Cracks are never uniform, and the width of the crack or separation plane between materials may vary, either as a result of the shape of the fractured interfaces or from particle accumulation within the void. The void may also have become contaminated with debris carried into the gap under the action of on going movement e.g.
Thermal expansion and contraction, particularly in wet or damp conditions,
Flexing of walls such as wind loads, both positive and negative,
Vibration e.g. church bells, traffic vibration,
Settlement, from ground heave, foundation settlement etc.
Water carrying dust or fragments into open spaces and voids
Jacking between interfaces can occur as a result of ice or salt crystallization
Free flowing gravity grout through a Syringe inserted into the target point or through multiple points, with minimal pressure, to avoid physical stress to already separated layers of materials is the most common method to secure plasters and renders that may have historical importance. In some instances, grouting to secure rather than repair may be the valid option say on a through coloured finish that would be difficult if not impossible to match in over a large area and will in its self be a financially viable alternative to repair or complete replacement.
Within a unitary masonry element such as a piece of fractured natural stone, the grout is normally applied under pressure (1Bar and higher) such that the grout fully penetrates the entire depth and volume of the crack. These types of grouts require the particle sizes generally less than 3 microns, often described as Nano Grouts5
5They are actually very finely dispersed lime particles with additions to make them perform in specific ways. A micron is 1/1,000 of a millimeter and a Nanometer is 1/1,000 of a Micron so in practice ultra fine particle grouts are mostly in the range of what would be at best classed as coarse particles in the grouping of nanoparticles – but they do not really meet the criteria of measurement to be called true Nano particles. None-the-less our super fine grouts are on average less than 3 microns or 3/1,000 of a millimeter.