Choosing the Best Historic Masonry Contractor
Choosing the right masonry contractor in Lancaster, Pa. has gotten a whole lot easier if you have an historic home built before 1900. Brookline has all the historic trades in-house to completely restore your historic home. Brookline is a masonry contractor in Lancaster, PA. that has skilled masons fully trained in the use of historically-compatible mortars, including locally-made lime mortar.
Lime mortar is not a simple matter. Lime was the only binder for mortars 120 years ago, and is superior to Portland cement in every way except one: speed of hardening. Portland cement was invented and adopted for one reason only, and that is that masons can lay more wall in a day’s time because it sets up quickly.A very important question that must be asked when interviewing potential masonry contractors for your historic building is “what kind of mortar are you going to use”, then no matter the answer, follow up with a second question “why”? If the answer is “because this is the mortar we always use”, or “this is what is sold at the local masonry supply house”, then it is time to move along and find another mason. The correct answer to these questions are as follows, “We design the mortar for your historic building”.
Speed is an advantage. Being able to go fast is superior. But in every other way, Portland cement is inferior to a lime-sand mortar. Portland seals water into a wall, deteriorating the masonry and other building components. Portland destroys historic homes fast.
Having a masonry contractor who knows this is priceless. So many contractors today are uneducated about lime mortar, and how important it is for historic brick and stone. Brookline uses lime putty from Lancaster Lime Works in Lancaster, PA. to make all of its mortars for restoration of historic homes. The training that Lancaster Lime Works provides can help any masonry contractor to understand the full options when it comes to mortar.
There are many ways to make mortar besides opening a bag and adding sand and water. Knowing the different binders that are available today allows a masonry contractor to make intelligent choices about the mortar that is used in a particular construction situation.
The most important factor in a mortar is breathability. Masonry contractors must have an idea of the vapor permeability of the product they are using for restoration of historic masonry. Using Portland cement to restore any historic masonry will cause damage that cannot be repaired. That is why it is so important to choose a masonry contractor that knows historic mortars.
Contact Brookline a specialized masonry contractor in Lancaster, Pa. Experts in historic masonry restoration and historic mortars.
What Makes a Great Lime for True Lime Mortars?
Higher calcium content leads to quicker carbonation and a more durable mortar. Magnesium and other impurities either don’t react or react slowly with CO2. But is high-calcium lime enough to make a good lime-only mortar? No. Food-grade calcium hydrate is pure, but does not have high surface area for CO2 reactivity. A calcium hydroxide slurry for deacidification is more likely to have high-surface area, but may not be high-calcium. Limes manufactured for Portland cement need to be “dead burned” (sintered) so the lime and Portland (hydraulic) reactions don’t compete.
A “soft-burned” lime (maintained at 900°C) has a porous “coral-like” surface that is derived from firing the lime optimally to drive off CO2 (see below). Lime for Portland-based masonry mixes continues to be fired beyond this stage until the pores close up again. This material adds plasticity to stiff and brittle Portland, but would not provide porosity or reactivity for lime-only mortar.
When a soft, self-healing, porous, and breathable surface is desired, then a high-calcium, “softburned” lime is ideal. Only a purpose-made architectural lime putty will have high surface area (greater than 30 m2/g) and porosity that comes from “soft burning” and the chemical purity of a consistently high calcium content. Together these characteristics ensure all of the lime is acting as a binder (cementing the aggregate particles). Architectural lime putty makes great mortar without Portland cement or pozzolans. And only lime mortars are consistently compatible with historic lime-only mortars that rarely exceed 200 p.s.i. and allow full water and vapor movement to the exterior of a wall. Remember, the role of mortar is to cushion the masonry units and accommodate movement while eliminating point-loading. Lime mortars meet this requirement fully.
The lime cycle is a closed loop. Calcium Carbonate [CaCO3, i.e. limestone, oyster shell] is fired to drive off the CO2, making Calcium Oxide [CaO, quicklime]. When water is introduced during the volatile process called slaking, calcium hydroxide is formed [Ca(OH)2]. Lime putty is calcium hydroxide. As water leaves the lime, carbon dioxide from the air begins to take its place, once again reacting with the lime to create Calcium Carbonate again. There is an increase in mass associated with carbonation that counteracts shrinkage from water loss.What if we slake calcium oxide ourselves? Good quality control in manufacturing Sample A produced an optimum lime putty. The same oxide slaked by hand without precise control of water feed rate and temperature created the inferior putties, C and D. Reactivity is created through careful control in firing and slaking. Small particles and high surface area create longer decantation times (the time necessary for a particle to settle out). Viscosity is related to surface area and plasticity. Greater plasticity and water-holding capacity mean better working properties and contribute to low shrinkage from water loss.
True lime mortars set (carbonate or “cure”) by reacting with atmospheric CO2, creating a sort of man-made limestone. Hydraulic mortars (Portland cement, bagged mortar mixes, and hydraulic limes and cements) set by reacting with water. The firing of all mortar constituents drives off CO2, but the reabsorption of this CO2 by lime mortars makes it a more environmentally-responsible building choice. Lime mortars are also less fuel intensive to produce than Portland-based mortars.
Blog post attributed to www.preservationscience.com