the internal buffer system is composed of a buffering salt of mono -, di -, tri - and / or tetravalent acids . the salts are typically salts of mineral acids of the type [ me : h : anion ], wherein : me = na , k , mg , ca or similar elements ; anion = co 3 2 − , po 4 3 − , so 3 2 − , so 4 2 − , sio 4 4 − or fluoride ; and h = one or more hydrogen ions . in one embodiment the cbc material of the invention is combined with an added acid as dry component in inert particle porosities for a rapid change early in the hydration process , such as improved early - age mechanical properties , as described in wo 2005 / 039508 . the ph change over time is presented in fig1 . another advantage of a controlled ph reduction after a given period of time according to the invention is the possibility to optimize the hydration during the first 30 - 60 minutes , and thereafter obtaining a substantially reduced ph to a more biocompatible range . see the ph vs . time plot in fig2 . in such embodiment , in order to achieve an optimized hydration during the first 30 - 60 minutes , the buffering salt can be impregnated in porous granules of the component forming the chemically bonded ceramic material and / or in an additional porous inert phase . the diffusion of the buffer salt into the surrounding media will thereby be delayed . according to the present invention the initial hydration reaction of very alkaline systems , such as ca - aluminate and ca - silicate ( with an initial ph value on hydration of 11 - 12 . 5 ), can be effectively controlled to be in a reduced ph range of 7 . 5 - 10 already after 1 hour of hydration reaction , preferably within the range 7 . 5 to 9 . 5 , and more preferably within the range of 7 . 5 to 9 . 0 , by the use of an internal buffer system added to the powdered biomaterial of cbc type used . the internal buffer system is composed of a buffering salt of mono -, di -, tri - and / or tetravalent mineral acids . the salts are typically acidic salts of the type [ me : h : anion ], wherein : me = na , k , mg , ca and similar elements ; anion = co 3 2 − , po 4 3 − , so 3 2 − , so 4 2 − , sio 4 4 − and fluoride ; and h = one or more hydrogen ions . very attractive are the acid salts forming the ions hf 2 − , hco 3 − , hp 4 2 − and h 2 po 4 − in aqueous solutions . examples of most suitable salts for use as the buffer are : khf 2 , nahco 3 , mg ( ca ) hpo 4 , na 2 hpo 4 and nah 2 po 4 . the ph development has been found to depend on three major material sources ; the solid raw material ( cbc ), the general hydration liquid used , and the body liquid encountered after implantation . especially effective with high buffering capacity are the solid buffers according to the present invention . the buffering capacity can be expressed as mm oh —. the capacity is as high as 100 mm , corresponding to a ph change from approximately 13 to near neutral , i . e . ph 7 - 8 . the buffering capacity of body liquids are considerably lower due to two aspects ; the ion buffering capacity expressed as mm is considerably lower , and secondly the body liquids are external and thus ph reduction is diffusion - time restricted , as generally described in ceramics , cells and tissues , ed ravaglioli and krajewski , pp 200 - 201 , published by is - tec - cnr - faenza , may 2005 . the buffering salts are preferably added in solid form to the ceramic powder . however , the salts could also be dissolved in the hydrating liquid , in which case the required buffer salt could be added to the hydration liquid only , or to both liquid and powder in any desired ratio . the skilled person having read the present disclosure can readily establish the necessary amount of a given buffer salt to be added to a given amount of the composition in order to achieve a desired ph value on hydration , e . g . merely by routine ph measurements . with reference to table 3 it can be seen that the necessary amount is dependent upon the specific cbc material used . for example , the silicate material is more alkaline than the aluminate material and will consequently require a larger amount of any given buffer salt . in general the amount of the buffer is selected so as to obtain a ph value within the range of 7 . 5 to 10 after 1 hour of hydration reaction , preferably within the range 7 . 5 to 9 . 5 , and more preferably within the range of 7 . 5 to 9 . 0 . the amount of the buffer should not exceed 30 % by volume of the total volume of the constituents of the powder system in the dry state , since otherwise the mechanical properties of the bound material will be negatively effected . on the other hand , an amount of less than 3 % by volume of any buffer salt will not produce a sufficient ph controlling effect . with reference to tables 1 and 2 it can be seen that the ph in phosphate buffer solution ( i . e . a simulated body fluid ), declines more rapidly than in water . a volume of 3 % of the acid buffer salt of the invention is generally sufficient to produce a ph reduction at 1 hour of hydration of at least 0 . 5 . with reference to the total hydrating system , i . e . the powdery constituents together with hydration liquid , a preferred amount the acidic buffer salt is within the range of 5 - 20 % by weight , and more preferably 5 - 10 % by weight . suitable ca - aluminate and / or ca - silicate systems for implants which can be used in the present invention are described in e . g . swedish patent applications nos . : se 0200637 - 7 , se 0203223 - 3 and se 0203324 - 1 . additives which can be used are described in se 463493 , se 502987 , wo 00 / 21489 , wo 01 / 76534 , wo 01 / 76535 , wo 2004 / 037215 and wo 2003 / 041662 , the relevant contents of which are incorporated herein by reference . as disclosed in wo 2005 / 039508 , in order to improve the controllability of the composition &# 39 ; s initial viscosity and consistency , and early - age properties ( initial strength , pore closure , translucency and early obtained bioactivity ), a polycarboxylic acid or a copolymer or a salt or an ester thereof having a molecular weight of 100 - 250 , 000 , preferably 1000 - 100 , 000 could be added to the powder . the amount of the acid or the copolymer , salt or ester thereof must however be selected so as not interfere with the buffering action of the salt , or compromise the desired mechanical properties of the hardened material . a suitable amount is believed to be within the range of 1 - 15 % and preferably 2 - 5 % by weight , based on the powdered material including any dry additives . ca - aluminate ( ca ) and ca - silicate ( c 3 s ) were synthesized at 1410 and 1380 ° c ., respectively , in a conventional sintering furnace for 6 h . the materials were crushed and thereafter milled for 72 h in a rotating mill using ceramics containers and with the milling media of silicon nitride balls ( 15 mm in diameter ) and iso - propanol as liquid . after thin film evaporation the powder was pressed to small pellets . the pellet dimensions were : length , 1 = 6 mm ; and diameter , d = 4 mm . the compact density was approximately 59 %. in tables 1 and 2 ph development for a pure ca - aluminate and a pure ca - silicate phase ( 0 - 30 days with exchange of liquid after each test time ), respectively , are shown . in table 3 ph development for the systems in examples ( tables 1 and 2 ) with different amounts of three different internal buffer systems added ( khf 2 , nahco 3 and mghpo 4 and nah 2 po 4 ) according to table 3 below , are presented . the amount of added internal buffer was a = 5 , b = 10 and c = 20 wt -% of the weight of the hydrating system used .
the present invention deals with the initial hydration reaction of highly alkaline chemically bonded ceramic systems such as ca - aluminate and ca - silicate , exhibiting a controlled ph development , reduced from very high levels to be in a ph range of 7 - 9 by the use of an internal buffer system added to the cbc type biomaterial used . the invention is especially intended for endodontic , orthopaedic applications and / or soft tissue applications and / or drug delivery carrier applications .