oven at 40°C for a day.
Kaolin and products of synthesis were analysed by powder X-ray diffraction (XRPD); the instrument was a Siemens D5000 operating with a Bragg-Brentano geometry (CuKa=1.518 Å, 40 kV, 40mA, 2-45°, 2-90° 2theta scanning interval, step size 0.020° 2theta). Identification of Na-P and relative peak assignment was performed with reference to the following JCPDS code: 00-039-0219. Both the crystalline and amorphous phases in the synthesis powders were estimated using quantitative phase analysis (QPA) applying the combined Rietveld and reference intensity ratio (RIR) methods; corundum NIST 676a was added to each sample, amounting to 10% (according to the strategy proposed by Novembre et al.  and the powder mixtures were homogenized by hand-grinding in an agate mortar. Data for the QPA refinement were collected in the angular range 5-120° 2theta with steps of 0.02° and 10s step-1, a divergence slit of 0.5° and a receiving slit of 0.1mm.
Data were processed with the GSAS software  and the graphical interface  starting with the structural models proposed by Albert et al.  for Na-P1 and Gatta et al. for nepheline . The following parameters were refined: background parameters, zero shift, cell parameters and peak profiles.
Thermal stability and phase transformations were studied using high-temperature X-ray diffractometry with a PANalytical X’Pert PRO MPD (CuKα =1.518 A ˚, 45 kV, 40 mA, X’Celerator Detector with active length of 2.122 º, q/2q scan from 5 to 50º 2q with step size of 0.017º and measuring time of 100 seconds per step), equipped with a high temperature camera Anton Paar HTK1200N (thermocouple Pt 10% RhPt). The sample holder was a platform with a 16mm diameter equipped with a ceramic cup (0.8mm deep and 14mm inner diameter) for holding powder. The analyses were taken at different temperatures: from 28° up to 1000ºC, every 100ºC. Slope was 10ºC/min. The program pakage GSAS - EXPGUI was used for the calculation of cell parameters, using the Rietveld full-profile method starting with the structural models proposed by Albert et al.  for Na-P1 and Gatta et al.  for nepheline.
Morphological analyses were obtained by means of scanning electron microscopy (JEOL JSM-840 served by a LINK Microanalysis EDS system, with operating conditions of 15kV and window conditions ranging from18 to 22 mm) .
Induced coupled plasma optical emission spectroscopy technique (ICP-OES, Perkin Elmer Optima 3200 RL) was performed on synthesized powders through previous fusion (Pt meltpot) in lithium meta-tetra borate pearls and subsequent acid solubilisation and analytical determination .
Zeolite density was calculated by He-picnometry using an AccuPyc 1330 pycnometer. The specific surface and porosity were obtained by applying the BET (Brunauer-Emmett-Teller) method with N2 using a Micromeritics ASAP2010 instrument (operating from 10 to 127 kPa) .
The infrared analysis was performed with a spectrometer FTLA2000, served by a separator of KBr and a DTGS detector; the source of IR radiation was a SiC (Globar) filament. Samples were treated according the method of Novembre et al. [39-40] using powder pressed pellets (KBr/sample ratio of 1/100, pressure undergone prior determination 15t/cm2); spectra were processed with the program GRAMS-Al (GRAMS/AI TM Spectroscopy Software, Thermo Scientific Company).
Differential thermal analysis (DTA) and thermogravimetry (TG) were performed on the zeolitic powder using a Mettler TGA/SDTA851e instrument (10º/min, 30-1100ºC, sample mass of ~10 mg, Al2O3 crucible) (Mettler Toledo, Greifensee, Switzerland) .