ca concentration mg 2 concentration na sar concentration ca concentration mg 2 concentration na sar 2 as explained below the only other cat ion present in the pore waters of quartz sand with additive of sodium silicate glass will be ca footnote 3 i e mg equal or close to 0 the allowable concentration of ca to retain a sar value of 12 in the pore waters is indicated in the last column to the right in table 2 3 stemming from the cement paste of the concrete block the advantage of using sodium silicate as an additive is apart from a favourable sar value also a persistently high ph which in itself will help to prevent plants from getting established actual ph measurements of joint filling sands according to the invention see table 3 below after instalment into a pavement consisting of 10 cm by 20 cm concrete paving stones showed a consistent ph value of about 10 9 a relative high ph of 10 36 was found in quartz joint filling sand without additive when recently installed the high ph is being ascribed to the influence of calcium ions from the cement paste of the concrete paving stones this effect is reduced as carbonisation forms a skin on the surface of the concrete paving stones in northern europe the annual rainfall is about 80 cm which means that each square meter receives 800 litres per year infiltration into the ground from a paved area has been measured to be about 40 when surface run off and evaporation is.